Network Working Group                                Rolf Blom, Ericsson
INTERNET-DRAFT                              Elisabetta Carrara, Ericsson
Expires: April 2001                               Karl Norrman, Ericsson
                                                  Mats Naslund, Ericsson
                                                                  Sweden
                                                       November 15, 2000





                     RTP Encryption for 3G Networks
                    <draft-blom-rtp-encrypt-00.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 a method for confidentiality protection
   (encryption) of the payload in conversational multimedia applications
   running over the Real-time Transport Protocol [RTP].

   The proposal is based on the 3GPP (3rd Generation Partnership
   Proposal) confidentiality algorithm "f8", and the new Advanced
   Encryption Standard (AES). The proposed scheme satisfies all the
   requirements put forward in [CMSec], such as being error-robust and
   allowing for bandwidth-saving header compression. Most important, the
   solution is based on a security mechanism that has undergone public
   scrutiny, and is widely accepted to be secure.



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TABLE OF CONTENTS

   1. Introduction..................................................2
   1.1. Conventions.................................................3
   1.2. Notation....................................................3
   2. Background....................................................4
   2.1. 3GPP Confidentiality, the f8-algorithm......................4
   2.1.1 f8-mode of operation.......................................4
   2.1.2. Misty/Kasumi..............................................7
   2.1.3. Performance...............................................7
   2.2. AES: an alternative to Kasumi...............................7
   3. Proposal for RTP Payload Encryption...........................8
   3.1. F8-mode for RTP Payload Encryption..........................8
   3.1.1. Parameter negotiation.....................................9
   3.1.2. Cipher Initialization....................................10
   3.1.3. IV Calculation...........................................10
   3.1.4. Encryption at sending end................................11
   3.1.5. Decryption at receiving end..............................11
   3.1.6. Multicast................................................11
   4. Key Management...............................................11
   5. Security Considerations......................................13
   5.1. Confidentiality of the RTP Payload.........................13
   5.2. Confidentiality of the RTP Header..........................14
   5.3. Message Integrity..........................................14
   6. Implementation experience and simulation results.............16
   7. Conclusions..................................................17
   8. Intellectual Property Rights Statement.......................17
   9. Acknowledgments..............................................17
   10. Authors addresses...........................................18
   11. References..................................................18
   Appendix A. Example Test-vectors................................20


1. Introduction

   As discussed in [CMSec], there are a number of requirements that
   immediately arise when designing an encryption scheme for packet-
   switched, real-time data sent on wireless (unreliable) media. The
   requirements are:

   - The encryption scheme should avoid error-propagation (error-
   robustness)

   - The mechanism must be fast, and must be implemented efficiently in
   thin clients

   - The encryption scheme has to show a "fast-forward/rewind" property




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   - The encryption scheme should not expand the message size

   - To allow for header compression over the air link, e.g. [ROHC], it
     is necessary to avoid end-to-end (e2e) encryption of RTP headers
     (the security aspects of this are discussed below, in Section 5.)

   Therefore, as noted in [CMSec], this leads to the choice of employing
   e2e encryption only on the RTP payload, using either a block cipher
   operating in a suitable feedback mode, or a pure stream cipher with a
   random-access property into different locations of the keystream. We
   propose a scheme of the former type, and motivate this as follows.

   In terms of security, the components of the proposal have undergone a
   fair amount of public scrutiny without the detection of any
   weaknesses, see Section 5.

   In a feedback type mode, the central ingredient is the block-cipher
   used. As far as modern cryptology knows, the security basically
   stands (and falls) with the security of the block cipher if
   implemented wisely. This means that if a weakness is found, replacing
   the block cipher with a new one will most likely remedy the security
   problems.

   Good stream ciphers available for public use are rare: there are far
   more (presumed) secure block ciphers than stream ciphers in this
   cathegory. In particular, the "random-access" property into the
   keystream that we desire, disqualifies all but a few stream ciphers
   such as the Software Encryption Algorithm [SEAL].

   Finally, the encryption mechanism (mode of operation) we propose was
   tailor-made for confidentiality protection in 3G networks.

   In section 2, we describe a solution that is used in a similar
   environment. In section 3, we apply a modified version of this
   solution in the context of RTP payload encryption, and propose to use
   the AES algorithm as the cryptographic core of this solution.


1.1. Conventions

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" that
   appear in this document are to be interpreted as described in  [RFC-
   2119].


1.2. Notation

   Except when otherwise noted, we use the same nomenclature as in
   [CMSec].




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   The symbol || denotes concatenation of two binary strings, and XOR
   denotes bitwise addition modulo 2.

   The prefix 0x is used to denote hexadecimal numbers. Data and
   variables are presented with their most significant bytes (or bits)
   to the left, and bit (and byte) indices are 0,1,2,.. counting left to
   right. In general, X[i] denotes the ith bit of X, i.e. X = X[0] ||
   X[1] || X[2] ...


