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<front> <front>
<title abbrev="SDP UKS">Unknown Key Share Attacks on uses of TLS with the Se <title abbrev="SDP UKS">Unknown Key-Share Attacks on Uses of TLS with the Se
ssion Description Protocol (SDP)</title> ssion Description Protocol (SDP)</title>
<seriesInfo name="RFC" value="8844"/>
<author initials="M." surname="Thomson" fullname="Martin Thomson"> <author initials="M." surname="Thomson" fullname="Martin Thomson">
<organization>Mozilla</organization> <organization>Mozilla</organization>
<address> <address>
<email>mt@lowentropy.net</email> <email>mt@lowentropy.net</email>
</address> </address>
</author> </author>
<author initials="E." surname="Rescorla" fullname="Eric Rescorla"> <author initials="E." surname="Rescorla" fullname="Eric Rescorla">
<organization>Mozilla</organization> <organization>Mozilla</organization>
<address> <address>
<email>ekr@rftm.com</email> <email>ekr@rtfm.com</email>
</address> </address>
</author> </author>
<date month="May" year="2020"/>
<date year="2019" month="August" day="09"/> <keyword>Unknown Key-Share Attack</keyword>
<keyword>SDP</keyword>
<keyword>DTLS-SRTP</keyword>
<keyword>WebRTC</keyword>
<keyword>SIP identity</keyword>
<abstract> <abstract>
<t>This document describes unknown key-share attacks on the use of Datagram <t>This document describes unknown key-share attacks on the use of
Transport Layer Security for the Secure Real-Time Transport Protocol Datagram Transport Layer Security for the Secure Real-Time Transport
(DTLS-SRTP). Similar attacks are described on the use of DTLS-SRTP with the Protocol (DTLS-SRTP). Similar attacks are described on the use of
identity bindings used in Web Real-Time Communications (WebRTC) and SIP DTLS-SRTP with the identity bindings used in Web Real-Time
identity. These attacks are difficult to mount, but they cause a victim to be Communications (WebRTC) and SIP identity. These attacks are difficult
mislead about the identity of a communicating peer. Mitigation techniques are to mount, but they cause a victim to be misled about the identity of a
defined that implementations of RFC 8122 are encouraged to deploy.</t> communicating peer. This document defines mitigation techniques that
implementations of RFC 8122 are encouraged to deploy.</t>
</abstract> </abstract>
</front> </front>
<middle> <middle>
<section anchor="introduction" numbered="true" toc="default">
<section anchor="introduction" title="Introduction"> <name>Introduction</name>
<t>The use of Transport Layer Security (TLS) <xref target="RFC8446"
<t>The use of Transport Layer Security (TLS) <xref target="TLS13"/> with the Ses format="default"/> with the Session Description Protocol (SDP) <xref
sion target="RFC4566" format="default"/> is defined in <xref target="RFC8122"
Description Protocol (SDP) <xref target="SDP"/> is defined in format="default"/>. Further use with Datagram Transport Layer Security
<xref target="FINGERPRINT"/>. Further use with Datagram Transport Layer Securit (DTLS) <xref target="RFC6347" format="default"/> and the Secure
y Real-time Transport Protocol (SRTP) <xref target="RFC3711"
(DTLS) <xref target="DTLS"/> and the Secure Real-time Transport Protocol (SRTP) format="default"/> is defined as DTLS-SRTP <xref target="RFC5763"
<xref target="SRTP"/> is defined as DTLS-SRTP <xref target="DTLS-SRTP"/>.</t> format="default"/>.</t>
<t>In these specifications, key agreement is performed using TLS or DTLS,
<t>In these specifications, key agreement is performed using TLS or DTLS, with with
authentication being tied back to the session description (or SDP) through the authentication being linked to the session description (or SDP) through the
use of certificate fingerprints. Communication peers check that a hash, or use of certificate fingerprints. Communication peers check that a hash, or
fingerprint, provided in the SDP matches the certificate that is used in the TLS fingerprint, provided in the SDP matches the certificate that is used in the TLS
or DTLS handshake.</t> or DTLS handshake.</t>
<t>WebRTC identity (see Section 7 of <xref target="WEBRTC-SEC"/>) <t>WebRTC identity (see <xref target="RFC8827" sectionFormat="of" section=
and SIP identity <xref target="SIP-ID"/> both provide a mechanism that binds an "7" format="default"/>)
and SIP identity <xref target="RFC8224" format="default"/> both provide a mechan
ism that binds an
external identity to the certificate fingerprints from a session description. external identity to the certificate fingerprints from a session description.
However, this binding is not integrity-protected and therefore vulnerable to an However, this binding is not integrity protected and is therefore vulnerable to
identity misbinding attack - or unknown key-share (UKS) attack - where the an
identity misbinding attack, also known as an unknown key-share (UKS) attack, whe
re the
attacker binds their identity to the fingerprint of another entity. A attacker binds their identity to the fingerprint of another entity. A
successful attack leads to the creation of sessions where peers are confused successful attack leads to the creation of sessions where peers are confused
about the identity of the participants.</t> about the identity of the participants.</t>
<t>This document describes a TLS extension that can be used in combination
<t>This document describes a TLS extension that can be used in combination with with
these identity bindings to prevent this attack.</t> these identity bindings to prevent this attack.</t>
<t>A similar attack is possible with the use of certificate fingerprints alone. <t>A similar attack is possible with the use of certificate fingerprints a
Though attacks in this setting are likely infeasible in existing deployments due lone.
to the narrow conditions necessary (see <xref target="limits"/>), this document Though attacks in this setting are likely infeasible in existing
also deployments due to the narrow preconditions
(see <xref target="limits" format="default"/>), this document also
describes mitigations for this attack.</t> describes mitigations for this attack.</t>
<t>The mechanisms defined in this document are intended to strengthen the
<t>The mechanisms defined in this document are intended to strengthen the protoc protocol
ol by preventing the use of unknown key-share attacks in combination with other pro
by preventing the use of unknown key shares in combination with other protocol tocol
or implementation vulnerabilities. RFC 8122 <xref target="FINGERPRINT"/> is upd or implementation vulnerabilities. RFC 8122 <xref target="RFC8122" format="defa
ated by this ult"/> is updated by this
document to recommend the use of these mechanisms.</t> document to recommend the use of these mechanisms.</t>
<t>This document assumes that signaling is integrity protected. However,
<t>This document assumes that signaling is integrity protected. However, as as
Section 7 of <xref target="FINGERPRINT"/> explains, many deployments that use SD <xref target="RFC8122" sectionFormat="of" section="7" format="default"/> explain
P do not s, many deployments that use SDP do not
guarantee integrity of session signaling and so are vulnerable to other attacks. guarantee integrity of session signaling and so are vulnerable to other attacks.
<xref target="FINGERPRINT"/> offers key continuity mechanisms as a potential mea ns of <xref target="RFC8122" format="default"/> offers key continuity mechanisms as a potential means of
reducing exposure to attack in the absence of integrity protection. reducing exposure to attack in the absence of integrity protection.
<xref target="continuity"/> provides some analysis of the effect of key continui ty in <xref target="continuity" format="default"/> provides some analysis of the effec t of key continuity in
relation to the described attacks.</t> relation to the described attacks.</t>
<t>
<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQU
“SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this IRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
document are to be interpreted as described in BCP 14 <xref target="RFC2119"/> < NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>
xref target="RFC8174"/> RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
when, and only when, they appear in all capitals, as shown here.</t> "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
be interpreted as
</section> described in BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
<section anchor="uks" title="Unknown Key-Share Attack"> when, and only when, they appear in all capitals, as shown here.
</t>
<t>In an unknown key-share attack <xref target="UKS"/>, a malicious participant </section>
in a protocol <section anchor="uks" numbered="true" toc="default">
<name>Unknown Key-Share Attack</name>
<t>In an unknown key-share attack <xref target="UKS" format="default"/>, a
malicious participant in a protocol
claims to control a key that is in reality controlled by some other actor. This claims to control a key that is in reality controlled by some other actor. This
arises when the identity associated with a key is not properly bound to the key. </t> arises when the identity associated with a key is not properly bound to the key. </t>
<t>An endpoint that can acquire the certificate fingerprint of another ent
<t>An endpoint that can acquire the certificate fingerprint of another entity ca ity can
n advertise that fingerprint as their own in SDP.
advertise that fingerprint as their own in SDP. An attacker can use a copy of An attacker can use a copy of that fingerprint to cause a victim to
that fingerprint to cause a victim to communicate with another unaware victim, communicate with another unaware victim, even though the first victim believe
even though it believes that it is communicating with the attacker.</t> s
that it is communicating with the attacker.
<t>When the identity of communicating peers is established by higher-layer </t>
signaling constructs, such as those in SIP identity <xref target="SIP-ID"/> or W <t>When the identity of communicating peers is established by higher-layer
ebRTC signaling constructs, such as those in SIP identity <xref target="RFC8224" forma
<xref target="WEBRTC-SEC"/>, this allows an attacker to bind their own identity t="default"/> or WebRTC
to a session <xref target="RFC8827" format="default"/>, this allows an attacker to bind their
own identity to a session
with any other entity.</t> with any other entity.</t>
<t>The attacker obtains an identity assertion for an identity it controls,
<t>The attacker obtains an identity assertion for an identity it controls, but but
binds that to the fingerprint of one peer. The attacker is then able to cause a binds that to the fingerprint of one peer. The attacker is then able to cause a
TLS connection to be established where two victim endpoints communicate. The TLS connection to be established where two victim endpoints communicate. The
victim that has its fingerprint copied by the attack correctly believes that it victim that has its fingerprint copied by the attack correctly believes that it
is communicating with the other victim; however, the other victim incorrectly is communicating with the other victim; however, the other victim incorrectly
believes that it is communicating with the attacker.</t> believes that it is communicating with the attacker.</t>
<t>An unknown key-share attack does not result in the attacker having acce
<t>An unknown key-share attack does not result in the attacker having access to ss to any
any
confidential information exchanged between victims. However, the failure in confidential information exchanged between victims. However, the failure in
mutual authentication can enable other attacks. A victim might send information mutual authentication can enable other attacks. A victim might send information
to the wrong entity as a result. Where information is interpreted in context, to the wrong entity as a result. Where information is interpreted in context,
misrepresenting that context could lead to the information being misinterpreted. </t> misrepresenting that context could lead to the information being misinterpreted. </t>
<t>A similar attack can be mounted based solely on the SDP <tt>fingerprint
<t>A similar attack can be mounted based solely on the SDP <spanx style="verb">f </tt> attribute
ingerprint</spanx> attribute <xref target="RFC8122" format="default"/> without compromising the integrity of
<xref target="FINGERPRINT"/> without compromising the integrity of the signaling the signaling channel.</t>
channel.</t> <t>This attack is an aspect of SDP-based protocols upon which the techniqu
e known as
<t>This attack is an aspect of SDP-based protocols that the technique known as third-party call control (3PCC) <xref target="RFC3725" format="default"/> relies
third-party call control (3PCC) <xref target="RFC3725"/> relies on. 3PCC exploi . 3PCC exploits the
ts the
potential for the identity of a signaling peer to be different than the media potential for the identity of a signaling peer to be different than the media
peer, allowing the media peer to be selected by the signaling peer. peer, allowing the media peer to be selected by the signaling peer.
