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Network Working Group                                          D. McGrew
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                               July 2007
Expires: January 2, 2008


        An Interface and Algorithms for Authenticated Encryption
                      draft-mcgrew-auth-enc-05.txt

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   Copyright (C) The IETF Trust (2007).














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Abstract

   This document defines algorithms for authenticated encryption with
   associated 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 . . . . . . . . . . . .  9
     3.1.  Requirements on Nonce Generation . . . . . . . . . . . . .  9
     3.2.  Recommended Nonce Formation  . . . . . . . . . . . . . . . 10
       3.2.1.  Partially Implicit Nonces  . . . . . . . . . . . . . . 11
     3.3.  Construction of AEAD Inputs  . . . . . . . . . . . . . . . 11
     3.4.  Example Usage  . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Requirements on AEAD Algorithm Specifications  . . . . . . . . 14
   5.  AEAD Algorithms  . . . . . . . . . . . . . . . . . . . . . . . 16
     5.1.  AEAD_AES_128_GCM . . . . . . . . . . . . . . . . . . . . . 16
       5.1.1.  Nonce Reuse  . . . . . . . . . . . . . . . . . . . . . 16
     5.2.  AEAD_AES_256_GCM . . . . . . . . . . . . . . . . . . . . . 17
     5.3.  AEAD_AES_128_CCM . . . . . . . . . . . . . . . . . . . . . 17
       5.3.1.  Nonce Reuse  . . . . . . . . . . . . . . . . . . . . . 18
     5.4.  AEAD_AES_256_CCM . . . . . . . . . . . . . . . . . . . . . 18
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   7.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
   Intellectual Property and Copyright Statements . . . . . . . . . . 27







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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 [R02], adds
   the ability to check the integrity and authenticity of some
   Associated Data (AD), also called "additional authenticated 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).




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   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].







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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.

      A nonce N. Each nonce provided to distinct invocations of the
      Authenticated Encryption operation MUST be distinct, for any
      particular value of the key, unless each and every nonce is zero-
      length.  Applications that can generate distinct nonces SHOULD use
      the nonce formation method defined in Section 3.2, and MAY use any
      other method that meets the uniqueness requirement.  Other
      applications SHOULD use zero-length nonces.

      A plaintext P, which contains the data to be encrypted and
      authenticated,

      The associated 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.

      For any particular value of the key, either 1) each nonce provided
      to distinct invocations of the Authenticated Encryption operation



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      MUST be distinct, or 2) each and every nonce MUST be zero-length.
      If zero-length nonces are used with a particular key, then each
      and every nonce used with that key MUST have a length of zero.
      Otherwise, the number of octets in the nonce SHOULD be twelve
      (12).  Nonces with different lengths MAY be used with a particular
      key.  Some algorithms cannot be used with zero-length nonces, but
      others can; see Section 4.  Applications that conform to the
      recommended nonce length will avoid having to construct nonces
      with different lengths, depending on the algorithm that is in use.
      This guidance helps to keep algorithm-specific logic out of
      applications.

      The number of octets in the plaintext P MAY be zero.

      The number of octets in the associated 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 associated 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 plaintext.

   The secret key K MUST NOT be included in any of the other inputs (N,
   P, and A).  (This restriction does not mean that the values of those
   inputs must be checked to ensure that they do not include substrings



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   that match the key; instead, it means that the key must not be
   explicitly copied into those inputs.)

   The nonce is authenticated internally to the algorithm, and it is not
   necessary to include it in the AD input.  The nonce MAY be included
   in P or A if it is convenient to the application.

   The nonce MAY be stored or transported with the ciphertext, 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
   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 associated 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 the inputs N, P, and A were crafted by a nonce-
   respecting adversary that does not know the secret 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



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   AEAD algorithms.


















































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3.  Guidance on the use of AEAD algorithms

   This section provides advice that must be followed in order to use an
   AEAD algorithm securely.

   If an application is unable to meet the uniqueness requirement on
   nonce generation, then it MUST use a zero-length nonce.  Randomized
   or stateful algorithms, which are defined below, are suitable for use
   with such applications.  Otherwise, an application SHOULD use nonces
   with a length of twelve octets.  Since algorithms are encouraged to
   support that length, applications should use that length to aid
   interoperability.

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-
   volatile 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



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   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.

   Each AEAD algorithm SHOULD describe what security degradation would
   result from an inadvertent 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.  For each fixed key, the length of each of these
   fields, and thus the length of the nonce, is fixed.  Implementations
   SHOULD support 12-octet nonces in which the Counter field is four
   octets long.

       <----- variable ----> <----------- variable ----------->
      +---------------------+----------------------------------+
      |        Fixed        |              Counter             |
      +---------------------+----------------------------------+

                    Figure 1: Recommended nonce format.

   The Counter fields of successive nonces form a monotonically
   increasing sequence, when those fields are regarded as unsigned
   integers in network byte order.  The length of the Counter field MUST
   remain constant for all nonces that are generated for a given
   encryption device.  The Counter part SHOULD be equal to zero for the
   first nonce, and increment by one for each successive nonce that is
   generated.  However, any particular Counter value MAY be skipped
   over, and left out of the sequence of values that are used, if it is
   convenient.  For example, an application could choose to skip the
   initial Counter=0 value, and set the Counter field of the initial
   nonce to 1.  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.



