RTPSEC P. Zimmermann
Internet-Draft Zfone Project
Intended status: Informational A. Johnston, Ed.
Expires: January 10, 2008 Avaya
J. Callas
PGP Corporation
July 9, 2007
ZRTP: Media Path Key Agreement for Secure RTP
draft-zimmermann-avt-zrtp-04
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Abstract
This document defines ZRTP, a protocol for media path Diffie-Hellman
exchange to agree on a session key and parameters for establishing
Secure Real-time Transport Protocol (SRTP) sessions. 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.
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ZRTP does not assume a Public Key Infrastructure (PKI) infrastructure
or require the complexity of certificates in end devices. For the
media session, ZRTP provides confidentiality, protection against Man
in the Middle (MITM) attacks, and, in cases where a secret is
available from the signaling protocol, authentication. ZRTP can
utilize two Session Description Protocol (SDP) attributes to provide
discovery and authentication through the signaling channel. To
provide best effort SRTP, ZRTP utilizes normal RTP/AVP profiles.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Media Security Requirements . . . . . . . . . . . . . . . . . 5
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Key Agreement Modes . . . . . . . . . . . . . . . . . . . 7
4.1.1. Diffie-Hellman Mode . . . . . . . . . . . . . . . . . 7
4.1.2. Preshared Mode . . . . . . . . . . . . . . . . . . . . 9
5. Protocol Description . . . . . . . . . . . . . . . . . . . . . 9
5.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Commit Contention Resolution . . . . . . . . . . . . . . . 10
5.3. Shared Secret Determination . . . . . . . . . . . . . . . 11
5.3.1. Responder Behavior . . . . . . . . . . . . . . . . . . 11
5.3.2. Initiator Behavior . . . . . . . . . . . . . . . . . . 12
5.4. Diffie-Hellman Mode . . . . . . . . . . . . . . . . . . . 12
5.4.1. Hash Commitment . . . . . . . . . . . . . . . . . . . 13
5.4.2. Responder Behavior . . . . . . . . . . . . . . . . . . 13
5.4.3. Initiator Behavior . . . . . . . . . . . . . . . . . . 14
5.4.4. Shared Secret Calculation . . . . . . . . . . . . . . 14
5.5. Preshared Mode . . . . . . . . . . . . . . . . . . . . . . 16
5.5.1. Hash Commitment . . . . . . . . . . . . . . . . . . . 16
5.5.2. Responder Behavior . . . . . . . . . . . . . . . . . . 17
5.5.3. Initiator Behavior . . . . . . . . . . . . . . . . . . 17
5.5.4. Shared Secret Calculation . . . . . . . . . . . . . . 17
5.6. Key Generation . . . . . . . . . . . . . . . . . . . . . . 18
5.7. Confirmation . . . . . . . . . . . . . . . . . . . . . . . 19
5.8. Random Number Generation . . . . . . . . . . . . . . . . . 20
5.9. ZID and Cache Operation . . . . . . . . . . . . . . . . . 21
5.10. Terminating an SRTP Session or ZRTP Exchange . . . . . . . 21
6. ZRTP Messages . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1. ZRTP Message Formats . . . . . . . . . . . . . . . . . . . 24
6.1.1. Message Type Block . . . . . . . . . . . . . . . . . . 24
6.1.2. Hash Type Block . . . . . . . . . . . . . . . . . . . 26
6.1.3. Cipher Type Block . . . . . . . . . . . . . . . . . . 26
6.1.4. Auth Tag Block . . . . . . . . . . . . . . . . . . . . 26
6.1.5. Key Agreement Type Block . . . . . . . . . . . . . . . 27
6.1.6. SAS Type Block . . . . . . . . . . . . . . . . . . . . 27
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6.1.7. Signature Block . . . . . . . . . . . . . . . . . . . 28
6.2. Hello message . . . . . . . . . . . . . . . . . . . . . . 28
6.3. HelloACK message . . . . . . . . . . . . . . . . . . . . . 30
6.4. Commit message . . . . . . . . . . . . . . . . . . . . . . 31
6.5. DHPart1 message . . . . . . . . . . . . . . . . . . . . . 32
6.6. DHPart2 message . . . . . . . . . . . . . . . . . . . . . 33
6.7. Confirm1 and Confirm2 messages . . . . . . . . . . . . . . 34
6.8. Conf2ACK message . . . . . . . . . . . . . . . . . . . . . 36
6.9. Error message . . . . . . . . . . . . . . . . . . . . . . 37
6.10. ErrorACK message . . . . . . . . . . . . . . . . . . . . . 38
6.11. GoClear message . . . . . . . . . . . . . . . . . . . . . 39
6.12. ClearACK message . . . . . . . . . . . . . . . . . . . . . 39
7. Retransmissions . . . . . . . . . . . . . . . . . . . . . . . 40
8. Short Authentication String . . . . . . . . . . . . . . . . . 41
8.1. SAS Verified Flag . . . . . . . . . . . . . . . . . . . . 42
8.2. Signing the SAS . . . . . . . . . . . . . . . . . . . . . 43
8.3. Relaying the SAS through a PBX . . . . . . . . . . . . . . 43
9. Signaling Interactions . . . . . . . . . . . . . . . . . . . . 45
10. The ZRTP Disclosure flag . . . . . . . . . . . . . . . . . . . 49
11. Intermediary ZRTP Devices . . . . . . . . . . . . . . . . . . 50
12. RTP Header Extension Flag for ZRTP . . . . . . . . . . . . . . 52
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 52
14. Security Considerations . . . . . . . . . . . . . . . . . . . 53
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 57
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
16.1. Normative References . . . . . . . . . . . . . . . . . . . 57
16.2. Informative References . . . . . . . . . . . . . . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 59
Intellectual Property and Copyright Statements . . . . . . . . . . 61
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1. Introduction
ZRTP is a key agreement protocol which performs Diffie-Hellman key
exchange during call setup in the media path, and is transported over
the same port as the Real-time Transport Protocol (RTP) [2] media
stream which has been established using a signaling protocol such as
Session Initiation Protocol (SIP) [19]. This generates a shared
secret which is then used to generate keys and salt for a Secure RTP
(SRTP) [3] session. ZRTP borrows ideas from PGPfone [15]. A
reference implementation of ZRTP is available as Zfone [16].
The ZRTP protocol has some nice cryptographic features lacking in
many other approaches to media session encryption. Although it uses
a public key algorithm, it does not rely on a public key
infrastructure (PKI). In fact, it does not use persistent public
keys at all. It uses ephemeral Diffie-Hellman (DH) with hash
commitment, and allows the detection of Man in the Middle (MITM)
attacks by displaying a short authentication string for the users to
read and compare over the phone. It has perfect forward secrecy,
meaning the keys are destroyed at the end of the call, which
precludes retroactively compromising the call by future disclosures
of key material. But even if the users are too lazy to bother with
short authentication strings, we still get reasonable authentication
against a MITM attack, based on a form of key continuity. It does
this by caching some key material to use in the next call, to be
mixed in with the next call's DH shared secret, giving it key
continuity properties analogous to SSH. All this is done without
reliance on a PKI, key certification, trust models, certificate
authorities, or key management complexity that bedevils the email
encryption world. It also does not rely on SIP signaling for the key
management, and in fact does not rely on any servers at all. It
performs its key agreements and key management in a purely peer-to-
peer manner over the RTP packet stream.
If the endpoints have a mechanism for knowing or retrieving the other
endpoint's signature key, the short authentication string can be
authenticated by exchanging a signature over the short authentication
string.
ZRTP can be used and discovered without being declared or indicated
in the signaling path. This provides the a best effort SRTP
capability. Also, this reduces the complexity of implementations and
minimizes interdependency between the signaling and media layers.
When ZRTP is indicated in the signaling and the SDP attribute
extensions are used, ZRTP has additional useful properties. When the
signaling path has end-to-end integrity protection, the short
authentication string can be compared automatically by the ZRTP
endpoints. By sending a unique ZRTP Identifier (ZID) in the
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signaling, ZRTP provides a useful binding between the signaling and
media paths.
The following sections provide an overview of the ZRTP protocol,
describe the key agreement algorithm and RTP message formats.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 and
indicate requirement levels for compliant implementations [1].
3. Media Security Requirements
This section discuses how ZRTP meets all ten RTP security
requirements discussed in Section 4 of [13].
Since ZRTP is a media path key agreement approach, it meets the
following requirements:
R1: Forking and retargeting MUST work with all end-points being SRTP.
R2: Forking and retargeting MUST allow establishing SRTP or RTP with
a mixture of SRTP- and RTP-capable targets.
R3: With forking, only the entity to which the call is finally
established, MUST get hold of the media encryption keys.
Note: R4 is not present in [13].
R5: A solution SHOULD avoid clipping media before SDP answer without
additional signalling.
ZRTP's use of Diffie-Hellman key agreement allows it to meet these
requirements:
R6: A solution MUST provide protection against passive attacks.
R7: A solution MUST be able to support Perfect Forward Secrecy.
ZRTPs meet the following requirements with its handling of algorithm
lists:
R8: A solution MUST support algorithm negotiation without incurring
per-algorithm computational expense.
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R9: A solution MUST support multiple cipher suites without additional
computational expense.
The use of the a=zrtp-zid allows ZRTP to meet this requirement:
R10: Endpoint identification when forking.
The use of the optional signature block in the Confirm1 and Confirm2
messages allows ZRTP to meet this requirement:
R11: A solution MUST NOT require 3rd-party certs. If two parties
share an auth infrastructure they should be able to use it.
4. Overview
This section provides a description of how ZRTP works. This
description is non-normative in nature but is included to build
understanding of the protocol.
ZRTP is negotiated the same way a conventional RTP session is
negotiated in an offer/answer exchange using the standard AVP/RTP
profile. The ZRTP protocol begins after two endpoints have utilized
a signaling protocol such as SIP and are ready to send. If ICE [26]
is being used, ZRTP begins after ICE has completed its connectivity
checks.
ZRTP is multiplexed on the same ports as RTP. It uses a unique
header that makes it clearly differentiable from RTP or STUN.
In environments in which sending ZRTP packets to non-ZRTP endpoints
might cause problems and signaling path discovery is not an option,
ZRTP endpoints can include the RTP header extension flag in normal
RTP packets sent at the start of a session as a probe to discover if
the other endpoint supports ZRTP. If the flag is received from the
other endpoint, ZRTP messages can then be exchanged.
A ZRTP endpoint initiates the exchange by sending a ZRTP Hello
message to the other endpoint. The purpose of the Hello message is
to confirm the endpoint supports the protocol and to see what
algorithms the two ZRTP endpoints have in common.
The Hello message contains the SRTP configuration options, and the
ZID. Each instance of ZRTP has a unique 96-bit random ZRTP ID or ZID
that is generated once at installation time. ZIDs are discovered
during the Hello message exchange. The received ZID is used to look
up retained shared secrets from previous ZRTP sessions with the
endpoint.
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A response to a ZRTP Hello message is a ZRTP HelloACK message. The
HelloACK message simply acknowledges receipt of the Hello. Since RTP
commonly uses best effort UDP transport, ZRTP has retransmission
timers in case of lost datagrams. There are two timers, both with
exponential backoff mechanisms. One timer is used for
retransmissions of Hello messages and the other is used for
retransmissions of all other messages after receipt of a HelloACK.
4.1. Key Agreement Modes
After both endpoints exchange Hello and HelloACK messages, the key
agreement exchange can begin with the ZRTP Commit message. ZRTP
supports a number of key agreement modes including both Diffie-
Hellman and non-Diffie-Hellman modes as described in the following
sections.
4.1.1. Diffie-Hellman Mode
An example ZRTP call flow is shown in Figure 1 below. Note that the
order of the Hello/HelloACK exchanges in F1/F2 and F3/F4 may be
reversed. That is, either Alice or Bob might send the first Hello
message. Also, an endpoint that receives a Hello message and wishes
to immediately begin the ZRTP key agreement can omit the HelloACK and
send the Commit instead. In Figure 1, this would result in messages
F2, F3, and F4 being omitted. Note that the endpoint which sends the
Commit message is considered the initiator of the ZRTP session and
drives the key agreement exchange. The Diffie-Hellman public values
are exchanged in the DHPart1 and DHPart2 messages. SRTP keys and
salts are then calculated.
