Network Working Group D. McGrew Internet-Draft Cisco Systems, Inc. Expires: April 26, 2007 October 23, 2006 An Interface and Algorithms for Authenticated Encryption draft-mcgrew-auth-enc-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 April 26, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This draft defines algorithms for authenticated encryption with additional authenticated data (AEAD), and defines a uniform interface and a registry for such algorithms. The interface and registry can be used as an application independent set of cryptoalgorithm suites. This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. McGrew Expires April 26, 2007 [Page 1] Internet-Draft Authenticated Encryption October 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Benefits . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.4 Conventions Used In This Document . . . . . . . . . . . . 4 2. AEAD Interface . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Authenticated Encryption . . . . . . . . . . . . . . . . . 5 2.2 Authenticated Decryption . . . . . . . . . . . . . . . . . 7 2.3 Data Formatting . . . . . . . . . . . . . . . . . . . . . 7 3. Recommended Nonce Formation . . . . . . . . . . . . . . . . . 8 4. Requirements on the use of AEAD algorithms . . . . . . . . . . 10 5. Requirements on AEAD algorithms . . . . . . . . . . . . . . . 11 6. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 12 6.1 AEAD_AES_128_GCM . . . . . . . . . . . . . . . . . . . . . 12 6.1.1 AEAD_AES_256_GCM . . . . . . . . . . . . . . . . . . . 12 6.2 AEAD_AES_128_CCM . . . . . . . . . . . . . . . . . . . . . 12 6.2.1 AEAD_AES_256_CCM . . . . . . . . . . . . . . . . . . . 13 6.3 AEAD_AES_128_HMAC_SHA1 . . . . . . . . . . . . . . . . . . 13 6.3.1 Test Cases . . . . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8. Open Questions . . . . . . . . . . . . . . . . . . . . . . . . 17 9. Security Considerations . . . . . . . . . . . . . . . . . . . 19 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 20 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 11.1 Normative References . . . . . . . . . . . . . . . . . . . 21 11.2 Informative References . . . . . . . . . . . . . . . . . . 21 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 22 Intellectual Property and Copyright Statements . . . . . . . . 23 McGrew Expires April 26, 2007 [Page 2] Internet-Draft Authenticated Encryption October 2006 1. Introduction Authenticated encryption 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, adds the ability to check the integrity and authenticity of some additional "associated data" that is not encrypted. 1.1 Background Many cryptographic applications require both confidentiality and message authentication. Often an encryption method and a message authentication code (MAC) are used to provide those security services, with each algorithm using an independent key. More recently, the idea of providing both security services using a single cryptoalgorithm has become accepted. In this concept, the cipher and MAC are replaced by an Authenticated Encryption with Associated Data (AEAD) algorithm. Several crypto algorithms that implement AEAD algorithms have been defined, including block cipher modes of operation and dedicated algorithms. Some of these algorithms have been adopted and proven useful in practice. Additionally, AEAD is close to an 'idealized' view of encryption, such as those used in the automated analysis of cryptographic protocols. 1.2 Scope In this document we define an AEAD algorithm as an abstraction, by specifying an interface to an AEAD and defining an IANA registry for AEAD algorithms. We populate this registry with five AEAD algorithms: AES in Galois/Counter Mode [GCM] with 128 and 256 bit keys, AES in Counter and CBC MAC mode [CCM] with 128 and 256 bit keys, and an algorithm that composes AES-128 CBC and HMAC-SHA1. This approach enables applications that need cryptographic security services to more easily adopt those services. In the following, we define the AEAD interface (Section 2), and then outline the requirements that each AEAD algorithm must meet (Section 5) and provide guidance on the use of AEAD algorithms (Section 4). Then we define five AEAD algorithms (Section 6), and establish an IANA registry for AEAD algorithms (Section 7). Lastly, we discuss some open questions (Section 8). The AEAD interface specification does not address security protocol issues such as anti-replay services or access control decisions that are made on authenticated data. Instead, the specification aims to abstract the cryptography away from those issues. The interface, and the guidance about how to use it, are consistent with the McGrew Expires April 26, 2007 [Page 3] Internet-Draft Authenticated Encryption October 2006 recommendations from [EEM04]. 