Network Working Group D. McGrew Internet-Draft Cisco Systems, Inc. Intended status: Standards Track February 2007 Expires: August 5, 2007 An Interface and Algorithms for Authenticated Encryption draft-mcgrew-auth-enc-02.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 August 5, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). McGrew Expires August 5, 2007 [Page 1] Internet-Draft Authenticated Encryption February 2007 Abstract This document 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. 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. Guidance on the use of AEAD algorithms . . . . . . . . . . . . 8 3.1. Requirements on Nonce Generation . . . . . . . . . . . . . 8 3.2. Recommended Nonce Formation . . . . . . . . . . . . . . . 9 3.2.1. Partially Implicit Nonces . . . . . . . . . . . . . . 9 3.3. Construction of AEAD Inputs . . . . . . . . . . . . . . . 10 3.4. Example Usage . . . . . . . . . . . . . . . . . . . . . . 11 4. Requirements on AEAD algorithms . . . . . . . . . . . . . . . 12 5. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 14 5.1. AEAD_AES_128_GCM . . . . . . . . . . . . . . . . . . . . . 14 5.1.1. Nonce Reuse . . . . . . . . . . . . . . . . . . . . . 14 5.2. AEAD_AES_256_GCM . . . . . . . . . . . . . . . . . . . . . 15 5.3. AEAD_AES_128_CCM . . . . . . . . . . . . . . . . . . . . . 15 5.3.1. Nonce Reuse . . . . . . . . . . . . . . . . . . . . . 16 5.4. AEAD_AES_256_CCM . . . . . . . . . . . . . . . . . . . . . 16 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 7. Other Considerations . . . . . . . . . . . . . . . . . . . . . 18 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 10.1. Normative References . . . . . . . . . . . . . . . . . . . 21 10.2. Informative References . . . . . . . . . . . . . . . . . . 21 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 24 McGrew Expires August 5, 2007 [Page 2] Internet-Draft Authenticated Encryption February 2007 1. Introduction 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, adds the ability to check the integrity and authenticity of some additional authenticated data (also called "associated data") that is not encrypted. 1.1. Background Many cryptographic applications require both confidentiality and message authentication. Confidentiality is a security service that ensures that data is available only to those authorized to obtain it; usually it is realized through encryption. Message authentication is the service that ensures that data has not been altered or forged by unauthorized entities; it can be achieved by using a Message Authentication Code (MAC). This service is also called data integrity. Many applications use an encryption method and a MAC together to provide both of 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 (see, for example, Section 2.5 of [BOYD]). 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 four AEAD algorithms based on AES in Galois/Counter Mode [GCM] with 128 and 256 bit keys, and AES in Counter and CBC MAC mode [CCM] with 128 and 256 bit keys. In the following, we define the AEAD interface (Section 2), and then provide guidance on the use of AEAD algorithms (Section 3), and outline the requirements that each AEAD algorithm must meet (Section 4). Then we define several AEAD algorithms (Section 5), and establish an IANA registry for AEAD algorithms (Section 6). Lastly, we discuss some other considerations (Section 7). McGrew Expires August 5, 2007 [Page 3] Internet-Draft Authenticated Encryption February 2007 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 recommendations from [EEM04]. 1.3. Benefits The AEAD approach enables applications that need cryptographic security services to more easily adopt those services. It benefits the application designer by allowing them to focus on the important issues such as 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 more 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 August 5, 2007 [Page 4] Internet-Draft Authenticated Encryption February 2007 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 with other implementations. 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 3.2, 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 255. For each AEAD algorithm, the length of K MUST be fixed. The number of octets in the nonce MAY be zero, but SHOULD be twelve (12) octets. McGrew Expires August 5, 2007 [Page 5] Internet-Draft Authenticated Encryption February 2007 The number of octets in the plaintext P MAY be zero. The number of octets in the additional authenticated data A MAY be zero. The number of octets in the ciphertext C MAY be zero. This specification does not put a maximum length on the nonce, the plaintext, the ciphertext, nor the additional authenticated data. However, a particular AEAD algorithm MAY further restrict the lengths of those 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 stored or transported with the plaintext, or it MAY be reconstructed immediately prior to the authenticated decryption operation. It is sufficient to provide the decryption module with enough information to allow it to construct the nonce. (For example, a system could use a nonce consisting of a sequence number in a particular format, in which case it could be inferred from the order of the ciphertexts.) Because the authenticated decryption process detects incorrect nonce values, no security failure will result if a nonce is incorrectly reconstructed and fed into an authenticated McGrew Expires August 5, 2007 [Page 6] Internet-Draft Authenticated Encryption February 2007 decryption operation. Any nonce reconstruction method will need to take into account the possibility of loss or reorder of ciphertexts between the encryption and decryption processes. Applications MUST NOT assume any particular structure or formatting of the ciphertext. 