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Alice Bob
| |
| Alice and Bob establish a media session.|
| They initiate ZRTP on media ports |
| |
| Hello (version, options, Alice's ZID) F1|
|---------------------------------------->|
| HelloACK F2 |
|<----------------------------------------|
| Hello (version, options, Bob's ZID) F3 |
|<----------------------------------------|
| HelloACK F4 |
|---------------------------------------->|
| |
| Bob acts as the initiator |
| |
| Commit (Bob's ZID, options, hvi) F5 |
|<----------------------------------------|
| DHPart1 (pvr, shared secret hashes) F6 |
|---------------------------------------->|
| DHPart2 (pvi, shared secret hashes) F7 |
|<----------------------------------------|
| |
| Alice and Bob generate SRTP session key.|
| |
| SRTP begins |
|<=======================================>|
| |
| Confirm1 (HMAC, D,S,V flags, sig) F8 |
|---------------------------------------->|
| Confirm2 (HMAC, D,S,V flags, sig) F9 |
|<----------------------------------------|
| Confirm2ACK F10 |
|---------------------------------------->|
Figure 1. Establishment of an SRTP session using ZRTP
ZRTP authentication uses a Short Authentication String (SAS) which is
ideally displayed for the human user. Alternatively, the SAS can be
transported over the signaling channel in the SDP and compared
automatically, provided the signaling has end-to-end integrity
protection. Or, the SAS can be authenticated by exchanging a digital
signature (sig) over the short authentication string in the Confirm1
or Confirm2 messages.
The ZRTP Confirm1 and Confirm2 messages are sent for a number of
reasons. First, they confirm that all the key agreement calculations
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were successful and thus the encryption will work, and they enable
automatic detection of a DH MITM attack from a reckless attacker who
does not know the retained shared secret. Digital signatures over
the SAS can be exchanged to authenticate the exchange. And, they
enable ZRTP to transmit some parameters under cover of CFB
encryption, such as the Disclosure flag (D), the Allow Clear flag
(A), and most importantly the SAS Verified flag (V SAS Verified flag
(V), shielding it from a passive observer who would like to know if
the human users are in the habit of diligently verifying the SAS.
4.1.2. Preshared Mode
In the Preshared Mode, endpoints can skip the DH calculation if they
have a shared secret from a previous ZRTP session. Preshared mode is
indicated in the Commit message and results in the same call flow as
Figure 1. The DHPart1 and DHPart2 messages are exchanged so that the
set of shared secrets can be determined, but the pvr and pvi are
replaced by random nonces and no DH calculation is performed.
Instead nonces from the DHPart1 and DHPart2 are exchanged and used
along with the retained secrets to derive new unique session key
material. This mode could be useful for slow processor endpoints so
that a DH calculation does not need to be performed every session.
Or, this mode could be used to rapidly re-establish an earlier
session that was recently torn down or interrupted without the need
to perform another DH calculation. Since the cache is not affected
during this mode, multiple Preshared mode exchanges can be processed
at a time between two endpoints.
5. Protocol Description
ZRTP MUST be multiplexed on the same ports as the RTP media packets.
To support best effort encryption [13], ZRTP uses normal RTP/AVP
profile (AVP) media lines in the initial offer/answer exchange. The
ZRTP SDP attribute flag a=zrtp-id defined in Section 9 SHOULD be used
in all offers and answers to indicate support for the ZRTP protocol.
In subsequent offer/answer exchanges after a successful ZRTP exchange
has resulted in an SRTP session, the Secure RTP/AVP (SAVP) profile
MAY be used.
5.1. Discovery
During the ZRTP discovery phase, a ZRTP endpoint discovers if the
other endpoint supports ZRTP and the supported algorithms and
options. This information is transported in a Hello message.
ZRTP endpoints SHOULD include the SDP attribute a=zrtp-zid in offers
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and answers, as defined in Section 9. ZRTP MAY use an RTP [2]
extension field as a flag to indicate support for the ZRTP protocol
in RTP packets as described in Section 12.
The Hello message includes the ZRTP version, hash, cipher,
authentication method and tag length, key agreement type, and Short
Authentication String (SAS) algorithms that are supported. In
addition, each endpoint sends and discovers ZIDs. The received ZID
is used to retrieve previous retained shared secrets, rs1 and rs2.
If the endpoint has other secrets, then they are also collected.
Details on how to derive the signaling secret, sigs, and SRTP secret,
srtps, are in Section 9.
Additional shared secrets can be defined and used as other_secret.
If no secret of a given type is available, a random value is
generated and used for that secret to ensure a mismatch in the hash
comparisons in the DHPart1 and DHPart2 messages. This prevents an
eavesdropper from knowing how many shared secrets are available
between the endpoints.
A Hello message can be sent at any time, but is usually sent at the
start of an RTP session to determine if the other endpoint supports
ZRTP, and also if the SRTP implementations are compatible. A Hello
message is retransmitted using timer T1 and an exponential backoff
mechanism detailed in Section 7 until the receipt of a HelloACK
message or a Commit message.
5.2. Commit Contention Resolution
After receiving a Hello message from the other endpoint, a Commit
message can be sent to begin the ZRTP key exchange. The endpoint
that sends the Commit is known as the initiator, while the receiver
of the Commit is known as the responder.
If both sides send Commit messages initiating a secure session at the
same time, the Commit message with the lowest hvi value is discarded
and the other side is the initiator. This breaks the tie, allowing
the protocol to proceed from this point with a clear definition of
who is the initiator and who is the responder.
Because the DH exchange affects the state of the retained shared
secret cache, only one in-process ZRTP DH exchange may occur at a
time between two ZRTP endpoints. Otherwise, race conditions and
cache integrity problems will result. When multiple media streams
are established in parallel between the same pair of ZRTP endpoints
(determined by the ZIDs in the Hello Messages), only one can be
processed. Once that exchange completes with Confirm2 and Conf2ACK
messages, another ZRTP DH exchange can begin.
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In the event that Commit messages are sent by both ZRTP endpoints at
the same time, but are received in different media streams, the same
resolution rules apply - the Commit message with the lowest hvi value
is discarded and the other side is the initiator. The media stream
in which the Commit was sent will proceed through the ZRTP exchange
while the media stream with the discarded Commit must wait for the
completion of the other ZRTP exchange.
5.3. Shared Secret Determination
The following sections describe how ZRTP endpoints generate the set
of shared secrets s1, s2, s3, s4, and s5 through the exchange of the
DHPart1 and DHPart2 messages.
5.3.1. Responder Behavior
The responder calculates an HMAC keyed hash using the first retained
shared secret, rs1, as the key on the string "Responder" which
generates a retained secret ID, rs1IDr, which is truncated to the
leftmost 64 bits. HMACs are calculated in a similar way for
additional shared secrets:
rs1IDr = HMAC(rs1, "Responder")
rs2IDr = HMAC(rs2, "Responder")
sigsIDr = HMAC(sigs, "Responder")
srtpsIDr = HMAC(srtps, "Responder")
other_secretIDr = HMAC(other_secret, "Responder")
The set of keyed hashes (HMACs) are included by the responder in the
DHPart1 message.
The HMACs of the possible shared secrets received in the DHPart2 can
be compared against the HMACs of the local set of possible shared
secrets.
The expected HMAC values of the shared secrets are calculated (using
the string "Initiator" instead of "Responder") as in Section 5.3.2
and compared to the HMACs received in the DHPart2 message. The
secrets corresponding to matching HMACs are kept while the secrets
corresponding to the non-matching ones are replaced with a null,
which is assumed to have a zero length for the purposes of hashing
them later. The set of up to five actual shared secrets are then s1,
s2, s3, s4, and s5 - the order is that chosen by the initiator.
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5.3.2. Initiator Behavior
The initiator calculates an HMAC keyed hash using the first retained
shared secret, rs1, as the key on the string "Initiator" which
generates a retained secret ID, rs1IDi, which is truncated to the
leftmost 64 bits. HMACs are calculated in a similar way for
additional shared secrets:
rs1IDi = HMAC(rs1, "Initiator")
rs2IDi = HMAC(rs2, "Initiator")
sigsIDi = HMAC(sigs, "Initiator")
srtpsIDi = HMAC(srtps, "Initiator")
other_secretIDi = HMAC(other_secret, "Initiator")
These HMACs are included by the initiator in the DHPart2 message.
The initiator then calculates the set of secret IDs that are expected
to be received from the responder in the DHPart1 message by
substituting the string "Responder" instead of "Initiator" as in
Section 5.3.1.
The HMACs of the possible shared secrets received are compared
against the HMACs of the local set of possible shared secrets.
The secrets corresponding to matching HMACs are kept while the
secrets corresponding to the non-matching ones are replaced with a
null, which is assumed to have a zero length for the purposes of
hashing them later. The set of up to five actual shared secrets are
then s1, s2, s3, s4, and s5 - the order is that chosen by the
initiator.
For example, consider two ZRTP endpoints who share secrets rs1, rs2,
and other_secret, which is a hash of a secret passphrase. During the
comparison, rs1ID, rs2ID, and other_secretID will match but sigsID
and srtpsID will not. As a result, s1 = rs1, s2 = rs2, s5 =
other_secret, while s3 and s4 will be nulls.
5.4. Diffie-Hellman Mode
The purpose of the Diffie-Hellman exchange is for the two ZRTP
endpoints to generate a new shared secret, s0. In addition, the
endpoints discover if they have any shared secrets in common. If
they do, this exchange allows them to discover how many and agree on
an ordering for them: s1, s2, etc.
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5.4.1. Hash Commitment
From the intersection of the algorithms in the sent and received
Hello messages, the initiator chooses a hash, cipher, auth tag, key
agreement type, and SAS type to be used.
A Diffie-Hellman mode is selected by setting the Key Agreement Type
to DH4k or DH3k in the Commit. In this mode, the key agreement
begins with the initiator choosing a fresh random Diffie-Hellman (DH)
secret value (svi) based on the chosen key agreement type value, and
computing the public value. (Note that to speed up processing, this
computation can be done in advance.) For guidance on generating
random numbers, see the section on Random Number Generation. The
Diffie-Hellman secret value, svi, SHOULD be twice as long as the AES
key length. This means, if AES 128 is used, the DH secret value
SHOULD be 256 bits long. If AES 256 is used, the secret value SHOULD
be 512 bits long.
pvi = g^svi mod p
where g and p are determined by the key agreement type value.
The hash commitment is performed by the initiator of the ZRTP
exchange. The hash value of the initiator, hvi, includes a hash of
the entire DHPart2 message as shown in Figure 7 (which includes the
Diffie-Hellman public value, pvi), and the responder's Hello message:
hvi=hash(initiator's DHPart2 message | responder's Hello message)
Note that the Hello message includes the fields shown in Figure 3.
The information from the responder's Hello message is included in the
hash calculation to prevent a bid-down attack by modification of the
responder's Hello message.
The initiator sends hvi in the Commit message.
5.4.2. Responder Behavior
Upon receipt of the Commit message, the responder generates its own
fresh random DH secret value, svr, and computes the public value.
(Note that to speed up processing, this computation can be done in
advance.) For guidance on random number generation, see the section
on Random Number Generation. The Diffie-Hellman secret value, svr,
SHOULD be twice as long as the AES key length. This means, if AES
128 is used, the DH secret value SHOULD be 256 bits long. If AES 256
is used, the secret value SHOULD be 512 bits long.
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pvr = g^svr mod p
Upon receipt of the DHPart2 message, the responder checks that the
initiator's public DH value is not equal to 1 or p-1. An attacker
might inject a false DHPart2 packet with a value of 1 or p-1 for
g^svi mod p, which would cause a disastrously weak final DH result to
be computed. If pvi is 1 or p-1, the user should be alerted of the
attack and the protocol exchange must be terminated. Otherwise, the
responder computes its own value for the hash commitment using the
public DH value (pvi) received in the DHPart2 packet and its Hello
packet and compares the result with the hvi received in the Commit
packet. If they are different, a MITM attack is taking place and the
user is alerted and the protocol exchange terminated.
The responder then calculates the Diffie-Hellman result:
DHResult = pvi^svr mod p
5.4.3. Initiator Behavior
Upon receipt of the DHPart1 message, the initiator checks that the
responder's public DH value is not equal to 1 or p-1. An attacker
might inject a false DHPart1 packet with a value of 1 or p-1 for
g^svr mod p, which would cause a disastrously weak final DH result to
be computed. If pvr is 1 or p-1, the user should be alerted of the
attack and the protocol exchange must be terminated.
