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Versions: 00 01

Network Working Group                                          D. McGrew
Internet-Draft                                                  J. Foley
Intended status: Standards Track                           Cisco Systems
Expires: August 18, 2014                               February 14, 2014


         Authenticated Encryption with Replay prOtection (AERO)
                        draft-mcgrew-aero-01.txt

Abstract

   This document describes Authenticated Encryption with Replay
   prOtection (AERO), a cryptographic technique that provides all of the
   essential security services needed for communication security.  AERO
   offers several advantages over other methods: it has more compact
   messages, provides stronger misuse resistance, avoids the need to
   manage implicit state, and is simpler to use.  This document defines
   a particular AERO algorithm as well as a registry for such
   algorithms.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions used in this document  . . . . . . . . . . . .  3
     1.2.  Glossary . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Data types and notation  . . . . . . . . . . . . . . . . .  5
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Problems addressed by AERO . . . . . . . . . . . . . . . .  6
   3.  Interface  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Encryption Initialization  . . . . . . . . . . . . . . . .  9
     3.2.  Encryption . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Decryption Initialization  . . . . . . . . . . . . . . . . 10
     3.4.  Decryption . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.5.  Multiple senders and multiple receivers  . . . . . . . . . 10
     3.6.  AEAD interface . . . . . . . . . . . . . . . . . . . . . . 11
   4.  Implementation . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Contexts . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  Encryption Initialization  . . . . . . . . . . . . . . . . 13
     4.3.  Encryption . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.4.  Decryption Initialization  . . . . . . . . . . . . . . . . 14
     4.5.  Decryption . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.  Stateless encryption algorithms  . . . . . . . . . . . . . . . 17
     5.1.  Wide PRP (WPRP)  . . . . . . . . . . . . . . . . . . . . . 17
       5.1.1.  WPRP Requirements and survey . . . . . . . . . . . . . 19
     5.2.  Other cryptographic techniques . . . . . . . . . . . . . . 19
   6.  Loss, reorder, and resynchronization . . . . . . . . . . . . . 21
   7.  Usage and applicability  . . . . . . . . . . . . . . . . . . . 23
   8.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  Testing  . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   11. Security considerations  . . . . . . . . . . . . . . . . . . . 27
     11.1. Authentication and confidentiality . . . . . . . . . . . . 28
       11.1.1. Authentication strength  . . . . . . . . . . . . . . . 30
     11.2. Length hiding  . . . . . . . . . . . . . . . . . . . . . . 30
     11.3. Denial of Service (DoS)  . . . . . . . . . . . . . . . . . 30
     11.4. Multiple senders . . . . . . . . . . . . . . . . . . . . . 31
     11.5. Sequence number management failure . . . . . . . . . . . . 32
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     13.1. Normative references . . . . . . . . . . . . . . . . . . . 34
     13.2. Informative references . . . . . . . . . . . . . . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37




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1.  Introduction

   This document describes Authenticated Encryption with Replay
   prOtection (AERO), a cryptographic technique that provides all of the
   essential security services needed for communication security.

   AERO can be considered a stateful and self-synchronizing
   authenticated encryption method that provides protection from replay
   attacks as a side benefit.  Replay protection and authentication are
   provided through a single mechanism, in which a sequence number is
   encrypted along with a plaintext message.  If the ciphertext message
   is not tampered with, then this sequence number will be recovered by
   the decrypter, and verified to be valid.  If the ciphertext message
   is a replay of an earlier message, then the decrypter will detect
   this fact from the recovered sequence number.  If the ciphertext
   message is tampered with, or is crafted as a forgery, this fact will
   be apparent to the decrypter because the value that should be a valid
   sequence is instead a pseudorandom value.

   AERO makes use of an underlying stateless encryption algorithm.  This
   note defines a set of AERO algorithms that are based on an underlying
   Wide Pseudo-Random Permutation (WPRP).  AERO is not, by itself, a
   block cipher mode of operation.

   This note is structured as follows.  Section 1 introduces the basic
   idea of AERO, describes the conventions and notations used in this
   note, and provides a glossary of acronyms and variables.  Section 2
   provides background on the problems that AERO solves.  The interface
   and implementation are presented in Section 3 and Section 4,
   respectively.  The underlying cryptographic primitives are described
   in Section 5.  Section 6 describes synchronization between encrypters
   and decrypters, and Section 7 describes usage.  A rationale for the
   design details is offered in Section 8, and test considerations and
   test cases are presented in Section 9.  IANA considerations and
   security considerations follow in Section 10 and Section 11,
   respectively.

1.1.  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].

1.2.  Glossary

   This section describes the symbols and acronyms used in this
   document.




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      A - Associated Data.  An unencrypted value whose integrity and
      authenticity is to be protected.

      AERO - Authenticated Encryption with Replay prOtection.  A
      cryptographic technique that combines the three security services
      essential to communication security.

      C - Ciphertext.  An encrypted value that corresponds to a
      plaintext.

      FAIL - a value returned by the AERO decryption operation to
      indicate that a ciphertext message is not valid; it could be
      either a replay or a forgery attempt.

      K - Key. A secret key that is shared between the sender(s) and
      receiver(s).

      M - bitmask.  A bit vector used by the AERO decryption algorithm
      that holds information about the sequence numbers that have
      already been accepted.  If a message with the sequence number X
      has been accepted and the current sequence number S > X, then
      M[S-X] = 1; otherwise M[S-X] = 0.

      P - Plaintext.  An unencrypted value whose confidentiality,
      integrity and authenticity is to be protected.

      PMIN - minimum number of bytes of plaintext for a stateless
      encryption algorithm.  PMIN is a parameter of the stateless
      encryption algorithm, which is used in the padding logic.

      R - the last rejected candidate sequence number that is greater
      than the current sequence number S. A T-bit unsigned integer
      maintained by the AERO decryption algorithm.

      S - the current sequence number.  A T-bit unsigned integer
      maintained by the AERO encryption algorithm, where it represents
      the sequence number of the next message to be encrypted, and by
      the AERO decryption algorithm, where it represents the last
      candidate sequence number that was accepted.

      T - The number of bits in the sequence number that is internal to
      AERO encryption and decryption.

      W - Reorder tolerance parameter.  A parameter used in AERO
      decryption (but not encryption) that determines the amount of
      message loss or reorder that the algorithm will tolerate.  This
      value is 64 by default, but other values may be used as well.




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      V - Resynchronization parameter.  A parameter that determines the
      amount of message loss or reorder that the AERO decryption
      algorithm will tolerate during resynchronization.  This value is 8
      by default, but other values may be used as well.

      Z - the candidate sequence number.  A value used in AERO
      decryption that may be a sequence number, or alternately may be
      data that resulted from the actions of an attacker.