2. Background

2.1. 3GPP Confidentiality, the f8-algorithm

   To encrypt UMTS data, the 3GPP has developed a solution (see [ES3D])
   known as the f8-algorithm. On a high level, the proposed scheme is a
   variant of Output Feedback Mode (OFB), see [HAC], with a more
   elaborate initialization and feedback function. As in normal OFB, the
   core consists of a block cipher. 3GPP specifies this block cipher to
   be Kasumi, see [ES3D], which is a derivative of Matsui's Misty
   algorithm, [MAT]. Kasumi may, in principle, be replaced by any secure
   block cipher (possibly after adjusting some parameters).

   In the next sections, we describe f8, Kasumi, its precursor Misty and
   the recently selected AES.


2.1.1. f8-Mode of Operation

   Figure 1 shows the structure of an arbitrary b-bit block size cipher,
   E, running in what we shall call "f8-mode of operation". (In the 3GPP
   specifiaction, E is the 64-bit block cipher "Kasumi", see below).






















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                   IV

                    |
                    |
                   \|/
                +------+
                |      |
            --->|  E   |
           |    |      |
           |    +------+
           |        |
     m --> *        |---------------------------  ...     -------|
   _____   |    IV' |           |             |                  |
           |        |  ct=1 --> *    ct=2 --> *   ... ct=L-1 --> *
           |        |           |             |                  |
           |        |       --> *         --> *   ...        --> *
           |       \|/     |   \|/       |   \|/            |   \|/
           |    +------+   | +------+    | +------+         | +------+
           |    |      |   | |      |    | |      |         | |      |
     k -------->|  E   |   | |  E   |    | |  E   |         | |  E   |
                |      |   | |      |    | |      |         | |      |
                +------+   | +------+    | +------+         | +------+
                    |      |    |        |    |             |    |
                    |------     |--------     |    ...  ----     |
                    |           |             |                  |
                   \|/         \|/           \|/                \|/

                   S(0)        S(1)          S(2)  . . .       S(L-1)




   Figure 1. In the figure, asterisk, *, denotes bitwise XOR.



   Let E(k,B) be the b-bit output of E when applied to the l-bit key k
   and plaintext block B. Let ct, IV, IV', and S() denote b-bit
   quantities, and m be an l-bit string (determined below). The
   encryption of an n-bit plaintext P = P[0] || P[1] || .. || P[n-1], is
   then performed as follows.

   Set IV' = E(k XOR m, IV), and ct = S(-1) = 00..0. For j = 0,1,.., L-1
   where L = n/b(rounded up to nearest integer) compute

      S(j) = E(k,IV' XOR ct XOR S(j-1)),            (Eq. 1)
      ct   = ct + 1 mod 2^b                         (Eq. 2)






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   Let S = S(0) || S(1) || .. || S(L-1), the concatenation of successive
   outputs from E. Then, the ciphertext, C, is determined from the
   plaintext P and this S by

      C[i] = S[i] XOR P[i], i = 0,1,..,n-1           (Eq. 3)

   i.e. the normal bitwise XOR.

   Thus, in the figure, k is the key, and IV is the initialization
   vector. Notice that the IV is not used directly. Instead it is fed
   through E under another key to produce an internal, "salted" value
   (denoted IV'). The motivation for this is explained below. Thus, the
   input supplied to each consecutive application of E (besides the key
   k) consists of the salted IV', XORed with the b-bit block-counter,
   ct, and the previously generated keystream block (if available). The
   value m is the fixed mask 0x555...(repeated to give as many bits as
   the key-length). The IV is by 3GPP determined as

     IV = COUNT || BEARER || DIR || 00..0

   (padded with as many zeros as needed to fill the block length of E).
   COUNT is a 32-bit value derived from a layer 1 frame counter and a
   "hyper-frame" counter (to avoid reuse of key stream). BEARER is a 4-
   bit bearer identifier, and DIR, finally, is a single bit,
   distinguishing the up-link (terminal to radio base station) from the
   down-link (base station to terminal).

   If the supplied key k is shorter than what is prescribed by E, the
   key bit-pattern is repeated in the least significant bits to the
   desired length.

   It is clear that the f8-mode of operation has the same properties as
   normal OFB mode as far as error propagation and synchronization is
   concerned. From a security point of view, the modified feedback has
   been included to improve security. Basically, the first application
   of E to the supplied IV is done to reduce effects of the fact that
   the IV may be known or perhaps even controlled by an adversary. In
   fact, even if this is the case, without knowledge of the key, it is
   hard to predict the internal IV', that is then the initialization
   vector actually used. Thus, this so called "whitening" or "salting"
   makes it hard for an adversary to obtain known input-output pairs to
   E, and makes analysis more difficult. The incorporation of the block
   counter is there to prevent reuse of keystream (the sequence will not
   have a short period). (It is assumed by 3GPP that no data of length >
   5114 bits are to encrypted, and hence, in practice the block counter
   will never wrap modulo 2^64. In fact, ct may then be implemented as a
   7-bit counter.)