The initiator then sends a DHPart2 message containing the initiator's
public DH value and the set of calculated retained secret IDs as
described in 5.3.2.
The initiator calculates the same Diffie-Hellman result using:
DHResult = pvr^svi mod p
For both the initiator and responder, the DHResult is formatted as a
byte string of the length of the modulus p, in big-endian format.
For example, for a 3072-bit p, DHResult would be a 384 byte value,
with the first byte the most significant. If the result of the DH
calculation contains leading zeros, they are included in the
DHResult, to make it always come out to the same length as p.
5.4.4. Shared Secret Calculation
A hash of the received and sent ZRTP messages in the current ZRTP
exchange in the following order is calculated:
total_hash = hash(Hello of responder | Commit | DHPart1 | DHPart2)
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Note that only the ZRTP message (Figures 3, 5, 6, and 7), not the
entire ZRTP packets, are included in the total_hash.
The calculation of the final shared secret, s0, is in compliance with
the recommendations in sections 5.8.1 and 6.1.2.1 of NIST SP800-56A
[7]. This is done by hashing a concatenation of a number of items,
including the DHResult, the ZID's of the initiator and the responder,
the total_hash, and the set of non-null shared secrets as described
in 5.2.
In section 5.8.1 of NIST SP800-56A [7], NIST requires certain
parameters to be hashed together in a particular order, which NIST
refers to as: Z, AlgorithmID, PartyUInfo, PartyVInfo, SuppPubInfo,
and SuppPrivInfo. In our implementation, our DHResult corresponds to
Z, "ZRTP-HMAC-KDF" corresponds to AlgorithmID, our ZIDi and ZIDr
correspond to PartyUInfo and PartyVInfo, our total_hash corresponds
to SuppPubInfo, and s1 through s5 corresponds to SuppPrivInfo. NIST
also requires a 32-bit big-endian integer counter to be included in
the hash each time the hash is computed, which we have set to the
fixed value of 1, because we only compute the hash once.
s0 = hash( counter | DHResult | "ZRTP-HMAC-KDF" | ZIDi | ZIDr |
total_hash | len(s1) | s1 | len(s2) | s2 | len(s3) | s3 | len(s4) |
s4 | len(s5) | s5 )
The length of the DHResult field was implicitly agreed to by the
negotiated DH prime size. The length of total_hash is implicitly
determined by the negotiated hash algoritm. ZIDi is the initiator's
fixed-length ZID, and ZIDr is the responder's fixed-length ZID. All
of the explicit length fields in the above hash are 32-bit big-endian
integers, giving the length in octets of the field that follows.
Some of the shared secrets s1 through s5 may have lengths of zero if
they are null (not shared), and are each preceded by a 4-octet length
field. For example, if s4 is null, len(s4) is 00 00 00 00, and s4
itself would be absent from the hash calculation, which means len(s5)
would immediately follow len(s4). While inclusion of ZIDi and ZIDr
may be redundant, because they are implicitly included in the
total_hash, we explicitly include them here to follow NIST SP800-56A.
The string "ZRTP-HMAC-KDF" (not null-terminated) identifies what
purpose the resulting s0 will be used for, which is to serve as the
master key for the ZRTP HMAC-based key derivation function defined in
section 5.6.
A new rs1 is calculated from s0:
rs1 = HMAC(s0,"retained secret")
After a successful exchange of Confirm1 and Confirm2 messages
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described later in this document, both sides now discard the rs2
value and store rs1 as rs2.
5.5. Preshared Mode
The Preshared key agreement mode can be used to generate SRTP keys
and salts without a DH calculation, instead relying on one or more
shared secrets from previous DH calculations between the endpoints.
This key agreement mode is useful for efficiently adding another
media stream to an existing secure session, such as adding video to a
session that already has performed a DH key agreement for the audio
stream. It can also be used to rapidly re-establish a secure session
between two parties who have recently started and ended a secure
session that has already performed a DH key agreement, without
performing another lengthy DH calculation, which may be desirable on
slow processors in resource-limited environments.
5.5.1. Hash Commitment
This mode is selected by setting the Key Agreement Type to Preshared
in the Commit message. From the intersection of the algorithms in
the sent and received Hello messages, the initiator chooses a hash,
cipher, auth tag, key agreement type, and SAS type to be used.
The hash commitment is performed by the initiator of the ZRTP
exchange. The hash value of the initiator, hvi, includes a hash of
the entire DHPart2 message (which includes a random nonce instead of
a DH public value pvi), and the responder's Hello message:
hvi=hash(initiator's DHPart2 message | responder's Hello message)
Note that the Hello message includes the fields shown in Figure 3.
The information from the responder's Hello message is included in the
hash calculation to prevent a bid-down attack by modification of the
responder's Hello message.
The initiator sends hvi in the Commit message.
The DHPart1 and DHPart2 messages are exchanged in this mode so that
the shared secrets can be determined. If it is determined that the
endpoints have no shared DH secrets (i.e. either rs1 or rs2) the
exchange MUST be terminated. It is RECOMMENDED that Preshared mode
only be used when the SAS Verified flag is set.
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5.5.2. Responder Behavior
Since no DH calculation is performed in Preshared Mode, no pvr is
sent in the DHPart1 message. In place of pvr in the DHPart1, a
random number, noncer, 32 octets long is chosen. Its value MUST be
unique for all nonce values chosen for all ZRTP sessions between a
pair of endpoints since the last DH exchange. This ensures that a
unique SRTP key and salt pairs are generated for each media session.
If both nonces are reused, it would result in the same key and salt
being generated for multiple streams which would introduce a major
security weakness.
If a DHPart1 is received with a reused nonce value, the ZRTP exchange
MAY be immediately terminated. (We would say MUST be terminated, but
we recognize it may be hard to determine if the nonce was never used
before. In practical terms, a random nonce of this length has
effectively no chance of repeating by accident. Also, either party
may unilaterally force uniqueness by making their own nonce unique.)
5.5.3. Initiator Behavior
Since no DH calculation is performed in Preshared Mode, no pvi is
sent in the DHPart2 message. In place of pvi in the DHPart2, a
random number, noncei, 32 octets long is chosen. Its value MUST be
unique for all nonce values chosen for all ZRTP sessions between a
pair of endpoints since the last DH exchange. This ensures that a
unique SRTP key and salt pairs are generated for each media session.
If a nonce is reused, it would result in the same key and salt being
generated for multiple streams which would introduce a major security
weakness.
If a DHPart2 is received with a reused nonce value, the ZRTP exchange
MAY be immediately terminated. (We would say MUST be terminated, but
we recognize it may be hard to determine if the nonce was never used
before. In practical terms, a random nonce of this length has
effectively no chance of repeating by accident. Also, either party
may unilaterally force uniqueness by making their own nonce unique.)
5.5.4. Shared Secret Calculation
A hash of the received and sent ZRTP messages in the current ZRTP
exchange in the following order is calculated:
total_hash = hash (Hello of responder | Commit | DHPart1 | DHPart2 )
Note that only the ZRTP message (Figures 3, 5, 6, and 7), not the
entire ZRTP packets, are included in the hash.
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The final shared secret, s0, is calculated in Preshared mode in the
same way as it is done in Diffie-Hellman mode, except the DHResult is
omitted. The hash calculation of s0 includes the ZID's of the
initiator and the responder, the total_hash, and the set of non-null
shared secrets as described in 5.2. The noncei and noncer values are
implicitly included in the hash because they were included in the
total_hash. The inclusion of these nonces produces a unique s0 for
each session, despite the absence of a fresh DH calculation. Note
that it is unsafe to use preshared mode if there are no previously
shared secrets.
s0 = hash( counter | "ZRTP-HMAC-KDF" | ZIDi | ZIDr | total_hash |
len(s1) | s1 | len(s2) | s2 | len(s3) | s3 | len(s4) | s4 | len(s5) |
s5 )
For a full description of all of these fields, see section 5.4.4,
which describes the shared secret s0 calculation in Diffie-Hellman
mode. The only difference here is that we omit the DHResult from the
hash.
No new retained shared secret is derived, and the values of rs1 and
rs2 are unchanged during this mode.
5.6. Key Generation
Various keys, such as those used by SRTP, must be derived from the
shared secret s0. To do this, ZRTP uses an HMAC-based key derivation
function, keyed by s0, instead of simply drawing subkey material
directly from s0, as defined in NIST SP800-56A. The possibly greater
noninvertability of HMAC may add an extra measure of isolation for
the derived keys.
The SRTP master key and master salt are derived from s0. Separate
SRTP keys and salts are used in each direction for each media stream.
Unless otherwise specified, ZRTP uses SRTP with no MKI, 32 bit
authentication using HMAC-SHA1, AES-CM 128 or 256 bit key length, 112
bit session salt key length, 2^48 key derivation rate, and SRTP
prefix length 0.
The ZRTP initiator encrypts and the ZRTP responder decrypts packets
by using srtpkeyi and srtpsalti, which are generated by:
srtpkeyi = HMAC(s0,"Initiator SRTP master key")
srtpsalti = HMAC(s0,"Initiator SRTP master salt")
The key and salt values are truncated (taking the leftmost bits) to
the length determined by the chosen SRTP algorithm. The ZRTP
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responder encrypts and the ZRTP initiator decrypts packets by using
srtpkeyr and srtpsaltr, which are generated by:
srtpkeyr = HMAC(s0,"Responder SRTP master key")
srtpsaltr = HMAC(s0,"Responder SRTP master salt")
The HMAC keys are generated by:
hmackeyi = HMAC(s0,"Initiator HMAC key")
hmackeyr = HMAC(s0,"Responder HMAC key")
Note that these HMAC keys are used only by ZRTP and not by SRTP.
Note: Different HMAC keys are needed for the initiator and the
responder to ensure that GoClear messages in each direction are
unique and can not be cached by an attacker and reflected back to the
endpoint.
ZRTP keys are generated for the initiator and responder to use to
encrypt the Confirm1 and Confirm2 messages. They are truncated to
the same size as the negotiated SRTP key size.
zrtpkeyi = HMAC(s0,"Initiator ZRTP key")
zrtpkeyr = HMAC(s0,"Responder ZRTP key")
After s0 has been used to calculate all the subkeys that are derived
from it, it MUST be erased from memory. All other key material,
especially the SRTP keys and salts, MUST also be erased from memory
when they are no longer used, no later than the end of the call. The
only exceptions are the retained shared secrets, or other cached
secrets needed for future calls.
The Short Authentication String (SAS) value is calculated from the
HMAC of a fixed string, keyed with an HMAC key derived from the whole
key agreement.
sashash = HMAC(hmackeyi,"SAS")
sasvalue = sashash [truncated to leftmost 32 bits]
5.7. Confirmation
The Confirm1 and Confirm2 messages contain the cache expiration
interval for the newly generated retained shared secret. The
flagoctet is an 8 bit unsigned integer made up of the Disclosure flag
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(D), Allow clear flag (A), SAS Verified flag (V):
flagoctet = V * 2^2 + A * 2^1 + D * 2^0
Part of the Confirm1 and Confirm2 messages are encrypted using full-
block Cipher Feedback Mode, and contain a 128-bit random CFB
Initialization Vector (IV). The Confirm1 and Confirm2 messages also
contain an HMAC covering the encrypted part of the Confirm1 or
Confirm2 message which includes a string of zeros, the signature
length, flag octet, cache expiration interval, signature type block
(if present) and signature block (if present). For the responder
hmac = HMAC(hmackeyr, encrypted part of Confirm1)
For the initiator:
hmac = HMAC(hmackeyi, encrypted part of Confirm2 message)
The Conf2ACK message sent by the responder completes the exchange.
5.8. Random Number Generation
The ZRTP protocol uses random numbers for cryptographic key material,
notably for the DH secret exponents and nonces, which must be freshly
generated with each session. Whenever a random number is needed, all
of the following criteria must be satisfied:
It MUST be derived from a physical entropy source, such as RF noise,
acoustic noise, thermal noise, high resolution timings of
environmental events, or other unpredictable physical sources of
entropy. Chapter 10 of [14] gives a detailed explanation of
cryptographic grade random numbers and provides guidance for
collecting suitable entropy. The raw entropy must be distilled and
processed through a deterministic random bit generator (DRBG).