1.3.  Data types and notation

   An octet string is an ordered sequence of eight-bit values.  Note
   that these strings are not character strings, and issues such as
   character representation are out of scope for this note.

   len(X) denotes the number of bits in the string X, expressed as an
   unsigned integer in network byte order.

   The concatenation of two octet strings A and B is denoted as A || B

   Conversion between unsigned integers and octet strings is done as
   follows.  The Integer to String (I2S) algorithm takes as input an
   unsigned integer U, and converts it to network byte order (that is, a
   big-endian representation, in which the most significant octet is the
   initial one), and then returns the resulting octet string.  The
   String to Integer (S2I) algorithm takes as input an octet string S,
   considers it as an integer in network byte order, and then converts
   it into the internal representation for unsigned integers used by the
   system.






















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2.  Background

   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.

   AEAD is used in many communication security protocols, such as ESP,
   TLS, DTLS, IKE, and SRTP.  These protocols also incorporate replay
   protection using an sequence number that is carried in the packet,
   which is authenticated because it is part of the associated data.
   This security service enables the receiver of a message to detect
   when a received message is a duplicate of a previously received
   message.

   In order to minimize the data overhead, some protocols (such as SRTP
   and ESP) have the most significant part of the sequence number (that
   is, the high order bits) be implicit state, which is not sent on the
   wire.  The receiver constructs an estimate of the sequence number
   from the data on the wire and its current state.

   Authenticated Encryption with Replay prOtection (AERO) extends AEAD
   so that it provides a replay protection service.  It also enables a
   receiver to correctly sequence all of the messages that it receives
   into the same order that they were sent.  A replay protection service
   has a reorder tolerance parameter W; if a receiver processes a
   message that is more than W messages earlier than a previously
   processed message, then the service will reject the message even if
   it is not a replay.

2.1.  Problems addressed by AERO

   Many communication security protocols are operated in environments
   where bandwidth is a scarce resource, and thus they seek to minimize
   the data overhead, that is, the number of additional bytes that must
   be communicated in order to add confidentiality, authenticity, and
   replay protection to each message.  To authenticate a message,
   lengthening it is unavoidable.  However, traditional encryption and
   replay protection techniques add avoidable overhead.  Encryption
   methods methods such as AES-CBC encryption [SP800-38] have an average
   overhead of 24 bytes, and many uses of CTR, GCM [GCM], and CCM [CCM]
   have eight bytes of overhead because a nonce is carried in each
   message.  In addition to that overhead, replay protection typically
   adds its own overhead; the use of a four-byte sequence number, for
   instance, adds that number of bytes.



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   Some communication security protocol uses an implicit or partly
   implicit sequence number to reduce overhead.  This practice
   complicates the use of that protocol whenever there are multiple
   receivers, since it is necessary to (securely) communicate the
   current value of the implicit part of the sequence number to a new
   receiver at the time that the receiver joins the session.  For
   instance, [RFC4771] furnishes an example of the complications
   introduced by the need to manage implicit state.

   Several cryptographic algorithms require that a distinct nonce is
   input into each invocation of the encryption algorithm.  This
   requirement is difficult to satisfy if there are multiple senders, or
   multiple encryption units, using the same secret key.  Algorithms
   that require nonces introduce significant complexity into the systems
   that use them, as is revealed by the survey of usage of and issues
   with nonces in Internet protocols [IV-GEN].  One of the cannons of
   computer security is that "input is evil", that is, data that is
   accepted into a security module might be manipulated by an attacker
   and thus should not be trusted until it has been validated.  A
   cryptographic algorithm that accepts a nonce and requires it to be
   distinct introduces an opportunity for an attacker to defeat security
   through "evil" influence of that input.

   Several cryptographic algorithms that require a distinct nonce as an
   input have a serious security degradation if there is nonce misuse
   (that is, if a system failure results in two or more identical nonces
   being used for the same key).  CTR, CCM, and GCM all have a loss of
   confidentiality for each message that is associated with a misused
   nonce.  GCM, GMAC [GCM], UMAC [RFC4418], and POLY1305 leak their
   authentication keys after nonce misuse.  In scenarios in which it is
   not possible to provide high assurance that nonces will not be
   misused, these algorithms may not be appropriate.

   Both dedicated authenticated encryption algorithms, and the
   traditional method of combination of separate authentication and
   encryption algorithms, are often complex to use.  Nonces and
   initialization vectors, especially, are often a source of confusion
   for implementers and application designers.

   AERO solves these problems by using a technique that combines message
   authentication with replay protection, thus eliminating the data
   overhead associated with replay protection and any need for using
   implicit state.  AERO does not use a nonce as input, which addresses
   multiple issues: it reduces data overhead, it avoids all of the
   problems of coordinating nonces and the potential problems of nonce
   misuse, and it simplifies the jobs of the designers and implementers
   of applications and protocols.  AERO can easily be used in multiple-
   sender scenarios, because no coordination of nonces is needed, and in



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   multiple-receiver scenarios, because no coordination of implicit
   state is needed.

















































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3.  Interface

   An AERO algorithm has four operations: encryption, decryption,
   encryption initialization, and decryption initialization.  The inputs
   and outputs of these algorithms are defined below in terms of octet
   strings.  An octet string is an ordered sequence of eight-bit values.

   The terms "AERO encryption" and "AERO decryption" refer to algorithms
   that provide authenticated encryption with replay protection.  For
   brevity, these algorithms are simply called "encryption" and
   "decryption" when AERO is clear from the context.

   An implementation MAY accept additional inputs to these algorithms.
   For example, an optional input could be provided to allow the user to
   select between different implementation strategies.  However, such
   extensions MUST NOT affect interoperability with other
   implementations.

3.1.  Encryption Initialization

   The encryption initialization operation has one input:

      A secret key K, which MUST be generated in a way that is uniformly
      random or pseudorandom.  This input is an octet string.  The
      number of octets in K is fixed for each AERO algorithm, and is
      between 1 and 255.

   The operation has a single output, an AERO encryption context.

3.2.  Encryption

   The AERO encryption operation has three inputs, each of which is an
   octet string:

      An AERO encryption context.

      A plaintext P, which contains the data to be encrypted and
      authenticated.  The number of octets in P MAY be zero.

      The associated data A, which contains the data to be
      authenticated, but not encrypted.  The number of octets in A MAY
      be zero.

   There is a single output:

      A ciphertext C, which is an octet string that is at least as long
      as the plaintext.




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   A plaintext P and its associated data A is sometimes denoted together
   as (A, P), and that pair of elements is called a plaintext message.
   Similarly, a ciphertext C and its associated data A is denoted
   together as (A, C), and is called a ciphertext message.

3.3.  Decryption Initialization

   The decryption initialization operation has one or two inputs:

      A secret key K, which MUST match the secret key used in the
      corresponding encryption initialization.  This input is an octet
      string.

      The reorder tolerance parameter W, a non-negative integer which
      indicates the amount of reorder of messages that must be
      tolerated.  This input is optional; when it is not provided as an
      input, the default value of 64 SHOULD be used.