<xref target="byebye-3pcc"/> describes the consequences of the mitigations descr ibed here for <xref target="byebye-3pcc" format="default"/> describes the consequences of the mitigations described here for
systems that use 3PCC.</t> systems that use 3PCC.</t>
<section anchor="limits" numbered="true" toc="default">
<section anchor="limits" title="Limits on Attack Feasibility"> <name>Limits on Attack Feasibility</name>
<t>The use of TLS with SDP depends on the integrity of session signaling
<t>The use of TLS with SDP depends on the integrity of session signaling. Assum . Assuming
ing
signaling integrity limits the capabilities of an attacker in several ways. In signaling integrity limits the capabilities of an attacker in several ways. In
particular:</t> particular:</t>
<ol spacing="normal" type="1">
<t><list style="numbers"> <li>An attacker can only modify the parts of the session signaling tha
<t>An attacker can only modify the parts of the session signaling that they ar t they are
e responsible for producing, namely their own offers and answers.</li>
responsible for producing, namely their own offers and answers.</t> <li>No entity will successfully establish a session with a peer unless
<t>No entity will successfully establish a session with a peer unless they are they are
willing to participate in a session with that peer.</t> willing to participate in a session with that peer.</li>
</list></t> </ol>
<t>The combination of these two constraints make the spectrum of possibl
<t>The combination of these two constraints make the spectrum of possible attack e attacks
s
quite limited. An attacker is only able to switch its own certificate quite limited. An attacker is only able to switch its own certificate
fingerprint for a valid certificate that is acceptable to its peer. Attacks fingerprint for a valid certificate that is acceptable to its peer. Attacks
therefore rely on joining two separate sessions into a single session. <xref tar get="fp"/> therefore rely on joining two separate sessions into a single session. <xref tar get="fp" format="default"/>
describes an attack on SDP signaling under these constraints.</t> describes an attack on SDP signaling under these constraints.</t>
<t>Systems that rely on strong identity bindings, such as those defined
<t>Systems that rely on strong identity bindings, such as those defined in in
<xref target="WEBRTC"/> or <xref target="SIP-ID"/>, have a different threat mode <xref target="WEBRTC" format="default"/> or <xref target="RFC8224" format="defau
l, which admits the lt"/>, have a different threat model, which admits the
possibility of attack by an entity with access to the signaling channel. possibility of attack by an entity with access to the signaling channel.
Attacks under these conditions are more feasible as an attacker is assumed to be Attacks under these conditions are more feasible as an attacker is assumed to be
able to both observe and modify signaling messages. <xref target="id"/> describ es an attack able to observe and to modify signaling messages. <xref target="id" format="def ault"/> describes an attack
that assumes this threat model.</t> that assumes this threat model.</t>
</section>
</section> <section anchor="continuity" numbered="true" toc="default">
<section anchor="continuity" title="Interactions with Key Continuity"> <name>Interactions with Key Continuity</name>
<t>Systems that use key continuity (as defined in
<t>Systems that use key continuity (as defined in Section 15.1 of <xref target=" <xref target="RFC6189" sectionFormat="of" section="15.1" format="default"/>
ZRTP"/> or as recommended in <xref target="RFC8122" sectionFormat="of" section="7" forma
or as recommended in Section 7 of <xref target="FINGERPRINT"/>) might be able to t="default"/>) might be able to detect an
detect an
unknown key-share attack if a session with either the attacker or the genuine unknown key-share attack if a session with either the attacker or the genuine
peer (i.e., the victim whose fingerprint was copied by an attacker) was peer (i.e., the victim whose fingerprint was copied by an attacker) was
established in the past. Whether this is possible depends on how key continuity established in the past. Whether this is possible depends on how key continuity
is implemented.</t> is implemented.</t>
<t>Implementations that maintain a single database of identities with an
<t>Implementations that maintain a single database of identities with an index o index of
n
peer keys could discover that the identity saved for the peer key does not match peer keys could discover that the identity saved for the peer key does not match
the claimed identity. Such an implementation could notice the disparity between the claimed identity. Such an implementation could notice the disparity between
the actual keys (those copied from a victim) and the expected keys (those of the the actual keys (those copied from a victim) and the expected keys (those of the
attacker).</t> attacker).</t>
<t>In comparison, implementations that first match based on peer identit
<t>In comparison, implementations that first match based on peer identity could y could
treat an unknown key-share attack as though their peer had used a treat an unknown key-share attack as though their peer had used a
newly-configured device. The apparent addition of a new device could generate newly configured device. The apparent addition of a new device could generate
user-visible notices (e.g., “Mallory appears to have a new device”). However, user-visible notices (e.g., "Mallory appears to have a new device"). However,
such an event is not always considered alarming; some implementations might such an event is not always considered alarming; some implementations might
silently save a new key.</t> silently save a new key.</t>
</section>
<section anchor="byebye-3pcc" numbered="true" toc="default">
<name>Third-Party Call Control</name>
</section> <t>Third-party call control (3PCC) <xref target="RFC3725" format="defaul
<section anchor="byebye-3pcc" title="Third-Party Call Control"> t"/> is a technique where a signaling
<t>Third-party call control (3PCC) <xref target="RFC3725"/> is a technique where
a signaling
peer establishes a call that is terminated by a different entity. An unknown peer establishes a call that is terminated by a different entity. An unknown
key-share attack is very similar in effect to some 3PCC practices, so use of key-share attack is very similar in effect to some 3PCC practices, so use of
3PCC could appear to be an attack. However, 3PCC that follows RFC 3725 guidance 3PCC could appear to be an attack. However, 3PCC that follows RFC 3725 guidance
is unaffected, and peers that are aware of changes made by a 3PCC controller can is unaffected, and peers that are aware of changes made by a 3PCC controller can
correctly distinguish actions of a 3PCC controller from attack.</t> correctly distinguish actions of a 3PCC controller from an attack.</t>
<t>3PCC as described in RFC 3725 is incompatible with SIP identity <xref
<t>3PCC as described in RFC 3725 is incompatible with SIP identity <xref target= target="RFC8224" format="default"/>, as
"SIP-ID"/> as
SIP Identity relies on creating a binding between SIP requests and SDP. The SIP Identity relies on creating a binding between SIP requests and SDP. The
controller is the only entity that generates SIP requests in RFC 3725. controller is the only entity that generates SIP requests in RFC 3725.
Therefore, in a 3PCC context, only the use of the <spanx style="verb">fingerprin Therefore, in a 3PCC context, only the use of the <tt>fingerprint</tt> attribute
t</spanx> attribute without additional bindings or WebRTC identity <xref target="RFC8827" format="de
without additional bindings or WebRTC identity <xref target="WEBRTC-SEC"/> is po fault"/> is possible.</t>
ssible.</t> <t>The attack mitigation mechanisms described in this document will prev
ent the use
<t>The attack mitigation mechanisms described in this document will prevent the of 3PCC if peers have different views of the involved identities or the value
use of SDP <tt>tls-id</tt> attributes.</t>
of 3PCC if peers have different views of the involved identities, or the value <t>For 3PCC to work with the proposed mechanisms, TLS peers need to be a
of SDP <spanx style="verb">tls-id</spanx> attributes.</t> ware of the
<t>For 3PCC to work with the proposed mechanisms, TLS peers need to be aware of
the
signaling so that they can correctly generate and check the TLS extensions. For signaling so that they can correctly generate and check the TLS extensions. For
a connection to be successfully established, a 3PCC controller needs to either a connection to be successfully established, a 3PCC controller needs either to
forward SDP without modification, or to avoid modifications to <spanx style="ver forward SDP without modification or to avoid modifications to <tt>fingerprint</t
b">fingerprint</spanx>, t>,
<spanx style="verb">tls-id</spanx>, and <spanx style="verb">identity</spanx> att <tt>tls-id</tt>, and <tt>identity</tt> attributes. A controller that follows th
ributes. A controller that follows the best e best
practices in RFC 3725 is expected to forward SDP without modification, thus practices in RFC 3725 is expected to forward SDP without modification, thus
ensuring the integrity of these attributes.</t> ensuring the integrity of these attributes.</t>
</section>
</section> </section>
</section> <section anchor="id" numbered="true" toc="default">
<section anchor="id" title="Unknown Key-Share with Identity Bindings"> <name>Unknown Key-Share Attack with Identity Bindings</name>
<t>The identity assertions used for WebRTC
<t>The identity assertions used for WebRTC (Section 7 of <xref target="WEBRTC-SE (<xref target="RFC8827" sectionFormat="of" section="7" format="default"/>) and t
C"/>) and the he
SIP PASSPoRT used in SIP identity (<xref target="SIP-ID"/>, <xref target="PASSPo Personal Assertion Token (PASSporT) used in SIP identity (<xref target="RFC8224"
RT"/>) are bound format="default"/>, <xref target="RFC8225" format="default"/>) are bound
to the certificate fingerprint of an endpoint. An attacker can cause an identit y to the certificate fingerprint of an endpoint. An attacker can cause an identit y
binding to be created that binds an identity they control to the fingerprint of binding to be created that binds an identity they control to the fingerprint of
a first victim.</t> a first victim.</t>
<t>An attacker can thereby cause a second victim to believe that they are
<t>An attacker can thereby cause a second victim to believe that they are
communicating with an attacker-controlled identity, when they are really talking communicating with an attacker-controlled identity, when they are really talking
to the first victim. The attacker does this by creating an identity assertion to the first victim. The attacker does this by creating an identity assertion
that covers a certificate fingerprint of the first victim.</t> that covers a certificate fingerprint of the first victim.</t>
<t>A variation on the same technique can be used to cause both victims to
<t>A variation on the same technique can be used to cause both victims to both
believe they are talking to the attacker when they are talking to each other. believe they are talking to the attacker when they are talking to each other.
In this case, the attacker performs the identity misbinding once for each In this case, the attacker performs the identity misbinding once for each
victim.</t> victim.</t>
<t>The problem might appear to be caused by the fact that the authority that <t>The authority certifying the identity binding is not required to verify
certifies the identity binding is not required to verify that the entity that the entity
requesting the binding controls the keys associated with the fingerprints. requesting the binding actually controls the keys associated with the fingerprin
ts,
and this might appear to be the cause of the problem.