2.2. Authenticated Decryption The authenticated decryption operation has four inputs: K, N , 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, a nonce N, and additional authenticated data A are authentic for key K when C is generated by the encrypt operation with inputs K, N, P and A, for some values of N, 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 choosing the format of application data, 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 instance, if the nonce and ciphertext both appear in a packet, the former value should precede the latter. This rule facilitates efficient and simple hardware implementations of AEAD algorithms. McGrew Expires August 5, 2007 [Page 7] Internet-Draft Authenticated Encryption February 2007 3. Guidance on the use of AEAD algorithms This section provides advice that must be followed in order to use an AEAD algorithm securely. 3.1. Requirements on Nonce Generation It is essential for security that the nonces be constructed in a manner that respects the requirement that each nonce value be distinct for each invocation of the authenticated encryption operation, for any fixed value of the key. In this section we call attention to some consequences of this requirement in different scenarios. When there are multiple devices performing encryption using a single key, those devices must coordinate to ensure that the nonces are unique. A simple way to do this is to use a nonce format that contains a field that is distinct for each one of the devices, as described in Section 3.2. Note that there is no need to coordinate the details of the nonce format between the encrypter and the decrypter, as long the entire nonce is sent or stored with the ciphertext and is thus available to the decrypter. If the complete nonce is not available to the decrypter, then the decrypter will need to know how the nonce is structured so that it can reconstruct it. Applications SHOULD provide encryption engines with some freedom in choosing their nonces; for example, a nonce could contain both a counter and a field that is set by the encrypter but is not processed by the receiver. This freedom allows a set of encryption devices to more readily coordinate to ensure the distinctness of their nonces. If a secret key will be used for a long period of time, e.g. across multiple reboots, then the nonce will need to be stored in non- volatible memory. In such cases it is essential to use checkpointing of the nonce, that is, the current nonce value should be stored to provide the state information needed to resume encryption in case of unexpected failure. One simple way to provide a high assurance that a nonce value will not be used repeatedly is to wait until the encryption process receives confirmation from the storage process indicating that the succeeding nonce value has already been stored. Because this method may add significant latency, it may be desirable to store a nonce value that is several values ahead in the sequence. As an example, the nonce 100 could be stored, after which the nonces 1 through 99 could be used for encryption. The nonce value 200 could be stored at the same time that nonces 1 through 99 are being used, and so on. Many problems with nonce re-use can be avoided by changing a key in a situation in which nonce coordination is difficult. McGrew Expires August 5, 2007 [Page 8] Internet-Draft Authenticated Encryption February 2007 Each AEAD algorithm SHOULD describe what security degradation would result from an inadvertant re-use of a nonce value. 3.2. Recommended Nonce Formation 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 zero for the first nonce, and it increments by one for each successive nonce that is generated. The initial zero value MAY be omitted if it is convenient. Thus at most 2^(8*C) 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. 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 encrypters can share a key when the Fixed field is F octets in length. 3.2.1. Partially Implicit Nonces In some cases it is desirable to not transmit or store an entire nonce, but instead to reconstruct that value from contextual information immediately prior to decryption. As an example, ciphertexts could be stored in sequence on a disk, and the nonce for a particular ciphertext could be inferred from its location, as long as the rule for generating the nonces is known by the decrypter. We call the portion of the nonce that is stored or sent with the ciphertext the explicit part. We call the portion of the nonce that is inferred the implicit part. When part of the nonce is implicit, the following specialization of the above format is RECOMMENDED. The Fixed field is divided into two sub-fields: a Fixed-Common field and a Fixed-Distinct field. This format is shown in Figure 2. If different devices are performing encryption with a single key, then each distinct device MUST use a distinct Fixed-Distinct field. The Fixed-Common field is common to all nonces. The Fixed-Distinct field McGrew Expires August 5, 2007 [Page 9] Internet-Draft Authenticated Encryption February 2007 and the Counter field MUST be in the explicit part of the nonce. The Fixed-Common field MAY be in the implicit part of the nonce. These conventions ensure that nonce is easy to reconstruct from the explicit data. +-------------------+--------------------+---------------+ | Fixed-Common | Fixed-Distinct | Counter | +-------------------+--------------------+---------------+ <---- implicit ---> <------------ explicit ------------> Figure 2: Partially implicit nonce format The rationale for the partially implicit nonce format is as follows. This method of nonce construction incorporates the best known practice; 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. 