   The operation has a single output, an AERO decryption context.

3.4.  Decryption

   The AERO decryption operation has three inputs, each of which is an
   octet string:

      An AERO decryption context.

      An octet string C.

      The associated data A, which contains the data to be
      authenticated, but not encrypted.  The length of A MAY be zero.

   The output is one of these two alternatives:

      An octet string P and an unsigned integer indicating the order in
      which P appeared in the sequence of plaintexts input to the send
      algorithm, or

      The symbol FAIL, which indicates that either the value C was not
      output by the send algorithm, or that (A, C) matches a previously
      accepted pair of values.

3.5.  Multiple senders and multiple receivers

   A communications security protocol may have multiple senders
   (encrypters) as well as multiple receivers (decrypters).  A single
   AERO key MAY be used by multiple senders, in which case each sender
   MUST use a distinct AERO encryption context.



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   When multiple AERO encryption contexts are in use for a single key, a
   receiver must use a distinct AERO decryption context to process the
   messages encrypted with each distinct encryption context.  That is,
   the receiver maintains a decryption context for each sender.  This
   matches the conventional practice of communication security
   protocols, in which the receiver maintains a distinct security
   association for each sender.

   When processing a protected message, a receiver needs to select the
   appropriate AERO context to use in processing that message.  Thus,
   each protected message must be associated with some indication of its
   sender.  We call a field that contains this indication a Sender
   IDentifier (SID).

   When there are multiple senders, the values of the associated data
   input to the encryption algorithm for different senders SHOULD be
   distinct.  This practice ensures that no two messages are identical,
   and that fact ensures that the AERO system provides strong
   confidentiality.  An easy way to ensure the distinctness of
   Associated Data values is to include an SID in those values.  For
   example, this requirement is met if the associated data includes a
   message header, and that header contains a field that identifies the
   sender of the message.  Alternatively, the value of the plaintext can
   be distinct for each sender, for instance, if each plaintext includes
   an SID.

   If senders accidentally use the same SID value, the security of the
   session will degrade, but not catastrophically.  See section
   Section 11.4 for more details.

3.6.  AEAD interface

   AERO can be used through the IETF standard interface for
   authenticated encryption with associated data [RFC5116].  As AERO
   does not use a nonce or initialization vector, N_MAX = N_MIN = 0, and
   the application using the AEAD interface need not provide a nonce.
   Note that most of the usage guidance in RFC 5116 describes
   stipulations and cautions on the use of the nonces, so by eliminating
   the need for a nonce, AERO is simplifying the use of this standard.












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4.  Implementation

   AERO makes use of a technique that combines message authentication
   with replay protection, in which the sender maintains a sequence
   number, and the encrypted value of this sequence number is
   communicated to the receiver.  When the receiver decrypts a message,
   it parses out the data that would be equal to a sequence number if
   the message was sent by a legitimate sender, and verifies that this
   data is sensible.  This data is called a candidate sequence number,
   and is denoted as Z; the verification process is detailed below.  If
   the data is not sensible, then the decryption algorithm rejects the
   message.  The technique provides authenticity because Z is a
   pseudorandom function of both the ciphertext and associated data, and
   in a forgery attempt, Z will be rejected with overwhelming
   probability.

   AERO is a stateful encryption method.  It makes use of an underlying
   stateless encryption method; the interface to this method is defined
   below.  This interface allows an AERO implementation to separate out
   the authentication and replay protection logic from the cryptographic
   algorithms that provide pseudorandomness.  The interface also
   facilitates the use of different cryptographic techniques, thus
   allowing algorithm agility.

   The stateless encryption algorithm takes as input a secret key K, a
   plaintext P, a T-bit unsigned integer S, and an associated data A,
   and returns a ciphertext C. Here K, A, P, and C are all octet
   strings.

   The stateless decryption algorithm takes as input a secret key K, a
   ciphertext C, and an associated data A, and returns a plaintext P and
   a T-bit unsigned integer U.

   The stateless encryption algorithm may not be able to encrypt
   plaintexts that are shorter than a certain minimum value, so
   plaintexts are lengthend with the "pad" routine prior to encryption,
   and padding is removed after decryption with the "strip" routine.  We
   denote as PMIN the minimum number of bytes in a plaintext that the
   stateful encryption algorithm can accept.

   Note that the stateless encryption provides only confidentiality, and
   not authenticity.

4.1.  Contexts

   An encryption context includes a T-bit unsigned integer that holds
   the sequence number S.




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   A decryption context includes a W-bit bitmask M and two T-bit
   unsigned integers S and R, which record the highest candidate
   sequence number that has been accepted, and the last candidate
   sequence number that has been rejected, respectively.

4.2.  Encryption Initialization

   The encryption initialization operation does the following:

   1.  S is initialized to zero.

   2.  The value of the secret key K is stored in the context.

4.3.  Encryption

   To encrypt a message (A, P), the following steps are performed:

   1.  The current sequence number S is read from the context.  If S is
       greater than 2^T - 1, then the context cannot be used to encrypt
       any messages, so an indication of this error state is returned
       and the encryption processing halts.  Otherwise, processing
       continues as follows.

   2.  U is set to the value of a S converted from an unsigned integer
       to an octet string using the Integer to String (I2S) routine
       described in Section 1.3.

   3.  If PMIN * 8 > T, then P is lengthened by appending a padding
       string PS to it, to ensure that L = len(P) + len(PS) + T is at
       least 128.  The value of PS is as follows:

         PS = 00                               if len(P) + T >= 120,
         PS = 0101                             if len(P) + T = 112,
         PS = 020202                           if len(P) + T = 104,
         PS = 03030303                         if len(P) + T = 96,
         PS = 0404040404                       if len(P) + T = 88,
         PS = 050505050505                     if len(P) + T = 80,
         PS = 06060606060606                   if len(P) + T = 72,
         PS = 0707070707070707                 if len(P) + T = 64,
         PS = 080808080808080808               if len(P) + T = 56,
         PS = 09090909090909090909             if len(P) + T = 48,
         PS = 0A0A0A0A0A0A0A0A0A0A0A           if len(P) + T = 40,
         PS = 0B0B0B0B0B0B0B0B0B0B0B0B         if len(P) + T = 32,
         PS = 0C0C0C0C0C0C0C0C0C0C0C0C0C       if len(P) + T = 24,
         PS = 0D0D0D0D0D0D0D0D0D0D0D0D0D0D     if len(P) + T = 16,
         PS = 0E0E0E0E0E0E0E0E0E0E0E0E0E0E0E   if len(P) + T = 8.

       Note that padding MUST be appended to the plaintext if



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       PMIN * 8 > T, and otherwise padding MUST NOT be used.  As the
       parameter T is fixed for each particular key, either all of the
       messages processed by a particular key will use padding, or all
       will not.