Examples of DRBGs may be found in NIST SP 800-90 [6], and in [14].
It MUST be freshly generated, meaning that it must not have been used
in a previous calculation.
It MUST be greater than or equal to two, and less than or equal to
2^L - 1, where L is the number of random bits required.
It MUST be chosen with equal probability from the entire available
number space, e.g., [2, 2^L - 1].
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5.9. ZID and Cache Operation
Each instance of ZRTP has a unique 96-bit random ZRTP ID or ZID that
is generated once at installation time. It is used to look up
retained shared secrets in a local cache. A single global ZID for a
single installation is the simplest way to implement ZIDs. However,
it is specifically not precluded for an implementation to use
multiple ZIDs, up to the limit of a separate one per callee. This
then turns it into a long-lived "association ID" that does not apply
to any other associations between a different pair of parties. It is
a goal of this protocol to permit both options to interoperate
freely.
Each time a new s0 is calculated, a new retained shared secret rs1 is
generated and stored in the cache, indexed by the ZID of the other
endpoint. The previous retained shared secret is then renamed rs2
and also stored in the cache. For the new retained shared secret,
each endpoint chooses a cache expiration value which is an unsigned
32 bit integer of the number of seconds that this secret should be
retained in the cache. The time interval is relative to when the
Confirm1 message is sent or received.
The cache intervals are exchanged in the Confirm1 and Confirm2
messages. The actual cache interval used by both endpoints is the
minimum of the values from the Confirm1 and Confirm2 messages. A
value of 0 seconds means the secret should not be cached and the
current values of rs1 and rs2 MUST be maintained. A value of
0xFFFFFFFF means the secret should be cached indefinitely and is the
recommended value. If the ZRTP exchange results in no new shared
secret generation (i.e. Preshared Mode), the field in the Confirm1
and Confirm2 is set to 0xFFFFFFFF and ignored, and the cache is not
updated.
The expiration interval need not be used to force the deletion of a
shared secret from the cache when the interval has expired. It just
means the shared secret MAY be deleted from that cache at any point
after the interval has expired without causing the other party to
note it as an unexpected security event when the next key negotiation
occurs between the same two parties. This means there need not be
perfectly synchronized deletion of expired secrets from the two
caches, and makes it easy to avoid a race condition that might
otherwise be caused by clock skew.
5.10. Terminating an SRTP Session or ZRTP Exchange
The Error message is used to terminate an in-progress ZRTP exchange
due to an error. The Error message contains an integer Error Code
for debugging purposes. The termination of a ZRTP key agreement
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exchange results in no updates to the cached shared secrets and
deletion of all crypto context.
The GoClear message is used to switch from SRTP to RTP, usually
because the user has chosen to do that by pressing a button. The
GoClear uses an HMAC of the Message Type Block sent in the GoGlear
Message computed with the hmackey derived from the shared secret.
When sent by the initiator:
clear_hmac = HMAC(hmackeyi, "GoClear ")
When sent by the responder:
clear_hmac = HMAC(hmackeyr, "GoClear ")
A GoClear message which does not receive a ClearACK response must be
resent. If a GoClear message is received with a bad HMAC, it must be
ignored, and no ClearACK is sent.
A ZRTP endpoint MAY choose to accept GoClear messages after the
session has switched to SRTP, allowing the session to revert to RTP.
This is indicated in the Confirm1 or Confirm2 messages by setting the
Allow Clear flag (A). If the other endpoint set the Allow Clear (A)
flag in their confirm message, GoClear messages MAY be sent.
A ZRTP endpoint that receives a GoClear authenticates the message by
checking the clear_hmac. If the message authenticates, the endpoint
stops sending SRTP packets, generates a ClearACK in response, and
deletes all the crypto key material for the SRTP session. Until
confirmation from the user is received (e.g. clicking a button,
pressing a DTMF key, etc.), the ZRTP endpoint MUST NOT resume sending
RTP packets. The endpoint then renders an indication that the media
session has switched to clear mode to the user and waits for
confirmation from the user. To prevent pinholes from closing or NAT
bindings from expiring, the ClearACK message MAY be resent at regular
intervals (e.g. every 5 seconds) while waiting for confirmation from
the user. After confirmation of the notification is received from
the user, the sending of RTP packets may begin.
After sending a GoClear message, the ZRTP endpoint stops sending SRTP
packets. When a ClearACK is received, the ZRTP endpoint deletes the
crypto context for the SRTP session and may then resume sending RTP
packets.
In the event a ClearACK is not received before the retransmissions of
GoClear are exhausted, the key material is deleted.
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6. ZRTP Messages
All ZRTP messages use the message format defined in Figure 2. All
word lengths referenced in this specification are 32 bits or 4
octets. All integer fields are carried in network byte order, that
is, most significant byte (octet) first, commonly known as big-
endian.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Not Used (set to zero) | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ZRTP Magic Cookie (0x5a525450) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ZRTP Message (length depends on Message Type) |
| . . . |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CRC (1 word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. ZRTP Packet Format
The Sequence Number is a count that is incremented for each ZRTP
packet sent. The count is initialized to a random value. This is
useful in estimating ZRTP packet loss and also detecting when ZRTP
packets arrive out of sequence.
The ZRTP Magic Cookie is a 32 bit string that uniquely identifies a
ZRTP packet, and has the value 0x5a525450.
Source Identifier is the SSRC number of the RTP stream that this ZRTP
packet relates to. For cases of forking or forwarding, RTP and hence
ZRTP may arrive at the same port from several different sources -
each of these sources will have a different SSRC and may initiate an
independent ZRTP protocol session.
This format is clearly identifiable as non-RTP due to the first two
bits being zero which looks like RTP version 0, which is not a valid
RTP version number. It is clearly distinguishable from STUN since
the magic cookies are different. The 12 not used bits are set to
zero and MUST be ignored when received.
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The ZRTP Messages are defined in Figures 3 to 11 and are of variable
length.
The ZRTP protocol uses a 32 bit CRC checksum in each ZRTP packet as
defined in RFC 3309 [5] to detect transmission errors. ZRTP packets
are typically transported by UDP, which carries its own built-in 16-
bit checksum for integrity, but ZRTP does not rely on it. This is
because of the effect of an undetected transmission error in a ZRTP
message. For example, an undetected error in the DH exchange could
appear to be an active man-in-the-middle attack. The psychological
effects of a false announcement of this by ZTRP clients can not be
overstated. The probability of such a false alarm hinges on a mere
16-bit checksum that usually protects UDP packets, so more error
detection is needed. For these reasons, this belt-and-suspenders
approach is used to minimize the chance of a transmission error
affecting the ZRTP key agreement.
The CRC is calculated across the entire ZRTP packet shown in Figure
2, including the ZRTP Header and the ZRTP Message, but not including
the CRC field. If a ZRTP message fails the CRC check, it is silently
discarded.
6.1. ZRTP Message Formats
ZRTP messages are designed to simplify endpoint parsing requirements
and to reduce the opportunities for buffer overflow attacks (a good
goal of any security extension should be to not introduce new attack
vectors...)
ZRTP uses 8 octets (2 words) blocks to encode Message Type. 4 octets
(1 word) blocks are used to encode Hash Type, Cipher Type, and Key
Agreement Type, and Authentication Tag. The values in the blocks are
ASCII strings which are extended with spaces (0x20) to make them the
desired length. Currently defined block values are listed in Tables
1-6 below.
Additional block values may be defined and used.
ZRTP uses this ASCII encoding to simplify debugging and make it
"ethereal friendly".
6.1.1. Message Type Block
Currently 12 Message Type Blocks are defined - they represent the set
of ZRTP message primitives. ZRTP endpoints MUST support the Hello,
HelloACK, Commit, DHPart1, DHPart2, Confirm1, Confirm2, Conf2ACK,
Error and ErrorACK block types. ZRTP endpoints MAY support the
GoClear and ClearACK messages. Additional messages may be defined in
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extensions to ZRTP.
Message Type Block | Meaning
---------------------------------------------------
"Hello " | Hello Message
| defined in Section 6.2
---------------------------------------------------
"HelloACK" | HelloACK Message
| defined in Section 6.3
---------------------------------------------------
"Commit " | Commit Message
| defined in Section 6.4
---------------------------------------------------
"DHPart1 " | DHPart1 Message
| defined in Section 6.5
---------------------------------------------------
"DHPart2 " | DHPart2 Message
| defined in Section 6.6
---------------------------------------------------
"Confirm1" | Confirm1 Message
| defined in Section 6.7
---------------------------------------------------
"Confirm2" | Confirm2 Message
| defined in Section 6.8
---------------------------------------------------
"Conf2ACK" | Conf2ACK Message
| defined in Section 6.9
---------------------------------------------------
"Error " | Error Message
| defined in Section 6.10
---------------------------------------------------
"ErrorACK" | ErrorACK Message
| defined in Section 6.11
---------------------------------------------------
"GoClear " | GoClear Message
| defined in Section 6.12
---------------------------------------------------
"ClearACK" | ClearACK Message
| defined in Section 6.13
---------------------------------------------------
Table 1. Message Block Type Values
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6.1.2. Hash Type Block
Only one Hash Type is currently defined, SHA256, and all ZRTP
endpoints MUST support this hash. Additional Hash Types can be
registered and used.
Hash Type Block | Meaning
---------------------------------------------------
"S256" | SHA-256 Hash defined in [SHA-256]
---------------------------------------------------
Table 2. Hash Block Type Values
6.1.3. Cipher Type Block
All ZRTP endpoints MUST support AES128 and MAY support AES256. or
other Cipher Types. Also, if AES 128 is used, DH3k should be used.
If AES 256 is used, DH4k should be used.
Note: DH4k may be deprecated in the future in favor of elliptic curve
algorithms.
Cipher Type Block | Meaning
---------------------------------------------------
"AES1" | AES-CM with 128 bit keys
| as defined in RFC 3711
---------------------------------------------------
"AES2" | AES-CM with 256 bit keys
| as defined in RFC 3711
---------------------------------------------------
Table 3. Cipher Block Type Values
6.1.4. Auth Tag Block
All ZRTP endpoints MUST support HMAC-SHA1 authentication, 32 bit and
80 bit length tags as defined in RFC 3711.
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Auth Tag Block | Meaning
---------------------------------------------------
"HS32" | HMAC-SHA1 32 bit authentication
| tag as defined in RFC 3711
---------------------------------------------------
"HS80" | HMAC-SHA1 80 bit authentication
| tag as defined in RFC 3711
---------------------------------------------------
Table 4. Auth Tag Values
6.1.5. Key Agreement Type Block
All ZRTP endpoints MUST support DH3k and MAY support DH4k. ZRTP
endpoints MUST use the DH generator function g=2. The choice of AES
key length is coupled to the choice of key agreement type. If AES
128 is chosen, DH3k SHOULD be used. If AES 256 is chosen, DH4k
SHOULD be used. ZRTP also defines a non-DH mode, Preshared, which
SHOULD be supported. In Preshared mode, the SRTP key is derived from
the set of shared secrets and a pair of nonces.
Note: DH4k may be deprecated in the future in favor of elliptic curve
algorithms.
Key Agreement Type Block | Meaning
---------------------------------------------------
"DH3k" | DH mode with p=3072 bit prime
| as defined in RFC 3526
---------------------------------------------------
"DH4k" | DH mode with p=4096 bit prime
| as defined in RFC 3526
---------------------------------------------------
"Prsh" | Preshared Non-DH mode
| uses shared secrets.
---------------------------------------------------
Table 5. Key Agreement Block Type Values
6.1.6. SAS Type Block
All ZRTP endpoints MUST support the base32 and MAY support base256
Short Authentication String scheme, and other SAS rendering schemes.
The ZRTP SAS is described in Section 7.
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SAS Type Block | Meaning
---------------------------------------------------
"B32 " | Short Authentication String using
| base32 encoding defined in Section 8.
---------------------------------------------------
"B256" | Short Authentication String using
| base256 encoding defined in Section 8.
---------------------------------------------------
Table 6. SAS Block Type Values
The SAS Type determines how the SAS is rendered to the user so that
the user may compare it with his partner over the voice channel.
This allows detection of a man-in-the-middle (MITM) attack.
6.1.7. Signature Block
The signature type block is a 4 octet (1 word) block used to
represent the signature algorithm. Suggested signature algorithms
and key lengths are a future subject of standardization.