1.3 Benefits The approach benefits the application designer by allowing them to focus on the important issues of security services, canonicalization, and data marshaling, and relieving them of the need to design crypto mechanisms that meet their security goals. Importantly, the security of an AEAD algorithm can be analyzed independent from its use in a particular application. This property frees the user of the AEAD of the need to consider security aspects such as the relative order of authentication and encryption and the security of the particular combination of cipher and MAC, such as the potential loss of confidentiality through the MAC. The application designer that uses the AEAD interface need not select a particular AEAD algorithm during the design stage. Additionally, the interface to the AEAD is relatively simple, since it requires only a single key as input and it requires only a single identifier to indicate the algorithm in use in a particular case. The AEAD approach benefits the implementer of the crypto algorithms by making available optimizations that are otherwise not possible to reduce the amount of computation, the implementation cost, and/or the storage requirements. The simpler interface makes testing easier; this is a considerable benefit for a crypto algorithm implementation. By providing a uniform interface to access cryptographic services, the AEAD approach allows a single crypto implementation to easily support multiple applications. For example, a hardware module that supports the AEAD interface can easily provide crypto acceleration to any application using that interface, even to applications that had not been designed when the module was built. 1.4 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]. McGrew Expires April 26, 2007 [Page 4] Internet-Draft Authenticated Encryption October 2006 2. AEAD Interface An AEAD algorithm has two operations, authenticated encryption and authenticated decryption. The inputs and outputs of these algorithms are defined in terms of octet strings. An implementation MAY accept additional inputs. For example, an input could be provided to allow the user to select between different implementation strategies. However, such extensions MUST NOT affect interoperability. 2.1 Authenticated Encryption The authenticated encryption operation has four inputs, each of which is an octet string: A secret key K, which MUST be generated in a way that is uniformly random or pseudorandom. An nonce N. Each nonce provided to distinct invocations of the Authenticated Encryption operation MUST be distinct, for any particular value of the key; applications SHOULD use the nonce formation method defined in Section 3, and MAY use any other method that meets this requirement. A plaintext P, which contains the data to be encrypted and authenticated, The additional authenticated data A, which contains the data to be authenticated, but not encrypted. There is a single output: A ciphertext C, which is as least as long as the plaintext, or an indication that the requested encryption operation could not be performed. All of the inputs and outputs are variable-length octet strings, whose lengths obey the following restrictions: The number of octets in the key K is between one and 256. For each AEAD algorithm, the length of K MUST be fixed. The number of octets in the nonce is between one and 2^64 - 1, inclusive. However, the length SHOULD be twelve (12) octets. McGrew Expires April 26, 2007 [Page 5] Internet-Draft Authenticated Encryption October 2006 The number of octets in the plaintext P is between zero and 2^64 - 1, inclusive. The number of octets in the additional authenticated data AAD is between zero and 2^64 - 1, inclusive. The number of octets in the ciphertext C is between zero and 2^64 + 255. An AEAD algorithm MAY further restrict the lengths of its inputs and outputs. A particular AEAD implementation MAY further restrict the lengths of its inputs and outputs. If a particular implementation of an AEAD algorithm is requested to process an input that is outside the range of admissible lengths, or an input that is outside the range of lengths supported by that implementation, it MUST return an error code and it MUST NOT output any other information. In particular, partially encrypted or partially decrypted data MUST NOT be returned. Both confidentiality and message authentication