   We next discuss Kasumi and its precursor, Misty.





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2.1.2. Misty/Kasumi

   Misty is a block cipher having block length 64 bits and a 128-bit
   key. Thus, in OFB-type modes, 192 bits are needed to specify the key
   and the IV. The design criteria of Misty were speed and, most
   importantly, provable security against the most common cryptanalytic
   attacks on block ciphers: linear and differential attacks. See [HAC]
   for description of these attacks. There are currently no known
   feasible attacks against Misty.

   The differences between Misty and Kasumi are mainly that
   modifications were made to improve hardware gate count and
   performance, see [ES3E]. No real security weaknesses have been found,
   arising from these modifications, again see [ES3E].


2.1.3. Performance

   In [ES3D] it is claimed that the f8-design meets the following
   hardware requirements:

   - implementable in hardware in less than 10000 gates (an actual
   figure of 3000 gates for Kasumi is mentioned)

   - at least 2Mb/s encryption speed at 20MHz clock rate

   - (re-)initialization within a 10ms time frame

   An important thing to note is that most mobile terminals will have
   the Kasumi based f8-algorithm implemented in hardware, offering very
   good performance. It should be noted, however, that if we use f8 also
   for application layer encryption, then we may be competing with the
   UMTS air-link for the hardware resources. In particular, a fast
   context switch is needed.

   We give some actual performance figures in Section 6.


2.2. AES: an alternative to Kasumi

   NIST has recently selected the new Advanced Encryption Standard, AES.
   This is a block cipher known as "Rijndael" which has a block size of
   128 bits, and allows for 128, 192, and 256 bit keys. For further
   technical details, see [NIST,Ri]. We see AES/Rijndael as an excellent
   alternative to Kasumi for the cryptographic core of the f8-mode of
   operation.

   - During the last three years, it has undergone extensive cryptologic
   analysis, without revealing weaknesses

   - It is a very fast algorithm (see Section 6)



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   - Its larger block and key sizes have advantages (discussed later)


3. Proposal for RTP Payload Encryption

   With the above discussion in mind, we see an f8-type scheme as the
   best solution for confidentiality protection of conversational
   multimedia carried by the RTP protocol.

   1) There is no error propagation. Concerning the error rate of the
   channel, the only assumption is that packets that do arrive at the
   application layer have no errors in their headers. The error control
   is typically taken care of by the header compression scheme, [ROHC],
   and possibly UDP Lite [UDPL]. (Thus, the negligible residual bit
   error rate over the air link is even further decreased by the header
   compression.)

   2)There is no message expansion since there is no padding. Hence,
   bandwidth is conserved.

   3)As the mechanism is used on the application layer, additional IP
   level security association numbers and sequence numbers are avoided.

   4)Synchronization is easy to achieve via the use of the IV, giving
   the desired random-access property.

   5)It is flexible, as it can be based on any secure block cipher with
   varying key and block sizes. In particular the AES, Rijndael [Ri] is
   a good choice. Though slower than a designated stream cipher, speeds
   of several Mb/s are possible both in hardware and software based on
   benchmarks done on popular block ciphers, see e.g. [BOS,LIP] and
   Section 6.

   6)It is highly probable that hardware support for Rijndael will exist
   in terminals.

   Next, we discuss the adaptation of the f8-mode of operation for use
   with RTP.


3.1. F8-Mode for RTP Payload Encryption

   Assumptions:

   1. We assume that a key exchange/parameter negotiation protocol
   exists (see also Section 4).

   2. For simplicity, we assume the communication to be unicast.
   (Extension to multicast is mainly a key management problem and is
   discussed below.)



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   3. The transport layer uses UDP. Some modifications to the
   initialization steps may be needed if some other protocol is used,
   and we discuss them later.

   4. We assume that the application has knowledge of the 16-bit
   port_number, on which the RTP packets are received. Typically, port
   numbers are negotiated via a signalling protocol, such as SIP. We
   furthermore assume that the combination (port_number, SSRC), where
   SSRC is the 32-bit RTP Synchronization Source, is unique for each
   media flow belonging to the same multimedia session between the two
   communicating parties. This must be assured by the signalling
   protocol during set-up, otherwise security is compromised (see
   discussion below). Observe that the "key" may then be the same for
   all media types/flows belonging to the same multimedia session
   between the two parties, since the distinction responsibility lies on
   the (port_number, SSRC)-pair.

   In case the transport layer does not use UDP, a 16-bit quantity
   serving as "port_number" and having the uniqueness properties
   mentioned above must be agreed upon during initial set-up.

   The encryption shall be done using the following steps.