6.2. Hello message
The Hello message has the format shown in Figure 3. The Hello ZRTP
message begins with the preamble value 0x505a then a 16 bit length in
32 bit words. This length includes only the ZRTP message (including
the preamble and the length) but not the ZRTP header or CRC.
Next is the Message Type Block and a 4 character string containing
the version (ver) of ZRTP, currently "0.07". Next is the Client
Identifier string (cid) which is 3 words long and identifies the
vendor and release of the ZRTP software. The next parameter is the
ZID, the 96 bit long unique identifier for the ZRTP endpoint.
The next four bits contains flag bits. The Initiator flag (I) is for
a PBX to set, and is normally set to false for any other non-PBX ZRTP
endpoint. The I flag means this ZRTP endpoint (the PBX) would like
to be the initiator, and requests the other endpoint to not
immediately send a Commit packet, and thus not attempt to initiate
secure mode at the start of this call. This allows the PBX to be the
initiator at a time of the PBX's own choosing. If both sides set the
I flag, it's equivalent to neither side setting the I flag. See
section 8.3 for an example of how this I flag can be used by a PBX.
The Passive flag (P) is a Boolean normally set to False. A ZRTP
endpoint which is configured to never initiate secure sessions is
regarded as passive, and would set the P bit to True. The next 8
bits are unused. They should be set to zero when sent and ignored on
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receipt.
Next is a list of supported Hash Types, Cipher Types, Auth Tag, Key
Agreement Types, and SAS Type. The number of listed algorithms are
listed for each type: hc=hash count, cc=cipher count, ac=auth tag
count, kc=key agreement count, and sc=sas count. The values for
these algorithms are defined in Tables 2, 3, 4, 5, and 6. A count of
zero means that only the mandatory to implement algorithms are
supported. Mandatory algorithms MAY be included in the list. The
order of the list indicates the preferences of the endpoint. If a
mandatory algorithm is not included in the list, it is added to the
end of the list for preference.
Note: Implementers are encouraged to keep these algorithm lists small
- the list does not need to include every cipher and hash supported,
just the ones the endpoint would prefer to use for this ZRTP
exchange.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="Hello " (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version (1 word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Client Identifier (3 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ZID (3 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0|I|P| unused (zeros)| hc | cc | ac | kc | sc |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hash (0 to 7 values) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cipher (0 to 7 values) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| at (0 to 7 values) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| keya (0 to 7 values) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sas (0 to 7 values) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. Hello message format
6.3. HelloACK message
The HelloACK message is used to stop retransmissions of a Hello
message. A HelloACK is sent regardless if the version number in the
Hello is supported or the algorithm list supported. The receipt of a
HelloACK stops retransmission of the Hello message. The format is
shown in Figure 4 below. Note that a Commit message can be sent in
place of a HelloACK by an initiator.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=3 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="HelloACK" (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4. HelloACK message format
6.4. Commit message
The Commit message is sent to initiate the key agreement process
after receiving a Hello message. The Commit message contains the
initiator's ZID and a list of selected algorithms (hash, cipher, at,
keya, sas), the ZRTP mode, and hvi, a hash of the DHPart2 of the
initiator and the algorithm list from the responder's Hello message.
The Commit Message format is shown in Figure 5.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=19 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="Commit " (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ZID (3 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cipher |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| at |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| keya |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SAS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| hvi (8 words) |
| . . . |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. Commit message format
6.5. DHPart1 message
The DHPart1 message begins the DH exchange. The format is shown in
Figure 5 below. The DHPart1 message is sent if a valid Commit
message is received. The length of the pvr value depends on the Key
Agreement Type chosen. If DH4k is used, the pvr will be 128 words
(512 octets) and the length of this message will be 141 words. If
DH3k is used, it is 96 words (384 octets) and the length of this
message will be 109 words. If the Key Agreement Type is Preshared,
then pvr is replaced by an 8 word noncer from the responder and the
length of this message will be 21 words.
The next five parameters are HMACs of potential shared secrets used
in generating the ZRTP secret. The first two, rs1IDr and rs2IDr, are
the HMACs of the responder's two retained shared secrets, truncated
to 64 bits. Next is sigsIDr, the HMAC of the responder's signaling
secret, truncated to 64 bits. Next is srtpsIDr, the HMAC of the
responder's SRTP secret, truncated to 64 bits. The last parameter is
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the HMAC of an additional shared secret. For example, if multiple
SRTP secrets are available or some other secret is used, it can be
used as the other_secret. The Message format for the DHPart1 message
is shown in Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=depends on KA Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="DHPart1 " (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rs1IDr (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rs2IDr (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sigsIDr (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| srtpsIDr (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| other_secretIDr (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| pvr (length depends on KA Type) or noncer (8 words) |
| . . . |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. DHPart1 message format
6.6. DHPart2 message
The DHPart2 message completes the DH exchange. A DHPart2 message is
sent if a valid DHPart1 message is received. The length of the pvi
value depends on the Key Agreement Type chosen. If DH4k is used, the
pvi will be 128 words (512 octets) and the length of this message
will be 141 words. If DH3k is used, it is 96 words (384 octets) and
the length of this message will be 109 words. If the Key Agreement
Type is Preshared, then pvi is replaced by an 8 word noncei from the
initiator and the length of this message will be 21 words.
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The next five parameters are HMACs of potential shared secrets used
in generating the ZRTP secret. The first two, rs1IDi and rs2IDi, are
the HMACs of the initiator's two retained shared secrets, truncated
to 64 bits. Next is sigsIDi, the HMAC of the initiator's signaling
secret, truncated to 64 bits. Next is srtpsIDi, the HMAC of the
initiator's SRTP secret, truncated to 64 bits. The last parameter is
the HMAC of an additional shared secret. For example, if multiple
SRTP secrets are available or some other secret is used, it can be
included. The message format for the DHPart2 message is shown in
Figure 7.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=depends on KA Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="DHPart2 " (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rs1IDi (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rs2IDi (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sigsIDi (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| srtpsIDi (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| other_secretIDi (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| pvi (length depends on KA Type) or noncei (8 words) |
| . . . |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7. DHPart2 message format
6.7. Confirm1 and Confirm2 messages
The Confirm1 message is sent in response to a valid DHPart2 message
after the SRTP session key and parameters have been negotiated. The
Confirm2 message is sent in response to a Confirm1 message. The
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format is shown in Figure 8 below. The message contains the Message
Type Block "Confirm1" or "Confirm2". Next is the HMAC, a keyed hash
over encrypted part of the message (shown enclosed by "===" in Figure
8.) The next 16 octets contain the CFB Initialization Vector. The
rest of the message is encrypted using CFB and protected by the HMAC.
The next 16 bits are not used. They SHOULD be set to zero and MUST
be ignored in received Confirm1 messages.
The next 8 bits contain the signature length. If no SAS signature
(described in Section 8.3) is present, all bits are set to zero. The
signature length is in words and includes the signature type block.
If the calculated signature octet count is not a multiple of 4, zeros
are added to pad it out to a word boundary. If no signature block is
present, the overall length of the Confirm1 or Confirm2 Message will
be set to 13 words.
The next 8 bits are used for flags. Undefined flags are set to zero
and ignored. Three flags are currently defined. The Disclosure Flag
(D) is a Boolean bit defined in Section 10. The Allow Clear flag (A)
is a Boolean bit defined in Section 5.6. The SAS Verified flag (V)
is a Boolean bit defined in Section 8. The cache expiration interval
is an unsigned 32 bit integer of the number of seconds that the newly
generated cached shared secret, rs1, should be stored.
The next 32 bit word contains the rendering scheme for the trusted
MiTM relayed sasvalue. This is only used in PBX environments which
treat the PBX as a trusted MiTM, and this field is zeroed out in any
oher situation. If this Confirm packet comes from a trusted MiTM (a
trusted PBX), this field contains "B32 " or "B256", to indicate the
SAS rendering scheme used for the calling party. The next 32 bit
word contains the binary sasvalue relayed by the trusted MiTM from
the calling party.
If the signature length (in words) is non-zero, a signature type
block will be present along with a signature block. Next is the
signature block. The signature block includes the key used to
generate the signature.
CFB [8] mode is applied with a feedback length of 128-bits, a full
cipher block, and the final block is truncated to match the exact
length of the encrypted data. The CFB Initialization Vector is a 128
bit random nonce. The block cipher algorithm and the key size is the
same as what was negotiated for the media encryption. CFB is used to
encrypt the part of the Confirm1 message beginning after the CFB IV
to the end of the message (the encrypted region is enclosed by
"======" in Figure 8).
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The responder uses the zrtpkeyr to encrypt the Confirm1 message. The
initiator uses the zrtpkeyi to encrypt the Confirm2 message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="Confirm1" or "Confirm2" (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| CFB Initialization Vector (4 words) |
| |
| |
+===============================================================+
| Unused (Set to zero, ignored) | sig length |0 0 0 0 0|V|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cache expiration interval (1 word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rendering scheme of relayed sasvalue (1 word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Trusted MITM relayed sasvalue (1 word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional signature type block (1 word if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| optional signature block (variable length) |
| . . . |
| |
| |
+===============================================================+
Figure 8. Confirm1 and Confirm2 message format
6.8. Conf2ACK message
The Conf2ACK message is sent in response to a valid Confirm2 message.
The message format for the Conf2ACK is shown in Figure 9. The
receipt of a Conf2ACK stops retransmission of the Confirm2 message.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=3 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="Conf2ACK" (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9. Conf2ACK message format
6.9. Error message
The Error message is sent to terminate an in-process ZRTP key
agreement exchange due to an error. The format is shown in Figure 10
below. The use of the Error message is described in Section 5.10.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=4 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="Error " (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integer Error Code (1 word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10. Error message format
Defined hexadecimal values for the Error Code are listed in Table 7.
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Error Code | Meaning
---------------------------------------------------
0x10 | Malformed packet (CRC OK, but wrong structure)
---------------------------------------------------
0x20 | Critical software error
---------------------------------------------------
0x30 | Unsupported ZRTP version
---------------------------------------------------
0x40 | Hello components mismatch
---------------------------------------------------
0x51 | Hash type not supported
---------------------------------------------------
0x52 | Cipher type not supported
---------------------------------------------------
0x53 | Public key exchange not supported
---------------------------------------------------
0x54 | SRTP auth. tag not supported
---------------------------------------------------
0x55 | SAS scheme not supported
---------------------------------------------------
0x61 | DH Error: bad pvi or pvr ( == 1, 0, or p-1)
---------------------------------------------------
0x62 | DH Error: hvi != hashed data
---------------------------------------------------
0x70 | Auth. Error: Bad Confirm pkt HMAC
---------------------------------------------------
0x80 | Nonce reuse
---------------------------------------------------
0x90 | Equal ZIDs in Hello
---------------------------------------------------
Table 7. ZRTP Error Codes
6.10. ErrorACK message
The ErrorACK message is sent in response to an Error message. The
format is shown in Figure 11 below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=3 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="ErrorACK" (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 11. ErrorACK message format
6.11. GoClear message
The optional GoClear message is sent to switch from SRTP to RTP. The
format is shown in Figure 12 below. The clear_hmac is used to
authenticate the GoClear message so that bogus GoClear messages
introduced by an attacker can be detected and discarded. The use of
GoClear is described in Section 5.10.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=5 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="GoClear " (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| clear_hmac (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12. GoClear message format
6.12. ClearACK message
The optional ClearACK message is sent to acknowledge receipt of a
GoClear. A ClearACK is only sent if the clear_hmac from the GoClear
message is authenticated. Otherwise, no response is returned. The
format is shown in Figure 13.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0| length=3 words |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type Block="ClearACK" (2 words) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13. ClearACK message format
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7. Retransmissions
ZRTP uses two retransmission timers T1 and T2. T1 is used for
retransmission of Hello messages, when the support of ZRTP by the
other endpoint may not be known. T2 is used in retransmissions of
all the other ZRTP messages with the exception of GoClear.
All message retransmissions MUST be identical to the initial message
including nonces, public values, etc; otherwise, hashes of the
message sequences may not agree.
Practical experience has shown that RTP packet loss at the start of
an RTP session can be extremely high. Since the entire ZRTP message
exchange occurs during this period, the defined retransmission scheme
is defined to be aggressive. Since ZRTP packets with the exception
of the DHPart1 and DHPart2 messages are small, this should have
minimal effect on overall bandwidth utilization of the media session.