SIP and WebRTC identity providers are not required to perform this SIP and WebRTC identity providers are not required to perform this
validation. However, validation of keys by the identity provider is not validation. However, validation of keys by the identity provider is not
relevant because verifying control of the associated keys is not a necessary relevant because verifying control of the associated keys is not a necessary
condition for a secure protocol, nor would it be sufficient to prevent attack condition for a secure protocol, nor would it be sufficient to prevent attack
<xref target="SIGMA"/>.</t> <xref target="SIGMA" format="default"/>.</t>
<t>A simple solution to this problem is suggested by <xref target="SIGMA"
<t>A simple solution to this problem is suggested by <xref target="SIGMA"/>. Th format="default"/>. The identity of
e identity of
endpoints is included under a message authentication code (MAC) during the endpoints is included under a message authentication code (MAC) during the
cryptographic handshake. Endpoints then validate that their peer has provided cryptographic handshake. Endpoints then validate that their peer has provided
an identity that matches their expectations. In TLS, the Finished message an identity that matches their expectations. In TLS, the Finished message
provides a MAC over the entire handshake, so that including the identity in a provides a MAC over the entire handshake, so that including the identity in a
TLS extension is sufficient to implement this solution.</t> TLS extension is sufficient to implement this solution.</t>
<t>Rather than include a complete identity binding, which could be
<t>Rather than include a complete identity binding - which could be sizable, a collision- and preimage-resistant hash of the binding is included
sizeable - a collision- and pre-image-resistant hash of the binding is included in a TLS extension as described in <xref target="external_id_hash" format="defau
in a TLS extension as described in <xref target="external_id_hash"/>. Endpoints lt"/>. Endpoints then need
then need
only validate that the extension contains a hash of the identity binding they only validate that the extension contains a hash of the identity binding they
received in signaling. If the identity binding is successfully validated, the received in signaling. If the identity binding is successfully validated, the
identity of a peer is verified and bound to the session.</t> identity of a peer is verified and bound to the session.</t>
<t>This form of unknown key-share attack is possible without compromising
<t>This form of unknown key-share attack is possible without compromising signal signaling
ing integrity, unless the defenses described in <xref target="fp" format="default"/>
integrity, unless the defenses described in <xref target="fp"/> are used. In or are used. In order to
der to prevent both forms of attack, endpoints <bcp14>MUST</bcp14> use the <tt>external
prevent both forms of attack, endpoints MUST use the <spanx style="verb">externa _session_id</tt>
l_session_id</spanx> extension (see <xref target="external_session_id" format="default"/>) in additio
extension (see <xref target="external_session_id"/>) in addition to the <spanx s n to the <tt>external_id_hash</tt>
tyle="verb">external_id_hash</spanx> (<xref target="external_id_hash" format="default"/>) so that two calls between t
(<xref target="external_id_hash"/>) so that two calls between the same parties c he same parties can't be
an’t be
altered by an attacker.</t> altered by an attacker.</t>
<section anchor="id-example" numbered="true" toc="default">
<section anchor="id-example" title="Example"> <name>Example</name>
<t>In the example shown in <xref target="identity-attack" format="defaul
<t>In the example shown in <xref target="identity-attack"/>, it is assumed that t"/>, it is assumed that the attacker
the attacker
also controls the signaling channel.</t> also controls the signaling channel.</t>
<t>Mallory (the attacker) presents two victims, Norma and Patsy, with tw
<t>Mallory (the attacker) presents two victims, Norma and Patsy, with two separa o separate
te
sessions. In the first session, Norma is presented with the option to sessions. In the first session, Norma is presented with the option to
communicate with Mallory; a second session with Norma is presented to Patsy.</t> communicate with Mallory; a second session with Norma is presented to Patsy.</t>
<figure anchor="identity-attack">
<figure title="Example Attack on Identity Bindings" anchor="identity-attack"><ar <name>Example Attack on Identity Bindings</name>
twork><![CDATA[ <artwork align="left" alt=""><![CDATA[
Norma Mallory Patsy Norma Mallory Patsy
(fp=N) ----- (fp=P) (fp=N) ----- (fp=P)
| | | | | |
|<---- Signaling1 ------>| | |<---- Signaling1 ------>| |
| Norma=N Mallory=P | | | Norma=N Mallory=P | |
| |<---- Signaling2 ------>| | |<---- Signaling2 ------>|
| | Norma=N Patsy=P | | | Norma=N Patsy=P |
| | | |
|<=================DTLS (fp=N,P)=================>| |<=================DTLS (fp=N,P)=================>|
| | | |
(peer = Mallory!) (peer = Norma) (peer = Mallory!) (peer = Norma)
]]></artwork></figure> ]]></artwork>
</figure>
<t>The attack requires that Mallory obtain an identity binding for her own ident <t>The attack requires that Mallory obtain an identity binding for her o
ity wn identity
with the fingerprints presented by Patsy (P), which Mallory might have obtained with the fingerprints presented by Patsy (P), which Mallory might have obtained
previously. This false binding is then presented to Norma (Signaling1 in the previously. This false binding is then presented to Norma ('Signaling1' in
figure).</t> <xref target="identity-attack" format="default"/>).</t>
<t>Patsy could be similarly duped, but in this example, a correct bindin
<t>Patsy could be similarly duped, but in this example, a correct binding betwee g between
n Norma's identity and fingerprint (N) is faithfully presented by Mallory. This
Norma’s identity and fingerprint (N) is faithfully presented by Mallory. This session ('Signaling2' in <xref target="identity-attack" format="default"/>) can
session (Signaling2 in the figure) can be entirely legitimate.</t> be entirely legitimate.</t>
<t>A DTLS session is established directly between Norma and Patsy.
<t>A DTLS session is established directly between Norma and Patsy. In order for In order for this to happen, Mallory can substitute transport-level
this to happen Mallory can substitute transport-level information in both information in both sessions, though this is not necessary if Mallory
sessions to facilitate this, though this is not necessary if Mallory is on the is on the network path between Norma and Patsy.
network path between Norma and Patsy.</t> </t>
<t>As a result, Patsy correctly believes that she is communicating with
<t>As a result, Patsy correctly believes that she is communicating with Norma. Norma.
However, Norma incorrectly believes she is talking to Mallory. As stated in However, Norma incorrectly believes that she is talking to Mallory. As stated i
<xref target="uks"/>, Mallory cannot access media, but Norma might send informat n
ion to Patsy <xref target="uks" format="default"/>, Mallory cannot access media, but Norma mi
that is Norma might not intend or that Patsy might misinterpret.</t> ght send information to Patsy
that Norma might not intend or that Patsy might misinterpret.</t>
</section> </section>
<section anchor="external_id_hash" title="The external_id_hash TLS Extension"> <section anchor="external_id_hash" numbered="true" toc="default">
<name>The <tt>external_id_hash</tt> TLS Extension</name>
<t>The <spanx style="verb">external_id_hash</spanx> TLS extension carries a hash <t>The <tt>external_id_hash</tt> TLS extension carries a hash of the ide
of the identity assertion ntity assertion
that the endpoint sending the extension has asserted to its peer. Both peers that the endpoint sending the extension has asserted to its peer. Both peers
include a hash of their own identity assertion.</t> include a hash of their own identity assertion.</t>
<t>The <tt>extension_data</tt> for the <tt>external_id_hash</tt> extensi
on contains a
<tt>ExternalIdentityHash</tt> struct, described below using the syntax defined i
n
<xref target="RFC8446" sectionFormat="of" section="3" format="default"/>:</t>
<t>The <spanx style="verb">extension_data</spanx> for the <spanx style="verb">ex <sourcecode type="tls-presentation"><![CDATA[
ternal_id_hash</spanx> extension contains a
<spanx style="verb">ExternalIdentityHash</spanx> struct, described below using t
he syntax defined in
Section 3 of <xref target="TLS13"/>:</t>
<figure><artwork><![CDATA[
struct { struct {
opaque binding_hash<0..32>; opaque binding_hash<0..32>;
} ExternalIdentityHash; } ExternalIdentityHash;
]]></artwork></figure> ]]></sourcecode>
<t>Where an identity assertion has been asserted by a peer, this extensi
<t>Where an identity assertion has been asserted by a peer, this extension inclu on includes
des a SHA-&wj;256 hash of the assertion. An empty value is used to indicate support
a SHA-256 hash of the assertion. An empty value is used to indicate support for for
the extension.</t> the extension.</t>
<dl newline="false" spacing="normal">
<dt>Note:</dt>
<dd>
For both types of identity assertion, if SHA-&wj;256 should prove to be inadeq
uate
in the future (see <xref target="RFC7696" format="default"/>), a new TLS extensi
on
that uses a different hash function can be defined.</dd>
</dl>
<t><list style="hanging"> <t>Identity bindings might be provided by only one peer. An endpoint th
<t hangText='Note:'> at does not
For both types of identity assertion, if SHA-256 should prove to be inadequate produce an identity binding <bcp14>MUST</bcp14> generate an empty <tt>external_i
at some point in the future (see <xref target="AGILITY"/>), a new TLS extension d_hash</tt> extension
can be defined that uses a different hash function.</t> in its ClientHello or -- if a client provides the extension -- in ServerHello or
</list></t> EncryptedExtensions. An empty extension has a zero-length <tt>binding_hash</tt>
field.</t>
<t>Identity bindings might be provided by only one peer. An endpoint that does <t>A peer that receives an <tt>external_id_hash</tt> extension that does
not not match the
produce an identity binding MUST generate an empty <spanx style="verb">external_ value of the identity binding from its peer <bcp14>MUST</bcp14> immediately fail
id_hash</spanx> extension the TLS
in its ClientHello or - if a client provides the extension - in ServerHello or handshake with an <tt>illegal_parameter</tt> alert. The absence of an identity
EncryptedExtensions. An empty extension has a zero-length binding_hash field.</ binding
t>
<t>A peer that receives an <spanx style="verb">external_id_hash</spanx> extensio
n that does not match the
value of the identity binding from its peer MUST immediately fail the TLS
handshake with a illegal_parameter alert. The absence of an identity binding
does not relax this requirement; if a peer provided no identity binding, a does not relax this requirement; if a peer provided no identity binding, a
zero-length extension MUST be present to be considered valid.</t> zero-length extension <bcp14>MUST</bcp14> be present to be considered valid.</t>
<t>Implementations written prior to the definition of the extensions in
<t>Implementations written prior to the definition of the extensions in this this
document will not support this extension for some time. A peer that receives an document will not support this extension for some time. A peer that receives an
identity binding but does not receive an <spanx style="verb">external_id_hash</s panx> extension MAY accept identity binding but does not receive an <tt>external_id_hash</tt> extension <bc p14>MAY</bcp14> accept
a TLS connection rather than fail a connection where the extension is absent.</t > a TLS connection rather than fail a connection where the extension is absent.</t >
<t>Any validation performed of the <spanx style="verb">external_id_hash</spanx> <!-- [rfced] Please clarify what is doing the validation in the sentence
extension is done in addition below. Two of the tasks sound like they would be performed by a receiver
to the validation required by <xref target="FINGERPRINT"/> and any identity asse of the information (validating the external_hash_id, [FINGERPRINT]
rtion validation), while "identity assertion definition" sounds like something
definition.</t> the sender would do. Should "definition" be "verification"?
<t>An <spanx style="verb">external_id_hash</spanx> extension that is any length
other than 0 or 32 is invalid
and MUST cause the receiving endpoint to generate a fatal <spanx style="verb">de
code_error</spanx> alert.</t>
<t>In TLS 1.3, an <spanx style="verb">external_id_hash</spanx> extension sent by
a server MUST be sent in the
EncryptedExtensions message.</t>
<section anchor="calculating-externalidhash-for-webrtc-identity" title="Calculat Original:
ing external_id_hash for WebRTC Identity"> Any validation performed of the "external_id_hash" extension is done
in addition to the validation required by [FINGERPRINT] and any
identity assertion definition.
<t>A WebRTC identity assertion (Section 7 of <xref target="WEBRTC-SEC"/>) is pro Perhaps:
vided as a JSON The endpoint performs the validation of the external_id_hash extension
<xref target="JSON"/> object that is encoded into a JSON text. The JSON text is in addition to the validation required by [RFC8122] and any verification
encoded using UTF-8 <xref target="UTF8"/> as described by Section 8.1 of <xref t of the identity assertion.
arget="JSON"/>. -->
The content of the <spanx style="verb">external_id_hash</spanx> extension is pro <t>
duced by hashing the The endpoint performs the validation of the <tt>external_id_hash</tt> extensi
resulting octets with SHA-256 <xref target="SHA"/>. This produces the 32 octets on
of in addition to the validation required by <xref target="RFC8122" format="defa
the <spanx style="verb">binding_hash</spanx> parameter, which is the sole conten ult"/> and any verification
ts of the extension.</t> of the identity assertion <xref target="RFC8827" format="default"/> <xref tar
get="RFC8224" format="default"/>.
</t>
<t>The SDP <spanx style="verb">identity</spanx> attribute includes the base64 <x <t>An <tt>external_id_hash</tt> extension with a <tt>binding_hash</tt> f
ref target="BASE64"/> encoding of ield
the UTF-8 encoding of the same JSON text. The <spanx style="verb">external_id_h that is any length other than 0 or 32 is invalid
ash</spanx> extension is and <bcp14>MUST</bcp14> cause the receiving endpoint to generate a fatal <tt>dec
validated by performing base64 decoding on the value of the SDP <spanx style="ve ode_error</tt> alert.</t>
rb">identity</spanx> <t>In TLS 1.3, an <tt>external_id_hash</tt> extension sent by a server <
attribute, hashing the resulting octets using SHA-256, and comparing the results bcp14>MUST</bcp14> be sent in the
EncryptedExtensions message.</t>
<section anchor="calculating-externalidhash-for-webrtc-identity" numbere
d="true" toc="default">
<name>Calculating <tt>external_id_hash</tt> for WebRTC Identity</name>
<t>A WebRTC identity assertion
(<xref target="RFC8827" sectionFormat="of" section="7" format="default"/>) is pr
ovided as a JSON
<xref target="RFC8259" format="default"/> object that is encoded into a JSON tex
t. The JSON text is
encoded using UTF-8 <xref target="RFC3629" format="default"/> as described by
<xref target="RFC8259" sectionFormat="of" section="8.1" format="default"/>.
