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Versions: 00 01 02 03 04 05 RFC 5116

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

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   Copyright (C) The IETF Trust (2007).














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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







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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, 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).




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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.

      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.





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      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



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   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.


















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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.

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.



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   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



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   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.






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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.






















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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



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   particular security model, because several different models have been
   used in the literature.  The security analysis SHOULD define or
   reference a security model.
















































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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], 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.




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   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





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      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.





























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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.







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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.




















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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 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.





















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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]      "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.




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   [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.


















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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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