Network Working Group D. McGrew Internet Draft Cisco Systems, Inc. Intended Status: Informational K.M. Igoe Expires: December 27, 2012 National Security Agency June 25, 2012 AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP) draft-ietf-avtcore-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). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current. 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." This Internet-Draft will expire on December 27, 2012. Copyright Notice Copyright (c) 2012 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Igoe and McGrew Informational [Page 1] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 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.....................................................3 2. Conventions Used In This Document................................3 3. Overview of the SRTP/SRTCP Security Architecture.................4 4. Terminology......................................................4 5. Generic AEAD Processing..........................................5 5.1. Types of Input Data.........................................5 5.2. AEAD Invocation Inputs and Outputs..........................5 5.2.1. Encrypt Mode...........................................5 5.2.2. Decrypt Mode...........................................6 5.3. Handling of AEAD Authentication.............................6 6. Counter Mode Encryption..........................................6 7. Unneeded SRTP/SRTCP Fields.......................................7 7.1. SRTP/SRTCP Authentication Field.............................7 7.2. RTP Padding.................................................7 8. AES-GCM/CCM processing for SRTP..................................7 8.1. SRTP IV formation for AES-GCM and AES-CCM...................7 8.2. Data Types in SRTP Packets..................................8 8.3. Prevention of SRTP IV Reuse.................................9 9. AES-GCM/CCM Processing of SRTCP Compound Packets................10 9.1. SRTCP IV formation for AES-GCM and AES-CCM.................10 9.2. Data Types in Encrypted SRTCP Compound Packets.............10 9.3. Data Types in Unencrypted SRTCP Compound Packets...........12 9.4. Prevention of SRTCP IV Reuse...............................13 10. Constraints on AEAD for SRTP and SRTCP.........................13 10.1. Generic AEAD Parameter Constraints........................13 10.2. AES-GCM for SRTP/SRTCP....................................14 10.3. AES-CCM for SRTP/SRTCP....................................14 11. Key Derivation Functions.......................................15 12. Security Considerations........................................15 12.1. Handling of Security Critical Parameters..................15 12.2. Size of the Authentication Tag............................15 13. IANA Considerations............................................16 13.1. SDES......................................................16 13.2. DTLS......................................................17 13.3. MIKEY.....................................................19 14. Parameters for use with MIKEY..................................19 15. Acknowledgements...............................................20 16. References.....................................................21 16.1. Normative References......................................21 16.2. Informative References....................................22 Igoe and McGrew Informational [Page 2] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 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). It is important to note that the outgoing SRTP packets from a single endpoint may be originating from several independent data sources. 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 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 Counter with Cipher Block Chaining-Message Authentication Code (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 of operation (GCM) and the Counter with Cipher Block Chaining-Message Authentication Code mode of operation (CCM) are both 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 hardware, 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. 2. Conventions Used In This Document 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]. Igoe and McGrew Informational [Page 3] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 3. Overview of the SRTP/SRTCP Security Architecture SRTP/SRTCP security is based upon the following principles: a) Both privacy and authentication are based upon the use of symmetric algorithms. An AEAD algorithm such as AES-CCM and AES-GCM combines privacy and authentication into a single process. b) A secret master key is shared by all participating endpoints, both those originating SRTP/SRTCP packets and those receiving these packets. Any given master key MAY be used simultaneously by several endpoints to originate SRTP/SRTCP packets (as well one or more endpoints using this master key to process inbound data). c) A Key Derivation Function is applied to the shared master key value to form separate encryption keys, authentication keys and salting keys for