The content of the <tt>external_id_hash</tt> extension is produced by hashing th
e
resulting octets with SHA-&wj;256 <xref target="RFC6234" format="default"/>. Th
is produces the 32 octets of
the <tt>binding_hash</tt> parameter, which is the sole contents of the extension
.</t>
<t>The SDP <tt>identity</tt> attribute includes the base64 <xref targe
t="RFC4648" format="default"/> encoding of
the UTF-8 encoding of the same JSON text. The <tt>external_id_hash</tt> extensi
on is
validated by performing base64 decoding on the value of the SDP <tt>identity</tt
>
attribute, hashing the resulting octets using SHA-&wj;256, and comparing the res
ults
with the content of the extension. In pseudocode form, using the with the content of the extension. In pseudocode form, using the
<spanx style="verb">identity-assertion-value</spanx> field from the <spanx style <tt>identity-assertion-value</tt> field from the SDP <tt>identity</tt> attribute
="verb">identity</spanx> attribute grammar as grammar as
defined in <xref target="WEBRTC-SEC"/>:</t> defined in <xref target="RFC8827" format="default"/>:</t>
<sourcecode type="pseudocode"><![CDATA[
<t><spanx style="verb">
external_id_hash = SHA-256(b64decode(identity-assertion-value)) external_id_hash = SHA-256(b64decode(identity-assertion-value))
</spanx></t> ]]></sourcecode>
<dl newline="false" spacing="normal">
<t><list style="hanging"> <dt>Note:</dt>
<t hangText='Note:'> <dd>
The base64 of the SDP <spanx style="verb">identity</spanx> attribute is decode The base64 of the SDP <tt>identity</tt> attribute is decoded to avoid capturin
d to avoid capturing g
variations in padding. The base64-decoded identity assertion could include variations in padding. The base64-decoded identity assertion could include
leading or trailing whitespace octets. WebRTC identity assertions are not leading or trailing whitespace octets. WebRTC identity assertions are not
canonicalized; all octets are hashed.</t> canonicalized; all octets are hashed.</dd>
</list></t> </dl>
</section>
</section> <section anchor="calculating-externalidhash-for-passport" numbered="true
<section anchor="calculating-externalidhash-for-passport" title="Calculating ext " toc="default">
ernal_id_hash for PASSPoRT"> <name>Calculating external_id_hash for PASSporT</name>
<t>Where the compact form of PASSporT <xref target="RFC8225" format="d
<t>Where the compact form of PASSPoRT <xref target="PASSPoRT"/> is used, it MUST efault"/>
be expanded is used, it <bcp14>MUST</bcp14> be expanded
into the full form. The base64 encoding used in the SIP Identity (or ‘y’) into the full form. The base64 encoding used in the SIP Identity (or 'y')
header field MUST be decoded then used as input to SHA-256. This produces the header field <bcp14>MUST</bcp14> be decoded then used as input to SHA-&wj;256.
32 octet <spanx style="verb">binding_hash</spanx> value used for creating or val This produces the
idating the extension. In 32-octet <tt>binding_hash</tt> value used for creating or validating the extensi
pseudocode, using the <spanx style="verb">signed-identity-digest</spanx> field f on. In
rom the <spanx style="verb">Identity</spanx> grammar pseudocode, using the <tt>signed-identity-digest</tt> parameter from the <tt>Ide
defined <xref target="SIP-ID"/>:</t> ntity</tt> header field grammar
defined <xref target="RFC8224" format="default"/>:</t>
<t><spanx style="verb"> <sourcecode type="pseudocode"><![CDATA[
external_id_hash = SHA-256(b64decode(signed-identity-digest)) external_id_hash = SHA-256(b64decode(signed-identity-digest))
</spanx></t> ]]></sourcecode>
</section>
</section> </section>
</section> </section>
</section> <section anchor="fp" numbered="true" toc="default">
<section anchor="fp" title="Unknown Key-Share with Fingerprints"> <name>Unknown Key-Share Attack with Fingerprints</name>
<t>An attack on DTLS-SRTP is possible because the identity of peers involv
<t>An attack on DTLS-SRTP is possible because the identity of peers involved is ed is not
not
established prior to establishing the call. Endpoints use certificate established prior to establishing the call. Endpoints use certificate
fingerprints as a proxy for authentication, but as long as fingerprints are used fingerprints as a proxy for authentication, but as long as fingerprints are used
in multiple calls, they are vulnerable to attack.</t> in multiple calls, they are vulnerable to attack.</t>
<t>Even if the integrity of session signaling can be relied upon, an attac
<t>Even if the integrity of session signaling can be relied upon, an attacker mi ker might
ght
still be able to create a session where there is confusion about the still be able to create a session where there is confusion about the
communicating endpoints by substituting the fingerprint of a communicating communicating endpoints by substituting the fingerprint of a communicating
endpoint.</t> endpoint.</t>
<t>An endpoint that is configured to reuse a certificate can be attacked i
<t>An endpoint that is configured to reuse a certificate can be attacked if it i f it is
s
willing to initiate two calls at the same time, one of which is with an willing to initiate two calls at the same time, one of which is with an
attacker. The attacker can arrange for the victim to incorrectly believe that attacker. The attacker can arrange for the victim to incorrectly believe that
it is calling the attacker when it is in fact calling a second party. The it is calling the attacker when it is in fact calling a second party. The
second party correctly believes that it is talking to the victim.</t> second party correctly believes that it is talking to the victim.</t>
<t>As with the attack on identity bindings, this can be used to cause two
<t>As with the attack on identity bindings, this can be used to cause two victim victims
s
to both believe they are talking to the attacker when they are talking to each to both believe they are talking to the attacker when they are talking to each
other.</t> other.</t>
<section anchor="fp-example" numbered="true" toc="default">
<section anchor="fp-example" title="Example"> <name>Example</name>
<t>To mount this attack, two sessions need to be created with the same e
<t>To mount this attack, two sessions need to be created with the same endpoint ndpoint at
at
almost precisely the same time. One of those sessions is initiated with the almost precisely the same time. One of those sessions is initiated with the
attacker, the second session is created toward another honest endpoint. The attacker, the second session is created toward another honest endpoint. The
attacker convinces the first endpoint that their session with the attacker has attacker convinces the first endpoint that their session with the attacker has
been successfully established, but media is exchanged with the other honest been successfully established, but media is exchanged with the other honest
endpoint. The attacker permits the session with the other honest endpoint to endpoint. The attacker permits the session with the other honest endpoint to
complete only to the extent necessary to convince the other honest endpoint to complete only to the extent necessary to convince the other honest endpoint to
participate in the attacked session.</t> participate in the attacked session.</t>
<t>In addition to the constraints described in <xref target="limits" for
<t>In addition to the constraints described in <xref target="limits"/>, the atta mat="default"/>, the attacker in this
cker in this
example also needs the ability to view and drop packets between victims. example also needs the ability to view and drop packets between victims.
That is, the attacker is on-path for media.</t> That is, the attacker needs to be on path for media.</t>
<t>The attack shown in <xref target="implausible-attack" format="default
<t>The attack shown in <xref target="implausible-attack"/> depends on a somewhat "/> depends on a somewhat implausible set
implausible set
of conditions. It is intended to demonstrate what sort of attack is possible of conditions. It is intended to demonstrate what sort of attack is possible
and what conditions are necessary to exploit this weakness in the protocol.</t> and what conditions are necessary to exploit this weakness in the protocol.</t>
<figure anchor="implausible-attack">
<figure title="Example Attack Scenario using Fingerprints" anchor="implausible-a <name>Example Attack Scenario Using Fingerprints</name>
ttack"><artwork><![CDATA[ <artwork align="left" alt=""><![CDATA[
Norma Mallory Patsy Norma Mallory Patsy
(fp=N) ----- (fp=P) (fp=N) ----- (fp=P)
| | | | | |
+---Signaling1 (fp=N)--->| | +---Signaling1 (fp=N)--->| |
+-----Signaling2 (fp=N)------------------------>| +-----Signaling2 (fp=N)------------------------>|
|<-------------------------Signaling2 (fp=P)----+ |<-------------------------Signaling2 (fp=P)----+
|<---Signaling1 (fp=P)---+ | |<---Signaling1 (fp=P)---+ |
| | | | | |
|=======DTLS1=======>(Forward)======DTLS1======>| |=======DTLS1=======>(Forward)======DTLS1======>|
|<======DTLS2========(Forward)<=====DTLS2=======| |<======DTLS2========(Forward)<=====DTLS2=======|
|=======Media1======>(Forward)======Media1=====>| |=======Media1======>(Forward)======Media1=====>|
|<======Media2=======(Forward)<=====Media2======| |<======Media2=======(Forward)<=====Media2======|
| | | | | |
|=======DTLS2========>(Drop) | |=======DTLS2========>(Drop) |
| | | | | |
]]></artwork></figure> ]]></artwork>
</figure>
<t>In this scenario, there are two sessions initiated at the same time by Norma. <t>In this scenario, there are two sessions initiated at the same time b
Signaling is shown with single lines (‘-‘), DTLS and media with double lines y Norma.
(‘=’).</t> Signaling is shown with single lines ('-'), DTLS and media with double lines
('=').</t>
<t>The first session is established with Mallory, who falsely uses Patsy’s <t>The first session is established with Mallory, who falsely uses Patsy
certificate fingerprint (denoted with ‘fp=P’). A second session is initiated 's
certificate fingerprint (denoted with 'fp=P'). A second session is initiated
between Norma and Patsy. Signaling for both sessions is permitted to complete.< /t> between Norma and Patsy. Signaling for both sessions is permitted to complete.< /t>
<t>Once signaling is complete on the first session, a DTLS connection is
<t>Once signaling is complete on the first session, a DTLS connection is established. Ostensibly, this connection is between Mallory and Norma, but
established. Ostensibly, this connection is between Mallory and Norma but
Mallory forwards DTLS and media packets sent to her by Norma to Patsy. These Mallory forwards DTLS and media packets sent to her by Norma to Patsy. These
packets are denoted ‘DTLS1’ because Norma associates these with the first packets are denoted 'DTLS1' because Norma associates these with the first
signaling session (‘signaling1’).</t> signaling session ('Signaling1').</t>
<t>Mallory also intercepts packets from Patsy and forwards those to Norm
<t>Mallory also intercepts packets from Patsy and forwards those to Norma at the a at the
transport address that Norma associates with Mallory. These packets are denoted transport address that Norma associates with Mallory. These packets are denoted
‘DTLS2’ to indicate that Patsy associates these with the second signaling 'DTLS2' to indicate that Patsy associates these with the second signaling
session (‘signaling2’), however Norma will interpret these as being associated session ('Signaling2'); however, Norma will interpret these as being associated
with the first signaling session (‘signaling1’).</t> with the first signaling session ('Signaling1').</t>
<t>The second signaling exchange ('Signaling2'), which is between Norma
<t>The second signaling exchange - ‘signaling2’, between Norma and Patsy - is and Patsy, is
permitted to continue to the point where Patsy believes that it has succeeded. permitted to continue to the point where Patsy believes that it has succeeded.