   4.  The stateless encryption algorithm is applied to U, P, and A,
       using the secret key from the context.

   5.  The sequence number in the context is incremented by one.

   6.  The ciphertext C resulting from the stateless encryption
       algorithm is returned.


                           A      P
                      +----|------|------------------+
                      |    |      |                  |
                      |    |      |       +------+   |
                      |    |      |       |      |   |
                      |    |      |       v   +----+ |
                      |    |      |       S ->| +1 | |
                      |    |      |       |   +----+ |
                      |    |      v       v          |
                      |    |   +-----+ +-----+       |     AERO
                      |    |   | pad | | I2S |       |  encryption
                      |    |   +-----+ +-----+       |
                      |    |      |       |          |
                      |    v      v       v          |
                      |  +------------------+        |
                  K ---->|    stateless     |        |
                      |  |    encryption    |        |
                      |  +------------------+        |
                      |           |                  |
                      +-----------|------------------+
                                  v
                                  C

         Figure 1: An illustration of the AERO encryption process.

4.4.  Decryption Initialization

   During the decryption initialization process,

   1.  S is initialized to the reorder tolerance parameter W,

   2.  R is initialized to 2^T - 1, and





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   3.  the bitmask M is initialized to the all-zero value.

4.5.  Decryption

   To decrypt an encrypted message (A, C),

   1.  The stateless decryption algorithm is applied to A and C, using
       the secret key K in the context, returning a candidate sequence
       number Z, which is a T-bit unsigned integer, and a plaintext Q,
       which is an octet string.

   2.  If PMIN * 8 > T, then padding is removed from Q as follows.  B is
       set to the value of the last octet of Q, converted to an 8-bit
       unsigned integer.  If B > (128 - T)/8, then the FAIL symbol is
       returned, and processing halts.  Otherwise, the octet string P is
       set to all but the final B + 1 octets of Q.

   3.  Z is converted from an octet string to an unsigned integer using
       the S2I routine defined in Section 1.3.

   4.  Z is processed as follows; see Figure 3 for an illustration.

       1.  If Z is between 0 and S-W, inclusive, then Z is rejected.

       2.  If Z is between S-W+1 and S, inclusive, then the bitmask M is
           checked.  If M[S-Z] = 0, then Z is accepted, M[S-Z] is set to
           1, and P is returned as the plaintext.  If M[S-Z] = 1, then Z
           is rejected.

       3.  If Z is between S+1 and S+W, inclusive, then Z is accepted, S
           is set to Z, and P is returned as the plaintext.  The bitmask
           is shifted by Z-S (in the direction of higher indicies).

       4.  If Z is between S+W+1 and R, inclusive, then Z is rejected
           and R is set to Z.

       5.  If Z is between R+1 and R+V, inclusive, then Z is accepted, S
           is set to Z, the bitmask M is set to the all-zero value, and
           P is returned as the plaintext.

       6.  If Z is between R+V+1 and 2^T+1, inclusive, then Z is
           rejected and R is set to Z.

   5.  When Z is rejected, then AERO decryption MUST return the FAIL
       symbol, which indicates that either the message was a replay or a
       forgery attempt.





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                             A       C
                      +------|-------|--------------+
                      |      v       v              |
                      |  +-------------+            |
                  K ---->|  stateless  |            |
                      |  |  decryption |            |
                      |  +-------------+            |
                      |      |       |              |
                      |      v       v              |
                      |   +-----+ +-----+           |
                      |   |strip| | S2I |           |
                      |   +-----+ +-----+           |     AERO
                      |      |       |              |  decryption
                      |      v       v              |
                      |      P       Z              |
                      |              |              |
                      |              v              |
                      |       +-------------+       |
                      |   S-->|    check    |<--R   |
                      |   ^   +-------------+   ^   |
                      |   |    |           |    |   |
                      |  +----------+  +----------+ |
                      |  |  accept  |  |  reject  | |
                      |  +----------+  +----------+ |
                      |        |           |        |
                      +--------|-----------|--------+
                               v           v
                               P   (or)   FAIL

         Figure 2: An illustration of the AERO decryption process.


               check         reject                 reject
              bitmask          and                    and
                 |          set R to Z             set R to Z
       reject    |  accept      |         accept       |
         |       |    |         |           |          |
         v       v    v         v           v          v
     +---------+----+----+----------------+---+----------------+-> Z
     |         |    |    |                |   |                |
     0        S-W   S   S+W               R  R+V             2^T-1

    Figure 3: An illustration of how the candidate sequence number Z is
   processed during AERO decryption.  The possible values of Z are shown
      on a number line from 0 to 2^T-1.   S and  R are taken from the
     decryption context, the region into which  Z falls determines how
    that value is processed.   The acceptance region between R and R+V
           provides resynchronization as described in Section 6.



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5.  Stateless encryption algorithms

5.1.  Wide PRP (WPRP)

   An Pseudo-Random Permutation (PRP) is a keyed function that is both
   invertible and pseudorandom; that is, when the key is chosen at
   random, an attacker cannot distinguish it from an invertible function
   selected at random.  For instance, a block cipher is a PRP with a
   fixed length (and narrow) input and output.

   An encryption algorithm that accepts plaintexts that have varying
   lengths can also be a PRP.  This type of algorithm is called a Wide
   Pseudo-Random Permutation (WPRP), because the inputs can be wider
   than those of a typical block cipher.  For our purposes, the WPRP
   must also accept an associated data input.  For each distinct value
   of the associated data, the WPRP must realize a distinct pseudorandom
   function.  Informally, for each distinct value of A, the WPRP "looks
   like" a distinct PRP.

   A WPRP is used as the underlying stateful encryption encryption
   method as follows.  To encrypt an associated data A, a plaintext P,
   and a sequence number U with a secret key K, first U is appended to
   P, and the WPRP encryption is performed with the key K, the
   associated data A, and the plaintext Q = P || U resulting from the
   concatenation of P and U. The output of the WPRP encryption is
   returned as the ciphertext.  This process is shown in Figure 4.


                             A     P    U
                      +------|-----|----|-----+
                      |      |     |    |     |
                      |      |     v    v     |
                      |      |   +--------+   |
                      |      |   | append |   |
                      |      |   +--------+   |  stateless
                      |      |       |        |  encryption
                      |      |       v        |
                      |      |       Q        |
                      |      |       |        |
                      |      v       v        |
                      |  +-----------------+  |
                  K ---->| WPRP encryption |  |
                      |  +-----------------+  |
                      |           |           |
                      +-----------|-----------+
                                  v
                                  C




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     Figure 4: Using a WPRP as a stateless encryption method in AERO.

   To decrypt a ciphertext C with associated data A with a secret key K,
   first the WPRP decryption is applied to C, using the key K. Then the
   resulting output Q is split into two separate octet strings.  The
   final T bits of the output is returned as U, and the initial len(Q) -
   T bits of Q are returned as P. This process is shown in Figure 5.


