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Versions: (draft-mcgrew-avt-srtp) 00 01 02 03 04 05 06 07 08 09 RFC 3711

   Internet Engineering Task Force                    Baugher, McGrew,
   AVT Working Group                                      Oran (Cisco)
   INTERNET-DRAFT                              Blom, Carrara, Naslund,
   EXPIRES: September 2002                          Norrman (Ericsson)
                                                              May 2002

                The Secure Real Time Transport Protocol
                      <draft-ietf-avt-srtp-04.txt>



Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/lid-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html



Abstract

   This document describes the Secure Real Time Transport Protocol
   (SRTP), a profile of the Real Time Transport Protocol (RTP), which
   can provide confidentiality, message authentication, and replay
   protection to the RTP/RTCP traffic.

   SRTP can achieve high throughput and low packet expansion. SRTP
   proves to be a suitable protection for heterogeneous environments,
   i.e., environments including both wired and wireless links. To get
   such features, default transforms are described, based on an
   additive stream cipher for encryption, a keyed-hash based function
   for message authentication, and an "implicit" index for
   sequencing/synchronization based on the RTP sequence number for SRTP
   and an index number for Secure RTCP (SRTCP).





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   TABLE OF CONTENTS

1. Notational Conventions............................................3
2. Goals and Features................................................3
3. SRTP Framework....................................................5
 3.1 SRTP Cryptographic Contexts.....................................6
   3.1.1 Transform-independent parameters............................7
   3.1.2 Transform-dependent parameters..............................9
   3.1.3 Mapping SRTP Packets to Cryptographic Contexts..............9
 3.2 SRTP Packet Processing.........................................10
   3.2.1 Packet Index Determination, and ROC, s_l Update............11
   3.2.2 Replay Protection..........................................13
 3.3 Secure RTCP....................................................14
4. Pre-Defined Cryptographic Transforms.............................17
 4.1 Encryption.....................................................17
   4.1.1 AES in Counter Mode........................................19
   4.1.2 AES in f8-mode.............................................20
   4.1.3 NULL Cipher................................................22
 4.2 Message Authentication and Integrity...........................22
   4.2.1. HMAC-SHA1.................................................23
 4.3 Key Derivation.................................................23
   4.3.1 Key Derivation Algorithm...................................23
   4.3.2 SRTCP Key Derivation.......................................25
   4.3.3 AES-CM PRF.................................................25
5. Default and mandatory-to-implement Transforms....................26
 5.1 Encryption: AES-CM and NULL....................................26
 5.2 Message Authentication/Integrity: HMAC-SHA1....................26
 5.3 Key Derivation: AES-CM PRF.....................................26
6. Adding SRTP Transforms...........................................26
7. Rationale........................................................27
 7.1 Key derivation.................................................27
 7.2 Salting key....................................................28
 7.3 Message Integrity from Universal Hashing.......................28
 7.4 Data Origin Authentication Considerations......................28
8. Key Management Considerations....................................29
 8.1. Re-keying.....................................................30
 8.2. Key Management parameters.....................................30
9. Security Considerations..........................................31
 9.1 SSRC collision and two-time pad................................31
 9.2 Key Usage......................................................32
 9.3 Confidentiality of the RTP Payload.............................34
 9.4 Confidentiality of the RTP Header..............................34
 9.5 Integrity of the RTP payload and header........................35
10. Interaction with Forward Error Correction mechanisms............36



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11. Scenarios.......................................................36
 11.1 Unicast.......................................................36
 11.2 Multicast.....................................................37
   11.2.1 Small multicast with one sender...........................37
   11.2.2 Large multicast with one sender...........................38
 11.3 Re-keying and access control..................................38
 11.4 Summary of basic scenarios....................................39
12. IANA Considerations.............................................40
13. Acknowledgements................................................40
14. Author's Addresses..............................................40
15. References......................................................41
Appendix A: Pseudocode for Index Determination......................43
Appendix B: Test Vectors............................................44
 B.1 AES-f8 Test Vectors............................................44
 B.2 AES-CM Test Vectors............................................45
 B.3 Key Derivation and PRF Test Vectors............................45


1. Notational Conventions

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].
   The terminology conforms to [RFC2828].

   By convention, the adopted representation is the network byte order,
   i.e. the left most bit (octet) is the most significant one. By XOR
   we mean bitwise addition modulo 2 of binary strings, and || denotes
   concatenation. In other words, if C = A || B, then the most
   significant bits of C are the bits of A, and the least significant
   bits of C equal the bits of B. Hexadecimal numbers are prefixed by
   0x.

   With slight abuse of notation, we refer in some cases to integrity
   protection of the message as "message authentication".  The word
   "encryption" includes also use of the NULL algorithm (which
   in practice does leave the data in the clear).


2. Goals and Features

   The security goals for SRTP are to ensure:

   * the confidentiality of the RTP and RTCP payloads, and





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   * the integrity of the entire RTP and RTCP packets, together with
     protection against replayed packets.

   These security services are optional and independent from each
   other, except that SRTCP integrity protection is mandatory.

   Other, functional, goals for the protocol are:

   * a framework that permits upgrading with new cryptographic
     transforms,

   * low bandwidth cost, i.e., a framework preserving RTP header
     compression efficiency,

   and, asserted by the pre-defined transforms:

   * a low computational cost,

   * a small footprint (i.e. small code size and data memory for keying
     information and replay lists),

   * limited packet expansion to support the bandwidth economy goal,

   * independence from the underlying transport, network, and physical
     layers used by RTP, in particular high tolerance to packet loss
     and re-ordering, and robustness to transmission bit-errors in the
     encrypted payload.

   These properties ensure that SRTP is a suitable protection scheme
   for RTP/RTCP in both wired and wireless scenarios.

2.1 Features

   Besides the above mentioned direct goals, SRTP provides for some
   additional features. They have been introduced to lighten the burden
   on key management and to further increase security. They include:

   *  A single so-called master key provides keying material for
     confidentiality and integrity protection, both for the SRTP stream
     and the corresponding SRTCP stream. This is achieved due to a key
     derivation function (see Section 4.3), providing so-called session
     keys for the respective security primitive, securely derived from
     the master key. Under additional SSRC uniqueness requirements, a
     single master key can even protect several SRTP streams, see
     Section 9.1.

   * In addition, the key derivation can be configured to periodically
     "refresh" the session keys, which limits the amount of ciphertext



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     produced by a fixed key, available for an adversary to
     cryptanalyze.

   * So-called salting keys are used to protect against off-line pre-
     computation attacks.

   Detailed rationale for these features can be found in Section 7.


3. SRTP Framework

   RTP is the Real Time Transport Protocol [RFC1889]. We define SRTP as
   a profile of RTP, in a way analogous to RFC1890 which defines the
   audio/video profile for RTP. Conceptually, we consider it to be a
   "bump in the stack" implementation which resides between the RTP
   application and the transport layer. SRTP intercepts RTP packets and
   then forwards an equivalent SRTP packet on the sending side, and
   which intercepts SRTP packets and passes an equivalent RTP packet up
   the stack on the receiving side.

   The format of an SRTP packet is illustrated in Figure 1.

   The Encrypted Portion of an SRTP packet consists of the encryption
   of the RTP payload (including RTP padding when present) of the
   equivalent RTP packet. (Note: our use of the word "encryption"
   includes also the possibility of a "NULL" encryption, and we
   therefore speak of the "Encrypted Portion" even if only the NULL-
   encryption has been applied.)

   The optional MKI and optional authentication tag are the only fields
   defined by SRTP that are not in RTP. Only 8-bit alignment is
   assumed.

   MKI (Master Key Identifier): variable length, OPTIONAL
          The MKI is defined, signaled, and used by key management.
          The MKI identifies the master key from which the session
          key(s) were derived that authenticate and/or encrypt the
          particular packet. Note that the MKI SHALL NOT identify the
          SRTP cryptographic context, which is identified according to
          Section 3.1.3. The MKI MAY be used by key management for the
          purposes of re-keying, identifying a particular master key
          within the cryptographic context (Section 3.1.1).

   Authentication tag: variable length, OPTIONAL
          The authentication tag is used to carry message
          authentication data. The Authenticated Portion of an SRTP
          packet consists of the RTP header followed by the Encrypted
          Portion of the SRTP packet. Thus, note that if both



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          encryption and authentication are applied, encryption SHALL
          be applied before authentication on the sender side and
          conversely on the receiver side. The authentication tag
          provides authentication of the RTP header and payload, and it
          indirectly provides replay protection by authenticating the
          sequence number. Note that the MKI is not integrity protected
          as this does not provide any extra protection.

     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
+-->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   |V=2|P|X|  CC   |M|     PT      |       sequence number         |
|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   |                           timestamp                           |
|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   |           synchronization source (SSRC) identifier            |
|   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|   |            contributing source (CSRC) identifiers             |
|   |                               ....                            |
|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   |                   RTP extension (OPTIONAL)                    |
| +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |                                                               |
| | |                           payload                             |
| | |                             ....                              |
+-+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                     SRTP MKI (OPTIONAL)                       ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                  authentication tag (OPTIONAL)                ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| +- Encrypted Portion
+---- Authenticated Portion

   Figure 1.  The format of an SRTP packet.

   Secure RTCP (SRTCP) is described in Section 3.3. SRTCP adds fields
   to the RTCP packets in a similar way.

3.1 SRTP Cryptographic Contexts

   Each SRTP stream requires the sender and receiver to maintain
   cryptographic state information. This information is called the
   "cryptographic context".

   SRTP uses two types of keys: session keys and master keys. By a
   "session key", we mean a key which is used directly in a
   cryptographic transform (e.g. encryption or message authentication),



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   and by a "master key", we mean a random bit string (given by the key
   management protocol) from which session keys are derived in a
   cryptographically secure way.

3.1.1 Transform-independent parameters

   "Transform-independent parameters" are present in the cryptographic
   context independently of the particular encryption or authentication
   transforms that are used. The transform-independent parameters of
   the cryptographic context for SRTP consist of:

   * a 32-bit unsigned rollover counter (ROC), which records how many
     times the 16-bit RTP sequence number has been reset to zero after
     passing through 65,535. Unlike the sequence number (SEQ), which
     SRTP extracts from the RTP packet header, the ROC is maintained by
     SRTP as described in Section 3.2.1.

     We define the index of the SRTP packet corresponding to a given
     ROC and RTP sequence number to be the 48-bit quantity

         i = 2^16 * ROC + SEQ.