3.1.1. Parameter negotiation

   Using the key-exchange protocol, the following parameters shall be
   agreed:

   cipher: the algorithm E. The AES algorithm must be supported, other
   algorithms, with block size at least 64, may be supported.

   block_size: almost always determined uniquely by "cipher". It is 64
   in the case of Kasumi, and usually 128 for AES (though Rijndael
   supports larger block sizes) and is therefore normally implicit.
   Note: as this parameter varies, different numbers of output keystream
   bits will be generated per application of "cipher", depending on what
   algorithm is used. This means that a 128-bit block algorithm will be
   about twice as fast in encrypting a fixed amount of data as a 64-bit
   algorithm. Notice that since AES must be supported, its definition
   implies that block_size 128 must be supported in connection with AES.

   Note that the internal block counter (this is "ct" in Figure 1)
   should be implemented as a counter of block_size bits. However, if it
   is known that the application never generates RTP packets of
   (payload) size exceeding 2^t times the block size (bits), ct may be
   implemented as a t-bit counter and then extended with leading zeros
   to fill the block_size before use. In almost all cases, t = 32 or
   even 16 will suffice.




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   key_size: the length of the key. The default shall be 128.

   key: a (pseudo)randomly chosen binary string of key_size bits to be
   used as the key. Important: we stress that the key must be random or
   pseudo-random, otherwise security is compromised. For details on
   pseudo-random generators and guidelines for key generation, see
   [HAC,FIPS140].

   If the key size is not in the set of key lengths supported by the
   cipher, the key shall be appended to itself as many times as neeeded
   to obtain, possibly after a final truncation to the most significant
   bits, a length which is the set of supported key sizes (this may be
   due to export restrictions). For example, if the minimum key size is
   16 bits, the key 0x123 (of length 12) is first turned into 0x123123
   (of length 24), and then truncated to 0x1231 which has the desired
   length.


3.1.2. Cipher Initialization

   Given the key and the IV, initialization is performed as in Figure 1,
   computing the internal IV'. If the cipher allows, pre-computing the
   cipher's key-schedules (one with the key, and one with key XOR
   0x555...) may offer performance enhancement on the actual encryptions
   that is follow. Thus, it remains to specify how the IV is formed.


3.1.3. IV Calculation

   For each RTP packet to be transmitted, the following encryption shall
   be performed. The payload of each RTP packet (i.e. starting
   immediately after its RTP header) shall be encrypted by taking
   information from the RTP header of said RTP packet, and the 16-bit
   receiving port number (as negotiated inside the signalling protocol),
   thereby forming the IV:

   IV = port_number || SSRC || SEQ || TS || M || PT || 00...0

   The SEQ (Sequence Number, 16 bits), SSRC (Synchronization Source, 32
   bits), TS (Timestamp, 32 bits), PT (Payload Type, 7 bits), and M
   (Marker Bit, 1 bit) fields are as specified by [RTP]. If the
   block_size is between 64 and 104, the IV shall be truncated after the
   corresponding number of bits in the generic IV (for example, if the
   block_size is 64, then IV = port_number || SSRC || SEQ). If the
   block_size is greater than 104, as many zeros as needed to fill the
   block_size shall be appended.

   By this choice of IV and the assumptions on the (port_number,SSRC)-
   pairs made above, the flow from A -> B will automatically be
   encrypted by a keystream distinct from the flow in the opposite
   direction, B -> A. Some care is thus needed in avoiding reuse of the



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   same IV inside a multimedia session, in case the latter shares the
   same key between its RTP sessions. See Sections 4 and 5 for further
   considerations.

   This IV shall then be entered as the IV to the f8-scheme and the
   internal IV' is computed as described earlier (c.f. Figure 1).


3.1.4. Encryption at sending end

   The RTP-payload only (not the RTP-header) shall then be encrypted in
   accordance with Eq. 1, 2, and 3 above, i.e. be bitwise XORed with the
   output keystream of the f8-scheme. That is, if the payload length is
   n, n/"block_size" (rounded up) applications of "cipher" is needed to
   produce sufficient keystream data. If the packet length is not an
   integer multiple of block_size, any extra keystream bits generated
   shall be ignored.


3.1.5. Decryption at receiving end

   As the XOR operation is an involution, decryption shall be performed
   in exactly the same way by the receiver.


3.1.6. Multicast

   Observe that in a multicast scenario, each packet belonging to a
   media flow will be sent to the same port_number for all intended
   receivers. Thus, as long as the receiving parties share a key with
   the sender, the above method (IV formation) may be used.

   This concludes the formal description of the proposed scheme. Example
   test vectors of the AES based implementation can be found in Appendix
   A.


4. Key Management

   Though beyond the scope of this draft, we make the following
   observations that must be taken into account when finding a suitable
   key-exchange protocol.

