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rfc:rfc5649

Network Working Group R. Housley Request for Comments: 5649 Vigil Security Category: Informational M. Dworkin

                                                                  NIST
                                                           August 2009
 Advanced Encryption Standard (AES) Key Wrap with Padding Algorithm

Abstract

 This document specifies a padding convention for use with the AES Key
 Wrap algorithm specified in RFC 3394.  This convention eliminates the
 requirement that the length of the key to be wrapped be a multiple of
 64 bits, allowing a key of any practical length to be wrapped.

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright and License Notice

 Copyright (c) 2009 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
 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 BSD License.

Housley & Dworkin Informational [Page 1] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

1. Introduction

 Management of cryptographic keys often leads to situations where one
 symmetric key is used to encrypt and integrity-protect another key,
 which can be either a symmetric key or an asymmetric key.  The
 operation is often called key wrapping.
 This document specifies an extension of the Advanced Encryption
 Standard (AES) Key Wrap algorithm [AES-KW1, AES-KW2].  Without this
 extension, the input to the AES Key Wrap algorithm, called the key
 data, must be a sequence of two or more 64-bit blocks.
 The AES Key Wrap with Padding algorithm can be used to wrap a key of
 any practical size with an AES key.  The AES key-encryption key (KEK)
 must be 128, 192, or 256 bits.  The input key data may be as short as
 one octet, which will result in an output of two 64-bit blocks (or 16
 octets).  Although the AES Key Wrap algorithm does not place a
 maximum bound on the size of the key data that can be wrapped, this
 extension does so.  The use of a 32-bit fixed field to carry the
 octet length of the key data bounds the size of the input at 2^32
 octets.  Most systems will have other factors that limit the
 practical size of key data to much less than 2^32 octets.
 A message length indicator (MLI) is defined as part of an
 "Alternative Initial Value" in keeping with the statement in Section
 2.2.3.2 of [AES-KW1], which says:
    Also, if the key data is not just an AES key, it may not always be
    a multiple of 64 bits.  Alternative definitions of the initial
    value can be used to address such problems.

2. Notation and Definitions

 The following notation is used in the algorithm descriptions:
    MSB(j, W)   Return the most significant j bits of W
    LSB(j, W)   Return the least significant j bits of W
    ENC(K, B)   AES Encrypt the 128-bit block B using key K
    DEC(K, B)   AES Decrypt the 128-bit block B using key K
    V1 | V2     Concatenate V1 and V2
    K           The key-encryption key
    m           The number of octets in the key data
    n           The number of 64-bit blocks in the padded key data
    Q[i]        The ith plaintext octet in the key data
    P[i]        The ith 64-bit plaintext block in the padded key data
    C[i]        The ith 64-bit ciphertext data block
    A           The 64-bit integrity check register

Housley & Dworkin Informational [Page 2] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

3. Alternative Initial Value

 The Alternative Initial Value (AIV) required by this specification is
 a 32-bit constant concatenated to a 32-bit MLI.  The constant is (in
 hexadecimal) A65959A6 and occupies the high-order half of the AIV.
 Note that this differs from the high order 32 bits of the default IV
 in Section 2.2.3.1 of [AES-KW1], so there is no ambiguity between the
 two.  The 32-bit MLI, which occupies the low-order half of the AIV,
 is an unsigned binary integer equal to the octet length of the
 plaintext key data, in network order -- that is, with the most
 significant octet first.  When the MLI is not a multiple of 8, the
 key data is padded on the right with the least number of octets
 sufficient to make the resulting octet length a multiple of 8.  The
 value of each padding octet shall be 0 (eight binary zeros).
 Notice that for a given number of 64-bit plaintext blocks, there are
 only eight values of MLI that can have that outcome.  For example,
 the only MLI values that are valid with four 64-bit plaintext blocks
 are 32 (with no padding octets), 31 (with one padding octet), 30, 29,
 28, 27, 26, and 25 (with seven padding octets).  When the unwrapping
 process specified below yields n 64-bit blocks of output data and an
 AIV, the eight valid values for the MLI are 8*n, (8*n)-1, ..., and
 (8*n)-7.  Therefore, integrity checking of the AIV, which is
 contained in a 64-bit register called A, requires the following
 steps:
 1) Check that MSB(32,A) = A65959A6.
 2) Check that 8*(n-1) < LSB(32,A) <= 8*n.  If so, let
    MLI = LSB(32,A).
 3) Let b = (8*n)-MLI, and then check that the rightmost b octets of
    the output data are zero.
 If all three checks pass, then the AIV is valid.  If any of the
 checks fail, then the AIV is invalid and the unwrapping operation
 must return an error.