SRTP and for SRTCP (a total of six keys). This process is described in sections 4.3.1 and 4.3.3 of [RFC3711]. Since AEAD algorithms such as AES-CCM and AES-GCM combine encryption and authentication into a single process, AEAD algorithms do not make use of the authentication keys. The master key MUST be at least as large as the encryption key derived from it. 4. Terminology The following terms have very specific meanings in the context of this RFC: Crypto Context For the purposes of this document, a crypto context is the outcome of any process which results in authentication of each endpoint in the SRTP session and possession by each endpoint of a shared secret master key. Various encryption keys, authentication keys and salts are derived from the master key. Aside from making modifications to IANA registries to allow AES-GCM and AES-CCM to work with SDES, DTLS and MIKEY, the details of how the master key is established are outside the scope of this document. Similarly any mechanism for rekeying an existing Cipher Context is outside the scope of the document. Instantiation In AEAD, an instantiation is an (Encryption_key, salt) pair together with all of the data structures (for example, counters) needed for it to function properly. In SRTP/SRTCP, each endpoint will need two instantiations of the AEAD algorithm for each master key in its possession, Igoe and McGrew Informational [Page 4] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 one for SRTP and one for SRTCP. 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. In many applications, each endpoint will have one master key for processing outbound data but may have one or more separate master keys for processing inbound data. 5. Generic AEAD Processing 5.1. Types of Input Data Associated Data This is data that is to be authenticated but not encrypted. Plaintext Data that is to be both encrypted and authenticated. Raw Data Data that is to be neither encrypted nor authenticated. Which portions of SRTP/SRTCP packets that are to be treated as associated data, which are to be treated as plaintext, and which are to be treated as raw data are covered in sections 8.2, 9.2 and 9.3. 5.2. AEAD Invocation Inputs and Outputs 5.2.1. Encrypt Mode Inputs: Encryption_key Octet string, either 16 or 32 octets long Initialization_Vector Octet string, 12 octets long Associated_Data Bit string of variable length Plaintext Bit string of variable length Tag_Size_Flag (CCM only*) One Octet Outputs Ciphertext Bit string, length = length(ciphertext)-tag_length (*) For GCM, the algorithm choice determines the tag size. AES-CCM uses a Tag_Size_Flag that has the hex value 5A if an 8-octet Igoe and McGrew Informational [Page 5] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 authentication tag is used, 6A if a 12-octet authentication tag is used, and 7A if a 16-octet authentication tag is used. 5.2.2. Decrypt Mode Inputs: Encryption_key Octet string, either 16 or 32 octets long Initialization_Vector octet string, 12 octets long Associated_Data Bit string of variable length Ciphertext Bit string of variable length Tag_Size_Flag (CCM only*) One Octet Outputs Plaintext Bit string, length = length(ciphertext)-tag_length Validity_Flag Boolean, TRUE if valid, FALSE otherwise (*) For GCM, the algorithm choice determines the tag size. AES-CCM uses a Tag_Size_Flag that 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. 5.3. Handling of AEAD Authentication AEAD requires that all incoming packets MUST pass AEAD authentication before any other action takes place. The ciphertext MUST NOT be decrypted until the AEAD tag has been validated. The associated data MUST NOT be released until the AEAD tag has been validated. Should the AEAD tag prove to be invalid, the incoming data is to be discarded and appropriate error flags raised. Local policy determines how these flags are to be handled and are outside the scope of this document. 6. Counter Mode Encryption Each outbound packet uses a 12 octet IV and encryption key to form a keystream of bits which is XORed to the plaintext to form cipher. Using the 12-octet IV and a 4-octet block counter, the keystream is formed in 16-octet blocks until it is at least as long as the plaintext, and any excess keystream bits are discarded. At the start of each packet, the block counter is initialized to 0x0000 for AES-CCM and to 0x0001 for AES-GCM. A key block is formed by key_block = AES_ENC( IV || block_counter; key=Encryption_key ) Igoe and McGrew Informational [Page 6] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 and the block counter is incremented. This allows for a per packet keystream of length of up to 2^36 octets for AES-CCM and up to 2^36-16 octets for AES-GCM. With any counter mode, if the same (IV, Encryption_key) pair is used twice, precisely the same keystream is formed. As explained in section 9.1 of RFC 3711, this is a cryptographic disaster. For AES-GCM, the consequences of such a reuse are even worse than explained in RFC 3711 because it would completely compromise the AES-GCM authentication mechanism. 