Network Working Group D. McGrew Internet Draft Cisco Systems, Inc. Intended Status: Informational March 06, 2009 Expires: September 7, 2009 AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP) draft-mcgrew-srtp-aes-gcm-01 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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 to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 7, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. McGrew Informational [Page 1] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 Abstract This document defines how AES-GCM, AES-CCM, and other Authenticated Encryption with Associated Data (AEAD) algorithms, can be used to provide confidentiality and data authentication mechanisms in the SRTP protocol. Table of Contents 1. Introduction.....................................................2 1.1. Conventions Used In This Document...........................3 1.2. AEAD processing for SRTP....................................4 1.2.1. AEAD Authentication versus SRTP Authentication.........4 1.2.2. Initialization Vectors for SRTP........................5 1.2.3. Initialization Vectors for SRTCP.......................5 1.2.4. Format for SRTP Packet.................................6 1.2.5. Packet Format for SRTCP................................7 1.2.5.1. Encrypted SRTCP packets...........................7 1.2.5.2. Unencrypted SRTCP packets.........................7 2. AEAD parameters for SRTP and SRTCP...............................9 2.1. Generic AEAD Parameter Constraints..........................9 2.2. AES-GCM for SRTP/SRTCP.....................................10 2.3. AES-CCM for SRTP/SRTCP.....................................11 3. Security Considerations.........................................12 4. IANA Considerations.............................................13 5. Acknowledgements................................................13 6. References......................................................13 6.1. Normative References.......................................13 6.2. Informative References.....................................14 1. Introduction The Secure Real-time Transport Protocol (SRTP) is a profile of the Real-time Transport Protocol (RTP), which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the control traffic for RTP, the Real-time Transport Control Protocol (RTCP). SRTP/SRTCP assumes that both the sender and recipient have a shared secret master key and a shared secret master salt. As described in sections 4.3.1 and 4.3.3 of [RFC3711], a Key Derivation Function is applied to these secret values to obtain separate encryption keys, authentication keys and salting keys for SRTP and for SRTCP. (Note: As will be explained below, AEAD SRTP/SRTCP does not make use of these authentication keys.) Authenticated encryption [BN00] is a form of encryption that, in addition to providing confidentiality for the plaintext that is encrypted, provides a way to check its integrity and authenticity. Authenticated Encryption with Associated Data, or AEAD [R02], adds McGrew Informational [Page 2] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 the ability to check the integrity and authenticity of some Associated Data (AD), also called "additional authenticated data", that is not encrypted. This specification makes use of the interface to a generic AEAD algorithm as defined in [RFC5116]. The Advanced Encryption Standard (AES) is a block cipher that provides a high level of security, and can accept different key sizes. Two families of AEAD algorithm families, AES Galois/Counter Mode (AES-GCM) and AES Cipher Block Chaining/Counter Mode (AES/CCM), are based upon AES. This specification makes use of the AES versions that use 128-bit and 256-bit keys, which we call AES-128 and AES-256, respectively. The Galois/Counter Mode (GCM) of operation and the Counter with CBC MAC (CCM) mode are AEAD modes of operation for block ciphers. Both use counter mode to encrypt the data, an operation that can be efficiently pipelined. Further, GCM authentication uses operations that are particularly well suited to efficient implementation in harware, making it especially appealing for high-speed implementations, or for implementations in an efficient and compact circuit. CCM is well suited for use in compact software implementations. This specification uses GCM and CCM with both AES-128 and AES-256. In summary, this document defines how to use AEAD algorithms, particularly AES-GCM and AES-CCM, to provide confidentiality and message authentication within SRTP and SRTCP packets. 