4. Specification of the AES Key Wrap with Padding Algorithm

 The AES Key Wrap with Padding algorithm consists of a wrapping
 process and an unwrapping process, both based on the AES codebook
 [AES].  It provides an extension to the AES Key Wrap algorithm
 [AES-KW1, AES-KW2] that eliminates the requirement that the length of
 the key to be wrapped be a multiple of 64 bits.  The next two
 sections specify the wrapping and unwrapping processes, called the

Housley & Dworkin Informational [Page 3] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

 extended key wrapping process and the extended key unwrapping
 process, respectively.  These names distinguish these processes from
 the ones specified in [AES-KW1] and [AES-KW2].

4.1. Extended Key Wrapping Process

 The inputs to the extended key wrapping process are the KEK and the
 plaintext to be wrapped.  The plaintext consists of between one and
 2^32 octets, containing the key data being wrapped.  The key wrapping
 process is described below.
 Inputs:  Plaintext, m octets {Q[1], Q[2], ..., Q[m]}, and
          Key, K (the KEK).
 Outputs: Ciphertext, (n+1) 64-bit values {C[0], C[1], ..., C[n]}.
 1) Append padding
    If m is not a multiple of 8, pad the plaintext octet string on the
    right with octets {Q[m+1], ..., Q[r]} of zeros, where r is the
    smallest multiple of 8 that is greater than m.  If m is a multiple
    of 8, then there is no padding, and r = m.
    Set n = r/8, which is the same as CEILING(m/8).
    For i = 1, ..., n
       j = 8*(i-1)
       P[i] = Q[j+1] | Q[j+2] | ... | Q[j+8].
 2) Wrapping
    If the padded plaintext contains exactly eight octets, then
    prepend the AIV as defined in Section 3 above to P[1] and encrypt
    the resulting 128-bit block using AES in ECB mode [Modes] with key
    K (the KEK).  In this case, the output is two 64-bit blocks C[0]
    and C[1]:
       C[0] | C[1] = ENC(K, A | P[1]).
    Otherwise, apply the wrapping process specified in Section 2.2.1
    of [AES-KW2] to the padded plaintext {P[1], ..., P[n]} with K (the
    KEK) and the AIV as defined in Section 3 above as the initial
    value.  The result is n+1 64-bit blocks {C[0], C[1], ..., C[n]}.

Housley & Dworkin Informational [Page 4] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

4.2. Extended Key Unwrapping Process

 The inputs to the extended key unwrapping process are the KEK and
 (n+1) 64-bit ciphertext blocks consisting of a previously wrapped
 key.  If the ciphertext is a validly wrapped key, then the unwrapping
 process returns n 64-bit blocks of padded plaintext, which are then
 mapped in this extension to m octets of decrypted key data, as
 indicated by the MLI embedded in the AIV.
 Inputs:  Ciphertext, (n+1) 64-bit blocks {C[0], C[1], ..., C[n]}, and
          Key, K (the KEK).
 Outputs: Plaintext, m octets {Q[1], Q[2], ..., Q[m]}, or an error.
 1) Key unwrapping
    When n is one (n=1), the ciphertext contains exactly two 64-bit
    blocks (C[0] and C[1]), and they are decrypted as a single AES
    block using AES in ECB mode [Modes] with K (the KEK) to recover
    the AIV and the padded plaintext key:
       A | P[1] = DEC(K, C[0] | C[1]).
    Otherwise, apply Steps 1 and 2 of the unwrapping process specified
    in Section 2.2.2 of [AES-KW2] to the n+1 64-bit ciphertext blocks,
    {C[0], C[1], ..., C[n]}, and to the KEK, K.  Define the padded
    plaintext blocks, {P[1], ..., P[n]}, as specified in Step 3 of
    that process, with A[0] as the A value.  Note that checking "If
    A[0] is an appropriate value" is slightly delayed to Step 2 below
    since the padded plaintext is needed to perform this verification
    when the AIV is used.
 2) AIV verification
    Perform the three checks described in Section 3 above on the
    padded plaintext and the A value.  If any of the checks fail, then
    return an error.
 3) Remove padding
    Let m = the MLI value extracted from A.
    Let P = P[1] | P[2] | ... | P[n].
    For i = 1, ... , m
     Q[i] = LSB(8, MSB(8*i, P))