   First note that as discussed, the IV forming allows us (if desired)
   to use only one key for the bi-directional flows, and for all media
   types in the overall multimedia session. Again, we stress that this
   is due to the fact that we assume that the (port_number/SSRC)-pairs
   are chosen uniquely during initialization, see above. If not, some
   media flow may be encrypted by the same keystream as another, and
   security is compromised. Distinct keys for different flows may of




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   course also be used, though efficiency will then be reduced due to
   frequent context switches.

   Key refresh: If a feedback type construction is used (as we propose
   to do), one must be aware that generating a very long keystream from
   the same IV/key pair, will eventually lead into "degeneration" of the
   keystream, making it possible to distinguish it from a random string.
   Whence, security might be compromised. The maximum allowable length
   (for a good block cipher) depends only on the block_size, b.
   Specifically, such bad behaviour will start to occur after about
   2^(b/2) iterative applications of the cipher. With b = 128 (for AES)
   or even b = 64 (Kasumi), this will in practice never happen, as
   typical packet sizes for, say conversational audio are typically 32
   bytes only. (The 3GPP f8-solution was designed with a maximum data
   length of 5000 bits in mind and thus, a large security margin
   exists.) If a shorter keystream is generated, there is still a slight
   chance of having collisions in the keystream. However, the
   probability of this occurring while encrypting a normal size packet
   is (with b = 64) on the order 2^(-52) (see [ES3E]).

   In [ES3E], it is also advised that the same key should not be used
   together with more than about 50 million IVs. It is noted that this
   corresponds to about 40 hours of data at 2Mbit/s.  However, in our
   case, since the sequence number (SEQ) is only 16 bits in length,
   after that many IVs (packets) the keystream will start to repeat
   itself. Therefore, if only the sequence number is used (as is the
   case for a 64-bit block size), the key must be refreshed at least for
   every 2^16 packets. Some practical implications of this can be
   examined. Consider for instance an audio stream with 50 packets/sec.
   The SEQ field will then wrap modulo 2^16 once every 20 minutes or so.
   If a larger block size is used, then also the 32-bit timestamp (TS)
   is included in the IV, thereby extending the "effective" sequence
   numbers to a (theoretical) maximum of 48-bits. This maximum may not
   always be achieved however. For instance, for an audio application
   with 20 ms samples, the RTP timestamp typically increments by 160 for
   each increment in SEQ. The combination SEQ || TS will then have the
   form (j mod 2^16) || (160j mod 2^32), j = 0,1,.., and hence have
   period 2^27 (the least common multiple of 2^16 and
   2^32/gcd(2^32,160)) rather than 2^48. Still, this is very large
   (about twice the recommended maximum of 50 million), and the need to
   refresh the key due to exhaustion of the IV-space should be rare. For
   other applications, e.g. video, the relation between SEQ and TS may
   look different.

   In summary, assisted by the key management functionality, the
   application must keep track of when a key-refresh is needed, either
   due to the fact that the RTP Sequence Number (possibly in combination
   with the RTP Timestamp) wraps/re-cycles, or because so many IVs (e.g.
   > 50 million) have been used with the same key that security may
   start to be endangered. Obviously, the mechanism must ensure that
   synchronization of the key refresh is obtained between



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   sender/receiver. It seems to be a reasonable solution to have the key
   exchange protocol exchange a "master secret", from which consecutive
   keys can be derived pseudo-randomly with the master secret as a seed,
   similar to what is done in [TLS, WTLS].


5. Security Considerations

   For general security overview of f8 (and Kasumi), we refer to the
   analysis in [ES3E] and for the AES algorithm, see [Ri,AES]. (If
   another block cipher is used, the security of that algorithm will of
   course be an issue.)

   However, there are some specific issues that arise in this
   application that we discuss in the next paragraphs.

   In a real-time scenario, key management must not be overloading, both
   for time and bandwidth reasons. To use a fixed key for all the RTP
   sessions which belong to the same multimedia session, special
   attention has to be placed on possible collisions in the way the IV
   is formed (see also above). The requirement is not to end up with two
   identical IVs, for the same key. The RTP standard defines some
   recommendations on how to use SSRC (unique inside a single RTP
   session, also accompanied by an anti-collision algorithm, and port
   numbers are recommended for demultiplexing). However, without special
   collision detection, there may be unlucky (port_number,SSRC)-
   combinations. We therefore believe it is up to the implementation not
   to end up in such a situation, and as noted above, we recommend a
   careful choice of the receiving ports (being negotiated inside the
   call control protocol, e.g. SIP). Again, the required property is
   that port numbers should be chosen to avoid the same combination of
   SSRC/port number inside RTP sessions which belong to the same
   multimedia session.

   Notice that the uniqueness requirement on the (port_number,SSRC)-
   combination allows for a theoretical maximum of 2^48 paralell flows,
   belonging to the same multimedia session.

   Security may, of course, also be obtained by asserting that distinct
   keys are used for the different streams (and their two directions)
   belonging to the same RTP session if one is willing to perform more
   frequent context switches.