ZRTP endpoints MUST NOT exceed the bandwidth of the resulting media
session as determined by the offer/answer exchange.
Hello ZRTP requests are retransmitted at an interval that starts at
T1 seconds and doubles after every retransmission, capping at 200ms.
A Hello message is retransmitted 20 times before giving up. T1 has a
recommended value of 50 ms. Retransmission of a Hello ends upon
receipt of a HelloACK or Commit message.
Non-Hello ZRTP requests are retransmitted only by the initiator -
that is, only Commit, DHPart2, Confirm2, and GoClear are
retransmitted if the corresponding message from the responder,
DHPart1, Confirm1, Conf2ACK, and ClearACK, are not received. Non-
Hello ZRTP messages are retransmitted at an interval that starts at
T2 seconds and doubles after every retransmission, capping at 600ms.
Only the ZRTP initiator performs retransmissions. Each message is
retransmitted 10 times before giving up and resuming a normal RTP
session. T2 has a default value of 150ms. Each message has a
response message that stops retransmissions, as shown in Table 8.
The high value of T2 means that retransmissions will likely only
occur with packet loss.
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Message Acknowledgement Message
------- -----------------------
Hello HelloACK or Commit
Commit DHPart1 or Confirm1
DHPart2 Confirm1
Confirm1 Confirm2
Confirm2 Conf2ACK
GoClear ClearACK
Error ErrorACK
Table 8. Retransmitted ZRTP Messages and Responses
8. Short Authentication String
This section will discuss the implementation of the Short
Authentication String, or SAS in ZRTP. The SAS can be verbally
verified by the human users reading the string aloud, exchanging and
comparing over an integrity-protected signaling channel using the
a=zrtp-sas attribute, or validating a digital signature exchanged in
the Confirm1 or Confirm2 messages.
The rendering of the SAS value to the user depends on the SAS Type
agreed upon in the Commit message. For the SAS Type of base32, the
leftmost 20 bits of the 32-bit sasvalue are rendered as a form of
base32 encoding known as z-base-32 [9]. The purpose of z-base-32 is
to represent arbitrary sequences of octets in a form that is as
convenient as possible for human users to manipulate. As a result,
the choice of characters is slightly different from base32 as defined
in RFC 3548. The leftmost 20 bits of the sasvalue results in four
base32 characters which are rendered to both ZRTP endpoints. For the
SAS Type of base256, the leftmost 16 bits of the 32-bit sasvalue are
rendered using the PGP Wordlist [10], [11]. Other SAS Types may be
defined to render the SAS value in other ways.
The SAS SHOULD be rendered to the user for authentication. In
addition, the SAS SHOULD be sent in a subsequent offer/answer
exchange (a re-INVITE in SIP) after the completion of ZRTP exchange
using the ZRTP SAS SDP attributes defined in Section 9.
The SAS is not treated as a secret value, but it must be compared to
see if it matches at both ends of the communications channel. The
two users read it aloud to their partners to see if it matches. This
allows detection of a man-in-the-middle (MITM) attack.
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8.1. SAS Verified Flag
The SAS Verified flag (V) is set based on the user indicating that
SAS comparison has been successfully performed. The SAS Verified
flag is exchanged securely in the Confirm1 and Confirm2 messages of
the next session. In other words, each party sends the SAS Verified
flag from the previous session in the Confirm message of the current
session. It is perfectly reasonable to have a ZRTP endpoint that
never sets the SAS Verified flag, because it would require adding
complexity to the user interface to allow the user to set it. The
SAS Verified flag is not required to be set, but if it is available
to the client software, it allows for the possibility that the client
software could render to the user that the SAS verify procedure was
carried out in a previous session.
Regardless of whether there is a user interface element to allow the
user to set the SAS Verified flag, it is worth caching a shared
secret, because doing so reduces opportunities for an attacker in the
next call.
If at any time the users carry out the SAS comparison procedure, and
it actually fails to match, then this means there is a very
resourceful man in the middle. If this is the first call, the MITM
was there on the first call, which is impressive enough. If it
happens in a later call, it also means the MITM must also know the
cached shared secret, because you could not have carried out any
voice traffic at all unless the session key was correctly computed
and is also known to the attacker. This implies the MITM must have
been present in all the previous sessions, since the initial
establishment of the first shared secret. This is indeed a
resourceful attacker. It also means that if at any time he ceases
his participation as a MITM on one of your calls, the protocol will
detect that the cached shared secret is no longer valid -- because it
was really two different shared secrets all along, one of them
between Alice and the attacker, and the other between the attacker
and Bob. The continuity of the cached shared secrets make it possible
for us to detect the MITM when he inserts himself into the ongoing
relationship, as well as when he leaves. Also, if the attacker tries
to stay with a long lineage of calls, but fails to execute a DH MITM
attack for even one missed call, he is permanently excluded. He can
no longer resynchronize with the chain of cached shared secrets.
Some sort of user interface element (maybe a checkbox) is needed to
allow the user to tell the software the SAS verify was successful,
causing the software to set the SAS Verified flag (V), which
(together with our cached shared secret) obviates the need to perform
the SAS procedure in the next call. An additional user interface
element can be provided to let the user tell the software he detected
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an actual SAS mismatch, which indicates a MITM attack. The software
can then take appropriate action, clearing the SAS Verified flag, and
erase the cached shared secret from this session. It is up to the
implementer to decide if this added user interface complexity is
warranted.
If the SAS matches, it means there is no MITM, which also implies it
is now safe to trust a cached shared secret for later calls. If
inattentive users don't bother to check the SAS, it means we don't
know whether there is or is not a MITM, so even if we do establish a
new cached shared secret, there is a risk that our potential attacker
may have a subsequent opportunity to continue inserting himself in
the call, until we finally get around to checking the SAS. If the
SAS matches, it means no attacker was present for any previous
session since we started propagating cached shared secrets, because
this session and all the previous sessions were also authenticated
with a continuous lineage of shared secrets.
8.2. Signing the SAS
The SAS MAY be signed and the signature sent using the Confirm1 or
Confirm2 messages. The signature algorithm is also sent in the
Confirm1 or Confirm2 message, along with the length of the signature.
The key types and signature algorithms are for future study. The
signature is calculated over the entire SAS hash result (sashash)
that was truncated down to derive the sasvalue. The signatures
exchanged in the encrypted Confirm1 or Confirm2 messages MAY be used
to authenticate the ZRTP exchange.
8.3. Relaying the SAS through a PBX
ZRTP is designed to use end-to-end encryption. The two parties'
verbal comparison of the short authentication string (SAS) depends on
this assumption.
But in some PBX environments, such as Asterisk, there are usage
scenarios that have the PBX acting as a trusted man in the middle
(MitM), which means there are two back-to-back ZRTP connections with
separate session keys and separate SAS's.
For example, imagine that Bob has a ZRTP-enabled VoIP phone that has
been registered with his company's PBX, so that it is regarded as an
extension of the PBX. Alice, whose phone is not associated with the
PBX, might dial the PBX from the outside, and a ZRTP connection is
negotiated between her phone and the PBX. She then selects Bob's
extension from the company directory in the PBX. The PBX makes a
call to Bob's phone (which might be offsite, many miles away from the
PBX through the Internet) and a separate ZRTP connection is
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negotiated between the PBX and Bob's phone. The two ZRTP sessions
have different session keys and different SAS's, which would render
the SAS useless for verbal comparison between Alice and Bob. They
might even mistakenly believe that a wiretapper is present because of
the SAS mismatch, causing undue alarm.
ZRTP has a mechanism for solving this problem by having the PBX relay
the Alice/PBX SAS to Bob, sending it through to Bob in the Confirm
(either Confirm1 or Confirm2) packet in the PBX/Bob ZRTP negotiation.
The Confirm packet protects it from tampering via the usual Confirm
packet's HMAC. Bob's ZRTP-enabled phone accepts the relayed SAS for
rendering only because Bob's phone had previously registered with the
PBX, establishing a special trusted relationship with the PBX. The
PBX is treated by Bob as a special trusted MitM. This results in
Alice's SAS being rendered to Bob, so that Alice and Bob may verbally
compare them and thus prevent a MitM attack by any other untrusted
MitM.
A real bad-guy MitM cannot exploit this protocol feature to mount a
MitM attack and relay Alice's SAS to Bob, because Bob has not
previously carried out a special registration ritual with the bad
guy. The relayed SAS would not be rendered by Bob's phone, because
it did not come from a trusted PBX. In fact, it should cause Bob's
phone to automatically raise an attack-detection alarm, because a
relayed SAS is only expected from a specially trusted ZRTP endpoint.
The recognition of the special trust relationship is achieved with
the prior establishment of a special shared secret between Bob and
his PBX, which is called the trusted MiTM key.
The trusted MiTM key can be stored in a special cache at the time of
the initial enrollment (which is carried out only once for Bob's
phone), and Bob's phone associates this key with the ZID of the PBX,
while the PBX associates it with the ZID of Bob's phone. After the
enrollment has establishedand stored this trusted MiTM key, it can be
detected during subsequent ZRTP call negotiations between the PBX and
Bob's phone, because the PBX SHOULD pass the hash of the trusted MiTM
key in the other_secret (s5) position in the DH packet.
To implement this feature, the Confirm packet will have a field to
hold the relayed SAS. This field will only be defined in the
appropriate usage scenario. The relayed SAS field will contain the
SAS type and the binary 32-bit sasvalue. The receiver MUST NOT
render the relayed SAS if it does not come from a specially trusted
ZRTP endpoint. In that case, the relayed SAS type and SAS fields are
both set to zero. If the relayed SAS type is zero, the relayed SAS
value is ignored. See the Confirm packet definition for further
details.
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The PBX, Alice, and Bob MUST all use the same SAS rendering scheme.
This can be done by the PBX using the Initiator (I) flag in the Hello
packet to delay the ZRTP negotiations with Bob until after he has
dialed the extension he wants, and both parties are on the line. The
PBX can then be the initiator with both parties, and choose one of
Alice's SAS rendering schemes for Bob via the Commit message from the
PBX, and to choose the same scheme with Alice with a Commit message
from the PBX. Another way to do it would be to constrain them both
via the list of acceptable SAS rendering schemes in the PBX's Hello
messages. The PBX should be configured to not offer the outside
caller any SAS rendering scheme in the Hello packet unless it is
supported by all of the internal phones registered to the PBX. It
can then offer the internal phone only one SAS rendering scheme in
the PBX Hello message, which was already negotiated with the calling
party.
During the initial enrollment that establishes a special shared
secret between the PBX and Bob's phone, a ZRTP session is established
between Bob's phone and the PBX, and then this trusted MiTM key is
derived in this manner:
trustedmitmkey = HMAC(hmackeyi,"Trusted MiTM key")
The hmackeyi is used here instead of s0 because this calculation may
be done at some later time in the session, after s0 has been erased.
Both the PBX and the endpoint need to know when enrollment is taking
place. One way of doing this is to setup an enrollment extension on
the PBX which a newly configured endpoint would call and establish a
ZRTP session. The PBX would then play media indicating that
enrollment has been completed. The PBX would know by the dialed
extension that the Trusted MiTM key should be derived from this
session. The endpoint requires a configuration setting or control so
the human user can tell the endpoint that enrollment is complete and
to derive and store the Trusted MiTM key.
Note: The enrollment is deliberately not signaled in the ZRTP
protocol - to do so would invite attackers to attempt to be added as
trusted PBX. The out-of-band coordination of this enrollment is
reasonable since it is a very rare (typically one time in the life of
the endpoint) event.
9. Signaling Interactions
This section discusses how ZRTP, SIP, and SDP work together.
The signaling secret (sigs) can be derived from SIP signaling and
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passed from the signaling protocol used to establish the RTP session
to ZRTP. Its the dialog identifier of a Secure SIP (sips) session: a
string composed of Call-ID and the local and remote tags. It can be
considered a secret because it is always transported using TLS and is
randomly generated for each SIP call. The local and remote tags are
sorted in ascending order in the hash. From the definitions in RFC
3261 [19]:
sigs = hash(call-id | tag1 | tag2)
Note: the dialog identifier of a non-secure SIP session should not be
considered a signaling secret as it has no confidentiality
protection.
Note: The signaling secret may not be regarded as having adequate
entropy for cryptographic protection without augmentation by key
material from other sources.