   * for the receiver only, a 16-bit sequence number s_l, which is the
     highest received RTP sequence number (possibly authenticated, if
     message authentication is provided),

   * an identifier for the encryption algorithm, i.e., the cipher and
     its mode of operation,

   * an identifier for the message authentication algorithm (when
     authentication is provided),

   * a replay list, maintained by the receiver only (when
     authentication and replay protection are provided), containing
     indices of recently received and authenticated SRTP packets,

   * an MKI indicator (0/1) as to whether an MKI is present in SRTP and
     SRTCP packets,

   * if the MKI indicator is set to one, the length (in octets) of the
     MKI field, and (for the sender) the actual value of the currently
     active MKI, (the value of the MKI indicator and length MUST be
     kept fixed for the lifetime of the context),

   * the master key(s), which MUST be random and kept secret,






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   * for each master key, there is a counter of the number of SRTP
     packets that has been processed (sent) with that master key
     (essential for security, see Sections 3.2.1 and 9),

   * non-negative integers n_e, and n_a, determining the length of the
     session keys for encryption, and message authentication.

   In addition, for each master key, an SRTP stream MAY use the
   following associated values:

   * a master salt, to be used in the key derivation of session keys.
     This value, when used, MUST be random, but MAY be public. Use of
     master salt is strongly RECOMMENDED, see Section 9.2. A "NULL"
     salt is treated as 00...0.

   * an integer in the set {1,2,4,...,2^24}, the "key_derivation_rate",
     where an unspecified value is treated as zero,

   * <"From", "To"> values, specifying the lifetime for a master key,
    expressed in terms of the two 48-bit index values inside whose
    range (including the range end-points) the master key is valid.
    These values are absolute quantities, not relative. Whenever this
    field is unspecified, the related master key is valid "from the
    first observed packet" to "until further notice" (with maximum
    lifetime as specified in Section 3.2.1).

   SRTCP SHALL by default share the crypto context with SRTP and uses
   the same cryptographic context parameters, except:

   * no rollover counter and s_l-value need to be maintained as the
     RTCP index is explicitly carried in each SRTCP packet,

   * a separate replay list is maintained (when replay protection is
     provided),

   * SRTCP maintains a separate counter for its master key (even if the
     master key is the same as that for SRTP, see below), as a mean to
     maintain a count of the number of SRTCP packets that have been
     processed with that key.

   Note in particular that the master key(s) MAY be shared between SRTP
   and SRTCP, if the pre-defined transforms (including the key
   derivation) are used but the session key(s) MUST NOT be so shared.

   In addition, there can be cases (see Sections 8 and 9.1) where
   several SRTP streams, identified by their SSRCs, share most of the
   crypto context parameters (including master keys). In such cases,
   just as in the normal SRTP/SRTCP parameter sharing above, separate



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   replay lists and packet counters for each stream (SSRC) MUST still
   be maintained, but the session keys MAY then be shared between SRTP
   streams.

   A summary of parameters, pre-defined transforms, and default values
   for the above parameters (and other SRTP parameters) can be found in
   Sections 5 and 8.2.

3.1.2 Transform-dependent parameters

   All encryption, authentication/integrity, and key derivation
   parameters are defined in the transforms section (Section 4).
   Typical examples of such parameters are block size of ciphers,
   session keys, data for IV formation, etc. Future SRTP transform
   specifications MUST include a section to list the additional
   cryptographic context's parameters for that transform, if any.

3.1.3 Mapping SRTP Packets to Cryptographic Contexts

   Recall that an RTP session for each participant is defined [RFC1889]
   by a pair of destination transport addresses (one network address
   plus a port pair for RTP and RTCP), and that a multimedia session is
   defined as a collection of RTP sessions. For example, a particular
   multimedia session could include an audio RTP session, a video RTP
   session, and a text RTP session.

   A cryptographic context SHALL be uniquely identified by the triplet
   context identifier:

   context id = <SSRC, destination network address, destination
   transport port number>

   where the destination network address and the destination transport
   port are the ones in the current RTP packet (for the sender) or SRTP
   packet (for the receiver). It is assumed that, when presented with
   this information, the key management returns a context with the
   information as described in Section 3.1.

   As noted above, SRTP and SRTCP by default share the bulk of the
   parameters in the cryptographic context. Thus, retrieving the crypto
   context parameters for an SRTCP stream in practice may imply a
   binding to the correspondent SRTP crypto context. It is up to the
   implementation to assure such binding, since the RTCP port may not
   be directly deducible from the RTP port only. Alternatively, the key
   management may choose to provide separate SRTP- and SRTCP-contexts,
   duplicating the common parameters (such as master key(s)). The
   latter approach then also enables SRTP and SRTCP to use, e.g.,




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   distinct transforms, if so desired. Similar considerations arise
   when multiple SRTP streams share keys and other parameters.

   If no valid context can be found for a packet corresponding to a
   certain context identifier, that packet MUST be discarded from
   further SRTP processing.

3.2 SRTP Packet Processing

   The following applies to SRTP. SRTCP is described in Section 3.3.

   Assuming initialization of the cryptographic context(s) has taken
   place via key management, the sender SHALL do the following to
   construct an SRTP packet:

   1. Determine which cryptographic context to use as described in
   Section 3.1.3.

   2. Determine the index of the SRTP packet using the rollover counter
   in the cryptographic context and the sequence number in the RTP
   packet, as described in Section 3.2.1.

   3. Determine the master key and master salt. This is done using the
   index determined in the previous step or the current MKI in the
   cryptographic context.

   4. Determine the session keys and session salt (if they are used by
   the transform) as described in Section 4.3, using master key, master
   salt, key_derivation_rate, and session key-lengths in the
   cryptographic context with the index, determined in Steps 2 and 3.

   5. Encrypt the RTP payload to produce the Encrypted Portion of the
   packet (see Section 4.1, for the defined ciphers). This step uses
   the encryption algorithm indicated in the cryptographic context, the
   session encryption key and the session salt (if used) found in Step
   4 together with the index found in Step 2.

   6. If the MKI indicator is set to one, append the MKI to the packet.

   7. If message authentication is provided, compute the authentication
   tag for the Authenticated Portion of the packet, as described in
   Section 4.2. This step uses the current rollover counter, the
   authentication algorithm indicated in the cryptographic context, and
   the session authentication key found in Step 4. Append the
   authentication tag to the packet.

   8. If necessary, update the ROC as in Section 3.2.1, using the
   packet index determined in Step 2.



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   To authenticate and decrypt an SRTP packet, the receiver SHALL do
   the following:

   1. Determine which cryptographic context to use as described in
   Section 3.1.3.

   2. Estimate the index of the SRTP packet using the rollover counter
   and highest sequence number in the cryptographic context with the
   sequence number in the SRTP packet, as described in Section 3.2.1.

   3. Determine the master key and master salt. If the MKI indicator in
   the context is set to one, use the MKI in the SRTP packet, otherwise
   use the index from the previous step.

   4. Determine the session keys, and session salt (if used by the
   transform) as described in Section 4.3, using master key, master
   salt, key_derivation_rate and session key-lengths in the
   cryptographic context with the index, determined in Steps 2 and 3.

   5. If message authentication and replay protection are provided,
   first check if the packet has been replayed (Section 3.2.2), using
   the Replay List and the index as determined in Step 2. If the packet
   is judged to be replayed, then the packet MUST be discarded, and the
   event SHOULD be logged.

   Next, perform verification of the authentication tag, using the
   rollover counter from Step 2, the authentication algorithm indicated
   in the cryptographic context, and the session authentication key
   from Step 4. If the result is "AUTHENTICATION FAILURE" (see Section
   4.2), the packet MUST be discarded from further processing and the
   event SHOULD be logged.

   6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
   the defined ciphers), using the decryption algorithm indicated in
   the cryptographic context, the session encryption key and salt (if
   used) found in Step 4 with the index from Step 2.

   7. Update the rollover counter and highest sequence number, s_l, in
   the cryptographic context as in Section 3.2.1, using the packet
   index estimated in Step 2. If replay protection is provided, also
   update the Replay List as described in Section 3.2.2.

   8. When present, remove the MKI and authentication tag fields from
   the packet.

3.2.1 Packet Index Determination, and ROC, s_l Update




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   SRTP implementations use an "implicit" packet index for sequencing,
   i.e., not all of the index is explicitly carried in the SRTP packet.
   For the pre-defined transforms, the index i is used in replay
   protection (Section 3.2.2), encryption (Section 4.1), message
   authentication (Section 4.2), and for the key derivation (Section
   4.3). The index MAY also be used to determine the correct master key
   when <"From", "To"> values are used to represent key lifetime
   (Section 3.1.1).

   When the session starts, the sender side MUST set the rollover
   counter, ROC, to zero. Each time the RTP sequence number, SEQ, wraps
   modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32
   (see security aspects below). The sender's packet index is then
   defined as

      i = 2^16 * ROC + SEQ.

   Receiver-side implementations use the RTP sequence number to estimate
   the correct index of a packet, which is the location of the packet in
   the sequence of all SRTP packets. A robust approach for the proper
   use of a rollover counter requires its handling and use to be well
   defined. In particular, out-of-order RTP packets with sequence
   numbers close to 2^16 or zero must be properly handled.

   The index estimate is based on the receiver's locally maintained ROC
   and s_l values. At the setup of the session, ROC MUST be set to zero.
   Receivers joining an on-going session MUST be given the current ROC
   value using out of band signaling. Furthermore, the receiver SHALL
   initialize s_l to the RTP sequence number (SEQ) of the first observed
   SRTP packet (unless the initial value is provided by key management).

   On consecutive SRTP packets, the receiver SHOULD estimate the index
   as

         i = 2^16 * v + SEQ,

   where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
   such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
   + s_l.

   After the packet has been processed using the estimated index, the
   receiver MUST decide if s_l and ROC should be updated. For instance,
   a simple (but not error robust) method is to simply set s_l to SEQ
   (if SEQ > s_l) and, if the value v = ROC+1 was used, to update ROC to
   v.






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   After a re-keying occurs (changing to a new master key), the
   rollover counter maintains its sequence of values, i.e., it MUST NOT
   be reset to zero, to avoid inconsistencies in key lifetimes.

   As the rollover counter is 32 bits long and the sequence number is
   16 bits long, the maximum number of packets belonging to a given
   SRTP stream that can be secured with the same key is 2^48 using the
   pre-defined transforms. After that number of SRTP packets have been
   sent with a given (master or session) key, the sender MUST NOT send
   any more packets with that key. (There exists a similar limit for
   SRTCP, which in practice may be more restrictive, see Section 9.2.)
   This limitation enforces a security benefit by providing an upper
   bound on the amount of traffic that can pass before cryptographic
   keys are changed. Re-keying (see Section 8.1) MUST be triggered,
   before this amount of traffic, and MAY be triggered earlier, e.g.,
   for increased security and access control to media. Recurring key
   derivation by means of a non-zero key_derivation_rate (see Section
   4.3), also gives stronger security but does not change the above
   absolute maximum value.