1.1. Conventions Used In This Document The following terms have very specific meanings in the context of this RFC: Crypto Context. For the purposes of this document a security association is the outcome of any process which results in authentication of each patricpant in the SRTP session and in their possesion of a shared secret master key and a shared secret master salt. Details of how the crypto context is established are outside the scope of this document. Instantiation. Once keys have been established, an instance of the AEAD algorithm is created using the appropriate key. In a point-to-point scenario, each participant in the SRTP/SRTCP session will need four instantiations of the AEAD algorithm; one for inbound SRTP traffic, one for outbound SRTP traffic source, one for inbound SRTCP traffic, and one for outbound SRTCP traffic source. Within a given crypto context, all of the encryption keys are derived from the crypto context's shared secret master key and all of the encryption salts are derived from the crypto context's shared secret master salt. McGrew Informational [Page 3] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 Invocation. SRTP/SRTCP data streams are broken into packets. Each packet is processed by a single invocation of the appropriate instantiation of the AEAD algorithm. Each AEAD instantiation has its own invocation counter which is incremented each time that particular instantiation is invoked. As we shall see below, the invocation counter is used to insure each invocation gets a unique initialization vector. The key words "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]. 1.2. AEAD processing for SRTP We first define how to use a generic AEAD algorithm in SRTP, then we describe the specific use of the AES-128-GCM and AES-256-GCM algorithms. The use of an AEAD algorithm is defined by expressing the AEAD encryption algorithm inputs in terms of SRTP fields and data structures. The AEAD encryption inputs are as follows: Key. This input is the SRTP encryption key (SRTP_encr_key) produced from the shared secret master key using the key derivation process. (Note that the SRTP_auth_key is not used). Associated Data. This is data that is to be authenticated but not encrypted. In SRTP, the associated data consists of the entire RTP header, including the list of CSRC identifiers (if present) and the RTP header extension (if present), as shown in Figure 2. Plaintext. This is data that is to be both authenticated and encrypted. In SRTP this consists of the RTP payload, and the RTP padding and RTP pad count fields (if the latter two fields are present), as shown in Figure 2. The padding service provided by RTP is not needed by the AEAD encryption algorithm, so the RTP padding and RTP pad count fields SHOULD be omitted. Initialization Vector (IV). Each SRTP/SRTCP packet has its own 12-octet initialization vector. Construction of this IV is covered in more detail below. The AEAD encryption algorithm accepts these four inputs and returns a Ciphertext field. 1.2.1. AEAD Authentication versus SRTP Authentication The reader is reminded that in addition to providing confidentiality for the plaintext that is encrypted, an AEAD algorithm also provides McGrew Informational [Page 4] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 a way to check the data integrity and authenticity of the plaintext and associated data. The AEAD integrity check is incorporated into the ciphertext field by RFC 5116, thus AEAD does not make use of the optional SRTP Authentication Tag field. (Note that this means that the cipher text will be longer than the plain text by precisely the length of the AEAD authentication tag.) The AEAD message authentication mechanism MUST be the primary message authentication mechanism for AEAD SRTP. Additional SRTP authentication mechanisms SHOULD NOT be used with any AEAD algorithm and the optional SRTP Authentication Tag SHOULD NOT be present. Rationale. Some applications use the Authentication Tag as a means of conveying additional information, notably [RFC4771]. This document retains the Authentication Tag field primarily to preserve compatibility with these applications. 1.2.2. Initialization Vectors for SRTP The initialization vector for an SRTP packet is formed from the 4-octet Synchronization Source identifier (SSRC), 4-octet Rollover Counter (ROC), the 2-octet RTP Sequence Number (SEQ), and a 12-octet SRTP session encryption salt produced by the SRTP Key Derivation Function (KDF) as shown in figure 1. (The concatenation of the ROC and SEQ serves as a 6-octet invocation counter.) First, a 2-octet string consisting of zeroes is prepended to the 4-octet SSRC, then the 4-octet ROC appended and 2-octet SEQ is appended to that octet string. The resulting 12-octet string is bitwise exclusive-ored into salt; the output of that process is the IV. The IV is always exactly 12 octets in length. 