                             A       C
                      +------|-------|--------+
                      |      |       |        |
                      |      |       |        |
                      |      v       v        |
                      |  +-----------------+  |
                  K ---->| WPRP decryption |  |
                      |  +-----------------+  |
                      |              |        |
                      |              v        |
                      |              Q        |  stateless
                      |              |        |  decryption
                      |              v        |
                      |          +-------+    |
                      |          | split |    |
                      |          +-------+    |
                      |           |     |     |
                      +-----------|-----|-----+
                                  v     v
                                  P     U

     Figure 5: Using a WPRP as a stateless decryption method in AERO.

   A WPRP has the property that each bit of its output is a pseudorandom
   function of each bit of both of its inputs, for both encryption and
   decryption.  A WPRP is very well suited for use in AERO.  The fact
   that encryption is a WPRP, combined with the use of a sequence
   number, ensures strong confidentiality of the plaintext.  The fact
   that the decryption algorithm is a WPRP ensures that an attacker
   cannot predict (much less control) the candidate sequence number,
   thus ensuring strong integrity, authenticity, and replay protection.

   WPRPs are used in disk-block encryption [1619.2], because they
   provide the best security possible in scenarios where the ciphertext
   cannot be any longer than the plaintext.

   When a WPRP algorithm is used as the EAD algorithm in AERO, we call
   the resulting combination AERO-WPRP.  An AERO-WPRP algorithm has very
   strong security properties, as described in Section 11.



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5.1.1.  WPRP Requirements and survey

   There are many WPRP designs in the cryptographic literature, driven
   largely by interest in disk-block encryption.  However, AERO has some
   requirements that are different from those of disk-block encryption.
   Most importantly, the WPRP encryption algorithm must be able to
   process plaintexts that vary in length, using a single key.  (In
   contrast, all of the blocks in a disk typically have the same length,
   allowing a WPRP design to be optimized for a fixed size.)

   The Advanced Encryption Standard (AES) eXtended Code Book (XCB) mode
   of operation, or AES-XCB [MF07], is a WPRP that can accept plaintexts
   with lengths of 16 bytes or higher.  Internally, it roughly consists
   of a layer of hashing, followed by a layer of counter mode, followed
   by another layer of hashing, with a small amount of additional
   processing.  It was first introduced in 2004, and a second version
   was introduced in 2007, and was standardized in the IEEE [1619.2].
   Recent work has raised questions about the security of the second
   version when used in scenarios in which the lengths of the plaintexts
   vary, which would make it inappropriate for AERO.

   Chakraborty and Sarkar presented HCH, an improved hash-couter-hash
   WPRP mode of operation in 2007, which reduces the amount of
   additional processing and improves the security bound, so that more
   data can be securely encrypted for a fixed key [CS07].

   Sarkar introduced yet another improvement in WPRPs in 2007,
   presenting a mode of operation based on a hash-encrypt-hash design
   [S07].

5.2.  Other cryptographic techniques

   Using a Wide PRP in AERO provides very strong security.  However, it
   is more computationally expensive than some other approaches.  Thus,
   it may be useful to use other techniques that trade some well-
   understood reduction in security properties for lessened
   computational cost.  We do not define any such techniques in this
   note, but we outline the tradeoffs that are possible.

   AEAD algorithms fit into two distinct categories: online and offline.
   With an online encryption algorithm, the ith ciphertext block can be
   output before the (i+1)st plaintext block has to be read (where a
   block is the size of the data blocks processed by a block cipher,
   typically) .  These algorithms make a single pass over the data, and
   implementations need not buffer a significant amount of state.  In
   contrast, with an offline encryption algorithm the entire plaintext
   must be read in before the first block of ciphertext can be output.
   Such algorithms must store state equal in length to the plaintext or



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   ciphertext that they are processing.  In a software environment, an
   offline algorithm is acceptable, as long it processes only plaintexts
   with lengths are short enough that they fit into high-speed random
   access memory.  However, offline algorithms cannot be implemented in
   a hardware pipeline, such as those used in high-speed network
   encryption (for protocols such as 802.1AE).

   A WPRP is necessarily an offline algorithm.  However, an online
   algorithm can implement a weakened variant of a WPRP: an Online
   Pseudo-Random Permutation (OPRP).  An online permutation is a length-
   preserving permutation such that the ith block of output depends only
   on the first i blocks of input, for all values of i.  An OPRP is an
   keyed online permutation that a computationally limited attacker
   cannot distinguish from an online permutation chosen uniformly at
   random from the set of all online permutations.  With an OPRP, an
   attacker will be able to recognize when two messages with common
   plaintext prefixes are encrypted with the same key, since the
   ciphertext prefixes will be identical.

   There are OPRPs in the cryptographic literature that are suitable for
   use in AERO, such as the Pipelineable Online Encryption (POE) mode of
   operation [AFFFLMW14], or the McOE-G [FFLW11] family of online
   authenticated encryption algorithms of Fleischmann, Forler, Lucks,
   and Wenzel.

   There are offline ciphers that are more efficient that WPRPs, such as
   the Synthetic Initialization Vector (SIV) mode of operation of
   Rogaway and Shrimpton [RFC5297].  SIV encryption makes two passes
   over the plaintext, first to compute a Message Authentication Code
   (MAC) of that plaintext, the second to encrypt the plaintext using
   counter mode, using the MAC as the initialization vector for the
   encryption process.  This two-pass encryption approach can be adapted
   for use in AERO, by having the IV computed by the tweakable
   encryption of the sequence number, using the MAC as the tweak.  Such
   a mode of operation could be computationally less costly than a WPRP,
   but would also lack some of the misuse-resistance properties of a
   WPRP.














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6.  Loss, reorder, and resynchronization

   AERO relies on the synchronization of the sequence number S between
   the sender (encryption algorithm) and the receiver (decryption
   algorithm).  If those values differ by more than W, then we say that
   the sender and receiver are desynchronized; if this occurs, then it
   is likely that the decryption algorithm will discard at least one
   legitimate message that it receives.  Desynchronization will occur
   whenever W+1 or more messages are lost by the communication channel
   between the sender and the receiver.

   When initializing an AERO decryption context, the reorder tolerance
   parameter W should be set to a value that is larger than the message
   reorder of the communication channel in use.  However, values of W
   larger than 256 are discouraged for security reasons; see Section 11.
   The default value of W=64 is recommended when AERO ciphertext
   messages are carried over an unreliable transport protocol.  When
   they are carried over a reliable transport protocol, W=1 is
   recommended.