Housley & Dworkin Informational [Page 5] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

5. Algorithm Identifiers

 Some security protocols employ ASN.1 [X.680] and employ algorithm
 identifiers to name cryptographic algorithms.  To support these
 protocols, the AES Key Wrap with Padding algorithm has been assigned
 the following algorithm identifiers, one for each AES KEK size.  The
 AES Key Wrap (without padding) algorithm identifiers are also
 included here for convenience.
    aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
              us(840) organization(1) gov(101) csor(3)
              nistAlgorithm(4) 1 }
    id-aes128-wrap     OBJECT IDENTIFIER ::= { aes 5 }
    id-aes128-wrap-pad OBJECT IDENTIFIER ::= { aes 8 }
    id-aes192-wrap     OBJECT IDENTIFIER ::= { aes 25 }
    id-aes192-wrap-pad OBJECT IDENTIFIER ::= { aes 28 }
    id-aes256-wrap     OBJECT IDENTIFIER ::= { aes 45 }
    id-aes256-wrap-pad OBJECT IDENTIFIER ::= { aes 48 }
 In all cases, the AlgorithmIdentifier parameter field MUST be absent.

6. Padded Key Wrap Examples

 The examples in this section were generated using the index-based
 implementation of the AES Key Wrap algorithm along with the padding
 approach specified in Section 4 of this document.  All values are
 shown in hexadecimal.
 The first example wraps 20 octets of key data with a 192-bit KEK.
    KEK   :  5840df6e29b02af1 ab493b705bf16ea1 ae8338f4dcc176a8
    Key   :  c37b7e6492584340 bed1220780894115 5068f738
    Wrap  :  138bdeaa9b8fa7fc 61f97742e72248ee 5ae6ae5360d1ae6a
          :  5f54f373fa543b6a
 The second example wraps 7 octets of key data with a 192-bit KEK.
    KEK   :  5840df6e29b02af1 ab493b705bf16ea1 ae8338f4dcc176a8
    Key   :  466f7250617369
    Wrap  :  afbeb0f07dfbf541 9200f2ccb50bb24f

Housley & Dworkin Informational [Page 6] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

7. Security Considerations

 Implementations must protect the key-encryption key (KEK).
 Compromise of the KEK may result in the disclosure of all keys that
 have been wrapped with the KEK, which may lead to the compromise of
 all traffic protected with those wrapped keys.
 The KEK must be at least as good as the keying material it is
 protecting.
 If the KEK and wrapped key are associated with different
 cryptographic algorithms, the effective security provided to data
 protected with the wrapped key is determined by the weaker of the two
 algorithms.  If, for example, data is encrypted with 128-bit AES and
 that AES key is wrapped with a 256-bit AES key, then at most 128 bits
 of protection is provided to the data.  If, for another example, a
 128-bit AES key is used to wrap a 4096-bit RSA private key, then at
 most 128 bits of protection is provided to any data that depends on
 that private key.  Thus, implementers must ensure that key-encryption
 algorithms are at least as strong as other cryptographic algorithms
 employed in an overall system.
 The AES Key Wrap and the AES Key Wrap with Padding algorithms use
 different constants in the initial value.  The use of different
 values ensures that the recipient of padded key data cannot
 successfully unwrap it as unpadded key data, or vice versa.  This
 remains true when the key data is wrapped using the AES Key Wrap with
 Padding algorithm but no padding is needed.
 The AES Key Wrap with Padding algorithm provides almost the same
 amount of integrity protection as the AES Key Wrap algorithm.
 A previous padding technique was specified for wrapping Hashed
 Message Authentication Code (HMAC) keys with AES [OLD-KW].  The
 technique in this document is more general; the technique in this
 document is not limited to wrapping HMAC keys.
 In the design of some high assurance cryptographic modules, it is
 desirable to segregate cryptographic keying material from other data.
 The use of a specific cryptographic mechanism solely for the
 protection of cryptographic keying material can assist in this goal.
 The AES Key Wrap and the AES Key Wrap with Padding are such
 mechanisms.  System designers should not use these algorithms to
 encrypt anything other than cryptographic keying material.