7. Unneeded SRTP/SRTCP Fields AEAD counter mode encryption removes the need for certain existing SRTP/SRTCP mechanisms. 7.1. SRTP/SRTCP Authentication Field The AEAD message authentication mechanism MUST be the primary message authentication mechanism for AEAD SRTP/SRTCP. Additional SRTP/SRTCP authentication mechanisms SHOULD NOT be used with any AEAD algorithm and the optional SRTP/SRTCP Authentication Tags are NOT RECOMMENDED and SHOULD NOT be present. Note that this contradicts section 3.4 of [RFC3711] which makes the use of the SRTCP Authentication field mandatory, but the presence of the AEAD authentication renders the older authentication methods redundant. Rationale. Some applications use the SRTP/SRTCP 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. 7.2. RTP Padding Neither AES-GCM not AES-CCM requires that the data be padded out to a specific block size, reducing the need to ude the padding mechanism provided by RTP. It is RECOMENDED that the RTP padding mechanism not be used unless it is necessary to disguise the length of the underlying plaintext. 8. AES-GCM/CCM processing for SRTP 8.1. SRTP IV formation for AES-GCM and AES-CCM The 12 byte initialization vector used by both AES-GCM and AES-CCM SRTP is formed by first concatenating 2-octets of zeroes, the 4-octet Igoe and McGrew Informational [Page 7] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 SSRC, the 4-octer Rollover Counter (ROS) and the two octet sequence number SEQ. The resulting 12-octet value is then XORed to the 12-octet salt to form the 12-octet IV. 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: AES-GCM and AES-CCM SRTP Initialization Vector formation. Using the terminology of section 8.2.1. of [GCM], the first six octets of the IV are the fixed field and the last six bytes are the invocation field. 8.2. Data Types in SRTP Packets All SRTP packets MUST be both authenticated and encrypted. The data fields within the SRTP packets are broken into Associated Data, Plaintext and Raw Data as follows (see figure 2): Associated Data The version (2 bits), padding flag (1 bit), extension flag (1 bit), CSRC count (4 bits), sequence number (16 bits), timestamp (32 bits), SSRC (32 bits), optional contributing source identifiers (CSRCs, 32 bits each), and optional RTP extension (32 bits). Plaintext The RTP payload (variable length), RTP padding (if used, variable length), and RTP pad count ( if used, 8 bits). Raw Data The optional 32-bit SRTP MKI and the 32-bit SRTP authentication tag (whose use is NOT RECOMMENDED). Igoe and McGrew Informational [Page 8] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R : SRTP MKI (optional) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R : authentication tag (NOT RECOMMENDED) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P = Plaintext (to be encrypted and authenticated) A = Associated Data (to be authenticated only) R = neither encrypted nor authenticated Note: The RTP padding and RTP padding count fields are optional and are not recommended Figure 2: AEAD inputs from an SRTP packet. 8.3. Prevention of SRTP IV Reuse In order to prevent IV reuse, we must ensure that the (ROC,SEQ,SSRC) triple is never used twice with the same master key. There are two phases to this issue. Counter Management A rekey MUST be performed to establish a new master key before the (ROC,SEQ) pair cycles back to its original value. SSRC Management The set of all SSRC values must be partitioned into disjoint pools, one pool for each endpoint using the master key to originate outbound data. Each such endpoint MUST only issue SSRC values from the pool it has been assigned. Further, each endpoint MUST maintain a history of outbound SSRC identifiers that it has issued within the lifetime of the current master key, and when a new synchronization source requests Igoe and McGrew Informational [Page 9] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 an SSRC identifier it MUST NOT be given an identifier that has been previously issued. A rekey MUST be performed before its pool of SSRC values is exhausted. 9. AES-GCM/CCM Processing of SRTCP Compound Packets All SRTCP compound packets MUST be authenticated, but unlike SRTP, SRTCP packet encryption is optional. A sender can select which packets to encrypt, and indicates this choice with a 1-bit encryption flag (located just before the 31-bit SRTCP index) 9.1. SRTCP IV formation for AES-GCM and AES-CCM The 12 byte initialization vector used by both AES-GCM and AES-CCM SRTCP is formed by first concatenating 2-octets of zeroes, the 4-octet Synchronization Source identifier (SSRC), 2-octets of zeroes, a single zero bit, and the 31-bit SRTCP Index. The resulting 12-octet value is then XORed to the 12-octet salt to form the 12-octet IV. 0 1 2 3 4 5 6 7 8 9 10 11 +--+--+--+--+--+--+--+--+--+--+--+--+ |00|00| SSRC |00|00|0+SRTCP Idx|---+ +--+--+--+--+--+--+--+--+--+--+--+--+ | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Encryption Salt |->(+) +--+--+--+--+--+--+--+--+--+--+--+--+ | | +--+--+--+--+--+--+--+--+--+--+--+--+ | | Initialization Vector |<--+ +--+--+--+--+--+--+--+--+--+--+--+--+ Figure 3: SRTCP Initialization Vector formation. Using the terminology of section 8.2.1. of [GCM], the first eight octets of the IV are the fixed field and the last four bytes are the invocation field. 