0 0 0 0 0 0 0 0 0 0 1 1 0 1 2 3 4 5 6 7 8 9 0 1 +--+--+--+--+--+--+--+--+--+--+--+--+ |00|00| SSRC | ROC | SEQ |---+ +--+--+--+--+--+--+--+--+--+--+--+--+ | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Encryption Salt |->(+) +--+--+--+--+--+--+--+--+--+--+--+--+ | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Initialization Vector |<--+ +--+--+--+--+--+--+--+--+--+--+--+--+ Figure 1: SRTP Initialization Vector formation. 1.2.3. Initialization Vectors for SRTCP The initialization vector for an SRTCP packet is formed from the 4-octet Synchronization Source identifier (SSRC), 31-bit SRTCP Index McGrew Informational [Page 5] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 (packed zero-filled, right justified into a 4-octet field), and a 12-octet SRTP session encryption salt produced by the SRTP Key Derivation Function (KDF) as described in [RFC 3711]. (The 31-bit SRTCP index serves as the invocation counter.) First a 12-octet string is formed by concatenating in order 2-octets of zeroes, the 4-octet SSRC, 2 more zero octets, and the 4-octet SRTCP index. The resulting 12-octet string is bitwise exclusive-ored into salt; the output of that process is the IV. The process is illustrated in Figure 3. The IV is always exactly 12 octets in length. 0 0 0 0 0 0 0 0 0 0 1 1 0 1 2 3 4 5 6 7 8 9 0 1 +--+--+--+--+--+--+--+--+--+--+--+--+ |00|00| SSRC |00|00|SRTCP Index|---+ +--+--+--+--+--+--+--+--+--+--+--+--+ | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Encryption Salt |->(+) +--+--+--+--+--+--+--+--+--+--+--+--+ | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Initialization Vector |<--+ +--+--+--+--+--+--+--+--+--+--+--+--+ Figure 2: SRTCP Initialization Vector formation. 1.2.4. Format for SRTP Packet All SRTP packets MUST be authenticated and encrypted. Figure 3 below shows which fields of AEAD SRTP packet are to be treated as plaintext, which are to be treated as additional authenticated data, and which fields are to be treated as additional authenticated data. McGrew Informational [Page 6] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A |V=2|P|X| CC |M| Packet Type | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ A | contributing source (CSRC) identifiers (optional) | A | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | RTP extension (OPTIONAL) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P | payload ... | P | +-------------------------------+ P | | RTP padding | RTP pad count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X : authentication tag (NOT RECOMMENDED) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P = Plaintext (to be encrypted and authenticated) A = Associated Data (to be authenticated only) X = neither encrypted nor authenticated Note: The RTP padding and RP pasdding count fields are optional and are not recommended Figure 3: AEAD inputs from an SRTP packet. 1.2.5. Packet Format for SRTCP Unlike SRTP, SRTCP packet encryption is optional (but authentication is mandatory). A sender can select which pakets to encrypt, and indicates this choice with a 1-bit encryption flag (located in the leftmost bit of the 32-bit word that contains the SRTCP index) 1.2.5.1. Encrypted SRTCP packets When the encryption flag is set to 1, the first 8-octets, the encrytion flag and SRTCP index are treated as AAD and eight octets and the encryption flag are treated as plaintext. Figure 4 below shows how fields of an RTCP packet are to be treated when the encryption flag is set to 1. 1.2.5.2. Unencrypted SRTCP packets When the encryption flag is set to 0, all of the data up to and including the SRTCP index is treated as AAD. Figure 5 shows how the McGrew Informational [Page 7] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 fields of an RTCP packet are to be treated when the encryption flag is set to 0. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A |V=2|P| RC | Packet Type | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | synchronization source (SSRC) of Sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P | sender info | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P | report block 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P | report block 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P |V=2|P| SC | Packet Type | length | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ P | SSRC/CSRC_1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P | SDES items | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ P | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ A |1| SRTCP index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X | SRTCP MKI (optional)index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X : authentication tag (NOT RECOMMENDED) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P = Plaintext (to be encrypted and authenticated) A = Associated Data (to be authenticated only) X = neither encrypted nor authenticated Figure 4: AEAD inputs for an encrypted SRTCP packet. McGrew Informational [Page 8] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A |V=2|P| RC | Packet Type | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | synchronization source (SSRC) of Sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | sender info | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | report block 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | report block 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A |V=2|P| SC | Packet Type | length | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ A | SSRC/CSRC_1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | SDES items | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ A | ... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ A |0| SRTCP index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X | SRTCP MKI (optional)index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X : authentication tag (NOT RECOMMENDED) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A = Associated Data (to be authenticated only) X = neither encrypted nor authenticated Figure 5: AEAD inputs for an unencrypted SRTCP packet. 2. AEAD parameters for SRTP and SRTCP In general, any AEAD algorithm can accept inputs with varying lengths, but each algorithm can accept only a limited range of lengths for a specific parameter. In this section, we describe the constraints on the parameter lengths that any AEAD algorithm must support to be used in AEAD-SRTP. Additionally we specify a complete parameter set for two specific AEAD algorithms, namely AES-GCM and AES-CCM. 2.1. Generic AEAD Parameter Constraints All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three constraints listed below: McGrew Informational [Page 9] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 PARAMETER Meaning Value A_MAX maximum additional MUST be at least 12 octets authenticated data length N_MIN minimum nonce (IV) MUST be no more than 12 octets length N_MAX maximum nonce (IV) MUST be at least 12 octets length C_MAX maximum ciphertext MUST be at most 2^16-40 octets length per invocation SHOULD be at least 2232 The upper bound on C_MAX is obtained by subtracting away a 20-octet IP header, an 8-octet UDP header, and a 12-octet RTP header out of the largest possible IP packet, the total length of which is 2^16 octets. Similarly the lower bound on C_MAX is based on the maximum transmission unit (MTU) of 2272 octets in IEEE 802.11. Because many RTP applications use very short payloads (for example, the G.729 codec used in VoIP can be as short as 20 octets), implementations that only support a maximum ciphertext length smaller than 2232 octets are permitted under this RFC. However, in the interest of maximizing interoperability between various AEAD implementations, the use of C_MAX values less than 2232 is discouraged. For sake of clarity we specify two additional parameters: Authentication Tag Length MUST be either 8, 12, or 16 octets Maximum number of invocations MUST be at most 2^48 for SRTP for a given instantiation MUST be at most 2^31 for SRTCP The reader is reminded that the plaintext is shorter than the ciphertext by exactly the length of the AEAD authentication tag. 2.2. AES-GCM for SRTP/SRTCP AES-GCM is a family of AEAD algorithms built around the AES block cipher algorithm. AES-GCM uses AES counter mode for encryption and Galois Message Authentication Code (GMAC) for authentication. A detailed description of the AES-GCM family can be found in [RFC5116]. The following members of the AES-GCM family may be used with SRTP/SRTCP: Table 1: AES-GCM algorithms for SRTP/SRTCP Name Key Size Auth. Tag Size Reference ================================================================ AEAD_AES_128_GCM 16 octets 16 octets [RFC5116] AEAD_AES_256_GCM 32 octets 16 octets [RFC5116] McGrew Informational [Page 10] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 AEAD_AES_128_GCM_8 16 octets 8 octets [RFC5282] AEAD_AES_256_GCM_8 32 octets 8 octets [RFC5282] AEAD_AES_128_GCM_12 16 octets 12 octets [RFC5282] AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282] Any implementation of AES-GCM SRTP MUST support both AEAD-AES-128-GCM-8 and AEAD-AES-256-GCM-8, ant it MAY support the four other variants shown in the table. In addition to the invocation counter used in the formation of IVs, each instantiation of AES-GCM has a block counter which is incremented each time AES is called to produce a 16-octet output block. The block counter is reset to "1" each time AES-GCM is invoked. 