5.1. Confidentiality of the RTP Payload

   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



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   bit of the keystream can be deduced. However, if the stream cipher is
   secure, knowledge of a few bits of the keystream will not aid an
   attacker in predicting the following keystream bits. Thus, the
   payload length (and information deducible from this) will leak, but
   nothing else.


5.2. Confidentiality of the RTP Header

   With our proposal, 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".

   Our proposal is a low-cost method, which allows header compression to
   reduce bandwidth. It is up to the endpoints policies to decide about
   the security scheme to employ. If the header compression is omitted,
   other solutions might be applicable, e.g. [IPsec]. In other words, we
   provide a solution that works in the most demanding scenario:
   conversational multimedia over low-bandwidth, unreliable media. Of
   course the solution will then also work in less restricted
   environments, but we suggest that if one really needs to protect
   headers, and is allowed to do so by the surrounding environment, then
   he should also look at alternatives. In addition, we strongly
   recommend the use of profiles to select the right trade-off for the
   required level of security.


5.3. Message Integrity

   The purpose of this draft is only to treat confidentiality, not
   integrity. However, we realize the importance of this cryptographic
   primitive, and a few things can be said.

   Adding a "full strength" message authentication code (MAC) to each
   packet is unacceptable for bandwidth reasons. For instance, a 160-bit
   HMAC [HAC] field will in principle double the data size for a typical
   audio packet. If size is reduced to bandwidth-acceptable values by
   truncating the MAC, perhaps to one or two bytes, a very low security-
   level is obtained.

   Moreover, due to possible errors induced by the transmission medium,
   an integrity check will put down packets that only have minor errors
   inflicted "non-adversarialy" by the channel, and will reduce the
   quality of the received signal.

   If bandwidth considerations allow it, adding a standard integrity
   mechanism is of course the right thing to do, but again, if we are in




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   this more favourable situation, one can perhaps use another, more
   complete security mechanism, e.g. [IPsec].

   Recall that we are aiming at providing security for conversational
   multimedia. What kind of "useful" attacks can be mounted to violate
   data integrity in such cases?

   First, notice that some amount of integrity for the RTP headers is
   obtained for the parts that enter into the IV-formation (SSRC, SEQ,
   TS, PT, and M). Namely, should any of these be modified during
   transmisson, the decryption will only produce "random garbage",
   reducing an attack to the integrity of these fields to pure denial-
   of-service (DoS) aspects.

   If an attacker records a session for a later replay-attack,
   possibilities of obtaining something useful seem remote. First of
   all, we may assume that the replay takes place at a later time within
   the same session. (Otherwise, with overwhelming probability, the new
   session will be running with a new distinct key and the receiver
   would then decrypt with the, for the attacker, wrong key and only
   "garbage" data is produced). But if the replay occurs within the same
   session, that would imply that the attacker needs to modify the
   header sequence numbers and timestamps. As these fields enter the
   encryption algorithm, an uncorrelated keystream will then be used by
   the receiver, and garbage will again be produced. We conclude that
   attempting to do a replay attack, and any attempt in altering
   important parts of the RTP header will mainly have DoS effects.

   In addition, due to the real-time aspects, there will be a natural
   window mechanism implemented by the application, making alterations
   of synchronization data (e.g. timestamps) to have mainly DoS aspects.

   If a window mechanism and/or buffering for received packets is used,
   a perhaps more serious attack (also of DoS type), would be to modify
   the sequence number and timestamp fields, causing the receiver to
   move his window too far away. The worst case seems to be if this
   attack is launched at the beginning of the session.

   Another attack could be to try to modify bits of the encrypted
   payload in real-time. Due to the absence of error propagation in the
   decryption process, it is indeed possible for an attacker to "flip"
   individual bits for the receiver. However, he does not know the
   outcome of the flip. Minor alterations (one or two bits) will
   probably still allow audio/video decoders to reconstruct data at an
   acceptable quality level for the receiver. However, we cannot exclude
   the fact that flipping bits will be of some use to an attacker, for
   instance if those bits have some "simple" meaning to the CODEC. We
   are aiming at a general solution for RTP data, and one therefore has
   to study the format of each possible payload type to understand
   exactly what can be gained by an attacker in each case. Moreover, we
   cannot foresee possible future formats, not yet defined.



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   Again, in most conversational multimedia applications we foresee, we
   feel that it will be hard to do harmful attacks against the
   integrity, since data is presented in real-time to the users, and
   since there is also real-time interaction between the communicating
   parties. One could say that there is, to some extent, an automatic
   and "manual" sanity-check done by the users as the data has direct
   meaning and can be interpreted by them.

   In summary, integrity protection may be needed, but it is to a large
   extent up to the application to decide this, taking bandwidth costs
   and quality into the budget. The only application-independent
   observation we can make at this point is that an attacker
   manipulating the data will be able to launch DoS attacks. However,
   adding a MAC field does not prevent this, it merely provides a
   detection method before data enters the CODEC.