Housley & Dworkin Informational [Page 7] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

8. References

8.1. Normative References

 [AES]     National Institute of Standards and Technology, FIPS Pub
           197: Advanced Encryption Standard (AES), 26 November 2001.
 [AES-KW1] National Institute of Standards and Technology, AES Key
           Wrap Specification, 17 November 2001.
           http://csrc.nist.gov/groups/ST/toolkit/documents/kms/
           AES_key_wrap.pdf
 [AES-KW2] Schaad, J. and R. Housley, "Advanced Encryption Standard
           (AES) Key Wrap Algorithm", RFC 3394, September 2002.
 [Modes]   Dworkin, M., "Recommendation for Block Cipher Modes of
           Operation -- Methods and Techniques",  NIST Special
           Publication 800-38A, 2001.
 [X.680]   ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
           Information technology - Abstract Syntax Notation One
           (ASN.1):  Specification of basic notation.

8.2. Informative References

 [AES-CMS] Schaad, J., "Use of the Advanced Encryption Standard (AES)
           Encryption Algorithm in Cryptographic Message Syntax
           (CMS)", RFC 3565, July 2003.
 [CMS-ASN] Schaad, J. and P. Hoffman, "New ASN.1 Modules for CMS and
           S/MIME", Work in Progress, August 2009.
 [OLD-KW]  Schaad, J. and R. Housley, "Wrapping a Hashed Message
           Authentication Code (HMAC) key with a Triple-Data
           Encryption Standard (DES) Key or an Advanced Encryption
           Standard (AES) Key", RFC 3537, May 2003.
 [X.681]   ITU-T Recommendation X.681 (2002) | ISO/IEC 8824-2:2002,
           Information Technology - Abstract Syntax Notation One:
           Information Object Specification.
 [X.682]   ITU-T Recommendation X.682 (2002) | ISO/IEC 8824-3:2002,
           Information Technology - Abstract Syntax Notation One:
           Constraint Specification.
 [X.683]   ITU-T Recommendation X.683 (2002) | ISO/IEC 8824-4:2002,
           Information Technology - Abstract Syntax Notation One:
           Parameterization of ASN.1 Specifications.

Housley & Dworkin Informational [Page 8] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

9. Acknowledgments

 Paul Timmel should be credited with the MLI and padding technique
 described in this document.

Housley & Dworkin Informational [Page 9] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

Appendix A. ASN.1 Modules

 This appendix includes two ASN.1 modules.  The first one makes use of
 the 1988 syntax, and the second one makes use of the 2002 ASN.1
 syntax.
 Appendix A.1 provides the normative ASN.1 definitions for the
 algorithm identifiers included in this specification using ASN.1 as
 defined in [X.680] using the 1988 ASN.1 syntax.
 Appendix A.2 provides informative ASN.1 definitions for the algorithm
 identifiers included in this specification using ASN.1 as defined in
 [X.680], [X.681], [X.682], and [X.683] using the 2002 ASN.1 syntax.
 This appendix contains the same information as Appendix A.1; however,
 Appendix A.1 takes precedence in case of conflict.  The content
 encryption and key wrap algorithm objects are defined in [CMS-ASN].
 The id-aes128-wrap, id-aes192-wrap, and id-aes256-wrap algorithm
 identifiers are defined in [AES-CMS].

A.1. 1988 ASN.1 Module

 AESKeyWrapWithPad-88 { iso(1) member-body(2) us(840) rsadsi(113549)
   pkcs(1) pkcs-9(9) smime(16) modules(0) 47 }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS ALL –
  1. - IMPORTS NONE –
  1. - AES information object identifiers –
 aes OBJECT IDENTIFIER ::= {
   joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
   csor(3) nistAlgorithms(4) 1 }
  1. - AES Key Wrap With Padding Algorithm Identifiers are to be used
  2. - with the Parameter field absent
 id-aes128-wrap-pad OBJECT IDENTIFIER ::= { aes 8 }
 id-aes192-wrap-pad OBJECT IDENTIFIER ::= { aes 28 }
 id-aes256-wrap-pad OBJECT IDENTIFIER ::= { aes 48 }
 END