This ensures that Patsy believes that she is communicating with Norma. In the This ensures that Patsy believes that she is communicating with Norma. In the
end, Norma believes that she is communicating with Mallory, when she is really end, Norma believes that she is communicating with Mallory, when she is really
communicating with Patsy. Just like the example in <xref target="id-example"/>, communicating with Patsy. Just like the example in <xref target="id-example" fo
Mallory rmat="default"/>, Mallory
cannot access media, but Norma might send information to Patsy that is Norma cannot access media, but Norma might send information to Patsy that Norma
might not intend or that Patsy might misinterpret.</t> might not intend or that Patsy might misinterpret.</t>
<t>Though Patsy needs to believe that the second signaling session has b
<t>Though Patsy needs to believe that the second signaling session has been een
successfully established, Mallory has no real interest in seeing that session successfully established, Mallory has no real interest in seeing that session
also be established. Mallory only needs to ensure that Patsy maintains the also be established. Mallory only needs to ensure that Patsy maintains the
active session and does not abandon the session prematurely. For this reason, active session and does not abandon the session prematurely. For this reason,
it might be necessary to permit the signaling from Patsy to reach Norma to allow it might be necessary to permit the signaling from Patsy to reach Norma in order to allow
Patsy to receive a call setup completion signal, such as a SIP ACK. Once the Patsy to receive a call setup completion signal, such as a SIP ACK. Once the
second session is established, Mallory might cause DTLS packets sent by Norma to second session is established, Mallory might cause DTLS packets sent by Norma to
Patsy to be dropped. However, if Mallory allows DTLS packets to pass, it is Patsy to be dropped. However, if Mallory allows DTLS packets to pass, it is
likely that Patsy will discard them as Patsy will already have a successful DTLS likely that Patsy will discard them as Patsy will already have a successful DTLS
connection established.</t> connection established.</t>
<t>For the attacked session to be sustained beyond the point that Norma
<t>For the attacked session to be sustained beyond the point that Norma detects detects
errors in the second session, Mallory also needs to block any signaling that errors in the second session, Mallory also needs to block any signaling that
Norma might send to Patsy asking for the call to be abandoned. Otherwise, Patsy Norma might send to Patsy asking for the call to be abandoned. Otherwise, Patsy
might receive a notice that the call is failed and thereby abort the call.</t> might receive a notice that the call has failed and thereby abort the call.</t>
<t>This attack creates an asymmetry in the beliefs about the identity of peers. <t>This attack creates an asymmetry in the beliefs about the identity of peers.
However, this attack is only possible if the victim (Norma) is willing to However, this attack is only possible if the victim (Norma) is willing to
conduct two sessions nearly simultaneously, if the attacker (Mallory) is on the conduct two sessions nearly simultaneously; if the attacker (Mallory) is on the
network path between the victims, and if the same certificate - and therefore network path between the victims; and if the same certificate -- and therefore
SDP <spanx style="verb">fingerprint</spanx> attribute value - is used by Norma f the SDP <tt>fingerprint</tt> attribute value -- is used by Norma for both sessio
or both sessions.</t> ns.</t>
<t>Where Interactive Connectivity Establishment (ICE) <xref target="RFC8
<t>Where ICE <xref target="ICE"/> is used, Mallory also needs to ensure that 445" format="default"/>
is used, Mallory also needs to ensure that
connectivity checks between Patsy and Norma succeed, either by forwarding checks connectivity checks between Patsy and Norma succeed, either by forwarding checks
or answering and generating the necessary messages.</t> or by answering and generating the necessary messages.</t>
</section>
</section> <section anchor="sess-id" numbered="true" toc="default">
<section anchor="sess-id" title="Unique Session Identity Solution"> <name>Unique Session Identity Solution</name>
<t>The solution to this problem is to assign a new identifier to communi
<t>The solution to this problem is to assign a new identifier to communicating cating
peers. Each endpoint assigns their peer a unique identifier during call peers. Each endpoint assigns their peer a unique identifier during call
signaling. The peer echoes that identifier in the TLS handshake, binding that signaling. The peer echoes that identifier in the TLS handshake, binding that
identity into the session. Including this new identity in the TLS handshake identity into the session. Including this new identity in the TLS handshake
means that it will be covered by the TLS Finished message, which is necessary to means that it will be covered by the TLS Finished message, which is necessary to
authenticate it (see <xref target="SIGMA"/>).</t> authenticate it (see <xref target="SIGMA" format="default"/>).</t>
<t>Successfully validating that the identifier matches the expected valu
<t>Successful validation that the identifier matches the expected value means th e means that
at
the connection corresponds to the signaled session and is therefore established the connection corresponds to the signaled session and is therefore established
between the correct two endpoints.</t> between the correct two endpoints.</t>
<t>This solution relies on the unique identifier given to DTLS sessions
<t>This solution relies on the unique identifier given to DTLS sessions using th using the
e SDP <tt>tls-id</tt> attribute <xref target="RFC8842" format="default"/>. This f
SDP <spanx style="verb">tls-id</spanx> attribute <xref target="DTLS-SDP"/>. Thi ield is
s field is
already required to be unique. Thus, no two offers or answers from the same already required to be unique. Thus, no two offers or answers from the same
client will have the same value.</t> client will have the same value.</t>
<t>A new <tt>external_session_id</tt> extension is added to the TLS or D
<t>A new <spanx style="verb">external_session_id</spanx> extension is added to t TLS handshake for
he TLS or DTLS handshake for
connections that are established as part of the same call or real-time session. connections that are established as part of the same call or real-time session.
This carries the value of the <spanx style="verb">tls-id</spanx> attribute and p rovides integrity This carries the value of the <tt>tls-id</tt> attribute and provides integrity
protection for its exchange as part of the TLS or DTLS handshake.</t> protection for its exchange as part of the TLS or DTLS handshake.</t>
</section>
</section> <section anchor="external_session_id" numbered="true" toc="default">
<section anchor="external_session_id" title="The external_session_id TLS Extensi <name>The external_session_id TLS Extension</name>
on"> <t>The <tt>external_session_id</tt> TLS extension carries the unique ide
ntifier that an
<t>The <spanx style="verb">external_session_id</spanx> TLS extension carries the endpoint selects. When used with SDP, the value <bcp14>MUST</bcp14> include the
unique identifier that an <tt>tls-id</tt>
endpoint selects. When used with SDP, the value MUST include the <spanx style="
verb">tls-id</spanx>
attribute from the SDP that the endpoint generated when negotiating the session. attribute from the SDP that the endpoint generated when negotiating the session.
This document only defines use of this extension for SDP; other methods of This document only defines use of this extension for SDP; other methods of
external session negotiation can use this extension to include a unique session external session negotiation can use this extension to include a unique session
identifier.</t> identifier.</t>
<t>The <tt>extension_data</tt> for the <tt>external_session_id</tt> exte
<t>The <spanx style="verb">extension_data</spanx> for the <spanx style="verb">ex nsion contains an
ternal_session_id</spanx> extension contains an
ExternalSessionId struct, described below using the syntax defined in ExternalSessionId struct, described below using the syntax defined in
<xref target="TLS13"/>:</t> <xref target="RFC8446" format="default"/>:</t>
<sourcecode type="tls-presentation"><![CDATA[
<figure><artwork><![CDATA[
struct { struct {
opaque session_id<20..255>; opaque session_id<20..255>;
} ExternalSessionId; } ExternalSessionId;
]]></artwork></figure> ]]></sourcecode>
<t>For SDP, the <tt>session_id</tt> field of the extension includes the
<t>For SDP, the <spanx style="verb">session_id</spanx> field of the extension in value of the
cludes the value of the <tt>tls-id</tt> SDP attribute as defined in <xref target="RFC8842" format="defau
<spanx style="verb">tls-id</spanx> SDP attribute as defined in <xref target="DTL lt"/>
S-SDP"/> (that is, the <tt>tls-id-value</tt> ABNF production). The value of the <tt>tls-
(that is, the <spanx style="verb">tls-id-value</spanx> ABNF production). The va id</tt>
lue of the <spanx style="verb">tls-id</spanx> attribute is encoded using ASCII <xref target="RFC0020" format="default"/>.</t>
attribute is encoded using ASCII <xref target="ASCII"/>.</t> <t>Where RTP and RTCP <xref target="RFC3550" format="default"/> are not
multiplexed, it is possible that the
<t>Where RTP and RTCP <xref target="RTP"/> are not multiplexed, it is possible t
hat the
two separate DTLS connections carrying RTP and RTCP can be switched. This is two separate DTLS connections carrying RTP and RTCP can be switched. This is
considered benign since these protocols are designed to be distinguishable as considered benign since these protocols are designed to be distinguishable as
SRTP <xref target="SRTP"/> provides key separation. Using RTP/RTCP multiplexing SRTP <xref target="RFC3711" format="default"/> provides key separation. Using R
<xref target="RTCP-MUX"/> further avoids this problem.</t> TP/RTCP multiplexing
<xref target="RFC5761" format="default"/> further avoids this problem.</t>
<t>The <spanx style="verb">external_session_id</spanx> extension is included in <t>The <tt>external_session_id</tt> extension is included in a ClientHel
a ClientHello and - if the lo, and if the
extension is present in the ClientHello - either ServerHello (for TLS and DTLS extension is present in the ClientHello, either ServerHello (for TLS and DTLS
versions less than 1.3) or EncryptedExtensions (for TLS 1.3).</t> versions older than 1.3) or EncryptedExtensions (for TLS 1.3).</t>
<t>Endpoints <bcp14>MUST</bcp14> check that the <tt>session_id</tt> para
<t>Endpoints MUST check that the <spanx style="verb">session_id</spanx> paramete meter in the extension that they
r in the extension that they receive includes the <tt>tls-id</tt> attribute value that they received in their
receive includes the <spanx style="verb">tls-id</spanx> attribute value that the peer's
y received in their peer’s
session description. Endpoints can perform string comparison by ASCII decoding session description. Endpoints can perform string comparison by ASCII decoding
the TLS extension value and comparing it to the SDP attribute value, or compare the TLS extension value and comparing it to the SDP attribute value or by compar ing
the encoded TLS extension octets with the encoded SDP attribute value. An the encoded TLS extension octets with the encoded SDP attribute value. An
endpoint that receives a <spanx style="verb">external_session_id</spanx> extensi endpoint that receives an <tt>external_session_id</tt> extension that is not ide
on that is not identical ntical
to the value that it expects MUST abort the connection with a fatal to the value that it expects <bcp14>MUST</bcp14> abort the connection with a fat
<spanx style="verb">illegal_parameter</spanx> alert.</t> al
<tt>illegal_parameter</tt> alert.</t>
<t>Any validation performed of the <spanx style="verb">external_session_id</span <t>
x> extension is done in The endpoint performs the validation of the <tt>external_id_hash</tt> extensi
addition to the validation required by <xref target="FINGERPRINT"/>.</t> on in
addition to the validation required by <xref target="RFC8122" format="default
<t>An endpoint that is communicating with a peer that does not support this "/>.