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 ----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | salt | | salt xor | block | | salt | xor | salt | invocation | counter | | | ssrc | | counter | | ----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Figure 5: AES Inputs for Counter Mode Encryption in GCM 2.3. AES-CCM for SRTP/SRTCP AES-CCM is another family of AEAD algorithms built around the AES block cipher algorithm. AES-GCM uses AES counter mode for encryption and AES Cipher Block Chaining Message Authentication Code (CBC MAC) for authentication. A detailed description of the AES-CCM family can be found in [RFC5116]. The following members of the AES-CCM family may be used with SRTP/SRTCP: Table 2: AES-CCM algorithms for SRTP/SRTCP Name Key Size Auth. Tag Size Reference ================================================================ AEAD_AES_128_CCM 16 octets 16 octets [RFC5116] AEAD_AES_256_CCM 32 octets 16 octets [RFC5116] Any implementation of AES-CCM SRTP/SRTCP MUST support both AEAD-AES-128-CCM and AEAD-AES-256-CCM. In addition to the invocation counter used in the formation of IVs, each instantiation of AES-CCM has a block counter which is incremented each time AES is called to produce a 16-octet output block. The block counter is reset to "0" each time AES-CCM is invoked. AES-CCM uses a flag octet that conveys information about the length of the authentication tag, length of the block counter, and presence McGrew Informational [Page 11] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 of additional authenticated data. For AES-CCM in SRTP/SRTCP, the flag octet has the hex value 5A if an 8-octet authentication tag is used, 6A if a 12-octet authentication tag is used, and 7A if a 16-octet authentication tag is used. The flag octet is one of the inputs to AES during the counter mode encryption of the plaintext (see Figure 6) 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 ----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | | salt | | salt xor | block | |Flag| salt | xor | salt | invocation | counter | | | | ssrc | | counter | | ----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Figure 6: AES Inputs for Counter Mode Encryption in CCM 3. Security Considerations We require that the AEAD authentication tag must be at least 8 octets, significantly reducing the probability of an adversary successfully introducing fraudulent data. The goal of an authentication tag is to minimize the probability of a successful forgery occurring anywhere in the network we are attempting to defend. There are three relevant factors: how low we wish the probability of successful forgery to be (prob_success), how many attempts the adversary can make (N_tries) and the size of the authentication tag in bits (N_tag_bits). Then prob_success < expected number of successes = N_tries * 2^-N_tag_bits. The table below summarizes the relationship between the authentication tag size, the probability of success, and the maximum numbers of forgery attempts that can be permitted on our network. |==================+========================================| | Authentication | Probability any Successful Forgeries | | Tag Size |-------------+-------------+------------| | (octets) | 2^-10 | 2^-20 | 2^-30 | |==================+=============+=============+============| | 4 | 2^22 tries | 2^12 tries | 2^2 tries | |==================+=============+=============+============| | 8 | 2^54 tries | 2^44 tries | 2^34 tries | |==================+=============+=============+============| | 12 | 2^86 tries | 2^76 tries | 2^66 tries | |==================+=============+=============+============| | 16 | 2^118 tries | 2^108 tries | 2^98 tries | |==================+=============+=============+============| Table 1: Maximum allowable number of forgery attempts for McGrew Informational [Page 12] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 a given tag size and probability of success. 