is provided on the plaintext P. When the length of P is zero, the AEAD algorithm acts as a Message Authentication Code on the input A. The additional authenticated data A is used to protect information that needs to be authenticated, but which does not need to be kept confidential. When using an AEAD to secure a network protocol, for example, this input could include addresses, ports, sequence numbers, protocol version numbers, and other fields that indicate how the plaintext or ciphertext should be handled, forwarded, or processed. In many situations, it is desirable to authenticate these fields, though they must be left in the clear to allow the network or system to function properly. When this data is included in the input A, authentication is provided without copying the data into the ciphertext. The nonce is authenticated internally to the algorithm, and it is not necessary to include it in the AAD input. The nonce MAY be included in P or A if it convenient to the application. The nonce MAY be transported along with the plaintext. The entire IV need not be transmitted; it is sufficient to provide the receiver with enough information to allow the nonce to be reconstructed. Because the authenticated decryption process detects incorrect nonce values, no security failure results when a receiver incorrectly reconstructs an IV. Any such reconstruction method will need to take into account the possible loss or reorder of ciphertexts between the encryption and decryption processes. McGrew Expires April 26, 2007 [Page 6] Internet-Draft Authenticated Encryption October 2006 Applications MUST NOT assume any particular structure or formatting of the ciphertext. 2.2 Authenticated Decryption The authenticated decryption operation has four inputs: K, IV , A and C, as defined above. It has only a single output, either a plaintext value P or a special symbol FAIL that indicates that the inputs are not authentic. A ciphertext C , nonce N , and additional authenticated data A are authentic for key K when IV, and C is generated by the encrypt operation with inputs K, IV, P and A, for some values of IV, P, and A. The authenticated decrypt operation will, with high probability, return FAIL whenever its inputs were not created by the encrypt operation with the identical key (assuming that the AEAD algorithm is secure). 2.3 Data Formatting This document does not specify any particular encoding for the AEAD inputs and outputs, since the encoding does not affect the security services provided by an AEAD algorithm. When formatting a ciphertext, an application SHOULD position the ciphertext C so that it appears after any other data that is needed to construct the other inputs to the Authenticated Decryption operation. For example, if the nonce and ciphertext both appear in a packet, the former value should precede the latter. This rule facilitates efficient and simple implementations of AEAD algorithms. McGrew Expires April 26, 2007 [Page 7] Internet-Draft Authenticated Encryption October 2006 3. Recommended Nonce Formation It is essential for security that the nonces be constructed in a manner that respects the requirements that each nonce value be distinct for each invocation of the Authenticated Encryption operation, for any fixed value of the key. Special attention must be paid to the case in which there are multiple encryption devices using a single key. The following method to construct nonces is RECOMMENDED. The nonce is formatted as illustrated in Figure 1, with the initial octets consisting of a Fixed field, and the final octets consisting of a Counter field. Both fields have variable lengths. <----- variable ----> <----------- variable -----------> +---------------------+----------------------------------+ | Fixed | Counter | +---------------------+----------------------------------+ Figure 1: Recommended nonce format. The Counter field is regarded as an unsigned integer in network byte order. The Counter part is equal to one for the first nonce, and it increments by one for each successive nonce that is generated. The integer part is never equal to the all-zero value. Thus at most 2^(8*C) - 1 nonces can be generated when the Counter field is C octets in length. The Fixed field MUST remain constant for all nonces that are generated for a given encryption device. However, if different devices are performing encryption with a single key, then each distinct device MUST use a distinct Fixed field, to ensure the uniqueness of the nonces. Thus at most 2^(8*F) distinct senders can share a key when the Fixed field is F octets in