   On the receiver side, there is a caveat to updating s_l and ROC: if
   message authentication is not present, neither the initialization of
   s_l, nor the ROC update can be made completely robust. The
   receiver's "implicit index" approach works for the pre-defined
   transforms as long as the reorder and loss of the packets are not
   too great and bit-errors do not occur in unfortunate ways. In
   particular, 2^15 packets would need to be lost, or a packet would
   need to be 2^15 packets out of sequence before synchronization is
   lost. Such drastic loss or reorder is likely to disrupt the RTP
   application itself.

   A more elaborate and more robust scheme is the handling of RTP's own
   "rollover counter", see Appendix A.1 of [RFC1889].


3.2.2 Replay Protection

   Secure replay protection is only possible when integrity protection
   is present. It is RECOMMENDED to use replay protection, both for RTP
   and RTCP, as integrity protection alone cannot assure security
   against replay attacks.

   A packet is "replayed" when it is stored by an adversary, and then
   re-injected into the network. When message authentication is
   provided, SRTP protects against such attacks through a "Replay
   List". Each SRTP receiver maintains a Replay List, which
   conceptually contains the indices of all of the packets which have
   been received and authenticated. In practice, the list can use a



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   "sliding window" approach, so that a fixed amount of storage
   suffices for replay protection. Packet indices which lag behind the
   packet index in the context by more than SRTP-WINDOW-SIZE can be
   assumed to have been received, where SRTP-WINDOW-SIZE is a receiver-
   side, implementation-dependent parameter and MUST be at least 64,
   but which MAY be set to a higher value.

   The receiver checks the index of an incoming packet against the
   replay list and the window. Only packets with index ahead of the
   window, or, inside the window but not already received, SHALL be
   accepted.

   After the packet has been authenticated (if necessary the window is
   first moved ahead), the replay list SHALL be updated with the new
   index.

   The Replay List can be efficiently implemented by using a bitmap to
   represent which packets have been received, as described in the
   Security Architecture for IP [RFC2401].


3.3 Secure RTCP

   Secure RTCP follows the definition of Secure RTP. SRTCP adds three
   mandatory new fields (the SRTCP index, an "encrypt-flag", and the
   authentication tag) and one optional field (the MKI) to the RTCP
   packet definition. The three mandatory fields MUST be appended to an
   RTCP packet in order to form an equivalent SRTCP packet. The added
   fields follow any other profile-specific extensions.

   According to [RFC1889] there is a "recommended" packet format for
   compound packets. SRTCP MUST be given packets according to that
   recommendation in the sense that the first part MUST be a sender
   report or a receiver report. However, the so-called encryption
   prefix (Section 6.1 of [RFC1889]), a random 32-bit quantity intended
   to deter known plaintext attacks, MUST NOT be used (see below).

   The Encrypted Portion of an SRTCP packet consists of the encryption
   (Section 4.1) of the RTCP payload of the equivalent compound RTCP
   packet, from the first RTCP packet, i.e., from the ninth (9) octet
   to the end of the compound packet. The Authenticated Portion of an
   SRTCP packet consists of the entire equivalent (eventually compound)
   RTCP packet, the E flag, and the SRTCP index (after any encryption
   has been applied to the payload).







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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
+-->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   |V=2|P|    RC   |   PT=SR or RR   |             length          |
|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   |                         SSRC of sender                        |
| +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| | ~                          sender info                          ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                         report block 1                        ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                         report block 2                        ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                              ...                              ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |V=2|P|    SC   |  PT=SDES=202  |             length            |
| | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| | |                          SSRC/CSRC_1                          |
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                           SDES items                          ~
| | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| | ~                              ...                              ~
| +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| | |E|                         SRTCP index                         |
+-|>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | ~                     SRTCP MKI (OPTIONAL)                      ~
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | :                     authentication tag                        :
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| +-- Encrypted Portion
+---- Authenticated Portion

   Figure 2.  An example of the format of a Secure RTCP packet,
   consisting of an underlying RTCP compound packet with a Report and
   SDES packet.

The added fields are:

   E-flag: 1 bit, REQUIRED
          The E-flag indicates if the current SRTCP packet is encrypted
          or unencrypted. Section 9.1 of [RFC1889] allows the split of
          a compound RTCP packet into two lower-layer packets, one to
          be encrypted and one to be sent in the clear. The E bit set
          to "1" indicates encrypted packet, and "0" indicates non-
          encrypted packet.

  SRTCP index: 31 bits, REQUIRED



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          The SRTCP index is a 31-bit counter for the SRTCP packet. The
          index is explicitly included in each packet, in contrast to
          the "implicit" index approach used for SRTP. The SRTCP index
          MUST be set to zero before the first SRTCP packet is sent,
          and MUST be incremented by one, modulo 2^31, after each SRTCP
          packet is sent. In particular, after a re-key, the SRTCP
          index MUST NOT be reset to zero again (Section 3.2.1).

   Authentication Tag: variable length, REQUIRED
          The authentication tag is used to carry message
          authentication data.

   MKI: variable length, OPTIONAL
          The MKI is the Master Key Indicator, and functions according
          to the MKI definition in Section 3.

   SRTCP uses the cryptographic context parameters and packet
   processing of SRTP by default, with the following changes:

   * The receiver does not need to "estimate" the index, as it is
   explicitly signaled in the packet.

   * If the MKI indicator in the cryptographic context is zero, the
   master key is determined by the current SRTP index when that key is
   shared between SRTP and SRTCP, even though SRTCP has its own index.
   Since the SRTCP source as with any SSRC in an SRTP session has its
   own sequence number space, the master key <"From", "To"> lifetime
   MUST be based on the SRTP master key lifetime when the master key is
   shared by both SRTP and SRTCP.  The concomitant re-keying issues are
   discussed in sections 8 and 9.

   * Pre-defined SRTCP encryption is as specified in Section 4.1, but
   using the definition of the SRTCP Encrypted Portion given in this
   section, and using the SRTCP index as the index i. The encryption
   transform and related parameters SHALL by default be the same
   selected for the protection of the associated SRTP stream(s), while
   the NULL algorithm SHALL be applied to the RTCP packets not to be
   encrypted.

   The E-flag is assigned a value by the sender depending on whether
   the packet was encrypted or not.

   * SRTCP decryption is performed as in Section 4, but only if the E
   flag is equal to 1. If so, the Encrypted Portion is decrypted, using
   the SRTCP index as the index i. In case the E-flag is 0, the payload
   is simply left unmodified.





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   * SRTCP replay protection is as defined in Section 3.2.2, but using
   the SRTCP index as the index i and a separate replay list that is
   specific to SRTCP.

   * The pre-defined SRTCP authentication tag is specified as in
   Section 4.2, but with the Authenticated Portion of the SRTCP packet
   given in this section (which includes the index). The authentication
   transform and related parameters (e.g., key size) SHALL by default
   be the same as selected for the protection of the associated SRTP
   stream(s) (except when SRTP is not authenticated).

   * In the last step of the processing, only the sender needs to
   update the value of the SRTCP index by incrementing it modulo 2^31
   and for security reasons the sender MUST also check the number of
   RTCP packets processed, see Section 9.2.


   As noted, the encryption prefix (Section 6.1 of RFC1889]) SHALL NOT
   be used, as it is not needed by the cryptographic mechanisms used in
   SRTP.

   Message authentication for RTCP is REQUIRED, as it is the control
   protocol (e.g., it has a BYE packet). Note also that the cost in
   total bandwidth for RTCP authentication is not as high as the one of
   RTP authentication, as the recommended session bandwidth allocated
   to RTCP is at most 5% and the RTCP packets are less frequent.
   However, when adding authentication to RTCP, the overhead in
   bandwidth SHOULD be considered and the total bandwidth for SRTCP
   SHOULD preserve the limit of 5%.


4. Pre-Defined Cryptographic Transforms

   While there are numerous encryption and message authentication
   algorithms that can be used in SRTP, we define below default
   algorithms in order to avoid the complexity of specifying the
   encodings for the signaling of algorithm and parameter identifiers.
   The defined algorithms have been chosen as they fulfill the goals
   listed in Section 2. Recommendations on how to extend SRTP with new
   transforms are given in Section 6.

4.1 Encryption

   The following parameters are common to both pre-defined, non-NULL,
   encryption transforms specified in this section.

   * BLOCK_CIPHER-MODE indicates the block cipher used and its mode of
    operation



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   * n_b is the bit-size of the block for the block cipher
   * k_e is the session encryption key
   * n_e is the bit-length of k_e
   * k_s is the session salting key
   * n_s is the bit-length of k_s
   * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix, an
    non-negative integer, specified by the message authentication code
    in use.

   The distinct session keys and salts for SRTP/SRTCP are by default
   derived as specified in Section 4.3.

   The encryption transforms defined in SRTP use a "seekable" segmented
   keystream generator (KG), which maps the SRTP packet index and
   secret key into a pseudorandom keystream segment. Each keystream
   segment encrypts a single RTP packet. The process of encrypting a
   packet consists of generating the keystream segment corresponding to
   the packet, and then bitwise exclusive-oring that keystream segment
   onto the payload of the RTP packet to produce the Encrypted Portion
   of the SRTP packet. Decryption is done the same way, but swapping
   the roles of the plaintext and ciphertext.

   The definition of how the keystream is generated, given the index,
   depends on the cipher and its mode of operation. Below, two such
   keystream generators are defined. The NULL cipher is also defined,
   to be used when encryption of RTP is not required.

   +----+   +------------------+---------------------------------+
   | KG |-->| Keystream Prefix |          Keystream Suffix       |---+
   +----+   +------------------+---------------------------------+   |
                                                                     |
                               +---------------------------------+   v
                               |     Payload of RTP Packet       |->(*)
                               +---------------------------------+   |
                                                                     |
                               +---------------------------------+   |
                               | Encrypted Portion of SRTP Packet|<--+
                               +---------------------------------+

   Figure 3: Default SRTP Encryption Processing. Here KG denotes the
   keystream generator, and (*) denotes bitwise exclusive-or.


   The SRTP definition of the keystream is illustrated in Figure 3. The
   initial octets of each keystream segment MAY be reserved for use in
   a message authentication code, in which case the keystream used for
   encryption starts immediately after the last reserved octet. The
   initial reserved octets are called the "keystream prefix" (not to be



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   confused with the so-called "encryption prefix" of [RFC1889, Section
   6.1]), and the remaining octets are called the "keystream suffix".
   The keystream prefix MUST NOT be used for encryption. The process is
   illustrated in Figure 3.

   The number of octets in the keystream prefix is denoted as
   SRTP_PREFIX_LENGTH. The keystream prefix is indicated by a positive,
   non-zero value of SRTP_PREFIX_LENGTH. This means that, even if
   confidentiality is not to be provided, the keystream generator
   output may still need to be computed for packet authentication, in
   which case the default keystream generator (mode) SHALL be used.