   Resynchronization occurs automatically in AERO, because of the
   processing of candidate sequence numbers between R+1 and R+V. If a
   burst of exactly J messages that are lost, where J > W+1, then the
   receiver will reject the first message received immediately after the
   burst of lost messages, and will set R to the value of the current
   sequence number S of the sender.  The candidate sequence number of
   the next message received will fall into the region between R+1 and
   R+V, since R=S and the candidate sequence number is S+1.  Thus, after
   a burst of J > W lost packets, resynchronization is obtained after
   the spurious rejection of a single packet.

   Most applications can accept the induced loss of a single message in
   situations where J > W or more messages have been lost, since this
   corresponds to an increase in packet loss less than 2% when the
   recommended value of W=64 is used.  However, an application MAY
   choose to cache a rejected message (A, C) and re-submit it to the
   AERO decryption algorithm if it appears to have been spuriously
   rejected during the resynchronization process, to eliminate the
   induced message loss.  An application can do this without any need
   for special processing from an AERO implementation.

   When there are multiple receivers, it may be the case that one of the
   receivers is a "late joiner" that does not receive the initial
   messages sent in a session, because it joined that session some time
   after it began.  The automatic resynchronization described above will
   enable a late joiner to synchronize with the sender after a single
   spurious rejection of a valid message.  If an application protocol is
   designed to allow a late joiner in a session, it is likely that the



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   application can tolerate this one spurious rejection as the cost of
   achieving synchronization without external coordination of state.

   The one free parameter in an AERO implementation is the number T of
   bits in the sequence number.  This parameter determines the strength
   of the authentication provided, as well as the number of messages
   that can be encrypted by a particular sender using a particular key.
   Securty is the main consideration in the selection of this parameter;
   this topic is covered in Section 11.1.1.










































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7.  Usage and applicability

   AERO addresses several outstanding issues in cryptography as it is
   used for communication security.  It is especially well suited for
   scenarios where there are bandwidth constrains or high costs
   associated with transmission and reception, as well as scenarios in
   which multiple senders or multiple receivers share an encryption key.
   It is also suitable for use when misuse resistance is desirable.

   AERO always detects replayed messages, as is expected of a replay
   protection service, but AERO may reject a message because it does not
   have all of the stored state that it needs in able to authoritatively
   determine that the message is not a replay.  In this way, it matches
   the conventional practice of replay protection using sequence
   numbers.

   The sequence number output by the decryption algorithm can be used by
   the application calling AERO to put the decrypted plaintexts into
   their correct order.  Some applications will not need this
   information, because they can rely on information in the associated
   data or plaintext to get information about the ordering of messages,
   or because they do not need to process the messages in order.

   When AERO ciphertext messages are carried above a reliable transport
   protocol such as TCP or SCCP, the reorder tolerance parameter W can
   be set to one and the resynchronization parameter V can be set to
   zero.  This has the effect of improving the strength of the
   authentication that is provided.

   AERO-WPRP should be used, unless it is not possible to buffer the
   entire plaintext or ciphertext in memory during the encryption and
   decryption processes, in which case an AERO-OPRP would be more
   appropriate.  Currently, there are no OPRP algorithms that can be
   implemented in an efficient pipeline, as is done with other AEAD
   algorithms such as AES-GCM, so this is an area of ongoing work.
















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8.  Rationale

   AES-XCB was chosen as it provides suitable security and performance,
   and it is a standard.  In the future, stateless encryption algorithms
   with performance or security benefits over AES-XCB may be developed,
   but it is essential to specify a particular algorithm in this note,
   in order to gain experience in the implementation and use of AERO.

   It is unfortunate, but unavoidable, that the plaintext must be
   padded.  It is unfortunate because of the slight additional overhead
   and implementation complexity.  It is unavoidable because there is no
   efficient way to encrypt fewer than b bytes of plaintext using a
   block cipher with a b-byte block, in a way that meets the security
   needs of AERO.  Also, there are not (yet) any dedicated encryption
   algorithms that are suitable.

   The padding and pad-stripping operations are included in the AERO
   encryption and decryption algorithms, respectively, because the pad-
   stripping operation also incorporates an authentication check.  This
   arrangement simplifies the stateless encryption algorithm, and also
   ensures that more algorithms can be used as stateless encryption
   methods.

   Incorporating the sequence number generation inside of AERO has the
   advantage of putting that security-critical operation inside of the
   cryptographic module, which often benefits from higher assurance than
   other system components, including testing, as observed by Sections 5
   and 6 of [IV-GEN].























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9.  Testing

   In the typical case in which the underlying stateless encryption
   algorithm is deterministic, both AERO encryption and AERO decryption
   are deterministic.  This makes it possible to use test cases for all
   of the operations supported by an AERO algorithm.













































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10.  IANA Considerations

   The Internet Assigned Numbers Authority (IANA) has defined the AERO
   algorithm registry described below.  An algorithm designer MAY
   register an algorithm in order to facilitate its use.  Additions to
   the AERO algorithm 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 "AERO_AES_128_XCB", that starts with the
      string "AERO",

      a positive number, and

      a reference to a specification that completely defines an AERO
      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].

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















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11.  Security considerations

   There are three AERO security services: confidentiality, message
   authentication, and replay protection.  We consider those services in
   that order.  We use a simplistic but realistic model in which
   pseudorandom values are treated as random values.

   A more precise analysis would define the advantage that an adversary
   has in distinguishing the pseudorandom output of the stateless
   encryption algorithm from a truly random value, then quantify an
   attacker's advantage at forging AERO messages or distinguishing the
   output of the decryption algorithm from random, and then show that
   the attacker's advantage against AERO is negligibly small as long as
   the advantage against the stateless encryption algorithm is
   negligibly small.  However, such a detailed analysis is beyond the
   scope of this note.

   Below we sketch out a formal security analysis for AERO, using the
   conventional definition of advantage as the absolute value of the
   difference between the probability that a distinguisher will have a
   true positive and the probability that it will have a false positive.
   AERO is secure in the following model, which captures the idea of a
   very strong adversary.  The attacker is assumed to be able to
   adaptively choose plaintext messages (A, P) and ciphertext messages
   (A, C), and obtain the corresponding ciphertext and plaintext
   messages.

   Assume that there is an attack that can distinguish AERO from a
   randomly chosen function with the same domain and range.  Then we can
   use this attack to build a distinguisher that can tell the underlying
   stateless encryption function from a truly random function, as
   follows.  Given access to an oracle that is either the stateless
   encryption function with a randomly chosen secret key, or is a truly
   random function, we use that oracle to construct an AERO instance.
   We then run the AERO-distinguishing attack against that AERO
   instance, responding to the the queries made by the attack by
   constructing the appropriate AERO queries.  If the attack returns a
   guess of "AERO", then we return a guess of "not random".  Otherwise,
   return a guess of "random".  This attack on the underlying encryption
   functions works with essentially the same advantage as the attack on
   AERO.