extension will receive a ClientHello, ServerHello or EncryptedExtensions that </t>
does not include this extension. An endpoint MAY choose to continue a session <t>If an endpoint communicates with a peer that does not support this
extension, it will receive a ClientHello, ServerHello, or EncryptedExtensions me
ssage that
does not include this extension. An endpoint <bcp14>MAY</bcp14> choose to conti
nue a session
without this extension in order to interoperate with peers that do not implement without this extension in order to interoperate with peers that do not implement
this specification.</t> this specification.</t>
<t>In TLS 1.3, an <tt>external_session_id</tt> extension sent by a serve
<t>In TLS 1.3, an <spanx style="verb">external_session_id</spanx> extension sent r <bcp14>MUST</bcp14> be sent in
by a server MUST be sent in
the EncryptedExtensions message.</t> the EncryptedExtensions message.</t>
<t>This defense is not effective if an attacker can rewrite <tt>tls-id</
<t>This defense is not effective if an attacker can rewrite <spanx style="verb"> tt> values in
tls-id</spanx> values in signaling. Only the mechanism in <tt>external_id_hash</tt> is able to defend ag
signaling. Only the mechanism in <spanx style="verb">external_id_hash</spanx> i ainst
s able to defend against
an attacker that can compromise session integrity.</t> an attacker that can compromise session integrity.</t>
</section>
</section> </section>
</section> <section anchor="concat" numbered="true" toc="default">
<section anchor="concat" title="Session Concatenation"> <name>Session Concatenation</name>
<t>Use of session identifiers does not prevent an attacker from establishi
<t>Use of session identifiers does not prevent an attacker from establishing two ng two
concurrent sessions with different peers and forwarding signaling from those concurrent sessions with different peers and forwarding signaling from those
peers to each other. Concatenating two signaling sessions in this way creates peers to each other. Concatenating two signaling sessions in this way creates
two signaling sessions, with two session identifiers, but only the TLS two signaling sessions, with two session identifiers, but only the TLS
connections from a single session are established as a result. In doing so, the connections from a single session are established as a result. In doing so, the
attacker creates a situation where both peers believe that they are talking to attacker creates a situation where both peers believe that they are talking to
the attacker when they are talking to each other.</t> the attacker when they are talking to each other.</t>
<t>In the absence of any higher-level concept of peer identity, the use of
<t>In the absence of any higher-level concept of peer identity, the use of sessi session
on
identifiers does not prevent session concatenation if the attacker is able to identifiers does not prevent session concatenation if the attacker is able to
copy the session identifier from one signaling session to another. This kind of copy the session identifier from one signaling session to another. This kind of
attack is prevented by systems that enable peer authentication such as WebRTC attack is prevented by systems that enable peer authentication, such as WebRTC
identity <xref target="WEBRTC-SEC"/> or SIP identity <xref target="SIP-ID"/>. H identity <xref target="RFC8827" format="default"/>
owever, session or SIP identity <xref target="RFC8224" format="default"/>. However, session
concatenation remains possible at higher layers: an attacker can establish two concatenation remains possible at higher layers: an attacker can establish two
independent sessions and simply forward any data it receives from one into the independent sessions and simply forward any data it receives from one into the
other.</t> other.</t>
<t>Use of the <tt>external_session_id</tt> does not guarantee that the ide
<t>Use of the <spanx style="verb">external_session_id</spanx> does not guarantee ntity of the
that the identity of the
peer at the TLS layer is the same as the identity of the signaling peer. The peer at the TLS layer is the same as the identity of the signaling peer. The
advantage an attacker gains by concatenating sessions is limited unless data is advantage that an attacker gains by concatenating sessions is limited unless dat
exchanged on the assumption that signaling and TLS peers are the same. If a a is
exchanged based on the assumption that signaling and TLS peers are the same. If
a
secondary protocol uses the signaling channel with the assumption that the secondary protocol uses the signaling channel with the assumption that the
signaling and TLS peers are the same then that protocol is vulnerable to attack. signaling and TLS peers are the same, then that protocol is vulnerable to attack
A signaling system that can defend against session concatenation, while out of .
scope for this document, requires that the signaling layer is authenticated and While out of scope for this document, a signaling system that can defend against
bound to any TLS connections.</t> session concatenation
requires that the signaling layer is authenticated and bound to any TLS connecti
<t>It is important to note that multiple connections can be created within the s ons.</t>
ame <t>It is important to note that multiple connections can be created within
the same
signaling session. An attacker might concatenate only part of a session, signaling session. An attacker might concatenate only part of a session,
choosing to terminate some connections (and optionally forward data) while choosing to terminate some connections (and optionally forward data) while
arranging to have peers interact directly for other connections. It is even arranging to have peers interact directly for other connections. It is even
possible to have different peers interact for each connection. This means that possible to have different peers interact for each connection. This means that
the actual identity of the peer for one connection might differ from the peer on the actual identity of the peer for one connection might differ from the peer on
another connection.</t> another connection.</t>
<t>Critically, information about the identity of TLS peers provides no ass
<t>Critically, information about the identity of TLS peers provides no assurance urances
s about the identity of signaling peers and does not transfer between TLS
about the identity of signaling peers and do not transfer between TLS
connections in the same session. Information extracted from a TLS connection connections in the same session. Information extracted from a TLS connection
therefore MUST NOT be used in a secondary protocol outside of that connection if therefore <bcp14>MUST NOT</bcp14> be used in a secondary protocol outside of tha t connection if
that protocol assumes that the signaling protocol has the same peers. that protocol assumes that the signaling protocol has the same peers.
Similarly, security-sensitive information from one TLS connection MUST NOT be Similarly, security-sensitive information from one TLS connection <bcp14>MUST NO T</bcp14> be
used in other TLS connections even if they are established as a result of the used in other TLS connections even if they are established as a result of the
same signaling session.</t> same signaling session.</t>
</section>
</section> <section anchor="security-considerations" numbered="true" toc="default">
<section anchor="security-considerations" title="Security Considerations"> <name>Security Considerations</name>
<t>When combined with identity assertions, the mitigations in this documen
<t>The mitigations in this document, when combined with identity assertions, ens t ensure
ure
that there is no opportunity to misrepresent the identity of TLS peers. This that there is no opportunity to misrepresent the identity of TLS peers. This
assurance is provided even if an attacker can modify signaling messages.</t> assurance is provided even if an attacker can modify signaling messages.</t>
<t>Without identity assertions, the mitigations in this document prevent the <t>Without identity assertions, the mitigations in this document prevent t
session splicing attack described in <xref target="fp"/>. Defense against sessi he
on session splicing attack described in <xref target="fp" format="default"/>. Defe
concatenation (<xref target="concat"/>) additionally requires protocol peers are nse against session
not able to concatenation (<xref target="concat" format="default"/>) additionally requires t
hat protocol peers are not able to
claim the certificate fingerprints of other entities.</t> claim the certificate fingerprints of other entities.</t>
</section>
</section> <section anchor="iana-considerations" numbered="true" toc="default">
<section anchor="iana-considerations" title="IANA Considerations"> <name>IANA Considerations</name>
<t>This document registers two extensions in the "TLS ExtensionType Values
<t>This document registers two extensions in the TLS “ExtensionType Values” "
registry established in <xref target="TLS13"/>:</t> registry established in <xref target="RFC8446" format="default"/>:</t>
<ul spacing="normal">
<t><list style="symbols"> <li>The <tt>external_id_hash</tt> extension defined in <xref target="ext
<t>The <spanx style="verb">external_id_hash</spanx> extension defined in <xref ernal_id_hash" format="default"/> has been
target="external_id_hash"/> has been assigned a code point of 55; it is recommended and is marked as "CH, EE"
assigned a code point of TBD; it is recommended and is marked as “CH, EE” in TLS 1.3.</li>
in TLS 1.3.</t> <li>The <tt>external_session_id</tt> extension defined in <xref target="
<t>The <spanx style="verb">external_session_id</spanx> extension defined in <x external_session_id" format="default"/> has
ref target="external_session_id"/> has been assigned a code point of 56; it is recommended and is marked as
been assigned a code point of TBD; it is recommended and is marked as "CH, EE" in TLS 1.3.</li>
“CH, EE” in TLS 1.3.</t> </ul>
</list></t> </section>
</section>
</middle> </middle>
<back> <back>
<displayreference target="RFC0020" to="ASCII"/>
<displayreference target="RFC3550" to="RTP"/>
<displayreference target="RFC3629" to="UTF8"/>
<displayreference target="RFC3711" to="SRTP"/>
<displayreference target="RFC4566" to="SDP"/>
<displayreference target="RFC4648" to="BASE64"/>
<displayreference target="RFC5761" to="RTCP-MUX"/>
<displayreference target="RFC5763" to="DTLS-SRTP"/>
<displayreference target="RFC6189" to="ZRTP"/>
<displayreference target="RFC6234" to="SHA"/>
<displayreference target="RFC6347" to="DTLS"/>
<displayreference target="RFC7696" to="AGILITY"/>
<displayreference target="RFC8122" to="FINGERPRINT"/>
<displayreference target="RFC8224" to="SIP-ID"/>
<displayreference target="RFC8225" to="PASSPORT"/>
<displayreference target="RFC8259" to="JSON"/>
<displayreference target="RFC8445" to="ICE"/>
<displayreference target="RFC8446" to="TLS13"/>
<displayreference target="RFC8827" to="WEBRTC-SEC"/>
<displayreference target="RFC8842" to="DTLS-SDP"/>
<references>
<name>References</name>
<references>
<name>Normative References</name>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8446.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4566.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8122.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.6347.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.3711.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.5763.xml"/>
<references title='Normative References'> <!-- draft-ietf-rtcweb-security-arch in C238; RFC 8827 -->
<reference anchor="RFC8827" target="https://www.rfc-editor.org/info/rfc8827">
<reference anchor="TLS13" target='https://www.rfc-editor.org/info/rfc8446'> <front>
<front> <title>WebRTC Security Architecture</title>
<title>The Transport Layer Security (TLS) Protocol Version 1.3</title> <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'><organization /> <organization/>
</author> </author>
<date year='2018' month='August' /> <date month='May' year='2020'/>
<abstract><t>This document specifies version 1.3 of the Transport Layer Security </front>
(TLS) protocol. TLS allows client/server applications to communicate over the <seriesInfo name="RFC" value="8827"/>
Internet in a way that is designed to prevent eavesdropping, tampering, and mess <seriesInfo name="DOI" value="10.17487/RFC8827"/>
age forgery.</t><t>This document updates RFCs 5705 and 6066, and obsoletes RFCs </reference>
5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
implementations.</t></abstract> ence.RFC.8224.xml"/>
</front> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
<seriesInfo name='RFC' value='8446'/> ence.RFC.2119.xml"/>
<seriesInfo name='DOI' value='10.17487/RFC8446'/> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
</reference> ence.RFC.8174.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
<reference anchor="SDP" target='https://www.rfc-editor.org/info/rfc4566'> ence.RFC.8225.xml"/>
<front> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
<title>SDP: Session Description Protocol</title> ence.RFC.8259.xml"/>
<author initials='M.' surname='Handley' fullname='M. Handley'><organization /></ <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
author> ence.RFC.3629.xml"/>
<author initials='V.' surname='Jacobson' fullname='V. Jacobson'><organization /> <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
</author> ence.RFC.6234.xml"/>
<author initials='C.' surname='Perkins' fullname='C. Perkins'><organization /></ <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
author> ence.RFC.4648.xml"/>
<date year='2006' month='July' /> <xi:include
<abstract><t>This memo defines the Session Description Protocol (SDP). SDP is i href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.