For the SRTP secret (srtps), it is the SRTP master key and salt.
This information may have been passed in the signaling using [22] or
[21], for example:
srtps = hash(SRTP master key | SRTP master salt)
Note that ZRTP may be implemented without coupling with the SIP
signaling. For example, ZRTP can be implemented as a "bump in the
wire" or as a "bump in the stack" in which RTP sent by the SIP UA is
converted to ZRTP. In these cases, the SIP UA will have no knowledge
of ZRTP. As a result, the signaling path discovery mechanisms
introduced in this section should not be definitive - they are a
hint. Despite the absence of an indication of ZRTP support in an
offer or answer, a ZRTP endpoint SHOULD still send Hello messages.
ZRTP endpoints which have control over the signaling path include a
ZRTP SDP attributes in their SDP offers and answers. The ZRTP
attribute, a=zrtp-id is a flag to indicate support for ZRTP. There
are a number of potential uses for this attribute. It is useful when
signaling elements would like to know when ZRTP may be utilized by
endpoints. It is also useful if endpoints support multiple methods
of SRTP key management. The ZRTP attribute can be used to ensure
that these key management approaches work together instead of against
each other. For example, if only one endpoint supports ZRTP but both
support another method to key SRTP, then the other method will be
used instead. When used in parallel, an SRTP secret carried in an
a=keymgt [22] or a=crypto [21] attribute can be used as a shared
secret for the srtp_secret. The ZRTP attribute is also used to
signal to an intermediary ZRTP device not to act as a ZRTP endpoint,
as discussed in Section 11.
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The a=zrtp-zid attribute can be included at a media level or at the
session level. It indicates support of ZRTP and provides the ZID
encoded in hex of the endpoint. When used at the media level, it
indicates that ZRTP is supported on this media stream. When used at
the session level, it indicates that ZRTP is supported in all media
streams in the session described by the offer or answer and that the
same ZID will be used for both streams.
In some scenarios, it is desirable for a signaling intermediary to be
able to validate the SAS on behalf of the user. This could be due to
an endpoint which has a user interface unable to render the SAS. Or,
this could be a protection by an organization against lazy users who
never check the SAS. Using either the ZRTP SAS or ZRTP SASvalue
attribute, the SAS check can be performed without requiring the human
users to speak the SAS. Note that this check can only be relied on
if the signaling path has end-to-end integrity protection.
The ZRTP SAS attribute a=zrtp-sas is a Media level SDP attribute that
can be used to carry the SAS string and value. The string is
identical to that rendered to the user while contents of the string
passed depends on the negotiated SAS Type. The value is the leftmost
64 bits of the SAS hash result (sashash) encoded as hex. Since the
SAS is not known at the start of a session, the a=zrtp-sas attribute
will never be present in the initial offer/answer exchange. After
the ZRTP exchange has completed, the SAS is known and can be
exchanged over the signaling using a second offer/answer exchange (a
re-INVITE in SIP terms). Note that the SAS is not treated as a
secret and as such does not need confidentiality protection when sent
over the signaling path.
The ABNF for the ZRTP attribute is as follows:
zrtp-attribute = "a=zrtp-zid:" zid-value
zid-value = 1*(HEXDIG)
The ABNF for the ZRTP SAS attribute is as follows:
zrtp-sas-attribute = "a=zrtp-sas:" sas-string sas-value
sas-string = non-ws-string
non-ws-string = 1*(VCHAR/%x80-FF)
;string of visible characters
sas-value = 1*(HEXDIG)
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Example of the ZRTP attribute in an initial SDP offer or answer used
at the session level:
v=0
o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
s=
c=IN IP4 client.biloxi.example.com
a=zrtp-zid:4cc3ffe30efd02423cb054e5
t=0 0
m=audio 3456 RTP/AVP 97 33
a=rtpmap:97 iLBC/8000
a=rtpmap:33 no-op/8000
Example of the ZRTP SAS and SASvalue attribute in a subsequent SDP
offer or answer used at the media level. Note that the a=zrtp-id
attribute doesn't provide any additional information when used with
the SAS and SASvalue attributes but does not do any harm:
v=0
o=bob 2890844527 2890844528 IN IP4 client.biloxi.example.com
s=
c=IN IP4 client.biloxi.example.com
a=zrtp-zid:4cc3ffe30efd02423cb054e5
t=0 0
m=audio 3456 RTP/AVP 97 33
a=rtpmap:97 iLBC/8000
a=rtpmap:33 no-op/8000
a=zrtp-sas: opzf 5e017f3a6563876a
Another example showing a second media stream being added to the
session. A second DH exchange is performed (instead of using the
Preshared mode) resulting in a second set of ZRTP SAS and SASvalue
attributes.
v=0
o=bob 2890844527 2890844528 IN IP4 client.biloxi.example.com
s=
c=IN IP4 client.biloxi.example.com
a=zrtp-zid:4cc3ffe30efd02423cb054e5
t=0 0
m=audio 3456 RTP/AVP 97 33
a=rtpmap:97 iLBC/8000
a=rtpmap:33 no-op/8000
a=zrtp-sas: opzf 5e017f3a6563876a
m=video 51372 RTP/AVP 31 33
a=rtpmap:31 H261/90000
a=rtpmap:33 no-op/8000
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a=zrtp-sas: gwif e1027fa9f865221c
10. The ZRTP Disclosure flag
There are no back doors defined in the ZRTP protocol specification.
The designers of ZRTP would like to discourage back doors in ZRTP-
enabled products. However, despite the lack of back doors in the
actual ZRTP protocol, it must be recognized that a ZRTP implementer
might still deliberately create a rogue ZRTP-enabled product that
implements a back door outside the scope of the ZRTP protocol. For
example, they could create a product that discloses the SRTP session
key generated using ZRTP out-of-band to a third party. They may even
have a legitimate business reason to do this for some customers.
For example, some environments have a need to monitor or record
calls, such as stock brokerage houses who want to discourage insider
trading, or special high security environments with special needs to
monitor their own phone calls. We've all experienced automated
messages telling us that "This call may be monitored for quality
assurance". A ZRTP endpoint in such an environment might
unilaterally disclose the session key to someone monitoring the call.
ZRTP-enabled products that perform such out-of-band disclosures of
the session key can undermine public confidence in the ZRTP protocol,
unless we do everything we can in the protocol to alert the other
user that this is happening.
If one of the parties is using a product that is designed to disclose
their session key, ZRTP requires them to confess this fact to the
other party through a protocol message to the other party's ZRTP
client, which can properly alert that user, perhaps by rendering it
in a GUI. The disclosing party does this by sending a Disclosure
flag (D) in Confirm1 and Confirm2 messages as described in Sections
6.7 and 6.8.
Note that the intention here is to have the Disclosure flag identify
products that are designed to disclose their session keys, not to
identify which particular calls are compromised on a call-by-call
basis. This is an important legal distinction, because most
government sanctioned wiretap regulations require a VoIP service
provider to not reveal which particular calls are wiretapped. But
there is nothing illegal about revealing that a product is designed
to be wiretap-friendly. The ZRTP protocol mandates that such a
product "out" itself.
You might be using a ZRTP-enabled product with no back doors, but if
your own GUI tells you the call is (mostly) secure, except that the
other party is using a product that is designed in such a way that it
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may have disclosed the session key for monitoring purposes, you might
ask him what brand of secure telephone he is using, and make a mental
note not to purchase that brand yourself. If we create a protocol
environment that requires such back-doored phones to confess their
nature, word will spread quickly, and the "unseen hand" of the free
market will act. The free market has effectively dealt with this in
the past.
Of course, a ZRTP implementer can lie about his product having a back
door, but the ZRTP standard mandates that ZRTP-compliant products
MUST adhere to the requirement that a back door be confessed by
sending the Disclosure flag to the other party.
There will be inevitable comparisons to Steve Bellovin's 2003 April
fool's joke, when he submitted RFC 3514 [24] which defined the "Evil
bit" in the IPV4 header, for packets with "evil intent". But we
submit that a similar idea can actually have some merit for securing
VoIP. Sure, one can always imagine that some implementer will not be
fazed by the rules and will lie, but they would have lied anyway even
without the Disclosure flag. There are good reasons to believe that
it will improve the overall percentage of implementations that at
least tell us if they put a back door in their products, and may even
get some of them to decide not to put in a back door at all. From a
civic hygiene perspective, we are better off with having the
Disclosure flag in the protocol.
If an endpoint stores or logs SRTP keys or information that can be
used to reconstruct or recover SRTP keys after they are no longer in
use (i.e. the session is active), or otherwise discloses or passes
SRTP keys or information that can be used to reconstruct or recover
SRTP keys to another application or device, the Disclosure flag D
MUST be set in the Confirm1 or Confirm2 message.
11. Intermediary ZRTP Devices
This section discusses the operation of a ZRTP endpoint which is
actually an intermediary. For example, consider a device which
proxies both signaling and media between endpoints. There are three
possible ways in which such a device could support ZRTP.
An intermediary device can act transparently to the ZRTP protocol.
To do this, a device MUST pass RTP header extensions and payloads (to
allow the ZRTP Flag) and non-RTP protocols multiplexed on the same
port as RTP (to allow ZRTP and STUN). This is the RECOMMENDED
behavior for intermediaries as ZRTP and SRTP are best when done end-
to-end.
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An intermediary device could implement the ZRTP protocol and act as a
ZRTP endpoint on behalf of non-ZRTP endpoints behind the intermediary
device. The intermediary could determine on a call-by-call basis
whether the endpoint behind it supports ZRTP based on the presence or
absence of the ZRTP SDP attribute flag (a=zrtp-id). For non-ZRTP
endpoints, the intermediary device could act as the ZRTP endpoint
using its own ZID and cache. This approach MUST only be used when
there is some other security method protecting the confidentiality of
the media between the intermediary and the inside endpoint, such as
IPSec or physical security.
The third mode, which is NOT RECOMMENDED, is for the intermediary
device to attempt to back-to-back the ZRTP protocol. The only
exception to this case is where the intermediary device is a trusted
element providing services to one of the endpoints - e.g. a Private
Branch Exchange or PBX. In this mode, the intermediary would attempt
to act as a ZRTP endpoint towards both endpoints of the media
session. This approach MUST NOT be used except in Section 8.3 as it
will always result in a detected Man-in-the-Middle attack and will
generate alarms on both endpoints and likely result in the immediate
termination of the session.
In cases where centralized media mixing is taking place, the SAS will
not match when compared by the humans. However, this situation is
known in the SIP signaling by the presence of the isfocus feature tag
[27]. As a result, when the isfocus feature tag is present, the SAS
can only be verified by comparison in the signaling or by validating
signatures in the Confirm. For example, consider a audio conference
call with three participants Alice, Bob, and Carol hosted on a
conference bridge in Dallas. There will be three ZRTP encrypted
media streams between each participant and Dallas. Each will have a
different SAS. Each participant will be able to validates their SAS
with the conference bridge using a=zrtp-sas or Confirm messages
containing signatures.
SIP feature tags can also be used to detect if a session is
established with an automaton such as an IVR, voicemail system, or
speech recognition system. The display of SAS strings to users
should be disabled in these cases.
It is possible that an intermediary device acting as a ZRTP endpoint
might still receive ZRTP Hello and other messages from the inside
endpoint. This could occur if there is another inline ZRTP device
which does not include the ZRTP SDP attribute flag. If this occurs,
the intermediary MUST NOT pass these ZRTP messages if it is acting as
the ZRTP endpoint.
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12. RTP Header Extension Flag for ZRTP
This specification defines a new RTP header extension used only for
discovery of support for ZRTP. No ZRTP data is transported in the
extension. When used, the X bit is set in the RTP header to indicate
the presence of the RTP header extension.
Section 5.3.1 in RFC 3550 defines the format of an RTP Header
extension. The Header extension is appended to the RTP header. The
first 16 bits are an identifier for the header extension, and the
following 16 bits are length of the extension header in 32 bit words.
The ZRTP flag RTP header extension has the value of 0x505A and a
length of 0. The format of the header extension is as shown in
Figure 14.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14. RTP Extension header format for ZRTP Flag
ZRTP endpoints SHOULD include the ZRTP Flag in RTP packets sent at
the start of a session. For example, including the flag in the first
1 second of RTP packets sent. The inclusion of the flag MAY be ended
if a ZRTP message (such as Hello) is received.