6. Implementation experience and simulation results

   We have implemented the f8-mode of operation using the AES as the
   underlying block cipher with 128 bit block size and 128 bit key size.
   Both algorithms were implemented in C (using the Microsoft Visual C++
   Compiler).

   The implementation of AES was made compact, that is, we only used
   tables for the S-box, its inverse and the round constants, for a
   total of 552 bytes. Everything else was calculated in real-time. With
   this approach we were able to encrypt at approximatly 22 Mbit/s on a
   Pentium II with a clock frequency of 266 Mhz.

   When measuring the encryption-speed of the f8-algorithm we used
   simulated RTP-packets with a typical 96-bit header (which was not
   encrypted) and a 32, 64 and 128 octet random payload (i.e 256, 512
   and 1024 bits). With this scenario we need one block encryption to
   set the internal IV, and two, four and eight block encryptions
   respectively, to produce the keystream for each packet. Heuristicly,
   this implies that the f8-algorithm should be able to encrypt
   somewhere around 16.5 Mbit/s for 32-byte packets. The actual
   throughput will be a bit less because of some overhead. Since
   encryption and decryption are the same operation, the same figures
   hold for decryption.

   We encrypted 10^6 packets of each size and took the average
   encryption times, which are presented in the table below.









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             Payload Size (bits)         Encrypt/Decrypt (Mbit/s)

                     256                           13.6
                     512                           15.9
                    1024                           17.9


               Table 1: Performance of AES-based f8.



   Calculation of the internal IV (copying of some data from the packet
   header and one block encryption) takes around 6 microseconds.

   Key-refresh and initialization of the keystream generator (we view
   initialization as a key-refresh from a non-existent key) takes
   approximatly 12 microseconds.

   For comparison, 20Mb/s at 100MHz is reported for Misty in [MIT]. This
   is about 3 times faster than typical (optimized) 3DES
   implementations.

   Estimates for hardware performance for AES can be obtained from
   [AES,Ri]. Similar estimates for Kasumi and f8 can be found in [ES3E].


7. Conclusions

   We have made a proposal for a secure, fast, and flexible encryption
   scheme that has the necessary robustness properties in terms of
   fault-tolerance needed in conversational multimedia applications over
   low-bandwidth, unreliable media.


8. Intellectual Property Rights Statement

   Pursuant to the provisions of [RFC-2026], the authors represent that
   they have disclosed the existence of any proprietary or intellectual
   property rights in the contribution that are reasonably and
   personally known to the authors. The authors do not represent that
   they personally know of all potentially pertinent proprietary and
   intellectual property rights owned or claimed by the organizations
   they represent or third parties.


9. Acknowledgments

   We thank Krister Svanbro and Vicknesan Ayadurai for instructing us on
   the inner workings of header compression, and Andras Mehes and Jari
   Arkko for helpful comments on this draft. We are grateful to Morgan



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   Lindqvist, Johan Sjoberg, Torbjorn Einarsson, and Magnus Westerlund
   for sorting out various RTP questions.


10. Authors' addresses


   Rolf Blom                Tel:    +46 8 58531707
   Ericsson Research
   Stockholm, Sweden        EMail:  rolf.blom@era.ericsson.se


   Elisabetta Carrara       Tel:    +46 8 50877040
   Ericsson Research
   Stockholm, Sweden        EMail:  elisabetta.carrara@era.ericsson.se


   Karl Norrman             Tel:    +46 8 58531225
   Ericsson Research
   Stockholm, Sweden        EMail:  karl.norrman@era.ericsson.se


   Mats Naslund             Tel:    +46 8 58533739
   Ericsson Research
   Stockholm, Sweden        EMail:  mats.naslund@era.ericsson.se



11. References

   [AES] NIST, "Advanced Encryption Standard (AES)",
   http://csrc.nist.gov/encryption/aes/

   [BOS] Bosselaers, A., "Fast Implementations on the Pentium",
   http://www.esat.kuleuven.ac.be/~bosselae/fast.html

   [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.

   [FIPS140] NIST, "Security Requirements for Cryptographic Modules",
   FIPS PUB 140-1

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



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   [IPsec] McGrew, D., Fluhrer, S., Peyravian, M.,  "The Stream Cipher
   Encapsulating Security Payload", Internet Draft, July 2000

   [LIP] Lipmaa, H., "AES Ciphers: speed",
   http://www.tml.hut.fi/~helger/aes/

   [MAT]  Matsui, M., "New  Block Encryption Algorthm MISTY". In Eli
   Biham (Ed.): Fast Software Encryption, 4th International Workshop,
   FSE '97. Proceedings. Lecture Notes in Computer Science, Vol. 1267,
   Springer-Verlag 1997, pp. 54-68.