   The default cipher is the Advanced Encryption Standard (AES), and we
   define two modes of running AES, Segmented Integer Counter Mode AES
   and AES in f8-mode. In the sequel, let E(k,x) be AES applied to key
   k and input block x.

4.1.1 AES in Counter Mode

   Conceptually, counter mode consists of encrypting successive
   integers. The actual definition is somewhat more complicated, in
   order to randomize the starting point of the integer sequence. Each
   packet is encrypted with a distinct keystream segment, which SHALL
   be computed as follows.

   A keystream segment SHALL be the concatenation of the 128-bit output
   blocks of the AES cipher in the encrypt direction, using key k =
   k_e, in which the block indices are in increasing order.
   Symbolically, each keystream segment looks like

      E(k, IV) || E(k, IV + 1 mod 2^128) || E(k, IV + 2 mod 2^128) ...

   where the 128-bit integer value IV SHALL be defined by the SSRC, the
   SRTP packet index i, and the SRTP session salting key k_s, as below.

        IV = (k_s * 2^16) XOR (SSRC * 2^64) XOR (i * 2^16)

   The inclusion of the SSRC allows the use of the same key to protect
   distinct SRTP streams (see the security caveats in Section 9.1).

   In the case of SRTCP, the SSRC of the first header of the compound
   packet MUST be used, i SHALL be the 31-bit SRTCP index and k_e, k_s
   SHALL be replaced by the SRTCP session key and salt.

   Note that the initial value, IV, is fixed for each packet. The
   number of blocks of keystream generated for any fixed value of IV
   MUST NOT exceed 2^16. The AES has a block size of 128 bits, so 2^16
   output blocks are sufficient to generate the 2^23 bits of keystream



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   needed to encrypt the largest possible RTP packet (except for IPv6
   "jumbograms" [RFC2675], which are not likely to be used for RTP-
   based multimedia traffic). This restriction on the maximum bit-size
   of the packet that can be encrypted ensures the security of the
   encryption method by limiting the effectiveness of probabilistic
   attacks [BDJR].

4.1.2 AES in f8-mode

                    IV
                    |
                    |
                    v
                +------+
                |      |
           +--->|  E   |
           |    |      |
           |    +------+
           |        |
     m -> (*)       +-----------+-------------+--  ...     ------+
           |    IV' |           |             |                  |
           |        |   j=1 -> (*)    j=2 -> (*)   ...  j=L-1 ->(*)
           |        |           |             |                  |
           |        |      +-> (*)       +-> (*)   ...      +-> (*)
           |        |      |    |        |    |             |    |
           |        v      |    v        |    v             |    v
           |    +------+   | +------+    | +------+         | +------+
           |    |      |   | |      |    | |      |         | |      |
    k_e ---+--->|  E   |   | |  E   |    | |  E   |         | |  E   |
                |      |   | |      |    | |      |         | |      |
                +------+   | +------+    | +------+         | +------+
                    |      |    |        |    |             |    |
                    +------+    +--------+    +--  ...  ----+    |
                    |           |             |                  |
                    v           v             v                  v
                   S(0)        S(1)          S(2)  . . .       S(L-1)


   Figure 4. f8-mode of operation (asterisk, (*), denotes bitwise XOR).
   The figure represents the KG in Figure 3, when AES-f8 is used.

   To encrypt UMTS (Universal Mobile Telecommunications System, as 3G
   networks) data, a solution (see [ES3D]) known as the f8-algorithm
   has been developed. On a high level, the proposed scheme is a
   variant of Output Feedback Mode (OFB) [HAC], with a more elaborate
   initialization and feedback function. As in normal OFB, the core
   consists of a block cipher. We also define here the use of AES as a
   block cipher to be used in f8-mode for RTP encryption. The AES f8-



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   mode SHALL use the same default sizes for session key and salt as
   AES counter mode.

   Figure 4 shows the structure of block cipher, E, running in what we
   shall call "f8-mode of operation".

4.1.2.1 f8 Keystream Generation

   The Initialization Vector (IV) SHALL be determined as described in
   Section 4.1.2.2.

   Let IV', S(j), and m denote n_b-bit blocks. The keystream, S(0) ||
   ... || S(L-1), for an N-bit message SHALL be defined by setting IV'
   = E(k_e XOR m, IV), and S(-1) = 00..0. For j = 0,1,..,L-1 where L =
   N/n_b (rounded up to nearest integer) compute

            S(j) = E(k_e, IV' XOR j XOR S(j-1))

   Notice that the IV is not used directly. Instead it is fed through E
   under another key to produce an internal, "masked" value (denoted
   IV') to prevent an attacker from gaining known input/output pairs.
   The role of the internal counter, j, is to prevent short keystream
   cycles. The value of the key mask m SHALL be

           m = k_s || 0x555..5,

   i.e. the session salting key, appended by the binary pattern 0101..
   to fill out the entire desired key size, n_e.

   The maximum allowable packet size can be determined as follows. The
   AES has a block size of 128 bits, and assuming that AES behaves like
   a random function, it is (heuristically) secure to generate less
   than 2^64 output blocks, we suggest a maximum of 2^32 blocks, which
   is sufficient to generate 2^39 bits of keystream. For practical
   sizes of the RTP packets, much fewer blocks are required though, and
   the counter j will often be sufficient if implemented as a 16-bit
   counter.

4.1.2.2 f8 SRTP IV Formation

   The purpose of the following IV formation is to provide a feature
   which we call implicit header authentication (IHA), see Section 9.5.

   The SRTP IV for 128-bit block AES-f8 SHALL be formed in the
   following way:





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        IV = 0x00 || M || PT || SEQ || TS || SSRC || ROC

   M, PT, SEQ, TS, SSRC SHALL be taken from the RTP header; ROC is from
   the cryptographic context.

   The presence of the SSRC as part of the IV allows AES-f8 to be used
   when a master key is shared between multiple streams, see Section
   9.1.

4.1.2.3 f8 SRTCP IV Formation

   The SRTCP IV for 128-bit block AES-f8 SHALL be formed in the
   following way:

   IV = 0...0 || E || SRTCP index || V || P || RC || PT || length ||
   SSRC

   where V, P, RC, PT, length, SSRC SHALL be taken from the first
   header in the RTCP compound packet. E and SRTCP index are the 1-bit
   and 31-bit fields added to the packet.

4.1.3 NULL Cipher

   The NULL cipher is used when no confidentiality for RTP/RTCP is
   requested. The keystream can be thought of as "000..0", i.e. the
   encryption SHALL simply copy the plaintext input into the ciphertext
   output.

4.2 Message Authentication and Integrity

   Throughout this section, M will denote data to be integrity
   protected: in the case of SRTP, M SHALL consist of the Authenticated
   Portion of the packet (as specified in Figure 1) concatenated with
   the ROC, M = Authenticated Portion || ROC; in the case of SRTCP, M
   SHALL consist of the Authenticated Portion (as specified in Figure
   2) only.

   Common parameters:

   * AUTH_ALG is the authentication algorithm
   * k_a is the session message authentication key
   * n_a is the bit-length of the authentication key
   * n_tag is the bit-length of the output authentication tag
   * SRTP_PREFIX_LENGTH is the octet length of the keystream prefix as
     defined above, a parameter of AUTH_ALG

   The distinct session authentication keys for SRTP/SRTCP are by
   default derived as specified in Section 4.3.



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   The values of n_a, n_tag, and SRTP_PREFIX_LENGTH MUST be fixed for
   any particular fixed value of the key.

   We describe the process of computing authentication tags as follows.
   The sender computes the tag of M and appends it to the packet. The
   SRTP receiver verifies a message/authentication tag pair by
   computing a new authentication tag over M using the selected
   algorithm and key, and then compares it to the tag associated with
   the received message. If the two tags are equal, then the
   message/tag pair is valid; otherwise, it is invalid and the error
   audit message "AUTHENTICATION FAILURE" MUST be returned.

4.2.1. HMAC-SHA1

   When HMAC-SHA1 is used, the SRTP_PREFIX_LENGTH (Figure 3) SHALL be
   0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to the
   session authentication key and M as specifed above, i.e. HMAC(k_a,
   M). The HMAC output SHALL then be truncated to the n_tag left-most
   bits.


4.3 Key Derivation

4.3.1 Key Derivation Algorithm

   Regardless of the encryption or message authentication transform
   that is employed (it may be an SRTP pre-defined transform or newly
   introduced according to Section 6), interoperable SRTP
   implementations MAY use the SRTP key derivation to generate session
   keys. Once the key derivation rate is properly signaled at the start
   of the session, there is no need for extra communication between the
   parties that use SRTP key derivation.

                            packet index ---+
                                            |
                                            |
                                            v
                  +-----------+ master  +--------+ session encr_key
                  | ext       | key     |        |---------->
                  | key mgmt  |-------->|  key   | session auth_key
                  | (optional |         | deriv  |---------->
                  | rekey)    |-------->|        | session salt_key
                  |           | master  |        |---------->
                  +-----------+ salt    +--------+

   Figure 5: SRTP key derivation.




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   At least one initial key derivation SHALL be performed by SRTP,
   i.e., the first key derivation is REQUIRED. Further applications of
   the key derivation MAY be performed, according to the
   "key_derivation_rate" value in the cryptographic context. The key
   derivation function SHALL be initially invoked before the first
   packet and then, if derivation rate is r > 0, further invoked on
   every r-th packet, and produce session keys according to the non-
   zero key derivation rate. This can be thought of as "refreshing" the
   session keys. The value of "key_derivation_rate" MUST be kept fixed
   for the lifetime of the associated master key.

   Interoperable SRTP implementations MAY also derive session salting
   keys for encryption transforms, as is done in both of the pre-
   defined transforms.

   Let m and n be positive integers. A pseudo-random function family is
   a set of keyed functions {PRF_n(k,x)} such that for the (secret)
   random key k, given m-bit x, PRF_n(k,x) is an n-bit string,
   computationally indistinguishable from random n-bit strings, see
   [HAC]. For the purpose of key derivation in SRTP, a secure PRF with
   m = 128 (or more) is needed, and a default PRF transform is defined
   in Section 4.3.3.

   Let "a DIV t" denote integer division of a by t, rounded down, and
   with the convention that "a DIV 0 = 0" for all a. We also make the
   convention of treating "a DIV t" as a bit string of the same length
   as a, and thus "a DIV t" will in general have leading zeros.

   Key derivation SHALL be defined as follows in terms of <label>, an
   8-bit constant (see below), master_salt and key_derivation_rate, as
   determined in the cryptographic context, and index, the packet index
   (i.e., the 48-bit ROC || SEQ for SRTP):

   * Let r = index DIV key_derivation_rate (with DIV as defined above).