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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
   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 the 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 AD 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 AD



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   input in an authenticated encryption operation in such a way that the
   data elements are different, but the AD input is unchanged.  This
   requirement is trivially met if the AD is composed of fixed-width
   elements.  If the AD contains a variable-length string, for example,
   this requirement can be met by also including the length of the
   string in the AD.

   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 included in the AD may be
   used when parsing the plaintext, though of course since the AD is not
   encrypted there is a potential loss of confidentiality when
   information about the plaintext is included in the AD.

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 = RestOfPayloadData || TFCpadding || Padding || PadLength ||
      NextHeader

      N = Salt || IV

      A = SPI || SequenceNumber

   where the symbol "||" denotes the concatenation operation, and the
   fields RestOfPayloadData, TFCpadding, Padding, PadLength, NextHeader,
   SPI, and SequenceNumber are as defined in [RFC4303] and the fields
   Salt and IV are as defined in [RFC4106].  The field RestOfPayloadData
   contains the plaintext data that is described by the NextHeader
   field, and no other data.  (Recall that the PayloadData field
   contains both the IV and the RestOfPayloadData; see Figure 2 of
   [RFC4303] for an illustration.)

   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



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   authentication tag).  Please note that here we have not described the
   use of the ESP Extended Sequence Number.

















































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4.  Requirements on AEAD Algorithm Specifications

   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.  The value of P_MAX MUST be larger than zero, and
   SHOULD be at least 65,536 (2^16) octets.  This size is a typical
   upper limit for network data packets.  Other applications may use
   even larger values of P_MAX, so it is desirable for general-purpose
   algorithms to support higher values.

   Each AEAD algorithm MUST accept any associated data with a length
   between zero and A_MAX octets, inclusive, where the value A_MAX is
   specific to that algorithm.  The value of A_MAX MUST be larger than
   zero, and SHOULD be at least 65,536 (2^16) octets.  Other
   applications may use even larger values of A_MAX, so it is desirable
   for general-purpose algorithms to support higher values.

   Each AEAD algorithm MUST accept any nonce 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.  The values of N_MAX and N_MIN MAY be
   equal.  Each algorithm SHOULD accept a nonce with a length of twelve
   (12) octets.  Randomized or stateful algorithms, which are described
   below, MAY have an N_MAX value of zero.

   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 AEAD algorithm of this sort is called randomized; though
   note that only encryption is random, and decryption is always
   deterministic.  A randomized algorithm MAY have a value of N_MAX that
   is equal to zero.

   An Authenticated Encryption algorithm MAY incorporate internal state
   information that is maintained between invocations of the encrypt
   operation, e.g. to allow for the construction of distinct values that
   are used as internal nonces by the algorithm.  An AEAD algorithm of
   this sort is called stateful.  This method could be used by an



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   algorithm to provide good security even when the application inputs
   zero-length nonces.  A stateful algorithm 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 largest 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.  This relation MUST NOT
   depend on external parameters, such as an authentication strength
   parameter (e.g. authentication tag length).  That sort of dependence
   would complicate the use of the algorithm by creating a situation in
   which the information from the AEAD registry was not sufficient to
   ensure interoperability.

   EACH AEAD algorithm specification SHOULD describe what security
   degradation would result from an inadvertent 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
   particular security model, because several different models have been
   used in the literature.  The security analysis SHOULD define or
   reference a security model.

   An algorithm that is randomized or stateful, as defined above, SHOULD
   describe itself using those terms.























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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], using AES-128 as the block cipher, by providing
   the key, nonce, and plaintext, and associated 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 and N_MAX are both 12 octets, and

      C_MAX is 2^36 - 15 octets.

   An AEAD_AES_128_GCM ciphertext is exactly 16 octets longer than its
   corresponding 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.

   The security consequences are quite serious if an attacker observes
   two ciphertexts that were created using the same nonce and key
   values, unless the plaintext and AD values in both invocations of the
   encrypt operation were identical.  First, a loss of confidentiality



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   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], using AES-128 as the block cipher, by providing
   the key, nonce, associated 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

      C_MAX is 2^24 + 15 octets.

   An AEAD_AES_128_CCM ciphertext is exactly 16 octets longer than its



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   corresponding 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 an adversary 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.































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6.  IANA Considerations

   The Internet Assigned Numbers Authority (IANA) will define the "AEAD
   Registry" described below.  An algorithm designer MAY register an
   algorithm in order to facilitate its use.  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.
   Submitters SHOULD have their requests reviewed by the IRTF Crypto
   Forum Research Group (CFRG) at cfrg@ietf.org.  Interested applicants
   that are unfamiliar with IANA processes should visit
   http://www.iana.org.

   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 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         |
          +------------------+-------------+--------------------+




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   An IANA registration of an AEAD does not constitute an endorsement of
   that algorithm or its security.

















































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7.  Other Considerations

   Directly testing a randomized AEAD encryption algorithm using 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 using the following technique.  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.




















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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 in that scenario 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 perceived 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.





















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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.














































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10.  References

10.1.  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.

10.2.  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",



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              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.

   [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.









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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








































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Full Copyright Statement

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