length. When the number of encrypters is less than this value, the initial octets of the Fixed fields MUST be chosen so that they are common across all encrypters. The final octets of the Fixed field will need to be distinct across all encrypters. This substructure is shown in Figure 2. In some cases it is desirable to not transmit or store an entire nonce, but instead to reconstruct that value immediately prior to decryption. In these cases, it is RECOMMENDED that the distinct final octets of the Fixed field, and the Counter field, be explicitly transmitted or stored, while the common initial octets of the Fixed field be stored with the key. The explicit information is shown in Figure 2. However, applications MAY use any method of reconstructing nonces that is convenient. McGrew Expires April 26, 2007 [Page 8] Internet-Draft Authenticated Encryption October 2006 <------------- Fixed field ------------> +-------------------+--------------------+---------------+ | Fixed (common) : Fixed (distinct) | Counter | +-------------------+--------------------+---------------+ <------------ explicit -------------> Figure 2: Nonce structure and reconstruction Rationale. This method of nonce construction incorporates the best known practice, and facilitates the reconstruction of nonces from partial explicit information. It is used by both GCM ESP [RFC4106] and CCM ESP [RFC4309], in which the Fixed field contains the Salt value and the lowest eight octets of the nonce are explicitly carried in the ESP packet. In GCM ESP, the Fixed field must be at least four octets long, so that it can contain the Salt value. In CCM ESP, the Fixed field must be at least three octets long for the same reason. This nonce generation method is also used by several counter mode variants including CTR ESP. McGrew Expires April 26, 2007 [Page 9] Internet-Draft Authenticated Encryption October 2006 4. Requirements on the use of AEAD algorithms This section provides advice that must be followed in order to use an AEAD algorithm securely. If the AAD input is constructed out of multiple data elements, then it is essential that it be unambiguously parseable into its constituent elements, without the use of any unauthenticated data in the parsing process. This requirement ensures that an attacker cannot fool a receiver into accepting a bogus set of data elements as authentic by altering a set of data elements that were used to construct an AAD input in an authenticated encryption operation in such a way that the data elements are different, but the AAD input is unchanged. This requirement is trivially met if the AAD is composed of fixed-width elements. If the AAD contains a variable-length string, for example, this requirement can be met by also including the length of the string in the AAD. Similarly, if the plaintext is constructed out of multiple data elements, then it is essential that it be unambiguously parseable into its constituent elements, without using any unauthenticated data in the parsing process. Note that data that included in the AAD may be used when parsing the plaintext, though of course since the AAD is not encrypted there is a potential loss of confidentiality when information about the plaintext is included in the AAD. McGrew Expires April 26, 2007 [Page 10] Internet-Draft Authenticated Encryption October 2006 5. Requirements on AEAD algorithms Each AEAD algorithm MUST only accept keys with a fixed key length K_LEN, and MUST NOT require any particular data format for the keys provided as input. An algorithm that requires such structure internally (e.g. one with subkeys in a particular parity-check format) will need to provide it internally. Each AEAD algorithm MUST accept any plaintext with a length between zero and P_MAX octets, where the value P_MAX is specific to that algorithm. Each AEAD algorithm MUST accept any additional authenticated data with a length between zero and A_MAX octets, where the value A_MAX is specific to that algorithm. Each AEAD algorithm MUST accept any IV with a length between N_MIN and N_MAX octets, where the values of N_MIN and N_MAX are specific to that algorithm. Each algorithm SHOULD accept an IV with a length of twelve (12) octets. An AEAD algorithm MAY structure its ciphertext output in any way; for example, the ciphertext can incorporate an authentication tag. Each algorithm SHOULD choose a structure that is amenable to efficient processing. An Authenticated Encryption algorithm MAY incorporate a random source, e.g. for the generation of an internal initialization vector that is incorporated into the ciphertext