   * Let key_id = <label> || r.

   * Let x = key_id XOR master_salt, where key_id and master_salt are
     aligned so that their least significant bits agree (right-
     alignment).

   The n-bit SRTP key (or salt) for this packet SHALL then be

      PRF_n(k_master, x).

   (The PRF may internally specify additional formatting and padding
   of x, see e.g. Section 4.3.3 for the default PRF.)



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   The session keys and salt SHALL now be derived using:

   - k_e (SRTP encryption): <label> = 0x00, n = n_e.

   - k_a (SRTP message authentication): <label> = 0x01, n = n_a.

   - k_s (SRTP salting key) <label> = 0x02, n = n_s.

   where n_e, n_s, and n_a are from the cryptographic context.

   The master key and master salting key MUST be random, but the master
   salt MAY be public.

   Note that for a key_derivation_rate of 0, the initial application of
   the key derivation SHALL take place exactly once. The derivation
   operation is facilitated if the non-zero rates are chosen to be
   powers of 256.

   The upper limit on the number of packets that can be secured using
   the same master key (see Section 9.2) is independent of the key
   derivation.

4.3.2 SRTCP Key Derivation

   SRTCP SHALL by default use the same master key (and master salt) as
   SRTP. To do this securely, the following changes SHALL be done to
   the definitions in Section 4.3.1 when applying session key
   derivation for SRTCP.

   Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index
   (i.e. excluding the E-bit, replacing it with a fixed 0-bit), and use
   <label> = 0x03 for the SRTCP encryption key, <label> = 0x04 for the
   SRTCP authentication key, and, <label> = 0x05 for the SRTCP salting
   key.

4.3.3 AES-CM PRF

   The currently defined PRF, keyed by 128 to 256 bit master key, has
   input block size m = 128 and can produce n-bit outputs for n up to
   2^23. PRF_n(k_master,x) SHALL be AES in Counter Mode as described in
   Section 4.1.1, applied to key k_master, and IV equal to (x*2^16),
   and with the output keystream truncated to the n first (left-most)
   bits. (Requiring n/128, rounded up, applications of AES.)







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5. Default and mandatory-to-implement Transforms

   The default transforms also are mandatory-to-implement transforms in
   SRTP. Of course, "mandatory-to-implement" does not imply "mandatory-
   to-use". Table 1 summarizes the pre-defined transforms.


                         mandatory-to-impl.   optional     default

   encryption            AES-CM, NULL         AES-f8       AES-CM
   message integrity     HMAC-SHA1              -          HMAC-SHA1
   key derivation (PRF)  AES-CM                 -          AES-CM


   Table 1: Mandatory-to-implement, optional and default transforms in
   SRTP.

5.1 Encryption: AES-CM and NULL

   AES running in Segmented Integer Counter Mode, as defined in Section
   4.1.1, SHALL be the default encryption algorithm. The default key
   lengths SHALL be 128-bit for the session encryption key (n_e). The
   default session salt key-length (n_s) SHALL be 112 bits.

   The NULL cipher SHALL also be mandatory-to-implement.

5.2 Message Authentication/Integrity: HMAC-SHA1

   HMAC-SHA1, as defined in Section 4.2.1, SHALL be the default message
   authentication code. The default session authentication key-length
   (n_a) SHALL be 128 bits, the default authentication tag length
   (n_tag) SHALL be 32 bits, and the SRTP_PREFIX_LENGTH SHALL be zero
   for HMAC-SHA1.

5.3 Key Derivation: AES-CM PRF

   The AES Counter Mode based key derivation and PRF defined in
   Sections 4.3.1 to 4.3.3, using a 128-bit master key, SHALL be the
   default method for generating session keys. The default master salt
   length SHALL be 112 bits and the default key-derivation rate SHALL
   be zero.


6. Adding SRTP Transforms

   Section 4 provides examples of the level of detail needed for
   defining transforms. Whenever a new transform is to be added to
   SRTP, a companion IETF RFC MUST be written to exactly define how the



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   new transform can be used with SRTP (and SRTCP). Such a companion
   RFC SHOULD avoid to overlap with the SRTP protocol document. Note
   however, that it MAY be necessary to extend the SRTP or SRTCP
   cryptographic context definition with new parameters (including
   fixed or default values), or add steps to the packet processing. The
   companion RFC SHALL explain any known issues regarding interactions
   between the transform and other aspects of SRTP.

   Each new transform document SHOULD specify its key attributes, e.g.,
   size of keys (minimum, maximum, recommended), format of keys,
   recommended/required processing of input keying material,
   requirements/recommendations on re-keying and key derivation, etc.


7. Rationale

7.1 Key derivation

   Key derivation reduces the burden on the key establishment. As many
   as six different keys are needed to protect the RTP/RTCP session
   (SRTP and SRTCP encryption keys and salts, SRTP and SRTCP
   authentication keys), but these are derived from a single master key
   in a cryptographically secure way. Thus, the key management protocol
   needs to exchange only one master key (plus master salt when
   required), and then SRTP itself derives all the necessary session
   keys (via the first, mandatory application of the key derivation
   function). Note however that the key management protocol may provide
   SRTP with more than one master key in advance, e.g., multiple
   distinct master keys with their respective lifetime. Each of these
   lifetimes MUST NOT be overlapping with the lifetime of the other
   master keys, so that one and only one master key is active at each
   point in time. Providing arrays of master keys in advance is for
   example used when a certain rate of re-keying is wanted.

   Multiple applications of the key derivation function are optional,
   but will give security benefits when enabled. They prevent an
   attacker from obtaining large amounts of ciphertext produced by a
   single fixed session key. If the attacker was able to collect a
   large amount of ciphertext for a certain session key, he might be
   helped in mounting certain attacks.

   Multiple applications of the key derivation function provide
   backwards and forward security in the sense that a compromised
   session key does not compromise other session keys derived from the
   same master key. This means that the attacker who is able to recover
   a certain session key, is anyway not able to have access to messages
   secured under previous and later session keys (derived from the same




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   master key). (Note that, of course, a leaked master key reveals all
   the session keys derived from it.)

   Considerations arise with high-rate key-refresh, especially in large
   multicast settings, see Section 11.

7.2 Salting key

   The master salt guarantees security against off-line key-collision
   attacks on the key derivation that might otherwise reduce the
   effective key size.

   The derived session salting key used in the encryption, has been
   introduced to protect against some attacks on additive stream
   ciphers, see Section 9.2. The explicit inclusion method of the salt
   in the IV has been selected for ease of hardware implementation.

7.3 Message Integrity from Universal Hashing

   The particular definition of the keystream given in Section 4.1 (the
   keystream prefix) is to give provision for particular universal hash
   functions, suitable for message authentication in the Wegman-Carter
   paradigm [WC81]. Such functions are provably secure, simple, quick,
   and especially appropriate for Digital Signal Processors and other
   processors with a fast multiply operation.

   No authentication transforms are currently provided in SRTP other
   than HMAC-SHA1. Future transforms, like the above mentioned
   universal hash functions, MAY be added following the guidelines in
   Section 6.

7.4 Data Origin Authentication Considerations

   Note that in unicast, integrity and data origin authentication are
   provided together. However, in group scenarios where the keys are
   shared between members, the MAC tag only proves that a member of the
   group sent the packet, but does not prevent against a member
   impersonating another. Data origin authentication (DOA) for
   multicast and group RTP sessions is a hard problem that needs a
   solution; while some promising proposals are being investigated
   [PCST1, PCST2], more work is needed to rigorously specify these
   technologies. Thus SRTP data origin authentication in groups is for
   further study.

   DOA can be done otherwise using signatures. However, this has high
   impact in terms of bandwidth and processing time, therefore we do
   not offer this form of authentication in the pre-defined packet-
   integrity transform.



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   The presence of mixers and translators does not allow data origin
   authentication in case the RTP payload and/or the RTP header are
   manipulated. Note that these types of middle entities also disrupt
   end-to-end confidentiality (as the IV formation depends e.g. on the
   RTP header preservation). A certain trust model may choose to trust
   the mixers/translators to decrypt/re-encrypt the media (this would
   imply breaking the end-to-end security, with related security
   implications).


8. Key Management Considerations

   For initialization, an interoperable SRTP implementation SHOULD be
   given the SSRC and MAY be given the initial RTP sequence number for
   the RTP stream by key management (thus, key management has a
   dependency on RTP operational parameters).

   If the pre-defined transforms are used, a particular key management
   system might allow different RTP sessions to share the same
   cryptographic master keys. The SRTP sender and receiver typically
   share a master key to derive session keys for encryption/decryption
   and authentication; SRTCP sources will typically derive keys from
   the same master key used by the correspondent SRTP. Sharing also
   between SRTP streams is secure if the design of the synchronization
   mechanism, i.e., the IV, avoids keystream re-use (the two-time pad,
   Section 9.1). If this feature is used, the SSRCs MUST be unique
   between all the RTP streams sharing the same master key. In other
   words, when a master key is shared among RTP sessions, SRTP/SRTCP
   cryptographic transforms are vulnerable to unfortunate SSRC
   collisions owing to normal operation of a compliant RTP
   implementation. SRTP implementations that share master keys
   introduce a non-standard constraint on RTP operation: SSRC values
   must be unique among RTP sessions that share an SRTP master key (see
   Section 9.1).

   The same considerations apply to message authentication: SRTP
   streams authenticated under the same key MUST have a distinct SSRC.

   Whenever uniqueness between the SSRCs can not be guaranteed, the
   same master key MUST NOT be shared among the streams.

   Note that sharing between SRTP and the corresponding SRTCP is made
   possible by the pre-defined key-derivation, and poses no
   restrictions on the SSRCs for SRTP and SRTCP; SRTP and SRTCP MAY
   (and typically will) use the same SSRC.





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8.1. Re-keying

   A particular key management system might choose to provide re-key:

   - by associating a master key for a crypto context with an MKI,

   or

   - by associating a master key for a crypto context directly with a
   pair of index (sequence number and ROC) values, <"From", "To">. In
   this case, the MKI is not included. Note that the range <"From",
   "To"> gives also the lifetime of the master key itself. <"From",
   "To"> are specified in the crypto context for a given master key, or
   the default values, "from the first observed packet" and "until
   further notice", respectively, are used. Also, in case the default
   values are used, the SRTP implementation MUST never exceed the
   maximum limit of SRTP/SRTCP packets sent for each given
   master/session key.

   The first method (using the MKI) has the advantage of easier master
   key retrieval (see Scenarios in Section 11), but has the
   disadvantage of adding extra bits to each packet. Using the MKI does
   not exclude using <"From", "To"> key lifetime simultaneously. This
   can for instance be useful to signal at which point in time an MKI
   is to be made active.