3.3. Construction of AEAD Inputs 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 August 5, 2007 [Page 10] Internet-Draft Authenticated Encryption February 2007 3.4. Example Usage To make use of an AEAD algorithm, an application must define how the encryption algorithm's inputs are defined in terms of application data, and how the ciphertext and nonce are conveyed. The clearest way to do this is to express each input in terms of the data that form it, then to express the application data in terms of the outputs of the AEAD encryption operation. For example, AES-GCM ESP [RFC4106] can be expressed as follows. The AEAD inputs are P = PayloadData || Padding || PadLength || NextHeader N = Salt || IV A = SPI || SequenceNumber where the symbol "||" denotes the concatenation operation, and the fields PayloadData, Padding, PadLength, NextHeader, SPI, and SequenceNumber are as defined in [RFC4303] and the fields Salt and IV are as defined in [RFC4106]. The format of the ESP packet can be expressed as ESP = SPI || SequenceNumber || IV || C where C is the AEAD ciphertext (which in this case incorporates the Integrity Check Value). Please note that here we have not described the use of the ESP Extended Sequence Number. McGrew Expires August 5, 2007 [Page 11] Internet-Draft Authenticated Encryption February 2007 4. 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 (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, inclusive, 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, inclusive, 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, inclusive, 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 or make use of 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. An AEAD algorithm of this sort is called randomized; though none that only encryption is random, and decryption is alwasy deterministic. 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. EACH AEAD algorithm specification SHOULD describe what security degradation would result from an inadvertant re-use of a nonce value. Each AEAD algorithm specification SHOULD provide a reference to a detailed security analysis. This document does not specify a McGrew Expires August 5, 2007 [Page 12] Internet-Draft Authenticated Encryption February 2007 particular security model, because several different models have been used in the literature. The security analysis SHOULD define or reference a security model. McGrew Expires August 5, 2007 [Page 13] Internet-Draft Authenticated Encryption February 2007 5. AEAD Algorithms This section defines four AEAD algorithms; two are based on AES GCM, two are based on AES CCM. Each pair includes an algorithm with a key size of 128 bits and one with a key size of 256 bits. 5.1. 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 a nonce length of 12 octets, since with GCM is there is a performance penalty for using other lengths, C_MAX is 2^36 - 15 octets. Note that AES GCM implementations that are performance-oriented MAY only support nonce lengths of exactly 12 octets. The AES GCM ciphertext is exactly the same length as its plaintext. A security analysis of GCM is available in [MV04]. 5.1.1. Nonce Reuse Inadvertent reuse of the same nonce by two invocations of the GCM encryption operation, with the same key, undermines the security of all of the subsequent uses of that key. For this reason, GCM should only be used whenever nonce uniqueness can be provided with assurance. The design feature that GCM uses to achieve minimal latency causes the vulnerabilities on the subsequent uses of the key. Note that it is acceptable to input the same nonce value multiple times to the decryption operation. McGrew Expires August 5, 2007 [Page 14] Internet-Draft Authenticated Encryption February 2007 The security consequences if an attacker observes two encryptions of the different plaintexts are quite serious. First, a loss of confidentiality ensues because he will be able to reconstruct the bitwise exclusive-or of the two plaintext values. Second, a loss of integrity ensues because the attacker will be able to recover the internal hash key used to provide data integrity. Knowledge of this key makes subsequent forgeries trivial. 5.2. 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. 5.3. 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 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 of 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 McGrew Expires August 5, 2007 [Page 15] Internet-Draft Authenticated Encryption February 2007 C_MAX is 2^24 + 15 octets. The AES CCM ciphertext is exactly the same length as its plaintext. A security analysis of AES CCM is available in [J02]. 5.3.1. Nonce Reuse Inadvertent reuse of the same nonce by two invocations of the CCM encryption operation, with the same key, undermines the security for the messages processed with those invocations. A loss of confidentiality ensues because he will be able to reconstruct the bitwise exclusive-or of the two plaintext values. 5.4. 