9.2. Data Types in Encrypted SRTCP Compound Packets When the encryption flag is set to 1, the SRTCP packet is broken into plaintext, associated data, and raw (untouched) data as listed below (see figure 4): Associated Data The packet version (2 bits), padding flag (1 bit), reception report count (5 bits), packet type (8 Igoe and McGrew Informational [Page 10] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 bits), length (2 octets), SSRC (4 octets), encryption flag (1 bit) and SRTCP index (31 bits). Raw Data The 32-bit optional SRTCP MKI index and 32-bit SRTCP authentication tag (whose use is NOT RECOMMENDED). Plaintext All other data. Note that the plaintext comes in one contiguous field. Since the AEAD cipher is larger than the plaintext by exactly the length of the AEAD authentication tag, the corresponding STRCP encrypted packet replaces the plaintext field with a slightly larger field containing the cipher. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R | SRTCP MKI (optional) index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R : authentication tag (NOT RECOMMENDED) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ P = Plaintext (to be encrypted and authenticated) A = Associated Data (to be authenticated only) R = neither encrypted nor authenticated Figure 4: AEAD SRTCP inputs when encryption flag = 1. Igoe and McGrew Informational [Page 11] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 9.3. Data Types in Unencrypted SRTCP Compound Packets When the encryption flag is set to 0, the SRTCP compound packet is broken into plaintext, associated data, and raw (untouched) data as follows (see figure 5): Plaintext None. Raw Data The 32-bit optional SRTCP MKI index and 32-bit SRTCP authentication tag (whose use is NOT RECOMMENDED). Associated Data All other data. Even though there is no plaintext in this RTCP packet, AEAD encryption returns a cipher field which is precisely the length of the AEAD authentication tag. This cipher is to be placed before the Encryption flag and the SRTCP index in the authenticated SRTCP packet. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R | SRTCP MKI (optional)index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R : authentication tag (NOT RECOMMENDED) : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A = Associated Data (to be authenticated only) R = neither encrypted nor authenticated Igoe and McGrew Informational [Page 12] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 Figure 5: AEAD SRTCP inputs when encryption flag = 0. 9.4. Prevention of SRTCP IV Reuse A new master key MUST be established before the 31-bit SRTCP index cycles back to its original value. Ideally a rekey performed should be performed and a new master key in place and well before the SRTCP index overflows. The comments on SSRC management in section 8.3 also apply. 10. Constraints on AEAD 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. 10.1. Generic AEAD Parameter Constraints All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three constraints listed below: 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 2^36-16 octets length per invocation C_max values less than 2232 are discouraged The upper bound on C_MAX are based on purely cryptographic considerations. The lower bound on C_MAX is obtained by subtracting away a 20-octet IP header, 8-octet UDP header, and 12-octet RTP header from the maximum transmission unit (MTU) of 2272. 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 Igoe and McGrew Informational [Page 13] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 for a given instantiation MUST be at most 2^31 for SRTCP Block Counter size MUST be 32 bits The reader is reminded that the plaintext is shorter than the ciphertext by exactly the length of the AEAD authentication tag. 10.2. 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] 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 SHOULD support both AEAD_AES_128_GCM_8 and AEAD_AES_256_GCM_8, and it MAY support the four other variants shown table 1. 10.3. AES-CCM for SRTP/SRTCP AES-CCM is another family of AEAD algorithms built around the AES block cipher algorithm. AES-CCM 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 8, 12 or 16 octets [RFC5116] AEAD_AES_256_CCM 32 octets 8, 12 or 16 octets [RFC5116] Any implementation of AES-CCM SRTP/SRTCP SHOULD support both AEAD_AES_128_CCM and AEAD_AES_256_CCM. AES-CCM uses a flag octet that conveys information about the length of the authentication tag, length of the block counter, and presence of additional authenticated data. For AES-CCM in SRTP/SRTCP, the Igoe and McGrew Informational [Page 14] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 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. 