   The key management specification may therefore require the SRTP
   implementation to check the index of an incoming SRTP packet against
   the interval for the master key in the context before using the key.

   SRTP senders SHALL count the amount of SRTP and SRTCP traffic being
   used for a master key and invoke key management to re-key if needed.
   These interactions are defined by the key management interface to
   SRTP and are not defined by this protocol specification.

8.2. Key Management parameters


   The table below lists all SRTP parameters that key management may
   need to supply. For reference, it also provides a summary of the
   default and mandatory-to-support values for an SRTP implementation
   as described in Section 5.









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   Parameter                     Mandatory-to-support    Default
   ---------                     --------------------    -------

   SRTP and SRTCP encr transf.       AES_CM, NULL         AES_CM
   (Other possible values: AES_f8)

   SRTP and SRTCP auth transf.       HMAC-SHA1           HMAC-SHA1

   SRTP and SRTCP auth params:
     n_tag (tag length)                 32                 32
     SRTP prefix_length                  0                  0

   Key derivation PRF                 AES_CM              AES_CM

   Key material params
   (for each master key):
     master key
     master key length                 128                128
     n_e (encr session key length)     128                128
     n_a (auth session key length)     128                128
     master salt key
     length of the master salt         112                112
     n_s (session salt key length)     112                112
     key derivation rate                 0                  0
     <"From", "To">
     MKI indicator                       0                  0
     length of the MKI                   0                  0
     value of the MKI

   Crypto context index params:
     SSRC value
     ROC
     SEQ
     Transport address
     Port number

   Relation to other RTP profiles:
     sender's order between FEC and SRTP               FEC-SRTP


9. Security Considerations

9.1 SSRC collision and two-time pad

   Any fixed keystream output, generated from the same key and index
   should only be used to encrypt once. Re-using such keystream
   (jokingly called a "two-time pad" system by cryptographers), can
   seriously compromise security. The NSA's VENONA project [C99]



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   provides a historical example of such a compromise. In SRTP, a "two-
   time pad" is avoided by requiring the key, or some other parameter
   of cryptographic significance, to be unique per RTP stream and
   packet. The pre-defined SRTP transforms accomplish packet-uniqueness
   by including the packet index and stream-uniqueness by inclusion of
   the SSRC.

   The pre-defined transforms (AES-CM and AES-f8) allow master keys to
   be shared across streams by the inclusion of the SSRC in the IV.
   Sharing a key among RTP sessions, however, requires the added
   constraint that SSRC values be unique across RTP sessions (see
   Section 8).

   Thus, the SSRC MUST be unique between all the RTP streams and
   sessions sharing the same master key. It is incumbent upon SRTP
   implementations to ensure SSRC uniqueness across RTP streams that
   share a master key, to avoid unfortunate IV combinations and end up
   in a two-time pad. Even with distinct SSRCs, extensive use of the
   same key might improve chances of probabilistic collision and time-
   memory-tradeoff attacks succeeding.

   It is RECOMMENDED that RTP senders on different hosts not use the
   same master key to send.  When a local host shares a master key
   among its RTP/RTCP streams to an RTP session, it MUST check for
   collisions among the SSRCs it is using at the time of SSRC
   generation and generate a unique SSRC before sending the value in an
   SRTP or SRTCP message.  When a local host shares a master key among
   RTP/RTCP streams in multiple RTP sessions (e.g. in a multimedia
   session), it MUST check for collisions among the SSRCs it is using
   for those sessions and enforce SSRC uniqueness even though SSRC
   uniqueness among RTP sessions is not an RTP requirement.  When
   master keys are shared between RTP hosts, the effect of an eventual
   RTP SSRC collision detection MUST be taken into account, as a
   collision could duplicate the SSRC leading temporarily to a two-time
   pad before the collision is detected. SRTP implementations SHOULD
   obtain unique SSRCs from key management when they share a master
   key. Failing this, an SRTP implementation MUST obtain a new master
   key from key management for any session that experiences an SSRC
   collision.

   Manual keying SHOULD NOT be used in SRTP.

9.2 Key Usage

   The effective key size is determined (upper bounded) by the size of
   the master key and, for encryption, the size of the salting key. Any
   additive stream cipher is vulnerable to attacks that use statistical
   knowledge about the plaintext source to enable key collision and



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   time-memory tradeoff attacks [MF00,H80]. These attacks take
   advantage of commonalities among plaintexts, and provide a way for a
   cryptanalyst to amortize the computational effort of decryption over
   many keys, thus reducing the effective key size of the cipher. A
   detailed analysis of these attacks and their applicability to the
   encryption of Internet traffic is provided in [MF00]. In summary,
   the effective key size of SRTP when used in a security system in
   which m distinct keys are used, is equal to the key size of the
   cipher less the logarithm (base two) of m. Protection against such
   attacks can be provided simply by increasing the size of the keys
   used, which here can be accomplished by the use of the salting key.
   Note that the salting key MUST be random but MAY be public. A salt
   size of (the suggested) size 112 bits protects against attacks in
   scenarios where at most 2^112 keys are in use. This is sufficient
   for all practical purposes.

   Implementations SHOULD use keys that are as large as possible.
   Please note that in many cases increasing the key size of a cipher
   does not affect the throughput of that cipher.

   The use of the SRTP and SRTCP indexes in the pre-defined transforms
   fixes the maximum number of packets that can be secured with the
   same key. This limit is fixed to 2^48 SRTP packets for an SRTP
   stream, and 2^31 SRTCP packets, when SRTP and SRTCP are considered
   independently. Due to for example re-keying, reaching this limit may
   or may not coincide with wrapping of the indices, and thus the
   sender MUST keep packet counts. However, when the session keys for
   related SRTP and SRTCP streams are derived from the same master key
   (the default behavior, Section 4.3), the upper bound that has to be
   considered is in practice the minimum of the two quantities. That
   is, when 2^48 SRTP packets or 2^31 SRTCP packets have been secured
   with the same key (whichever occurs before), the key management MUST
   be called to provide new master key(s) (previously stored and used
   keys MUST NOT be used again), or the session MUST be terminated. If
   a sender of RTCP discovers that the sender of SRTP (or SRTCP) has
   not updated the master or session key prior to sending 2^48 SRTP (or
   2^31 SRTCP) packets belonging to the same SRTP (SRTCP) stream, it is
   up to the security policy of the RTCP sender how to behave, e.g.
   whether an RTCP BYE-packet should be sent and/or if the event should
   be logged.

   Note: in most typical applications (assuming at least one RTCP
   packet for every 128,000 RTP packets), it will be the SRTCP index
   that first reaches the upper limit, although the time until this
   occurs is very long: even at 200 SRTCP packets/sec, the 2^31 index
   space of SRTCP is enough to secure approximately 4 months of
   communication.




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   Note that if the master key is to be shared between SRTP streams
   having distinct SSRCs (Section 9.1), although the above bounds are
   on a per stream (i.e. per SSRC) basis, the sender MUST base re-key
   decision on the stream whose sequence number space is the first to
   be exhausted.

   Key derivation limits the amount of plaintext that is encrypted with
   a fixed session key, and made available to an attacker for analysis,
   but key derivation does not extend the master key's lifetime. To see
   this, simply consider our requirements to avoid two-time pad: two
   distinct packets MUST either be processed with distinct IVs, or with
   distinct session keys, and both the distinctness of IV and of the
   session keys are (for the pre-defined transforms) dependent on the
   distinctness of the packet indicies.

9.3 Confidentiality of the RTP Payload

   By using "seekable" stream ciphers, SRTP avoids the denial of
   service attacks that are possible on stream ciphers that lack this
   property. It is important to be aware that, as with any stream
   cipher, the exact length of the payload is revealed by the
   encryption. This means that it may be possible to deduce certain
   "formatting bits" of the payload, as the length of the codec output
   might vary due to certain parameter settings etc. This, in turn,
   implies that the corresponding bit of the keystream can be deduced.
   However, if the stream cipher is secure (counter mode and f8 are
   provably secure under certain assumptions [BDJR,KSYH]), knowledge of
   a few bits of the keystream will not aid an attacker in predicting
   subsequent keystream bits. Thus, the payload length (and information
   deducible from this) will leak, but nothing else.

   As some RTP packet could contain highly predictable data, e.g. SID,
   it is important to use a cipher designed to resist known plaintext
   attacks (which is the current practice).

9.4 Confidentiality of the RTP Header

   In SRTP, RTP headers are sent in the clear to allow for header
   compression. This means that data such as payload type,
   synchronization source identifier, and timestamp are available to an
   eavesdropper. Moreover, since RTP allows for future extensions of
   headers, we cannot foresee what kind of possibly sensitive
   information might also be "leaked".

   SRTP is a low-cost method, which allows header compression to reduce
   bandwidth. It is up to the endpoints' policies to decide about the
   security protocol to employ. If one really needs to protect headers,




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   and is allowed to do so by the surrounding environment, then one
   should also look at alternatives, e.g., IPsec.

9.5 Integrity of the RTP payload and header

   Additive stream ciphers do not provide any security service other
   than confidentiality. In particular, they do not provide message
   authentication (see [RK99] or [HAC] for a discussion of this
   security service). SRTP uses a message authentication code to
   provide a message authentication service.

   HMAC is a well-studied message authentication code that is based on
   a provably secure construction. The security against MAC forgery
   depends on the key-size and the size of the output tags (or for some
   attacks, half the size of the tag due to the "birthday-paradox").

   The default tag size for SRTP HMAC is 32 bits. Other size values MAY
   be chosen (via the key management protocol). The use of a truncated
   size is motivated by the fact that it may be desirable, e.g., in
   wireless environments, to save bandwidth. The choice of such a
   truncation MUST be evaluated to the reduction in security it
   implies. The default 32-bit size is a compromise, offering a
   reasonable level of security, taking into account the real-time
   aspects of the protected protocol. High security applications SHOULD
   however use larger tags.

   The fact that message authentication is optional (for SRTP) is
   motivated by the fact that, while the function is typically highly
   desired, there are certain cases (notably in cellular environments)
   where it has an impact in terms of cost, e.g. for bandwidth
   consumption. Also, independently of the tag length, a single
   transmission bit error in the protected part of the packet or in the
   tag itself forces the entire packet to be dropped. Given a fixed
   quality of service, it implies the necessity of higher protection of
   the transmitted unit, hence higher cost. In those cases, it is up to
   the user's security profile to request authentication.

   The use of error detection mechanism (e.g., Unequal Error Detection,
   UED and UEP) is compatible with SRTP and the pre-defined encryption
   transforms, since stream ciphers operate on each bit individually.
   However, the use of UED/UEP may be difficult to combine with
   authentication because any bit error will cause authentication to
   fail.