output. An algorithm that uses an internal random initialization vector in this manner MAY have a value of N_MAX that is equal to zero. The specification of an AEAD algorithm MUST include the values of K_LEN, P_MAX, A_MAX, N_MIN, and N_MAX defined above. Additionally, it MUST specify the number of octets in the largeset possible ciphertext, which we denote C_MAX. Each AEAD algorithm MUST provide a description relating the length of the plaintext to that of the ciphertext. McGrew Expires April 26, 2007 [Page 11] Internet-Draft Authenticated Encryption October 2006 6. AEAD Algorithms This section defines five AEAD algorithms; two are based on AES GCM, two are based on AES CCM, and one is based on a composition of AES CBC and HMAC SHA1. Each pair includes an algorithm with a key size of 128 bits and one with a key size of 256 bits. 6.1 AEAD_AES_128_GCM The AEAD_AES_128_GCM authenticated encryption algorithm works as specified in [GCM], by providing the key, nonce, and plaintext, and additional authenticated data to that mode of operation. An authentication tag with a length of 16 octets (128 bits) is used. The AEAD_AES_128_GCM ciphertext is formed by appending the authentication tag provided as an output to the GCM encryption operation to the ciphertext that is output by that operation. Test cases are provided in the appendix of [GCM]. The input and output lengths are as follows: K_LEN is 16 octets, P_MAX is 2^36 - 31 octets, A_MAX is 2^61 - 1 octets, N_MIN is 1 (one) octet and N_MAX is 2^61 -1 octets; applications SHOULD use an nonce length of 12 octets, since GCM is optimized for that length, C_MAX is 2^36 - 15 octets. 6.1.1 AEAD_AES_256_GCM This algorithm is identical to AEAD_AES_128_GCM, but with the following differences: K_LEN is 32 octets, instead of 16 octets, and AES-256 GCM is used instead of AES-128 GCM. 6.2 AEAD_AES_128_CCM The AEAD_AES_128_CCM authenticated encryption algorithm works as specified in [CCM], by providing the key, nonce, additional authenticated data, and plaintext to that mode of operation. The formatting and counter generation function are as specified in McGrew Expires April 26, 2007 [Page 12] Internet-Draft Authenticated Encryption October 2006 Appendix A of that reference, and the values of the parameters identified in that appendix are as follows: the nonce length n is 12, the tag length t is 16, and the value q is 3. An authentication tag with a length of 16 octets (128 bits) is used. The AEAD_AES_128_CCM ciphertext is formed by appending the authentication tag provided as an output to the CCM encryption operation to the ciphertext that is output by that operation. Test cases are provided in [CCM]. The input and output lengths are as follows: K_LEN is 16 octets, P_MAX is 2^24 - 1 octets, A_MAX is 2^64 - 1 octets, N_MIN and N_MAX are both 12 octets, and C_MAX is 2^24 + 15 octets. 6.2.1 AEAD_AES_256_CCM This algorithm is identical to AEAD_AES_128_CCM, but with the following differences: K_LEN is 32 octets, instead of 16, and AES-256 CCM is used instead of AES-128 CCM. 6.3 AEAD_AES_128_HMAC_SHA1 This algorithm random and stateless. It is based on the "generic composition" of CBC encryption with HMAC authentication, with the the encrypt-then-MAC method [AE]. It uses the HMAC message authentication code [RFC2104] with the SHA-1 hash function [SHA1] to provide message authentication. Test cases for HMAC_SHA1 are provided in [RFC2202]. For encryption, it uses AES-128 in the cipher block chaining (CBC) mode of operation as defined in Section 6.2 of [MODES], with the padding method defined by Appendix A of the same reference. The input key is 128 bits long, and the CBC IV is McGrew Expires April 26, 2007 [Page 13] Internet-Draft Authenticated Encryption October 2006 generated uniformly at random, and is also 128 bits long. The authenticated encryption algorithm is as follows, or uses an equivalent set of steps: 1. Generate the secondary keys MAC_KEY and ENC_KEY from the input key K as follows: MAC_KEY = HMAC_SHA1(K, "MAC"); ENC_KEY = leftmost(HMAC_SHA1(K, "ENC"), 128); where the function leftmost(X, m) accepts a bitstring X and a non-negative integer m and returns the bitstring containing the leftmost m bits of X. MAC_KEY is 160 bits long, and ENC_KEY is 128 bits long. 2. Generate a 128-bit IV uniformly at random. (A pseudorandom process MAY be used if its strength is equivalent to AES-128.) Note that this IV is distinct from the nonce provided as an input to the authenticated encryption operation. 