11. Key Derivation Functions A Key Derivation Function (KDF) is used to derive all of the required encryption and authentication keys from a secret value shared by the endpoints. Both the AEAD_AES_128_GCM algorithms and the AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key Derivation Function described in [RFC3711]. Both the AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST use the AES_256_CM_PRF Key Derivation Function described in [RFC 6188]. 12. Security Considerations 12.1. Handling of Security Critical Parameters As with any security process, the implementer must take care to ensure cryptographically sensitive parameters are properly handled. Many of these recommendations hold for all SRTP cryptographic algorithms, but we include them here to emphasize their importance. - If the master salt is to be kept secret, it MUST be properly erased when no longer needed. - The secret master key and all keys derived from it MUST be kept secret. All keys MUST be properly erased when no longer needed. - At the start of each packet, the block counter MUST be reset (to 0 for CCM, to 1 for GCM). The block counter is incremented after each block key has been produced, but it MUST NOT be allowed to exceed 2^32-1. - Each time a rekey occurs, the initial values of the SRTCP index and the values of all the SEQ counters MUST be saved. - Processing MUST cease if the 48-bit Packet Counter or the 31-bit SRTCP index cycles back to its initial value. Processing MUST NOT resume until a new SRTP/SRTCP session has been established using a new SRTP master key. Ideally, a rekey should be done well before either of these counters cycle. 12.2. Size of the Authentication Tag 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 Igoe and McGrew Informational [Page 15] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 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. Suppose an adversary wishes to introduce a forged or altered packet into a target network by randomly selecting an authentication value until by chance they hit a valid authentication tag. The table below summarizes the relationship between the number of forged packets the adversary has tried, the size of the authentication tag, and the probability of a compromise occurring (i.e. at least one of the attempted forgeries having a valid authentication tag). The reader is reminded that the forgery attempts can be made over the entire network, not just a single link, and that frequently changing the key does not decrease the probability of a compromise occurring. +==================+========================================+ | Authentication | Probability of a Compromise Occurring | | Tag | for a given number of forgery attempts | | Size |------------+-------------+-------------| | (octets) | prob=2^-30 | prob=2^-20 | prob=2^-10 | |==================+=============+=============+============| | 4 | 2^2 tries | 2^12 tries | 2^22 tries | |==================+============+=============+=============| | 8 | 2^34 tries | 2^44 tries | 2^54 tries | |==================+============+=============+=============| | 12 | 2^66 tries | 2^76 tries | 2^86 tries | |==================+============+=============+=============| | 16 | 2^98 tries | 2^108 tries | 2^118 tries | +=================+============+=============+==============+ Table 3: Probability of a compromise occurring for a given number of forgery attempts and tag size. 13. IANA Considerations 13.1. SDES Security description [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" / Igoe and McGrew Informational [Page 16] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 "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 13.2. DTLS DTLS-SRTP [RFC5764] 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, we request IANA register the following SRTP Protection Profiles: AEAD_AES_128_CCM cipher: AES_128_CCM cipher_key_length: 128 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_256_CCM cipher: AES_256_CCM cipher_key_length: 256 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_128_CCM_8 cipher: AES_128_CCM cipher_key_length: 128 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_256_CCM_8 cipher: AES_256_CCM cipher_key_length: 256 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_128_CCM_12 cipher: AES_128_CCM cipher_key_length: 128 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets Igoe and McGrew Informational [Page 17] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 AEAD_AES_256_CCM_12 cipher: AES_256_CCM cipher_key_length: 256 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_128_CCM cipher: AES_128_CCM cipher_key_length: 128 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_256_CCM cipher: AES_256_CCM cipher_key_length: 256 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_128_GCM_8 cipher: AES_128_GCM cipher_key_length: 128 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_256_GCM_8 cipher: AES_256_GCM cipher_key_length: 256 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_128_GCM_12 cipher: AES_128_GCM cipher_key_length: 128 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets AEAD_AES_256_GCM_12 cipher: AES_256_GCM cipher_key_length: 256 bits cipher_salt_length: 96 bits maximum lifetime: at most 2^31 SRTCP packets and at most 2^48 SRTP packets Note that these