   The IV formation of the f8-mode gives implicit authentication (IHA)
   of the RTP header, even if no cryptographic integrity protection is
   present. This means that modifying bits of the RTP header will cause




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   the decryption process at the receiver to produce essentially random
   garbage.


10. Interaction with Forward Error Correction mechanisms

   The default processing when using Forward Error Correction (e.g. RFC
   2733) processing with SRTP SHALL be to perform FEC processing prior
   to SRTP processing on the sender side and to perform SRTP processing
   prior to FEC processing on the receiver side.  Any change to this
   ordering (reversing it, or, placing FEC between SRTP encryption and
   SRTP authentication) SHALL be signaled out of band.


11. Scenarios

   SRTP can be used as security protocol for the RTP/RTCP traffic in
   many different scenarios. SRTP has a number of configuration
   options, and can have impact on the total performance of the
   application according to the way it is used. Hence, the use of SRTP
   is dependent on the kind of scenario and application it is used
   with. In the following, we briefly illustrate some use cases for
   SRTP, and give some guidelines for recommended setting of its
   options.

11.1 Unicast

   A typical example would be a voice call or video-on-demand
   application.

   Consider one bi-directional RTP stream. It is possible for the two
   parties to share the same master key in the two directions. The
   first round of the key derivation splits the master key into any or
   all of the following session keys (according to the provided
   security functions):

   SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.

   (For simplicity, we omit discussion of the salts, which are also
   derived.) In this scenario, it will in most cases suffice to have a
   single master key with unspecified lifetime (i.e. unrestricted key
   lifetime, not using explicit <"From", "To"> values). This guarantees
   sufficiently long lifetime of the keys and a minimum set of keys in
   place for most practical purposes. Also, in this case RTCP
   protection can be applied smoothly. Under these assumptions, use of
   the MKI can be omitted. As the key-derivation in combination with
   large difference in the packet rate in the respective directions may




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   require simultaneous storage of several session keys, if storage is
   an issue, we recommended to use low-rate key derivation.

   The same considerations can be extended to the unicast scenario with
   multiple RTP sessions sharing the master key if particular care is
   taken to guarantee unique SSRCs for the streams.

11.2 Multicast

   Just as with (unprotected) RTP, a scalability issue arises in big
   groups due to the possibly very large amount of SRTCP Receiver
   Reports that the sender might need to process. In SRTP, the sender
   may have to keep state (the cryptographic context) for each
   receiver, or more precisely, for the SRTCP used to protect Receiver
   Reports. The overhead increases proportionally to the size of the
   group. In particular, re-keying requires special concern, see below.

   We describe in the following multicast for small groups, and give
   guidelines for use of SRTP/SRTCP with large group multicast.


11.2.1 Small multicast with one sender

   The sender secures his RTP stream using one cryptographic context.
   The sender's RTP and RTCP are secured with the same master key. Key
   derivation gives the necessary session keys, i.e.

   SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.

   If there are multiple RTP streams, their SSRCs MUST (as noted) be
   unique to avoid two-time pad (see Section 9.1), or else distinct
   per-stream master keys MUST be used.

   There are a few possible setups with the distribution of master keys
   among the receivers. One possibility is that the receivers share the
   same master key to secure all their respective RTCP traffic. This
   shared master key could then be the same one used by the sender to
   protect its outbound traffic. Alternatively, it could be a master key
   shared only among the receivers and used solely for their SRTCP
   traffic. Both alternatives requires the receivers to trust each
   other.

   Considering SRTCP and key storage, it is recommended to use low-rate
   (or zero) key_derivation (except the mandatory initial one), so that
   the sender does not need to store too many session keys (each SRTCP
   stream might otherwise have a different session key at a given point
   in time, as the SRTCP sources send at different times). Thus, in
   case key derivation is wanted for SRTP, the cryptographic context



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   for SRTP can be kept separate from the SRTCP crypto context, so that
   it is possible to have a key_derivation_rate of 0 for SRTCP and a
   non-zero value for SRTP.

   Re-keying gives two problems: the number of master keys stored at
   the sender side, and re-keying triggering. Forcing re-keying using
   the <"From", "To"> fields creates the problem that the sender needs
   to maintain multiple master keys, as the re-keying will typically
   happen at different times on each SRTCP stream from the receivers
   (because each SSRC defines a sequence number space). Also, problems
   may occur in retrieving the current master key for the SRTCP packets
   in some cases, since that is done based on SRTP index, not on the
   SRTCP index. Use of the MKI for re-keying is recommended for most
   applications (see Section 8.1).

   If there are more than one outgoing SRTP stream sharing master key,
   the upper limit of 2^48 SRTP packets / 2^31 SRTCP packets means
   that, before one of the streams reaches such maximum number of
   packets, re-keying MUST be triggered on ALL streams sharing the
   master key. (From strict security point of view, only the stream
   reaching the maximum would need to be re-keyed, but then the streams
   would no longer be sharing master key, which is the intention.) A
   local policy at the sender side should force rekeying in a way that
   the maximum packet limit is not reached on any of the streams. The
   MKI or <"From", "To"> fields may be employed for key synchronization
   during changeover to a new key, see Section 11.3.

11.2.2 Large multicast with one sender

   The same considerations as for the small group multicast hold. The
   biggest issue in this scenario is the additional load placed at the
   sender side, due to the state (cryptographic contexts) that has to
   be maintained for each receiver, sending back RTCP Receiver Reports.
   At minimum, a replay window might need to be maintained for each
   RTCP source. Therefore, with big groups and where the load at the
   sender is considered not acceptable, an RTP sender may choose not to
   authenticate or protect against replay for incoming SRTCP messages
   (nor to negotiate encryption for them). Of course, security impacts
   of neglecting to authenticate certain packets MUST be carefully
   considered. This is therefore strongly NOT RECOMMENDED from a
   security point of view, but may appear a reasonable compromise in
   order to have at least security guaranteed on the outgoing RTP
   traffic.

11.3 Re-keying and access control

   Re-keying may occur due to access control (e.g., when a member is
   removed during a multicast RTP session), or, for pure cryptographic



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   reasons (e.g. the key is at the end of its lifetime). When using
   SRTP default transforms, the master key MUST be replaced before any
   of the index spaces are exhausted for any of the streams protected
   by one and the same master key.

   How key management rekeys SRTP implementations is out of our scope,
   but it is clear that there are straightforward ways to manage keys
   for a multicast group. In one-sender multicast, for example, it is
   typically the responsibility of the sender to determine when a new
   key is needed. The sender is the one entity that can keep track of
   when the maximum number of packets has been sent, as receivers may
   join and leave the session at any time, there may be packet loss and
   delay etc. In scenarios other than one-sender multicast, other
   methods can be used. Here, one must take into consideration that key
   exchange can be a costly operation, taking several seconds for a
   single exchange. Hence, some time before the master key is
   exhausted/expires, out-of-band key management is initiated,
   resulting in a new master key shared with the receiver(s). In any
   event, to maintain synchronization when switching to the new key,
   group policy might choose between using  the MKI or the <"From",
   "To">, as described in Section 8.1.

   For access control purposes, the <"From", "To"> periods are set at
   the desired granularity, dependent on the packet rate. High rate re-
   keying can be problematic for some large-group SRTP scenarios, with
   SRTCP. There are potential problems in using the SRTP index, rather
   than the SRTCP index, for determining the master key. In particular,
   for short periods during switching of master keys, it may be the
   case that SRTCP packets are not under the current master key of the
   correspondent SRTP. Therefore, using the MKI for re-keying in such
   scenarios is likely to produce better results.

11.4 Summary of basic scenarios

   The description of these scenarios highlights some recommendations
   on the use of SRTP, mainly related to re-keying and large scale
   multicast:

   - Do not use SRTP for fast re-keying using the <"From", "To">
     feature. It may, in particular, give problems in retrieving the
     correct SRTCP key, if an SRTCP packet arrives close to the re-
     keying time. The MKI SHOULD be used in this case.

   - If multiple SRTP streams share the same master key, also moderate
     rate re-keying MAY have the same problems, and the MKI SHOULD be
     used.





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   - Carefully consider the additional load at the sender side in
     multicast scenarios. Optionally, but NOT RECOMMENDED, SRTCP
     Receiver Reports' authentication could be left unverified by the
     sender (and SRTCP Receiver Reports' encryption not selected).

   - Though offering increased security, a non-zero key_derivation_rate
     is NOT RECOMMENDED when trying to minimize the number of keys in
     use with multiple streams.


12. IANA Considerations

   The RTP specification establishes a registry of profile names for
   use by higher-level control protocols, such as the Session
   Description Protocol (SDP), to refer to transport methods. This
   profile registers the name "RTP/SAVP".

   SRTP uses cryptographic transforms, which a key management protocol
   signals. It is the task of each particular key management protocol
   to register the cryptographic transforms or suites of transforms
   with IANA. The key management protocol conveys these protocol
   numbers, not SRTP, and each key management protocol chooses the
   numbering scheme and syntax that it requires.

   Specification of a key management protocol for SRTP is out of scope
   here. Section 8.2, however, provides guidance on the parameters that
   need to be defined for the default and mandatory transforms.


13. Acknowledgements

   The authors would like to thank Magnus Westerlund, Brian Weis,
   Robert Fairlie-Cuninghame, Adrian Perrig, and the AVT WG for their
   reviews and comments.


14. Author's Addresses

   Questions and comments should be directed to the authors and
   avt@ietf.org:



      Mark Baugher
      Cisco Systems, Inc.
      5510 SW Orchid Street     Phone:  +1 408-853-4418
      Portland, OR 97219 USA    Email:  mbaugher@cisco.com




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      Rolf Blom
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 58531707
      Sweden                 EMail:  rolf.blom@era.ericsson.se

      Elisabetta Carrara
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 50877040
      Sweden                 EMail:  elisabetta.carrara@era.ericsson.se

      David A. McGrew
      Cisco Systems, Inc.
      San Jose, CA 95134-1706   Phone:  +1 301-349-5815
      USA                       EMail:  mcgrew@cisco.com

      Mats Naslund
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 58533739
      Sweden                 EMail:  mats.naslund@era.ericsson.se

      Karl Norrman
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 4044502
      Sweden                 EMail:  karl.norrman@era.ericsson.se

      David Oran
      Cisco Systems, Inc.
      San Jose, CA 95134-1706
      USA                       EMail:  oran@cisco.com


15. References

   Normative

   [AES] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
         http://www.nist.gov/aes/

   [HMAC] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed-
         hashing for message authentication". IETF RFC 2104, February
         1997.

   [RFC1889] Schulzrinne, H., Casner, S., Frederick, R., Jacobson,V.,
           "RTP: A Transport Protocol for Real-Time Applications", IETF
           RFC 1889.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", IETF RFC 2119, March 1997.