ntended for describing multimedia sessions for the purposes of session announcem 0020.xml"/>
ent, session invitation, and other forms of multimedia session initiation. [STA <!-- draft-ietf-mmusic-dtls-sdp-32 in C238; RFC 8842 -->
NDARDS-TRACK]</t></abstract> <reference anchor="RFC8842" target="https://www.rfc-editor.org/info/rfc8842">
</front>
<seriesInfo name='RFC' value='4566'/>
<seriesInfo name='DOI' value='10.17487/RFC4566'/>
</reference>
<reference anchor="FINGERPRINT" target='https://www.rfc-editor.org/info/rfc8122
'>
<front>
<title>Connection-Oriented Media Transport over the Transport Layer Security (TL
S) Protocol in the Session Description Protocol (SDP)</title>
<author initials='J.' surname='Lennox' fullname='J. Lennox'><organization /></au
thor>
<author initials='C.' surname='Holmberg' fullname='C. Holmberg'><organization />
</author>
<date year='2017' month='March' />
<abstract><t>This document specifies how to establish secure connection-oriented
media transport sessions over the Transport Layer Security (TLS) protocol using
the Session Description Protocol (SDP). It defines the SDP protocol identifier
, 'TCP/TLS'. It also defines the syntax and semantics for an SDP 'fingerprint'
attribute that identifies the certificate that will be presented for the TLS ses
sion. This mechanism allows media transport over TLS connections to be establis
hed securely, so long as the integrity of session descriptions is assured.</t><t
>This document obsoletes RFC 4572 by clarifying the usage of multiple fingerprin
ts.</t></abstract>
</front>
<seriesInfo name='RFC' value='8122'/>
<seriesInfo name='DOI' value='10.17487/RFC8122'/>
</reference>
<reference anchor="DTLS" target='https://www.rfc-editor.org/info/rfc6347'>
<front>
<title>Datagram Transport Layer Security Version 1.2</title>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'><organization />
</author>
<author initials='N.' surname='Modadugu' fullname='N. Modadugu'><organization />
</author>
<date year='2012' month='January' />
<abstract><t>This document specifies version 1.2 of the Datagram Transport Layer
Security (DTLS) protocol. The DTLS protocol provides communications privacy fo
r datagram protocols. The protocol allows client/server applications to communi
cate in a way that is designed to prevent eavesdropping, tampering, or message f
orgery. The DTLS protocol is based on the Transport Layer Security (TLS) protoc
ol and provides equivalent security guarantees. Datagram semantics of the under
lying transport are preserved by the DTLS protocol. This document updates DTLS
1.0 to work with TLS version 1.2. [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='6347'/>
<seriesInfo name='DOI' value='10.17487/RFC6347'/>
</reference>
<reference anchor="SRTP" target='https://www.rfc-editor.org/info/rfc3711'>
<front>
<title>The Secure Real-time Transport Protocol (SRTP)</title>
<author initials='M.' surname='Baugher' fullname='M. Baugher'><organization /></
author>
<author initials='D.' surname='McGrew' fullname='D. McGrew'><organization /></au
thor>
<author initials='M.' surname='Naslund' fullname='M. Naslund'><organization /></
author>
<author initials='E.' surname='Carrara' fullname='E. Carrara'><organization /></
author>
<author initials='K.' surname='Norrman' fullname='K. Norrman'><organization /></
author>
<date year='2004' month='March' />
<abstract><t>This document describes the Secure Real-time Transport Protocol (SR
TP), a profile of the Real-time Transport Protocol (RTP), which can provide conf
identiality, message authentication, and replay protection to the RTP traffic an
d to the control traffic for RTP, the Real-time Transport Control Protocol (RTCP
). [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='3711'/>
<seriesInfo name='DOI' value='10.17487/RFC3711'/>
</reference>
<reference anchor="DTLS-SRTP" target='https://www.rfc-editor.org/info/rfc5763'>
<front>
<title>Framework for Establishing a Secure Real-time Transport Protocol (SRTP) S
ecurity Context Using Datagram Transport Layer Security (DTLS)</title>
<author initials='J.' surname='Fischl' fullname='J. Fischl'><organization /></au
thor>
<author initials='H.' surname='Tschofenig' fullname='H. Tschofenig'><organizatio
n /></author>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'><organization />
</author>
<date year='2010' month='May' />
<abstract><t>This document specifies how to use the Session Initiation Protocol
(SIP) to establish a Secure Real-time Transport Protocol (SRTP) security context
using the Datagram Transport Layer Security (DTLS) protocol. It describes a me
chanism of transporting a fingerprint attribute in the Session Description Proto
col (SDP) that identifies the key that will be presented during the DTLS handsha
ke. The key exchange travels along the media path as opposed to the signaling p
ath. The SIP Identity mechanism can be used to protect the integrity of the fin
gerprint attribute from modification by intermediate proxies. [STANDARDS-TRACK]
</t></abstract>
</front>
<seriesInfo name='RFC' value='5763'/>
<seriesInfo name='DOI' value='10.17487/RFC5763'/>
</reference>
<reference anchor="WEBRTC-SEC">
<front>
<title>WebRTC Security Architecture</title>
<author initials='E' surname='Rescorla' fullname='Eric Rescorla'>
<organization />
</author>
<date month='July' day='22' year='2019' />
<abstract><t>This document defines the security architecture for WebRTC, a proto
col suite intended for use with real-time applications that can be deployed in b
rowsers - "real time communication on the Web".</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-rtcweb-security-arch-20' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-rtcweb-security-a
rch-20.txt' />
</reference>
<reference anchor="SIP-ID" target='https://www.rfc-editor.org/info/rfc8224'>
<front>
<title>Authenticated Identity Management in the Session Initiation Protocol (SIP
)</title>
<author initials='J.' surname='Peterson' fullname='J. Peterson'><organization />
</author>
<author initials='C.' surname='Jennings' fullname='C. Jennings'><organization />
</author>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'><organization />
</author>
<author initials='C.' surname='Wendt' fullname='C. Wendt'><organization /></auth
or>
<date year='2018' month='February' />
<abstract><t>The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of the end use
rs that originate SIP requests, especially in an interdomain context. This docu
ment defines a mechanism for securely identifying originators of SIP requests.
It does so by defining a SIP header field for conveying a signature used for val
idating the identity and for conveying a reference to the credentials of the sig
ner.</t><t>This document obsoletes RFC 4474.</t></abstract>
</front>
<seriesInfo name='RFC' value='8224'/>
<seriesInfo name='DOI' value='10.17487/RFC8224'/>
</reference>
<reference anchor="RFC2119" target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner'><organization /></
author>
<date year='1997' month='March' />
<abstract><t>In many standards track documents several words are used to signify
the requirements in the specification. These words are often capitalized. This
document defines these words as they should be interpreted in IETF documents.
This document specifies an Internet Best Current Practices for the Internet Comm
unity, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
</reference>
<reference anchor="RFC8174" target='https://www.rfc-editor.org/info/rfc8174'>
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></auth
or>
<date year='2017' month='May' />
<abstract><t>RFC 2119 specifies common key words that may be used in protocol s
pecifications. This document aims to reduce the ambiguity by clarifying that on
ly UPPERCASE usage of the key words have the defined special meanings.</t></abs
tract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/>
</reference>
<reference anchor="PASSPoRT" target='https://www.rfc-editor.org/info/rfc8225'>
<front>
<title>PASSporT: Personal Assertion Token</title>
<author initials='C.' surname='Wendt' fullname='C. Wendt'><organization /></auth
or>
<author initials='J.' surname='Peterson' fullname='J. Peterson'><organization />
</author>
<date year='2018' month='February' />
<abstract><t>This document defines a method for creating and validating a token
that cryptographically verifies an originating identity or, more generally, a UR
I or telephone number representing the originator of personal communications. T
he Personal Assertion Token, PASSporT, is cryptographically signed to protect th
e integrity of the identity of the originator and to verify the assertion of the
identity information at the destination. The cryptographic signature is define
d with the intention that it can confidently verify the originating persona even
when the signature is sent to the destination party over an insecure channel.
PASSporT is particularly useful for many personal-communications applications ov
er IP networks and other multi-hop interconnection scenarios where the originati
ng and destination parties may not have a direct trusted relationship.</t></abst
ract>
</front>
<seriesInfo name='RFC' value='8225'/>
<seriesInfo name='DOI' value='10.17487/RFC8225'/>
</reference>
<reference anchor="JSON" target='https://www.rfc-editor.org/info/rfc8259'>
<front>
<title>The JavaScript Object Notation (JSON) Data Interchange Format</title>
<author initials='T.' surname='Bray' fullname='T. Bray' role='editor'><organizat
ion /></author>
<date year='2017' month='December' />
<abstract><t>JavaScript Object Notation (JSON) is a lightweight, text-based, lan
guage-independent data interchange format. It was derived from the ECMAScript P
rogramming Language Standard. JSON defines a small set of formatting rules for
the portable representation of structured data.</t><t>This document removes inco
nsistencies with other specifications of JSON, repairs specification errors, and
offers experience-based interoperability guidance.</t></abstract>
</front>
<seriesInfo name='STD' value='90'/>
<seriesInfo name='RFC' value='8259'/>
<seriesInfo name='DOI' value='10.17487/RFC8259'/>
</reference>
<reference anchor="UTF8" target='https://www.rfc-editor.org/info/rfc3629'>
<front>
<title>UTF-8, a transformation format of ISO 10646</title>
<author initials='F.' surname='Yergeau' fullname='F. Yergeau'><organization /></
author>
<date year='2003' month='November' />
<abstract><t>ISO/IEC 10646-1 defines a large character set called the Universal
Character Set (UCS) which encompasses most of the world's writing systems. The
originally proposed encodings of the UCS, however, were not compatible with many
current applications and protocols, and this has led to the development of UTF-
8, the object of this memo. UTF-8 has the characteristic of preserving the full
US-ASCII range, providing compatibility with file systems, parsers and other so
ftware that rely on US-ASCII values but are transparent to other values. This m
emo obsoletes and replaces RFC 2279.</t></abstract>
</front>
<seriesInfo name='STD' value='63'/>
<seriesInfo name='RFC' value='3629'/>
<seriesInfo name='DOI' value='10.17487/RFC3629'/>
</reference>
<reference anchor="SHA" target='https://www.rfc-editor.org/info/rfc6234'>
<front>
<title>US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)</title>
<author initials='D.' surname='Eastlake 3rd' fullname='D. Eastlake 3rd'><organiz
ation /></author>
<author initials='T.' surname='Hansen' fullname='T. Hansen'><organization /></au
thor>
<date year='2011' month='May' />
<abstract><t>Federal Information Processing Standard, FIPS</t></abstract>
</front>
<seriesInfo name='RFC' value='6234'/>
<seriesInfo name='DOI' value='10.17487/RFC6234'/>
</reference>
<reference anchor="BASE64" target='https://www.rfc-editor.org/info/rfc4648'>
<front>
<title>The Base16, Base32, and Base64 Data Encodings</title>
<author initials='S.' surname='Josefsson' fullname='S. Josefsson'><organization
/></author>
<date year='2006' month='October' />
<abstract><t>This document describes the commonly used base 64, base 32, and bas
e 16 encoding schemes. It also discusses the use of line-feeds in encoded data,
use of padding in encoded data, use of non-alphabet characters in encoded data,
use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK
]</t></abstract>
</front>
<seriesInfo name='RFC' value='4648'/>
<seriesInfo name='DOI' value='10.17487/RFC4648'/>
</reference>
<reference anchor="DTLS-SDP">
<front>
<title>Session Description Protocol (SDP) Offer/Answer Considerations for Datagr
am Transport Layer Security (DTLS) and Transport Layer Security (TLS)</title>
<author initials='C' surname='Holmberg' fullname='Christer Holmberg'>
<organization />
</author>
<author initials='R' surname='Shpount' fullname='Roman Shpount'>
<organization />
</author>
<date month='October' day='29' year='2017' />
<abstract><t>This document defines the Session Description Protocol (SDP) offer/
answer procedures for negotiating and establishing a Datagram Transport Layer S
ecurity (DTLS) association. The document also defines the criteria for when a n
ew DTLS association must be established. The document updates RFC 5763 and RFC
7345, by replacing common SDP offer/answer procedures with a reference to this s
pecification. This document defines a new SDP media-level attribute, 'tls-id'.