   Similarly, if there is an attack that can forge AERO messages with
   probability significantly greater than 1/2^T' (where T' is the
   effective authentication tag length defined below), then we can use
   that attack to build a distinguisher that can tell the underlying
   stateless encryption function from a truly random function, as
   follows.  Given access to an oracle that is either the stateless



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   encryption function with a randomly chosen secret key, or is a truly
   random function, we use that oracle to construct an AERO instance.
   We then run the forgery attack against that AERO instance, responding
   to the queries made by the attack by constructing the appropriate
   AERO queries.  If the attack against AERO succeeds in making a
   forgery, then we return a guess of "not random".  Otherwise, return a
   guess of "random".  This distinguishing attack works with advantage P
   - 1/2^T', where P is the probability that the AERO-forgery attack
   will succeed in a forgery attempt.

   AERO is a stateful authenticated encryption method.  Bellare, Kohno,
   and Namprempre have studied the security of stateful encrption and
   decryption in the context of the Secure Shell (SSH) protocol [BKN04].

11.1.  Authentication and confidentiality

   The confidentiality properties of AERO follow directly from that of
   the stateless encryption algorithm that it incorporates.  When a WPRP
   is used, each WPRP plaintext will be distinct, because of the
   uniqueness of the sequence numbers.  This fact ensures the
   indistinguishability of AERO encryption from a random function, and
   thus the strength of the confidentiality that it provides.

   AERO decryption can detect forgery attempts in two ways: the
   processing of the candidate sequence number, and the strip function
   that removes padding from the plaintext.  The candidate sequence
   number is a pseudorandom function of both the AERO ciphertext and the
   AERO associated data, as is the post-decryption padding string PS.
   To an attacker that cannot distinguish the pseudorandom value from a
   random one, the probability p that a forgery attempt will not be
   detected by the candidate sequence number processing is



                 (2*W + V)
            p = ------------.
                    2^T

   To an attacker that cannot distinguish the post-decryption value of
   PS from a random string, the probability ps that a forgery attempt
   will pass through the "strip" function undetected is

                 (PMIN - T/8)
           ps = -------------.
                     256

   Thus, the probability that a forgery attempt will go undetected is
   1/2^T', where



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           T' = T + 8 - lg((2*W + V) - lg(PMIN - T/8))

   and lg(X) denotes the logarithm base two of X. As revealed by the
   equation above, AERO provides integrity and authentication protection
   that is essentially equivalent to an ideal message authentication
   code with a tag length of T' which is slightly less than T. The
   following table shows T' as a function of T, using the default values
   of W=64 and V=8 with PMIN=16, to three significant figures.  The
   third column shows the number of bits in the data "on the wire"
   required by AERO.  The final column shows the delta between the
   number of bits on the wire and the equivalent tag size T'.

                    +-----+------+-----------+-------+
                    |   T |   T' | Data size | Delta |
                    +-----+------+-----------+-------+
                    | 128 |  121 |       128 |   7.0 |
                    |     |      |           |       |
                    | 120 |  121 |       128 |   7.0 |
                    |     |      |           |       |
                    | 112 |  112 |       120 |   8.0 |
                    |     |      |           |       |
                    | 104 |  103 |       112 |   9.0 |
                    |     |      |           |       |
                    |  96 | 94.9 |       104 |   9.1 |
                    |     |      |           |       |
                    |  88 | 86.6 |        96 |   9.4 |
                    |     |      |           |       |
                    |  72 | 70.1 |        80 |   9.9 |
                    |     |      |           |       |
                    |  64 | 61.9 |        72 |  10.1 |
                    |     |      |           |       |
                    |  56 | 53.7 |        64 |  10.3 |
                    |     |      |           |       |
                    |  48 | 45.6 |        56 |  10.4 |
                    |     |      |           |       |
                    |  40 | 37.5 |        48 |  10.5 |
                    |     |      |           |       |
                    |  32 | 29.3 |        40 |  10.7 |
                    |     |      |           |       |
                    |  28 | 25.3 |        32 |   6.7 |
                    +-----+------+-----------+-------+

   The delta values can be considered to be the data overhead inherent
   in AERO, that is, the amount of additional data that must be included
   in a message in order to provide the replay protection, sequencing
   information, and self-synchronization capability.  For higher
   authentication strengths, this overhead is roughly one byte;
   otherwise, it is slightly higher.



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   When AERO is used over a reliable transport protocol, then the
   reorder tolerance parameter W SHOULD be set to one, and the
   resynchronization parameter V SHOULD be set to zero.  When this is
   done, the delta values are about 7 bits smaller, and for a given
   amount of data overhead, the probability of a successful forgery is
   smaller by a factor of about 1/128.

11.1.1.  Authentication strength

   As always, stronger authentication, and thus larger values of T',
   should be preferred over weaker authentication, and smaller values
   of T'.  We recommend the use of T' = 121 (with 128 bits on the wire)
   whenever possible.  When bandwidth is at a premium, it may be
   acceptable to use T' = 53.7 (64 bits on the wire).

   When the plaintext is audio or video data to be converted to an
   analog signal and played out of an audio speaker or video monitor, it
   may be acceptable to use even weaker authentication, whenever the
   consequences of a single forgery are limited to a localized "glitch"
   in the audio or video output.  In such cases, an authentication
   strength of T' = 25.3 (32 bits on the wire) may be acceptable, but
   users are cautioned to verify these criteria before using such weak
   authentication.  With these parameters, the likelihood of a
   successful forgery is one in 32 million.

11.2.  Length hiding

   In some applications, it is desirable to hide the length of the
   plaintext from an attacker, in order to prevent the attacker from
   being able to make inferences about the plaintext based on those
   lengths.  The plaintext padding scheme defined in this note is not
   suitable for this purpose and it MUST NOT be used as such.  An
   application that seeks to hide plaintext lengths should instead
   process the plaintexts with a fragmentation and encoding scheme prior
   to encrypting them.  Schemes of this sort are out of scope of this
   note.

11.3.  Denial of Service (DoS)

   An attacker can potentially flood a communications channel with a set
   of bogus ciphertext messages in order to consume the computational
   resources of the receiver.  In this section we review the
   vulnerability of AERO to this type of attack.

   AERO synchronization is robust even in the face of a DoS attack.  An
   AERO sender and receiver will stay in synchronization unless either
   1) the attacker succeeds in forging a message, or 2) a burst of at
   least B messages have been lost.  The forgery likelihood is



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   determined by the parameters T, V, and W, and it can be set high
   enough that the attacker's chance of success is negligible.  A DoS
   attack may cause bursts of lost messages, by causing congestion on
   communication links.  But AERO recovers quickly once that loss is
   over.