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   [RFC2401] Kent, S., and R. Atkinson, "Security Architecture for IP",
          IETF RFC 2401, November 1998.

   [RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms",
          IETF RFC 2675, August 1999.

   [RFC2828] Shirey, R., "Internet Security Glossary", IETF RFC 2828,
            May 2000.


  Informative

   [BDJR] Bellare, M., Desai, A., Jokipii, E., and Rogaway, P.,
          "A Concrete Treatment of Symmetric Encryption: Analysis of
            DES Modes of Operation", Proceedings 38th IEEE FOCS,
          pp. 394-403, 1997.

   [C99]  Crowell, W. P., "Introduction to the VENONA Project",
          http://www.nsa.gov:8080/docs/venona/index.html.

   [CTR] Morris Dworkin, NIST Special Publication 800-38A,
         "Recommendation for Block Cipher Modes of Operation: Methods
         and Techniques",  2001.  Online at
         http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-
         38a.pdf.

   [ES3D] ETSI SAGE 3GPP Standard Algorithms Task Force, "Security
          Algorithms Group of Experts (SAGE); General Report on the
          Design, Specification and Evaluation of 3GPP Standard
          Confidentiality and Integrity Algorithms", Public report,
          Draft Version 1.0, Dec 1999.

   [ES3E] ETSI SAGE 3GPP Standard Algorithms Task Force, "Security
          Algorithms Group of Experts (SAGE) Report on the Evaluation
          of 3GPP Standard Confidentiality and Integrity Algorithms",
          Public report, Draft Version 1.0, Dec 1999.

   [HAC]  Menezes, A., Van Oorschot, P., and Vanstone, S., "Handbook of
          Applied Cryptography", CRC Press, 1997, ISBN 0-8493-8523-7.

   [H80]  Hellman, M. E., "A cryptanalytic time-memory trade-off",
          IEEE Transactions on Information Theory, July 1980,
          pp. 401-406.

   [KSYH] Kang, J-S., Shin, S-U., Hong, D., and Yi, O., "Provable
          Security of KASUMI and 3GPP Encryption Mode f8",




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          Proceedings Asiacrypt 2001, Springer Verlag LNCS 2248,
          pp. 255-271, 2001.

   [MF00] McGrew, D., and Fluhrer, S., "Attacks on Encryption of
         Redundant Plaintext and Implications on Internet Security",
         the Proceedings of the Seventh Annual Workshop on Selected
         Areas in Cryptography (SAC 2000), Springer-Verlag.

   [RK99] Rescorla, E., and Korver, B., "Guidelines for Writing RFC
         Text on Security Considerations," draft-rescorla-sec-cons-
         00.txt

   [PCST1] Perrig, A., Canetti, R., Tygar, D., Song, D., "Efficient and
         Secure Source Authentication for Multicast", in Proc. of
         Network and Distributed System Security Symposium NDSS 2001,
         pp. 35-46, 2001.

   [PCST2] Perrig, A., Canetti, R., Tygar, D., Song, D., "Efficient
           Authentication and Signing of Multicast Streams over Lossy
          Channels", in Proc. of IEEE Security and Privacy Symposium
          S&P2000, pp. 56-73, 2000.

   [WC81] M. N. Wegman and J. L. Carter, "New Hash Functions and Their
         Use in Authentication and Set Equality", JCSS 22, 265-279,
         1981.


Appendix A: Pseudocode for Index Determination

   The following is an example of pseudocode for the algorithm to
   determine the index i of an SRTP packet with sequence number SEQ. In
   the following, signed arithmetic is assumed.


         if (s_l < 32,768)
            if (SEQ - s_l > 32,768)
               set v to (ROC-1) mod 2^32
            else
               set v to ROC
            endif
         else
            if (s_l - 32,768 > SEQ)
               set v to (ROC+1) mod 2^32
            else
               set v to ROC
            endif
         endif
         return SEQ + v*65,536



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Appendix B: Test Vectors

   All values are in hexadecimal.

B.1 AES-f8 Test Vectors


   SRTP PREFIX LENGTH  :   0

   RTP packet header   :   806e5cba50681de55c621599

   RTP packet payload  :   70736575646f72616e646f6d6e657373
                           20697320746865206e65787420626573
                           74207468696e67

   ROC                 :   d462564a
   key                 :   234829008467be186c3de14aae72d62c
   salt key            :   32f2870d
   key-mask (m)        :   32f2870d555555555555555555555555
   key XOR key-mask    :   11baae0dd132eb4d3968b41ffb278379

   IV                  :   006e5cba50681de55c621599d462564a
   IV'                 :   595b699bbd3bc0df26062093c1ad8f73

   j                   :   0
   IV' XOR j           :   595b699bbd3bc0df26062093c1ad8f73
   S(-1)               :   00000000000000000000000000000000
   S(-1) XOR IV' XOR j :   595b699bbd3bc0df26062093c1ad8f73
   S(0)                :   71ef82d70a172660240709c7fbb19d8e
   plaintext           :   70736575646f72616e646f6d6e657373
   ciphertext          :   019ce7a26e7854014a6366aa95d4eefd

   j                   :   1
   IV' XOR j           :   595b699bbd3bc0df26062093c1ad8f72
   S(0)                :   71ef82d70a172660240709c7fbb19d8e
   S(0) XOR IV' XOR j  :   28b4eb4cb72ce6bf020129543a1c12fc
   S(1)                :   3abd640a60919fd43bd289a09649b5fc
   plaintext           :   20697320746865206e65787420626573
   ciphertext          :   1ad4172a14f9faf455b7f1d4b62bd08f

   j                   :   2
   IV' XOR j           :   595b699bbd3bc0df26062093c1ad8f70
   S(1)                :   3abd640a60919fd43bd289a09649b5fc
   S(1) XOR IV' XOR j  :   63e60d91ddaa5f0b1dd4a93357e43a8c
   S(2)                :   584d14a591acfca846b3aa3a0ab50fec



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   plaintext           :   74207468696e67
   ciphertext          :   2c6d60cdf8c29b



B.2 AES-CM Test Vectors

   Keystream segment length: 1044512 octets (65282 AES blocks)
   Key:              2B7E151628AED2A6ABF7158809CF4F3C
   Rollover Counter: 00000000
   Sequence Number:  0000
   SSRC:             00000000
   Salt:             F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000
   Offset:           F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000

   Counter                            Keystream

   F0F1F2F3F4F5F6F7F8F9FAFBFCFD0000   E03EAD0935C95E80E166B16DD92B4EB4
   F0F1F2F3F4F5F6F7F8F9FAFBFCFD0001   D23513162B02D0F72A43A2FE4A5F97AB
   F0F1F2F3F4F5F6F7F8F9FAFBFCFD0002   41E95B3BB0A2E8DD477901E4FCA894C0
   ...                                ...
   F0F1F2F3F4F5F6F7F8F9FAFBFCFDFEFF   EC8CDF7398607CB0F2D21675EA9EA1E4
   F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF00   362B7C3C6773516318A077D7FC5073AE
   F0F1F2F3F4F5F6F7F8F9FAFBFCFDFF01   6A2CC3787889374FBEB4C81B17BA6C44

  Nota Bene: this test case is contrived so that the latter part of the
  keystream segment coincides with the test case in Section F.5.1 of
  [CTR].



B.3 Key Derivation and PRF Test Vectors

   This section provides test data for the default key derivation
   function, which uses AES-128 in Counter Mode. In the following, we
   walk through the initial key derivation for the AES-128 Counter Mode
   cipher, which requires a 16 octet key and a 14 octet salt, and an
   authentication function which requires a 94-octet authentication
   key. These values are called the cipher key, the cipher salt, and
   the auth key in the following. Since this is the initial key
   derivation, the value of (index DIV key_derivation_rate) is zero
   (actually, a six-octet string of zeros). In the following, we
   shorten key_derivation_rate to kdr.

   The inputs to the key derivation function are the 16 octet master
   key and the 14 octet master salt:

      master key:  E1F97A0D3E018BE0D64FA32C06DE4139



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      master salt: 0EC675AD498AFEEBB6960B3AABE6

   We first show how the cipher key is generated. The input block for
   AES is generated by exclusive-oring the master salt with the
   concatenation of the encryption key label 0x00 with (index DIV kdr),
   then padding on the right with two null octets (which implements the
   multiply-by-2^16 operation, see Section 4.3.3). The resulting value
   is then AES-128 encrypted using the master key to get the cipher
   key.

      index DIV kdr:                 000000000000
      label:                       00
      master salt:   0EC675AD498AFEEBB6960B3AABE6
      -----------------------------------------------
      xor:           0EC675AD498AFEEBB6960B3AABE6     (x, PRF input)

      x*2^16:        0EC675AD498AFEEBB6960B3AABE60000 (AES input)


      cipher key:    C61E7A93744F39EE10734AFE3FF7A087 (AES output)

   Next, we show how the cipher salt is generated.  The input block for
   AES is generated by exclusive-oring the master salt with the
   concatenation of the encryption salt label.  That value is padded
   and encrypted as above.

      index DIV kdr:                 000000000000
      label:                       02
      master salt:   0EC675AD498AFEEBB6960B3AABE6

      ----------------------------------------------
      xor:           0EC675AD498AFEE9B6960B3AABE6     (x, PRF input)

      x*2^16:        0EC675AD498AFEE9B6960B3AABE60000 (AES input)

                     30CBBC08863D8C85D49DB34A9AE17AC6 (AES ouptut)

      cipher salt:   30CBBC08863D8C85D49DB34A9AE1

   We now show how the auth key is generated.  The input block for
   AES is generated as above, but using the authentication key label.

      index DIV kdr:                   000000000000
      label:                         01
      master salt:     0EC675AD498AFEEBB6960B3AABE6
      -----------------------------------------------
      xor:             0EC675AD498AFEEAB6960B3AABE6     (x, PRF input)




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      x*2^16:          0EC675AD498AFEEAB6960B3AABE60000 (AES input)



   Below, the auth key is shown on the left, while the corresponding
   AES input blocks are shown on the right.

   auth key                           AES input blocks
   CEBE321F6FF7716B6FD4AB49AF256A15   0EC675AD498AFEEAB6960B3AABE60000
   6D38BAA48F0A0ACF3C34E2359E6CDBCE   0EC675AD498AFEEAB6960B3AABE60001
   E049646C43D9327AD175578EF7227098   0EC675AD498AFEEAB6960B3AABE60002
   6371C10C9A369AC2F94A8C5FBCDDDC25   0EC675AD498AFEEAB6960B3AABE60003
   6D6E919A48B610EF17C2041E47403576   0EC675AD498AFEEAB6960B3AABE60004
   6B68642C59BBFC2F34DB60DBDFB2       0EC675AD498AFEEAB6960B3AABE60005



   This Internet-Draft expires in November 2002.

































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