SRTP Protection Profiles do not specify an Igoe and McGrew Informational [Page 18] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 auth_function, auth_key_length, or auth_tag_length because all of these profiles use AEAD algorithms, and thus do not use a separate auth_function, auth_key, or auth_tag. 13.3. MIKEY In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830], IANA maintains several Payload Name Spaces under Multimedia Internet KEYing (MIKEY). This document requires dditions to two of the lists maintained under MIKEY Security Protocol Parameters. On the SRTP policy Type/Value list (derived from Table 6.10.1.a of [RFC3830]) we request the following addition: Type | Meaning | Possible values ---------------------------------------------------------------- TBD | AEAD authentication tag length | 8, 12, or 16 (in octets) On the Encryption Algorithm List (derived from Table 6.10.1.b of [RFC3830]) we request the following additions: SRTP encr alg. | Value | Default Session Encr. Key Length ----------------------------------------------------------- AES-CCM | TBD | 16 octets AES-GCM | TBD | 16 octets The SRTP encryption algorithm, session encryption key length, and AEAD authentication tag values received from MIKEY fully determine the AEAD algorithm (e.g., AEAD_AES_256_GCM_8). The exact mapping is described in section 14. 14. Parameters for use with MIKEY MIKEY specifies the algorithm family separately from the key length (which is specified by the Session Encryption key length ) and the authentication tag length (specified by AEAD Auth. tag length). +------------+-------------+---------------+ | Encryption | Encryption | AEAD Auth. | | Algorithm | Key Length | Tag Length | +============+=============+===============+ AEAD_AES_128_GCM | AES-GCM | 16 | 16 | +------------+-------------+---------------+ AEAD_AES_128_CCM | AES-CCM | 16 | 16 | +------------+-------------+---------------+ AEAD_AES_128_GCM_12 | AES-GCM | 16 | 12 | +------------+-------------+---------------+ AEAD_AES_128_CCM_12 | AES-CCM | 16 | 12 | Igoe and McGrew Informational [Page 19] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 +------------+-------------+---------------+ AEAD_AES_128_GCM_8 | AES-GCM | 16 | 8 | +------------+-------------+---------------+ AEAD_AES_128_CCM_8 | AES-CCM | 16 | 8 | +------------+-------------+---------------+ AEAD_AES_256_GCM | AES-GCM | 32 | 16 | +------------+-------------+---------------+ AEAD_AES_256_CCM | AES-CCM | 16 | 16 | +------------+-------------+---------------+ AEAD_AES_256_GCM_12 | AES-GCM | 32 | 12 | +------------+-------------+---------------+ AEAD_AES_256_CCM_12 | AES-CCM | 16 | 12 | +------------+-------------+---------------+ AEAD_AES_256_GCM_8 | AES-GCM | 32 | 8 | +------------+-------------+---------------+ AEAD_AES_256_CCM_8 | AES-CCM | 16 | 8 | +============+=============+===============+ Table 4: Mapping MIKEY parameters to AEAD algorithm Section 11 in this document restricts the choice of Key Derivation Function for AEAD algorithms. To enforce this restriction in MIKEY, we require that the SRTP PRF has value AES-CM whenever an AEAD algorithm is used. Note that, according to Section 6.10.1 in [RFC3830], the key length of the Key Derivation Function (i.e. the SRTP master key length) is always equal to the session encryption key length. This means, for example, that AEAD_AES_256_GCM will use AES_256_CM_PRF as the Key Derivation Function. 15. Acknowledgements The authors would like to thank Michael Peck, Michael Torla, Qin Wu, Magnus Westerland, Oscar Ohllson and many other reviewers who provided valuable comments on earlier drafts of this document. Igoe and McGrew Informational [Page 20] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 16. References 16.1. Normative References [CCM] Dworkin, M., "NIST Special Publication 800-38C: The CCM Mode for Authentication and Confidentiality", U.S. 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. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,and Norrman, K, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004. [RFC4568] Andreasen, F., Baugher, M., and D.Wing, "Session Description Protocol (SDP): Security Descriptions for Media Streams", RFC 4568, July 2006. [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. [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. [RFC6188] McGrew,D.,"The Use of AES-192 and AES-256 in Secure RTP" RFC 6811, March 2011 Igoe and McGrew Informational [Page 21] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 16.2. 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/ 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. Igoe and McGrew Informational [Page 22] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 [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. Igoe and McGrew Informational [Page 23] Internet Draft AES-GCM and AES-CCM for SRTP Jun 25, 2012 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 Kevin M. Igoe NSA/CSS Commercial Solutions Center National Security Agency EMail: kmigoe@nsa.gov Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Igoe and McGrew Informational [Page 24]