   AERO requires that all of the cryptographic processing be done during
   the decryption algorithm in order to detect a forgery attempt.  This
   is in contrast to other authenticated encryption algorithms (e.g.
   GCM) that can avoid doing decryption processing, since the
   authentication check can be completed before the encryption starts.
   When AERO is in use on a communication environment where the data
   rate of the communications greatly exceeds that of AERO decryption,
   it may be beneficial to use additional mechanisms that allow the
   receiver to reject bogus messages.  This could include having a
   fixed, unpredictable value in each message in order to foil off-path
   DoS attacks.  To defend against on-path attackers, cryptographic
   techniques can be used.  These techniques are beyond the scope of
   this document.

11.4.  Multiple senders

   Section 3.5 describes the sender uniqueness requirement: a set of
   senders sharing the same key are recommended to ensure that their
   associated data inputs to the encryption algorithm will be distinct,
   for instance by including information in the associated data that
   uniquely identifies each sender.  If the senders do not meet this
   requirement, then the security will degrade somewhat.  The details of
   this degradation depend on the details of the underlying encryption
   method.

   When a WPRP is used in AERO and the sender uniqueness requirement is
   not met, an attacker can recognize when the same exact plaintext
   message (A, P) is encrypted by different senders with the same
   sequence number.  Symbolically, if sender S1 and sender S2 both use
   the same associated data value A and plaintext value P in the ith
   message in sequence that they are each independently encrypting, then
   the resulting ciphertexts will be identical, and an attacker can
   infer that the plaintexts are matching.  In many real-world
   scenarios, it is highly unlikely that distinct senders will each
   encrypt the same plaintext value for the same sequence number, and
   thus the security degradation will be tolerable.  If, however, the
   plaintexts are either very short (such as eight or fewer octets) or
   very repetitive (there are only a million possible plaintexts, for
   instance), then the loss of confidentiality may be unacceptably high.

   When the sender uniqueness requirement is not met, the receiver will
   be processing messages from two or more senders, while believing it



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   to be from a single sender.  The decryption algorithm will
   synchronize with the highest sequence number of any sender, and
   reject the messages from other senders, if the highest sequence
   number is at least W greater than all other sequence numbers.  If the
   sequence numbers of the senders are close, then the receiver will
   interleave messages from different senders.  In the latter case, an
   attacker could potentially manipulate what the post-decryption
   plaintext looks like by selectively withholding messages from
   particular senders.  Note that this property is not particular to
   AERO; it holds for any authenticated encryption scheme in which
   multiple entities are sharing the encryption key, and the receiver
   has no way of determining which entity encrypted a particular
   ciphertext message.

   These security properties compare very favorably with those of
   conventional authenticated encryption schemes.  When the sender
   uniqueness requirement is not met with a nonce-based encryption
   scheme such as counter mode, CCM, GCM, Salsa or ChaCha, then
   essentially all of the confidentiality guarantees are lost.  For
   nonce-based authentication schemes such as GCM, GMAC, and UMAC, all
   of the authentication guarantees are lost.

11.5.  Sequence number management failure

   An AERO implementation might accidentally mismanage sequence numbers,
   due to faulty logic in the implementation, or due to external factors
   such as the cloning of the state of the virtual machine on which the
   implementation is running.  If this happens, security will degrade.
   In order to ensure that the degradation is not catastrophic, the
   underlying stateless encryption algorithm should be robust against
   nonce misuse, in the sense of [RFC5297].




















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12.  Acknowledgements

   The authors thank Dan Wing, Stefan Lucks, and Brian Weis for feedback
   and discussions, and Richard Barnes, Kenny Paterson, and Robert
   Moskowitz for suggestions and corrections.














































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13.  References

13.1.  Normative references

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

13.2.  Informative references

   [1619.2]   "IEEE 1619.2-2010: Standard for Wide-Block Encryption for
              Shared Storage Media", IEEE Computer Society https://
              standards.ieee.org/findstds/standard/1619.2-2010.html,
              2010.

   [AFFFLMW14]
              Abed, Fluhrer, Foley, Forler, Lucks, McGrew, and Wenzel,
              "Pipelineable Online Encryption", Proceedings of the 21st
              international conference on Fast Software Encryption (FSE
              2014) .

   [BKN04]    Bellare, Kohno, and Namprempre, "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, 7(2), May
              2004. http://homes.cs.washington.edu/~yoshi/papers/SSH/.

   [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.

   [CCM]      Dworkin, "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.

   [CS07]     Chakraborty, D. and P. Sarkar, "HCH: A New Tweakable
              Enciphering Scheme Using the Hash-Counter-Hash Approach",
              IACR eprint archive  http://eprint.iacr.org/2007/28, 2007.




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   [FFLW11]   Fleischmann, Forler, Lucks, and Wenzel, "McOE: a family of
              almost foolproof on-line authenticated encryption
              schemes", Proceedings of the 19th international conference
              on Fast Software Encryption (FSE 2011) .

   [FIPS197]  "FIPS 197: Advanced Encryption Standard (AES)", Federal
              Information Processing Standard
              (FIPS) http://www.itl.nist.gov/fipspubs/fip197.htm.

   [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.

   [IV-GEN]   McGrew, D., "Generation of Deterministic Initialization
              Vectors (IVs) and Nonces", IETF Internet
              Draft http://www.ietf.org/id/draft-mcgrew-iv-gen-03.txt,
              October 2013.

   [MF07]     McGrew, D. and S. Fluhrer, "The Security of the Extended
              Codebook (XCB) Mode of Operation", 14th Workshop on
              Selected Areas in Cryptography (SAC
              2007) http://eprint.iacr.org/2007/298, August 2007.

   [R02]      Rogaway, P., "Authenticated encryption with Associated-
              Data", Proceedings of the 2002 ACM Conference on Computer
              and Communication Security (CCS'02), pp. 98-107, ACM
              Press,
              2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.pdf.

   [RFC4418]  Krovetz, T., "UMAC: Message Authentication Code using
              Universal Hashing", RFC 4418, March 2006.

   [RFC4771]  Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
              Transform Carrying Roll-Over Counter for the Secure Real-
              time Transport Protocol (SRTP)", RFC 4771, January 2007.

   [RFC5297]  Harkins, D., "Synthetic Initialization Vector (SIV)
              Authenticated Encryption Using the Advanced Encryption
              Standard (AES)", RFC 5297, October 2008.

   [S07]      Sarkar, P., "Improving Upon the TET Mode of Operation",
              IACR eprint archive  http://eprint.iacr.org/2007/317,
              2007.

   [SP800-38]
              Dworkin, M., "NIST Special Publication 800-38:
              Recommendation for Block Cipher Modes of Operation", U.S.



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              National Institue of Standards and Technology http://
              csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf.

















































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Authors' Addresses

   David McGrew
   Cisco Systems
   13600 Dulles Technology Drive
   Herndon, VA  20171
   US

   Email: mcgrew@cisco.com
   URI:   http://www.mindspring.com/~dmcgrew/dam.htm


   John Foley
   Cisco Systems
   7025-2 Kit Creek Road
   Research Triangle Park, NC  14987
   US

   Email: foleyj@cisco.com
































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