Housley & Dworkin Informational [Page 10] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

A.2. 2002 ASN.1 Module

 AESKeyWrapWithPad-02 { iso(1) member-body(2) us(840) rsadsi(113549)
   pkcs(1) pkcs-9(9) smime(16) modules(0) 48 }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS ALL –
 IMPORTS
   AlgorithmIdentifier{}, CONTENT-ENCRYPTION, KEY-WRAP, SMIME-CAPS
   FROM AlgorithmInformation-2009  -- [CMS-ASN]
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-algorithmInformation-02(58) };
 AES-ContentEncryption CONTENT-ENCRYPTION ::= {
   cea-aes128-wrap-pad |
   cea-aes192-wrap-pad |
   cea-aes256-wrap-pad,
   ... }
 AES-KeyWrap KEY-WRAP ::= {
   kwa-aes128-wrap-pad |
   kwa-aes192-wrap-pad |
   kwa-aes256-wrap-pad,
   ... }
 SMimeCaps SMIME-CAPS ::= {
   cea-aes128-wrap-pad.&smimeCaps |
   cea-aes192-wrap-pad.&smimeCaps |
   cea-aes256-wrap-pad.&smimeCaps |
   kwa-aes128-wrap-pad.&smimeCaps |
   kwa-aes192-wrap-pad.&smimeCaps |
   kwa-aes256-wrap-pad.&smimeCaps,
   ... }
  1. - AES object identifier
 aes OBJECT IDENTIFIER ::= {
   joint-iso-itu-t(2) country(16) us(840) organization(1)
   gov(101) csor(3) nistAlgorithms(4) 1 }

Housley & Dworkin Informational [Page 11] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

  1. - Content Encryption Algorithms
 cea-aes128-wrap-pad CONTENT-ENCRYPTION ::= {
   IDENTIFIER id-aes128-wrap-pad
   PARAMS ARE absent
   SMIME-CAPS { IDENTIFIED BY id-aes128-wrap-pad } }
 cea-aes192-wrap-pad CONTENT-ENCRYPTION ::= {
   IDENTIFIER id-aes192-wrap-pad
   PARAMS ARE absent
   SMIME-CAPS { IDENTIFIED BY id-aes192-wrap-pad } }
 cea-aes256-wrap-pad CONTENT-ENCRYPTION ::= {
   IDENTIFIER id-aes256-wrap-pad
   PARAMS ARE absent
   SMIME-CAPS { IDENTIFIED BY id-aes256-wrap-pad } }
  1. - Key Wrap Algorithms
 kwa-aes128-wrap-pad KEY-WRAP ::= {
   IDENTIFIER id-aes128-wrap-pad
   PARAMS ARE absent
   SMIME-CAPS { IDENTIFIED BY id-aes128-wrap-pad } }
 id-aes128-wrap-pad OBJECT IDENTIFIER ::= { aes 8 }
 kwa-aes192-wrap-pad KEY-WRAP ::= {
   IDENTIFIER id-aes192-wrap-pad
   PARAMS ARE absent
   SMIME-CAPS { IDENTIFIED BY id-aes192-wrap-pad } }
 id-aes192-wrap-pad OBJECT IDENTIFIER ::= { aes 28 }
 kwa-aes256-wrap-pad KEY-WRAP ::= {
   IDENTIFIER id-aes256-wrap-pad
   PARAMS ARE absent
   SMIME-CAPS { IDENTIFIED BY id-aes256-wrap-pad } }
 id-aes256-wrap-pad OBJECT IDENTIFIER ::= { aes 48 }
 END

Housley & Dworkin Informational [Page 12] RFC 5649 AES Key Wrap with Padding Algorithm August 2009

Authors' Addresses

 Russell Housley
 Vigil Security, LLC
 918 Spring Knoll Drive
 Herndon, VA 20170
 USA
 EMail: housley@vigilsec.com
 Morris Dworkin
 National Institute of Standards and Technology
 100 Bureau Drive, Mail Stop 8930
 Gaithersburg, MD 20899-8930
 USA
 EMail: dworkin@nist.gov

Housley & Dworkin Informational [Page 13]

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