This document also defines how the 'tls-id' attribute can be used for negotiati
ng and establishing a Transport Layer Security (TLS) connection, in conjunction
with the procedures in RFC 4145 and RFC 8122.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-mmusic-dtls-sdp-32' />
<format type='TXT'
target='http://www.ietf.org/internet-drafts/draft-ietf-mmusic-dtls-sdp-3
2.txt' />
</reference>
<reference anchor="ASCII" target='https://www.rfc-editor.org/info/rfc20'>
<front>
<title>ASCII format for network interchange</title>
<author initials='V.G.' surname='Cerf' fullname='V.G. Cerf'><organization /></au
thor>
<date year='1969' month='October' />
</front>
<seriesInfo name='STD' value='80'/>
<seriesInfo name='RFC' value='20'/>
<seriesInfo name='DOI' value='10.17487/RFC0020'/>
</reference>
</references>
<references title='Informative References'>
<reference anchor="UKS" >
<front>
<title>Unknown Key-Share Attacks on the Station-to-Station (STS) Protocol</t
itle>
<author initials="S." surname="Blake-Wilson">
<organization></organization>
</author>
<author initials="A." surname="Menezes">
<organization></organization>
</author>
<date year="1999"/>
</front>
<seriesInfo name="Lecture Notes in Computer Science 1560, Springer, pp. 154–17
0" value=""/>
</reference>
<reference anchor="SIGMA" >
<front> <front>
<title>SIGMA: The ‘SIGn-and-MAc’approach to authenticated Diffie-Hellman and <title>Session Description Protocol (SDP) Offer/Answer Considerations for
its use in the IKE protocols</title> Datagram Transport Layer Security (DTLS) and Transport Layer Security
<author initials="H." surname="Krawczyk"> (TLS)</title>
<organization></organization>
</author> <author initials="C." surname="Holmberg" fullname="Christer Holmberg">
<date year="2003"/> <organization />
</front>
<seriesInfo name="Annual International Cryptology Conference, Springer, pp. 40
0-425" value=""/>
</reference>
<reference anchor="WEBRTC" >
<front>
<title>WebRTC 1.0: Real-time Communication Between Browsers</title>
<author initials="A." surname="Bergkvist">
<organization></organization>
</author>
<author initials="D." surname="Burnett">
<organization></organization>
</author>
<author initials="A." surname="Narayanan">
<organization></organization>
</author>
<author initials="C." surname="Jennings">
<organization></organization>
</author>
<author initials="B." surname="Aboba">
<organization></organization>
</author>
<author initials="T." surname="Brandstetter">
<organization></organization>
</author>
<author initials="J." surname="Bruaroey">
<organization></organization>
</author> </author>
<date year="2018" month="November" day="08"/>
</front>
<seriesInfo name="W3C Editor's Draft" value=""/>
</reference>
<reference anchor="RFC3725" target='https://www.rfc-editor.org/info/rfc3725'> <author initials="R." surname="Shpount" fullname="Roman Shpount">
<front> <organization />
<title>Best Current Practices for Third Party Call Control (3pcc) in the Session </author>
Initiation Protocol (SIP)</title>
<author initials='J.' surname='Rosenberg' fullname='J. Rosenberg'><organization
/></author>
<author initials='J.' surname='Peterson' fullname='J. Peterson'><organization />
</author>
<author initials='H.' surname='Schulzrinne' fullname='H. Schulzrinne'><organizat
ion /></author>
<author initials='G.' surname='Camarillo' fullname='G. Camarillo'><organization
/></author>
<date year='2004' month='April' />
<abstract><t>Third party call control refers to the ability of one entity to cre
ate a call in which communication is actually between other parties. Third part
y call control is possible using the mechanisms specified within the Session Ini
tiation Protocol (SIP). However, there are several possible approaches, each wi
th different benefits and drawbacks. This document discusses best current pract
ices for the usage of SIP for third party call control. This document specifies
an Internet Best Current Practices for the Internet Community, and requests dis
cussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='85'/>
<seriesInfo name='RFC' value='3725'/>
<seriesInfo name='DOI' value='10.17487/RFC3725'/>
</reference>
<reference anchor="ZRTP" target='https://www.rfc-editor.org/info/rfc6189'> <date month="May" year="2020" />
<front> </front>
<title>ZRTP: Media Path Key Agreement for Unicast Secure RTP</title> <seriesInfo name="RFC" value="8842" />
<author initials='P.' surname='Zimmermann' fullname='P. Zimmermann'><organizatio <seriesInfo name="DOI" value="10.17487/RFC8842"/>
n /></author>
<author initials='A.' surname='Johnston' fullname='A. Johnston' role='editor'><o
rganization /></author>
<author initials='J.' surname='Callas' fullname='J. Callas'><organization /></au
thor>
<date year='2011' month='April' />
<abstract><t>This document defines ZRTP, a protocol for media path Diffie-Hellma
n exchange to agree on a session key and parameters for establishing unicast Sec
ure Real-time Transport Protocol (SRTP) sessions for Voice over IP (VoIP) applic
ations. The ZRTP protocol is media path keying because it is multiplexed on the
same port as RTP and does not require support in the signaling protocol. ZRTP
does not assume a Public Key Infrastructure (PKI) or require the complexity of c
ertificates in end devices. For the media session, ZRTP provides confidentialit
y, protection against man-in-the-middle (MiTM) attacks, and, in cases where the
signaling protocol provides end-to-end integrity protection, authentication. ZR
TP can utilize a Session Description Protocol (SDP) attribute to provide discove
ry and authentication through the signaling channel. To provide best effort SRT
P, ZRTP utilizes normal RTP/AVP (Audio-Visual Profile) profiles. ZRTP secures me
dia sessions that include a voice media stream and can also secure media session
s that do not include voice by using an optional digital signature. This docume
nt is not an Internet Standards Track specification; it is published for inform
ational purposes.</t></abstract>
</front>
<seriesInfo name='RFC' value='6189'/>
<seriesInfo name='DOI' value='10.17487/RFC6189'/>
</reference>
<reference anchor="AGILITY" target='https://www.rfc-editor.org/info/rfc7696'>
<front>
<title>Guidelines for Cryptographic Algorithm Agility and Selecting Mandatory-to
-Implement Algorithms</title>
<author initials='R.' surname='Housley' fullname='R. Housley'><organization /></
author>
<date year='2015' month='November' />
<abstract><t>Many IETF protocols use cryptographic algorithms to provide confide
ntiality, integrity, authentication, or digital signature. Communicating peers
must support a common set of cryptographic algorithms for these mechanisms to wo
rk properly. This memo provides guidelines to ensure that protocols have the ab
ility to migrate from one mandatory-to-implement algorithm suite to another over
time.</t></abstract>
</front>
<seriesInfo name='BCP' value='201'/>
<seriesInfo name='RFC' value='7696'/>
<seriesInfo name='DOI' value='10.17487/RFC7696'/>
</reference> </reference>
<reference anchor="ICE" target='https://www.rfc-editor.org/info/rfc8445'> </references>
<front>
<title>Interactive Connectivity Establishment (ICE): A Protocol for Network Addr
ess Translator (NAT) Traversal</title>
<author initials='A.' surname='Keranen' fullname='A. Keranen'><organization /></
author>
<author initials='C.' surname='Holmberg' fullname='C. Holmberg'><organization />
</author>
<author initials='J.' surname='Rosenberg' fullname='J. Rosenberg'><organization
/></author>
<date year='2018' month='July' />
<abstract><t>This document describes a protocol for Network Address Translator (
NAT) traversal for UDP-based communication. This protocol is called Interactive
Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utili
ties for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN)
.</t><t>This document obsoletes RFC 5245.</t></abstract>
</front>
<seriesInfo name='RFC' value='8445'/>
<seriesInfo name='DOI' value='10.17487/RFC8445'/>
</reference>
<reference anchor="RTP" target='https://www.rfc-editor.org/info/rfc3550'> <references>
<front> <name>Informative References</name>
<title>RTP: A Transport Protocol for Real-Time Applications</title> <reference anchor="UKS">
<author initials='H.' surname='Schulzrinne' fullname='H. Schulzrinne'><organizat <front>
ion /></author> <title>Unknown Key-Share Attacks on the Station-to-Station (STS) Pro
<author initials='S.' surname='Casner' fullname='S. Casner'><organization /></au tocol</title>
thor> <author initials="S." surname="Blake-Wilson">
<author initials='R.' surname='Frederick' fullname='R. Frederick'><organization <organization/>
/></author> </author>
<author initials='V.' surname='Jacobson' fullname='V. Jacobson'><organization /> <author initials="A." surname="Menezes">
</author> <organization/>
<date year='2003' month='July' /> </author>
<abstract><t>This memorandum describes RTP, the real-time transport protocol. R <date year="1999" month="March"/>
TP provides end-to-end network transport functions suitable for applications tra </front>
nsmitting real-time data, such as audio, video or simulation data, over multicas <seriesInfo name="DOI" value="10.1007/3-540-49162-7_12"/>
t or unicast network services. RTP does not address resource reservation and do <refcontent>Public Key Cryptography</refcontent>
es not guarantee quality-of- service for real-time services. The data transport <refcontent>Lecture Notes in Computer Science</refcontent>
is augmented by a control protocol (RTCP) to allow monitoring of the data deliv <refcontent>Vol. 1560</refcontent>
ery in a manner scalable to large multicast networks, and to provide minimal con </reference>
trol and identification functionality. RTP and RTCP are designed to be independ
ent of the underlying transport and network layers. The protocol supports the u
se of RTP-level translators and mixers. Most of the text in this memorandum is i
dentical to RFC 1889 which it obsoletes. There are no changes in the packet for
mats on the wire, only changes to the rules and algorithms governing how the pro
tocol is used. The biggest change is an enhancement to the scalable timer algori
thm for calculating when to send RTCP packets in order to minimize transmission
in excess of the intended rate when many participants join a session simultaneou
sly. [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='STD' value='64'/>
<seriesInfo name='RFC' value='3550'/>
<seriesInfo name='DOI' value='10.17487/RFC3550'/>
</reference>
<reference anchor="RTCP-MUX" target='https://www.rfc-editor.org/info/rfc5761'> <reference anchor="SIGMA">
<front> <front>
<title>Multiplexing RTP Data and Control Packets on a Single Port</title> <title>SIGMA: The 'SIGn-and-MAc' Approach to Authenticated Diffie-He
<author initials='C.' surname='Perkins' fullname='C. Perkins'><organization /></ llman and Its Use in the IKE Protocols</title>
author> <author initials="H." surname="Krawczyk">
<author initials='M.' surname='Westerlund' fullname='M. Westerlund'><organizatio <organization/>
n /></author> </author>
<date year='2010' month='April' /> <date year="2003" month="August"/>
<abstract><t>This memo discusses issues that arise when multiplexing RTP data pa </front>
ckets and RTP Control Protocol (RTCP) packets on a single UDP port. It updates R <seriesInfo name="DOI" value="10.1007/978-3-540-45146-4_24"/>
FC 3550 and RFC 3551 to describe when such multiplexing is and is not appropriat <refcontent>Advances in Cryptology -- CRYPTO 2003</refcontent>
e, and it explains how the Session Description Protocol (SDP) can be used to sig <refcontent>Lecture Notes in Computer Science</refcontent>
nal multiplexed sessions. [STANDARDS-TRACK]</t></abstract> <refcontent>Vol. 2729</refcontent>
</front> </reference>
<seriesInfo name='RFC' value='5761'/>
<seriesInfo name='DOI' value='10.17487/RFC5761'/>
</reference>
<reference anchor="WEBRTC" target="https://www.w3.org/TR/2019/CR-webrtc-
20191213/">
<front>
<title>WebRTC 1.0: Real-time Communication Between Browsers</title>
<author initials="C." surname="Jennings">
<organization/>
</author>
<author initials="H." surname="Boström">
<organization/>
</author>
<author initials="J-I." surname="Bruaroey">
<organization/>
</author>
<date year="2019" month="December" day="13"/>
</front>
<refcontent>W3C Candidate Recommendation</refcontent>
</reference>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.3725.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.6189.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.7696.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8445.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.3550.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.5761.xml"/>
</references>
</references> </references>
<section anchor="acknowledgements" numbered="false" toc="default">
<name>Acknowledgements</name>
<section anchor="acknowledgements" title="Acknowledgements"> <t>This problem would not have been discovered if it weren't for
discussions with <contact fullname="Sam Scott"/>, <contact
<t>This problem would not have been discovered if it weren’t for discussions wit fullname="Hugo Krawczyk"/>, and <contact fullname="Richard Barnes"/>. A
h solution similar to the one presented here was first proposed by
Sam Scott, Hugo Krawczyk, and Richard Barnes. A solution similar to the one <contact fullname="Karthik Bhargavan"/>, who provided valuable input on
presented here was first proposed by Karthik Bhargavan who provided valuable this document. <contact fullname="Thyla van der Merwe"/> assisted with
input on this document. Thyla van der Merwe assisted with a formal model of the a formal model of the solution. <contact fullname="Adam Roach"/> and
solution. Adam Roach and Paul E. Jones provided significant review and input.</ <contact fullname="Paul E. Jones"/> provided significant review and
t> input.</t>
</section>
</section>
</back> </back>
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