   [MIT] Mistubishi Electric, "MISTY",
   http://www.mitsubishi.com/ghp_japan/misty/

   [RFC-2026]  Bradner, S., "The Internet Standards Process -- Revision
   3", RFC2026, October 1996.

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

   [Ri] Daemen, J. and Rijmen, V.: "AES Proposal: Rijndael", available
   at http://www.esat.kuleuven.ac.be/~rijmen/rijndael

   [ROHC] Burmeister, C., Clanton, C., Degermark, M., Fukushima, H.,
   Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z.,
   Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Zheng, H.,
   "RObust Header Compression (ROHC)", Internet Draft, October 2000

   [RTP] Schulzrinne, H., Casner, S., Frederick, R., and Jacobson, V.:
   "RTP: a Transport Protocol for Real-Time Applications", RFC 1889,
   Jan. 1996.

   [CMSec] Blom, R., Carrara, E., and Naslund, M.: "conversational
   Multimedia Security in 3G Networks", IETF Draft, November 2000.

   [SEAL] Rogaway, P. and Coppersmith, D.: "A Software-Optimized
   Encryption Algorithm", Journal of Cryptology, vol 11(4), 1998, 273-
   287.

   [TLS] Dierks, T., Allen, C., "The TLS Protocol", RFC 1998, Internet
   Draft, November 12

   [WTLS] Wireless Application Forum: "WAP WTLS, Wireless Application
   Protocol Wireless Transport Layer Security Specification", Version
   18-Feb-2000.








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Appendix A. Example Test-vectors

   Here is an example of the intermediate values during an encryption
   using the AES algorithm in f8-mode.

   The data encrypted is a single RTP-packet with a 256-bit (pseudo-
   randomly generated) payload. All values are in hex. Refer to Figure 1
   for notation.

   key:
     000102030405060708090a0b0c0d0e0f

   key XORed with 555... :
     55545756515053525d5c5f5e59585b5a

   Rijndael-internal expanded key:
     00010203 04050607 08090a0b 0c0d0e0f
     d6aa74fd d2af72fa daa678f1 d6ab76fe
     b692cf0b 643dbdf1 be9bc500 6830b3fe
     b6ff744e d2c2c9bf 6c590cbf 0469bf41
     47f7f7bc 95353e03 f96c32bc fd058dfd
     3caaa3e8 a99f9deb 50f3af57 adf622aa
     5e390f7d f7a69296 a7553dc1 0aa31f6b
     14f9701a e35fe28c 440adf4d 4ea9c026
     47438735 a41c65b9 e016baf4 aebf7ad2
     549932d1 f0855768 1093ed9c be2c974e
     13111d7f e3944a17 f307a78b 4d2b30c5

   Rijndael-internal expanded value of (key XOR 555_):
     55545756 51505352 5d5c5f5e 59585b5a
     3e6de99d 6f3dbacf 3261e591 6b39becb
     2ec3f6e2 41fe4c2d 739fa9bc 18a61777
     0e33034f 4fcd4f62 3c52e6de 24f4f1a9
     b992d079 f65f9f1b ca0d79c5 eef9886c
     30568051 c6091f4a 0c04668f e2fdeee3
     447e91c9 82778e83 8e73e80c 6c8e06ef
     1d114e99 9f66c01a 11152816 7d9b2ef9
     8920d766 1646177c 07533f6a 7ac81193
     7aa20bbc 6ce41cc0 6bb723aa 117f3239
     9e81193e f26505fe 99d22654 88ad146d


   RTP-packet header fields:
     version      = 2
     padding      = 0
     extension    = 0
     CSRC count   = 0
     marker bit   = 0
     payload type = 0
     sequence no. = 3e7a



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     timestamp    = 7e5d40a7
     SSRC         = 7b777a8f

   Receiver port number:
     abcd


   IV:
     abcd7b777a8f3e7a7e5d40a7003e0000

   IV':
     7f25578863921e41120b09ebfdd43f1c


   Encryption of bits 0 to 127 of the payload

   ct: 0
   S(-1)                 : 00000000000000000000000000000000
   S(-1) XOR IV'         : 7f25578863921e41120b09ebfdd43f1c
   plain text P[0..127]  : 9979b51c83ac87a3330a6178cc3b1aa6
   final keystream S(0)  : b57d2b3337b281f04645bed7082af95d
   cipher text C[0..127] : 2c049e2fb41e0653754fdfafc411e3fb


   Encryption of bits 128 to 255 of the payload

   ct: 1
   S(0)                   : b57d2b3337b281f04645bed7082af95c
   S(0) XOR IV'           : ca587cbb54209fb1544eb73cf5fec640
   plain text P[128..255] : 79cddb405384255385f11619ad46e86f
   final keystream S(1)   : e7d836679304e5c7f0881b067e3d682c
   cipher text C[128..255]: 9e15ed27c080c09475790d1fd37b8043


   This Internet-Draft expires in April 2001.



















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