3. Pad the plaintext by appending a single '1' bit to that data and then appending to the resulting string as few '0' bits as are necessary to make the number of bits in the plaintext into a multiple of 128. Note that padding MUST be added to the data; if the number of octets in the payload data is a multiple of 16, then 16 octets of padding will be added. 4. Encrypt the payload using AES-128 in CBC mode, using the ENC_KEY and the random IV. Form the ciphertext by prepending the IV to the CBC ciphertext outputs. 5. Compute the authentication tag by applying HMAC_SHA1 to the AAD, the length of the AAD expressed as a 64-bit unsigned integer in network byte order, the IV, and the ciphertext, in that order, using the MAC_KEY. 6. Return the ciphertext and the authentication tag. The authenticated decryption algorithm is as follows, or uses an equivalent set of steps: 1. Generate the secondary keys MAC_KEY and ENC_KEY from the input key K as follows: MAC_KEY = HMAC_SHA1(K, "MAC"); McGrew Expires April 26, 2007 [Page 14] Internet-Draft Authenticated Encryption October 2006 ENC_KEY = leftmost(HMAC_SHA1(K, "ENC"), 128); 2. Compute the MAC value by applying HMAC_SHA1 to the AAD, he length of the AAD expressed as a 64-bit unsigned integer in network byte order, the IV, and the ciphertext, in that order, using the MAC_KEY. Compare this value to the authentication tag. If they match, then continue with the processing. Otherwise, discard the data and return FAIL. 3. Decrypt the payload using AES-128 in CBC mode, with the ENC_KEY, using the first 128 bits of the ciphertext as the random IV. 4. Remove padding by deleting the final '1' bit and all of the following '0' bits. The remaining data forms the payload data. 5. Return the plaintext. The length of the ciphertext can be inferred from that of the plaintext. The number L of octets in the ciphertext is given by L = 16 * ( floor(M / 16) + 2) where M denotes the number of octets in the payload, and the function floor() rounds its argument down to the nearest integer. This fact is needed by the encoding function, since the length of the ciphertext, rather than the length of the payload, must be authenticated. The lengths of the inputs are restricted as follows: K_LEN is 16 octets, P_MAX is 2^32 - 1 octets, A_MAX is 2^64 - 1 octets, N_MIN and N_MAX are both zero octets, and C_MAX is 2^32 + 19 octets. 6.3.1 Test Cases A future version of this document will include test cases for AEAD_AES_128_HMAC_SHA1. McGrew Expires April 26, 2007 [Page 15] Internet-Draft Authenticated Encryption October 2006 7. IANA Considerations IANA will define the "AEAD Registry" described below. Additions and changes to the AEAD Registry are by expert review. Each entry in the registry contains the following elements: a short name, such as "AEAD_AES_128_GCM", that starts with the string "AEAD", a positive number, and a reference to a specification that completely defines an AEAD algorithm and provides test cases that can be used to verify the correctness of an implementation. Requests to add an entry to the registry MUST include the name and the reference. The number is assigned by IANA. These number assignments SHOULD use the smallest available positive number. IANA will add the following five entries to the AEAD Registry: +----------------------------+---------------+--------------------+ | Name | Reference | Numeric Identifier | +----------------------------+---------------+--------------------+ | AEAD_AES_128_GCM | Section 6.1 | 1 | | | | | | AEAD_AES_256_GCM | Section 6.1.1 | 2 | | | | | | AEAD_AES_128_CCM | Section 6.2 | 3 | | | | | | AEAD_AES_256_CCM | Section 6.2.1 | 4 | | | | | | AEAD_AES_128_CBC_HMAC_SHA1 | Section 6.3 | 5 | +----------------------------+---------------+--------------------+ McGrew Expires April 26, 2007 [Page 16] Internet-Draft Authenticated Encryption October 2006 8. Open Questions The additional authenticated data input is well suited to authenticating headers. Some cryptographic protocols have trailers that should be authenticated. For example, in the Secure RTP protocol the authenticated data consists of the RTP header, the ciphertext containing the encrypted payload, and some additional data, in that order. It is impossible for an AEAD to accommodate both the authenticated header and authenticated trailer without adding an additional input for the trailer. Because none of the specified AEAD algorithms can handle both a trailer and a footer, this specification does not include a trailer in its interface. We expect that protocols like SRTP will need to define different processing rules that include all of the authenticated-only data into a single AAD input in order to make use of this