13. IANA Considerations
This specification defines two new SDP [12] attributes in Section 9.
The IANA registration of ZRTP SDP attribute:
Contact name: Phil Zimmermann <prz@mit.edu>
Attribute name: "zrtp-zid".
Type of attribute: Session level or Media level.
Subject to charset: Not.
Purpose of attribute: The 'zrtp-zid' indicates that a UA supports the
ZRTP protocol and provides the ZID of the UA.
Allowed attribute values: Hex.
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IANA registration of the ZRTP SAS SDP attribute:
Contact name: Phil Zimmermann <prz@mit.edu>
Attribute name: "zrtp-sas".
Type of attribute: Media level.
Subject to charset: Yes.
Purpose of attribute: The 'zrtp-sas' is used to convey the ZRTP SAS
string and value. The string is identical to that
rendered to the users. The value is the 64 bit SAS
encoded as hex.
Allowed attribute values: String and Hex.
14. Security Considerations
This document is all about securely keying SRTP sessions. As such,
security is discussed in every section.
Most secure phones rely on a Diffie-Hellman exchange to agree on a
common session key. But since DH is susceptible to a man-in-the-
middle (MITM) attack, it is common practice to provide a way to
authenticate the DH exchange. In some military systems, this is done
by depending on digital signatures backed by a centrally-managed PKI.
A decade of industry experience has shown that deploying centrally
managed PKIs can be a painful and often futile experience. PKIs are
just too messy, and require too much activation energy to get them
started. Setting up a PKI requires somebody to run it, which is not
practical for an equipment provider. A service provider like a
carrier might venture down this path, but even then you have to deal
with cross-carrier authentication, certificate revocation lists, and
other complexities. It is much simpler to avoid PKIs altogether,
especially when developing secure commercial products. It is
therefore more common for commercial secure phones in the PSTN world
to augment the DH exchange with a Short Authentication String (SAS)
combined with a hash commitment at the start of the key exchange, to
shorten the length of SAS material that must be read aloud. No PKI
is required for this approach to authenticating the DH exchange. The
AT&T TSD 3600, Eric Blossom's COMSEC secure phones [17], PGPfone
[15], and CryptoPhone [18] are all examples of products that took
this simpler lightweight approach.
The main problem with this approach is inattentive users who may not
execute the voice authentication procedure, or unattended secure
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phone calls to answering machines that cannot execute it.
Additionally, some people worry about voice spoofing. But it is a
mistake to think this is simply an exercise in voice impersonation
(perhaps this could be called the "Rich Little" attack). Although
there are digital signal processing techniques for changing a
person's voice, that does not mean a man-in-the-middle attacker can
safely break into a phone conversation and inject his own short
authentication string (SAS) at just the right moment. He doesn't
know exactly when or in what manner the users will choose to read
aloud the SAS, or in what context they will bring it up or say it, or
even which of the two speakers will say it, or if indeed they both
will say it. In addition, some methods of rendering the SAS involve
using a list of words such as the PGP word list, in a manner
analogous to how pilots use the NATO phonetic alphabet to convey
information. This can make it even more complicated for the
attacker, because these words can be worked into the conversation in
unpredictable ways. Remember that the attacker places a very high
value on not being detected, and if he makes a mistake, he doesn't
get to do it over. Some people have raised the question that even if
the attacker lacks voice impersonation capabilities, it may be unsafe
for people who don't know each other's voices to depend on the SAS
procedure. This is not as much of a problem as it seems, because it
isn't necessary that they recognize each other by their voice, it's
only necessary that they detect that the voice used for the SAS
procedure matches the voice in the rest of the phone conversation.
A popular and field-proven approach is used by SSH (Secure Shell)
[20], which Peter Gutmann likes to call the "baby duck" security
model. SSH establishes a relationship by exchanging public keys in
the initial session, when we assume no attacker is present, and this
makes it possible to authenticate all subsequent sessions. A
successful MITM attacker has to have been present in all sessions all
the way back to the first one, which is assumed to be difficult for
the attacker. All this is accomplished without resorting to a
centrally-managed PKI.
We use an analogous baby duck security model to authenticate the DH
exchange in ZRTP. We don't need to exchange persistent public keys,
we can simply cache a shared secret and re-use it to authenticate a
long series of DH exchanges for secure phone calls over a long period
of time. If we read aloud just one SAS, and then cache a shared
secret for later calls to use for authentication, no new voice
authentication rituals need to be executed. We just have to remember
we did one already.
If we ever lose this cached shared secret, it is no longer available
for authentication of DH exchanges, so we would have to do a new SAS
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procedure and start over with a new cached shared secret. Then we
could go back to omitting the voice authentication on later calls.
A particularly compelling reason why this approach is attractive is
that SAS is easiest to implement when a GUI or some sort of display
is available, which raises the question of what to do when no display
is available. We envision some products that implement secure VoIP
via a local network proxy, which lacks a display in many cases. If
we take an approach that greatly reduces the need for a SAS in each
and every call, we can operate in GUI-less products with greater
ease.
It's a good idea to force your opponent to have to solve multiple
problems in order to mount a successful attack. Some examples of
widely differing problems we might like to present him with are:
Stealing a shared secret from one of the parties, being present on
the very first session and every subsequent session to carry out an
active MITM attack, and solving the discrete log problem. We want to
force the opponent to solve more than one of these problems to
succeed.
ZRTP can use different kinds of shared secrets. Each type of shared
secret is determined by a different method. All of the shared
secrets are hashed together to form a session key to encrypt the
call. An attacker must defeat all of the methods in order to
determine the session key.
First, there is the shared secret determined entirely by a Diffie-
Hellman key agreement. It changes with every call, based on random
numbers. An attacker may attempt a classic DH MITM attack on this
secret, but we can protect against this by displaying and reading
aloud a SAS, combined with adding a hash commitment at the beginning
of the DH exchange.
Second, there is an evolving shared secret, or ongoing shared secret
that is automatically changed and refreshed and cached with every new
session. We will call this the cached shared secret, or sometimes
the retained shared secret. Each new image of this ongoing secret is
a non-invertable function of its previous value and the new secret
derived by the new DH agreement. It's possible that no cached shared
secret is available, because there were no previous sessions to
inherit this value from, or because one side loses its cache.
There are other approaches for key agreement for SRTP that compute a
shared secret using information in the signaling. For example, [22]
describes how to carry a MIKEY (Multimedia Internet KEYing) [23]
payload in SDP [12]. Or [21] describes directly carrying SRTP keying
and configuration information in SDP. ZRTP does not rely on the
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signaling to compute a shared secret, but If a client does produce a
shared secret via the signaling, and makes it available to the ZRTP
protocol, ZRTP can make use of this shared secret to augment the list
of shared secrets that will be hashed together to form a session key.
This way, any security weaknesses that might compromise the shared
secret contributed by the signaling will not harm the final resulting
session key.
There may also be a static shared secret that the two parties agree
on out-of-band in advance. A hashed passphrase would suffice.
The shared secret provided by the signaling (if available), the
shared secret computed by DH, and the cached shared secret are all
hashed together to compute the session key for a call. If the cached
shared secret is not available, it is omitted from the hash
computation. If the signaling provides no shared secret, it is also
omitted from the hash computation.
No DH MITM attack can succeed if the ongoing shared secret is
available to the two parties, but not to the attacker. This is
because the attacker cannot compute a common session key with either
party without knowing the cached secret component, even if he
correctly executes a classic DH MITM attack. Mixing in the cached
shared secret for the session key calculation allows it to act as an
implicit authenticator to protect the DH exchange, without requiring
additional explicit HMACs to be computed on the DH parameters. If
the cached shared secret is available, a MITM attack would be
instantly detected by the failure to achieve a shared session key,
resulting in undecryptable packets. The protocol can easily detect
this. It would be more accurate to say that the MITM attack is not
merely detected, but thwarted.
When adding the complexity of additional shared secrets beyond the
familiar DH key agreement, we must make sure the lack of availability
of the cached shared secret cannot prevent a call from going through,
and we must also prevent false alarms that claim an attack was
detected.
An small added benefit of using these cached shared secrets to mix in
with the session keys is that it augments the entropy of the session
key. Even if limits on the size of the DH exchange produces a
session key with less than 256 bits of real work factor, the added
entropy from the cached shared secret can bring up all the subsequent
session keys to the full 256-bit AES key strength, assuming no
attacker was present in the first call.
We could have authenticated the DH exchange the same way SSH does it,
with digital signatures, caching public keys instead of shared
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secrets. But this approach with caching shared secrets seemed a bit
simpler, requiring less CPU time for low-powered mobile platforms
because it avoids an added digital signature step.
The ZRTP SDP attributes convey information through the signaling that
is already available in clear text through the media path. For
example, the ZRTP flag is equivalent to sending a ZRTP Hello message.
The SAS is calculated from a hash of material from ZRTP messages sent
over the media path. As a result, none of the ZRTP SDP attributes
require confidentiality from the signaling.
The ZRTP SAS attributes can use the signaling channel as an out-of-
band authentication mechanism. This authentication is only useful if
the signaling channel has end-to-end integrity protection. Note that
the SIP Identity header field [25] provides middle-to-end integrity
protection across SDP message bodies which provides useful protection
for ZRTP SAS attributes.
15. Acknowledgments
The authors would like to thank Bryce Wilcox-O'Hearn for his
contributions to the design of this protocol, and to thank Jon
Peterson, Colin Plumb, Hal Finney, Colin Perkins, and Dan Wing for
their helpful comments and suggestions. Also thanks to David McGrew,
Roni Even, Viktor Krikun, Werner Dittmann, Allen Pulsifer, Klaus
Peters, and Abhishek Arya for their feedback and comments.
16. References
16.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[3] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[4] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
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[5] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002.
[6] Barker, E. and J. Kelsey, "Recommendation for Random Number
Generation Using Deterministic Random Bit Generators", NIST
Special Publication 800-90 (Revised) March 2007.
[7] Barker, E., Johnson, D., and M. Smid, "Recommendation for Pair-
Wise Key Establishment Schemes Using Discrete Logarithm
Cryptography", NIST Special Publication 800-56A Revision 1,
March 2007.
[8] Dworkin, M., "Recommendation for Block Cipher: Methods and
Techniques", NIST Special Publication 800-38A 2001 Edition.
[9] Wilcox, B., "Human-oriented base-32 encoding",
http://zooko.com/repos/z-base-32/base32/DESIGN .
[10] Juola, P. and P. Zimmermann, "Whole-Word Phonetic Distances and
the PGPfone Alphabet", Proceedings of the International
Conference of Spoken Language Processing (ICSLP-96) .
[11] "PGP Wordlist", http://en.wikipedia.org/wiki/PGP_Words .
[12] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
16.2. Informative References
[13] Wing, D., "Requirements for a Media Security Key Management
Protocol", draft-wing-media-security-requirements-04 (work in
progress), June 2007.
[14] Ferguson, N. and B. Schneier, "Practical Cryptography", Wiley
Publishing 2003.
[15] Zimmermann, P., "PGPfone",
http://www.pgpi.org/products/pgpfone/ .
[16] Zimmermann, P., "Zfone", http://www.philzimmermann.com/zfone .
[17] Blossom, E., "The VP1 Protocol for Voice Privacy Devices
Version 1.2", http://www.comsec.com/vp1-protocol.pdf .
[18] "CryptoPhone", http://www.cryptophone.de/ .
[19] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
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Internet-Draft ZRTP July 2007
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[20] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol
Architecture", RFC 4251, January 2006.
[21] Andreasen, F., Baugher, M., and D. Wing, "Session Description
Protocol (SDP) Security Descriptions for Media Streams",
RFC 4568, July 2006.
[22] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
RFC 4567, July 2006.
[23] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[24] Bellovin, S., "The Security Flag in the IPv4 Header", RFC 3514,
April 1 2003.
[25] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.
[26] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Protocol for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", draft-ietf-mmusic-ice-16 (work in
progress), June 2007.
[27] Johnston, A. and O. Levin, "Session Initiation Protocol (SIP)
Call Control - Conferencing for User Agents", BCP 119,
RFC 4579, August 2006.
Authors' Addresses
Philip Zimmermann
Zfone Project
Email: prz@mit.edu
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Alan Johnston (editor)
Avaya
St. Louis, MO 63124
Email: alan@sipstation.com
Jon Callas
PGP Corporation
Email: jon@pgp.com
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