4. IANA Considerations RFC 4568 defines SRTP "crypto suites"; a crypto suite corresponds to a particular AEAD algorithm in SRTP. In order to allow SDP to signal the use of the algorithms defined in this document, IANA will register the following crypto suites into the subregistry for SRTP crypto suites under the SRTP transport of the SDP Security Descriptions: srtp-crypto-suite-ext = "AEAD_AES_128_GCM" / "AEAD_AES_256_GCM" / "AEAD_AES_128_GCM_8" / "AEAD_AES_256_GCM_8" / "AEAD_AES_128_GCM_12" / "AEAD_AES_256_GCM_12" / "AEAD_AES_128_CCM" / "AEAD_AES_256_CCM" / srtp-crypto-suite-ext draft-ietf-avt-dtls-srtp-05 defines a DTLS-SRTP "SRTP Protection Profile"; it also corresponds to the use of an AEAD algorithm in SRTP. In order to allow the use of the algorithms defined in this document in DTLS-SRTP, IANA will also register the following SRTP Protection Profiles: SRTP_AEAD_AES_128_GCM SRTP_AEAD_AES_256_GCM SRTP_AEAD_AES_128_GCM_8 SRTP_AEAD_AES_256_GCM_8 SRTP_AEAD_AES_128_GCM_12 SRTP_AEAD_AES_256_GCM_12 SRTP_AEAD_AES_128_CCM SRTP_AEAD_AES_256_CCM 5. Acknowledgements The author would like to thank Kevin Igoe and many other reviewers who provided valuable comments on earlier drafts of this document. 6. References 6.1. Normative References [CCM] Dworkin, M., "NIST Special Publication 800-38C: The CCM Mode for Authentication and Confidentiality", U.S. McGrew Informational [Page 13] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 National Institute of Standards and Technology http:// csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf. [GCM] Dworkin, M., "NIST Special Publication 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.", U.S. National Institute of Standards and Technology http:// csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption with Associated Data", RFC 5116, January 2008. [RFC5282] McGrew, D. and D. Black, "Using Authenticated Encryption Algorithms with the Encrypted Payload of the Internet Key Exchange version 2 (IKEv2) Protocol", RFC 5282, August 2008. 6.2. Informative References [BN00] Bellare, M. and C. Namprempre, "Authenticated encryption: Relations among notions and analysis of the generic composition paradigm", Proceedings of ASIACRYPT 2000, Springer-Verlag, LNCS 1976, pp. 531-545 http:// www-cse.ucsd.edu/users/mihir/papers/oem.html. [BOYD] Boyd, C. and A. Mathuria, "Protocols for Authentication and Key Establishment", Springer, 2003 . [CMAC] "NIST Special Publication 800-38B", http://csrc.nist.gov/ CryptoToolkit/modes/800-38_Series_Publications/ SP800-38B.pdf. [EEM04] Bellare, M., Namprempre, C., and T. Kohno, "Breaking and provably repairing the SSH authenticated encryption scheme: A case study of the Encode-then-Encrypt-and-MAC paradigm", ACM Transactions on Information and System Secu rity, http://www-cse.ucsd.edu/users/tkohno/papers/ TISSEC04/. [GR05] Garfinkel, T. and M. Rosenblum, "When Virtual is Harder than Real: Security Challenges in Virtual Machine Based Computing Environments", Proceedings of the 10th Workshop on Hot Topics in Operating Systems http:// www.stanford.edu/~talg/papers/HOTOS05/ virtual-harder-hotos05.pdf. [J02] Jonsson, J., "On the Security of CTR + CBC-MAC", Proceedings of the 9th Annual Workshop on Selected Areas on Cryptography, http://csrc.nist.gov/CryptoToolkit/modes/ McGrew Informational [Page 14] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 proposedmodes/ccm/ccm-ad1.pdf, 2002. [MODES] Dworkin, M., "NIST Special Publication 800-38: Recommendation for Block Cipher Modes of Operation", U.S. National Institute of Standards and Technology http:// csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf. [MV04] McGrew, D. and J. Viega, "The Security and Performance of the Galois/Counter Mode (GCM)", Proceedings of INDOCRYPT '04, http://eprint.iacr.org/2004/193, December 2004. [R02] Rogaway, P., "Authenticated encryption with Associated- Data", ACM Conference on Computer and Communication Security (CCS'02), pp. 98-107, ACM Press, 2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.html. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 4106, June 2005. [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", BCP 107, RFC 4107, June 2005. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP)", RFC 4309, December 2005. [RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity Transform Carrying Roll-Over Counter for the Secure Real- time Transport Protocol (SRTP)", RFC 4771, January 2007. McGrew Informational [Page 15] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 Author's Address David A. McGrew Cisco Systems, Inc. 510 McCarthy Blvd. Milpitas, CA 95035 US Phone: (408) 525 8651 Email: mcgrew@cisco.com URI: http://www.mindspring.com/~dmcgrew/dam.htm McGrew Informational [Page 16] Internet Draft AES-GCM and AES-CCM for SRTP Mar 06, 2009 Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). McGrew Informational [Page 17]