specification. However, new rules would need to be defined in order to use either GCM or CCM or any other AEAD transforms, so this need is not especially onerous. The TLS protocol as currently defined assumes that authentication will precede encryption. Thus, in order to accommodate this specification, new processing rules would need to be written that make no assumptions about the relative ordering of the cryptographic services. However, as above, these new rules would need to be defined anyway in order to use any AEAD algorithm. The AEAD algorithms selected reflect those that have been already adopted by standards. It is an open question as to what other AEAD algorithms should be added. Many variations on basic algorithms are possible, each with its own advantages. While it is desirable to admit any algorithms that are found to be useful in practice, it is also desirable to limit the total number of registered algorithms. The current specification requires that a registered algorithm provide a complete specification and a set of validation data; it is hoped that these prerequisites set the admission criteria appropriately. Some users may view an IANA assignment as a recommendation or an endorsement of a particular AEAD algorithm. Other users may desire to register an AEAD algorithm in order to allow for experimental or specialized use. Because of these conflicting perspectives, it may be desirable to allocate a second IANA registry for experimental use. It may be desirable to replace HMAC-SHA1 with AES CMAC [CMAC] in the generic composition algorithm, or to introduce an additional algorithm that does so. Directly testing a randomized AEAD encryption algorithm using a test McGrew Expires April 26, 2007 [Page 17] Internet-Draft Authenticated Encryption October 2006 cases with fixed inputs and outputs is not possible, since the encryption process is non-deterministic. However, it is easy to test a randomized AEAD algorithm against fixed test cases. The authenticated decryption algorithm is deterministic, and it can be directly tested. The authenticated encryption algorithm can be tested by encrypting a plaintext, decrypting the resulting ciphertext, and comparing the original plaintext to the post- decryption plaintext. Some of the terminology in this specification is unwieldy, and could perhaps be improved. For example, "AEAD algorithm" could be replaced with "crypto transform", which would be meaningful to a broader community, but is less precise. McGrew Expires April 26, 2007 [Page 18] Internet-Draft Authenticated Encryption October 2006 9. Security Considerations A future version of this document will define the security services that must be provided by an AEAD algorithm. McGrew Expires April 26, 2007 [Page 19] Internet-Draft Authenticated Encryption October 2006 10. Acknowledgments Many reviewers provided valuable comments on earlier drafts of this document. McGrew Expires April 26, 2007 [Page 20] Internet-Draft Authenticated Encryption October 2006 11. References 11.1 Normative References [CCM] "NIST Special Publication 800-38C: The CCM Mode for Authentication and Confidentiality", http://csrc.nist.gov/publications/nistpubs/SP800-38C.pdf. [GCM] McGrew, D. and J. Viega, "The Galois/Counter Mode of Operation (GCM)", Submission to NIST. http:// csrc.nist.gov/CryptoToolkit/modes/proposedmodes/gcm/ gcm-spec.pdf, January 2004. [MODES] "NIST Special Publication 800-38", Reccomendation for Block Cipher Modes of Operation http://csrc.nist.gov/ publications/nistpubs/800-38a/sp800-38a.pdf. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and HMAC- SHA-1", RFC 2202, September 1997. [RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 4106, June 2005. [RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP)", RFC 4309, December 2005. [SHA1] "FIPS 180-1: Secure Hash Standard,", Federal Information Processing Standard (FIPS) http://www.itl.nist.gov/fipspubs/fip180-1.htm. 11.2 Informative References [AE] "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. [CMAC] "NIST Special Publication 800-38B", http://csrc.nist.gov/ CryptoToolkit/modes/800-38_Series_Publications/ McGrew Expires April 26, 2007 [Page 21] Internet-Draft Authenticated Encryption October 2006 SP800-38B.pdf. [EEM04] "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 Security, http://www-cse.ucsd.edu/users/tkohno/ papers/TISSEC04/. 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 Expires April 26, 2007 [Page 22] Internet-Draft Authenticated Encryption October 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. McGrew Expires April 26, 2007 [Page 23]