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. McGrew Expires August 5, 2007 [Page 16] Internet-Draft Authenticated Encryption February 2007 6. IANA Considerations The Internet Assigned Numbers Authority (IANA) will define the "AEAD Registry" described below. Additions to the AEAD Registry require that a specification be documented in an Internet RFC or another permanent and readily available reference, in sufficient detail that interoperability between independent implementations is possible. 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. The numbers between 32,768 (binary 1000000000000000) and 65,535 (binary 1111111111111111) inclusive, will not be assigned by IANA, and are reserved for private use; no attempt will be made to prevent multiple sites from using the same value in different (and incompatible) ways [RFC2434]. IANA will add the following five entries to the AEAD Registry: +------------------+-------------+--------------------+ | Name | Reference | Numeric Identifier | +------------------+-------------+--------------------+ | AEAD_AES_128_GCM | Section 5.1 | 1 | | | | | | AEAD_AES_256_GCM | Section 5.2 | 2 | | | | | | AEAD_AES_128_CCM | Section 5.3 | 3 | | | | | | AEAD_AES_256_CCM | Section 5.4 | 4 | +------------------+-------------+--------------------+ An IANA registration of an AEAD does not constitute an endorsement of that algorithm or its security. McGrew Expires August 5, 2007 [Page 17] Internet-Draft Authenticated Encryption February 2007 7. Other Considerations Directly testing a randomized AEAD encryption algorithm using a test cases with fixed inputs and outputs is not possible, since the encryption process is non-deterministic. However, it is possible 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. Combining both of these tests covers both the encryption and decryption algorithms. 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. It may be desirable to define an AEAD algorithm that uses the generic composition with the encrypt-then-MAC method [BN00], combining a common encryption algorithm, such as CBC [MODES], with a common message authentication code, such as HMAC-SHA1 [RFC2104] or AES CMAC [CMAC]. An AEAD algorithm of this sort would reflect the best current practice, and might be more easily supported by crypto modules that lack support for other AEAD algorithms. McGrew Expires August 5, 2007 [Page 18] Internet-Draft Authenticated Encryption February 2007 8. Security Considerations This document describes authenticated encryption algorithms, and provides guidance on their use. While these algorithms make it easier, in some ways, to design a cryptographic application, it should be borne in mind that strong cryptographic security is difficult to achieve. While AEAD algorithms are quite useful, they do nothing to address the issues of key generation [RFC4086] and key management [RFC4107]. AEAD algorithms that rely on distinct nonces MAY NOT be appropriate for some applications or for some scenarios. Application designers should understand the requirements outlined in Section 3.1. A software implementation of the AEAD encryption operation in a Virtual Machine (VM) environment could inadvertently re-use a nonce due to a "rollback" of the VM to an earlier state [GR05]. Applications are encouraged to document potential issues to help the user of the application and the VM avoid unintentional mistakes of this sort. The possibility exists that an attacker can cause a VM rollback; threats and mitigations of are an area of active research. For perspective, we note that an attacker who can trigger such a rollback may have already succeeded in subverting the security of the system, e.g. by causing an accounting error. An IANA registration of an AEAD algorithm MUST NOT be regarded as an endorsement of its security. Furthermore, the percieved security level of an algorithm can degrade over time, due to cryptanalytic advances or to "Moore's Law", that is, the diminishing cost of computational resources over time. McGrew Expires August 5, 2007 [Page 19] Internet-Draft Authenticated Encryption February 2007 9. Acknowledgments Many reviewers provided valuable comments on earlier drafts of this document. Some fruitful discussions took place on the email list of the Crypto Forum Research Group in 2006. McGrew Expires August 5, 2007 [Page 20] Internet-Draft Authenticated Encryption February 2007 10. References 10.1. Normative References [CCM] "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/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. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 10.2. Informative References [BN00] "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] "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/. [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 Topcis in Operating Sytems 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. McGrew Expires August 5, 2007 [Page 21] Internet-Draft Authenticated Encryption February 2007 [MODES] Dworkin, M., "NIST Special Publication 800-38: Reccomendation for Block Cipher Modes of Operation", U.S. National Institue 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. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [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. McGrew Expires August 5, 2007 [Page 22] Internet-Draft Authenticated Encryption February 2007 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 August 5, 2007 [Page 23] Internet-Draft Authenticated Encryption February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). McGrew Expires August 5, 2007 [Page 24]