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INTERNET DRAFT                                               R. Housley
Intended Status: Informational                           Vigil Security
                                                             M. Dworkin
Expires: 29 July 2009                                   29 January 2009

   Advanced Encryption Standard (AES) Key Wrap Algorithm with Padding

Status of this Memo

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   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 key to be wrapped is a multiple of 64 bits,
   allowing a key of any practical length to be wrapped.

Housley & Dworkin                                               [Page 1]

INTERNET DRAFT                                              January 2009

1. Introduction

   Management of cryptographic keys often leads to situations where a
   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 the Advanced Encryption Standard
   (AES) Key Wrap algorithm [AES-KW1,AES-KW2].  Without this extension,
   the input to the AES Key Wrap algorithm must be a multiple of 64

   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-encrypting key (KEK)
   can be 128, 192, or 256 bits.  The AES Key Wrap algorithm requires
   the input to be at least two 64-bit blocks.  This specification
   allows inputs as short as 9 octets, which will result in 16 output
   octets or two 64-bit blocks.  Although the AES Key Wrap algorithm
   does not place a maximum bound on the number of blocks that can be
   wrapped, this specification does so.  The use of a 32-bit fixed field
   to carry the key length bounds the size of the input octet string 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 an "Alternative
   Initial Value" in keeping with the statement in 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
      B1 | B2       Concatenate B1 and B2
      K             The key-encryption key
      m             The number of octets in the key data
      n             The number of 64-bit key data blocks
      Q[i]          The ith plaintext octet in the key data
      P[i]          The ith plaintext 64-bit block in the padded key data
      C[i]          The ith ciphertext data block
      A             The 64-bit integrity check register

Housley & Dworkin                                               [Page 2]

INTERNET DRAFT                                              January 2009

3. Alternative Initial Value

   The Alternative Initial Value (AIV) required by this specification
   comprises 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 [AES-KW1] Section, so there is no
   ambiguity between the two.  The 32-bit MLI, which occupies the low-
   order half of the AIV, is a unsigned binary integer equal to the
   number of octets in the key data being wrapped.  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 a multiple 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 8 values of MLI that can have that outcome.  For example, the
   only MLI values that are valid with 4 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 AES Key Unwrap yields n
   64-bit blocks with an AIV, the eight valid values for the MLI are
   8*n, (8*n)-1, ..., and (8*n)-7.  Therefore, the integrity check for
   the AIV 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 plaintext 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 AES Key Unwrap operation
   must return an error.

4. Algorithms

   The specification of the key wrap algorithm requires the use of the
   AES codebook [AES] and provide a padding technique for use with the
   AES Key Wrap [AES-KW1,AES-KW2].  The next two sections describe the
   key wrap with padding algorithm and the key unwrap with padding

Housley & Dworkin                                               [Page 3]

INTERNET DRAFT                                              January 2009

4.1. Key Wrap with Padding

   The inputs to the key wrapping process are the KEK and the plaintext
   to be wrapped.  The plaintext consists of between 9 and 2^32 octets,
   containing the key data being wrapped.  The key wrapping process is
   described below.

   Inputs:      Plaintext, m octets {Q1, Q2, ..., Qm}, and
                Key, K (the KEK).
   Outputs:     Ciphertext, (n+1) 64-bit values {C0, C1, ..., Cn}.

   1) Initialize variables

       If m is not a multiple of 8, pad the plaintext octet string on
       the right with octets {Qm+1, ..., Qr} of zeros, where r is the
       smallest multiple of 8 that is greater than 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] .

       Set A = AIV, the Alternative Initial Value as defined in
       Section 3.

   2) Key wrapping

       Use the AES Key Wrap algorithm with the AIV as defined in
       Section 3, the padded plaintext {P1, ..., Pn}, and K (the KEK).
       The result is (n+1) 64-bit ciphertext blocks {C0, C1, ..., Cn}.

4.2  Key Unwrap with Padding

   The inputs to the key unwrap algorithm are the KEK and (n+1) 64-bit
   ciphertext blocks consisting of previously wrapped key.  The AES Key
   Unwrap returns n 64-bit plaintext blocks, which are then mapped to m
   octets of decrypted key data, as indicated by the MLI embedded in the

   Inputs:      Ciphertext, (n+1) 64-bit values {C0, C1, ..., Cn}, and
                Key, K (the KEK).
   Outputs:     Plaintext, m octets {Q1, Q2, ..., Qm}.

Housley & Dworkin                                               [Page 4]

INTERNET DRAFT                                              January 2009

   1) Key unwrapping

       Use the AES Key Unrap algorithm with the AIV as defined in
       Section 3, (n+1) 64-bit ciphertext blocks {C0, C1, ..., Cn}, and
       K (the KEK).  The result is the padded plaintext blocks
       {P1, ..., Pn}; also the A value is also needed to validate the
       AIV and remove the padding.

   2) AIV validation

       Perform the three checks described in Section 3.  If any of the
       checks fail, then return an error.

   3) Remove padding

       Let m = the MLI value extracted from A.

       For i = 1, ... , n
          j = 8*(i-1)
          Q[j+1] | Q[j+2] | ... | Q[j+8] = P[i]

5. Algorithm Identifiers

   Some security protocols employ ASN.1 [X.690], and these protocols
   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 TBD }

      id-aes192-wrap     OBJECT IDENTIFIER ::= { aes 25 }
      id-aes192-wrap-pad OBJECT IDENTIFIER ::= { aes TBD }

      id-aes256-wrap     OBJECT IDENTIFIER ::= { aes 45 }
      id-aes256-wrap-pad OBJECT IDENTIFIER ::= { aes TBD }

   In all cases, the AlgorithmIdentifier parameter field must be NULL.

Housley & Dworkin                                               [Page 5]

INTERNET DRAFT                                              January 2009

6. Padded Key Wrap Example

   The example in this section was generated using the index-based
   implementation of the AES Key Wrap algorithm along with the padding
   approach specified in Section 4 of this document.  The example wraps
   20 octets of Key Data with a 128-bit KEK.  All values are shown in

      KEK   :  5840df6e29b02af1 ab493b705bf16ea1 ae8338f4dcc176a8

      Key   :  c37b7e6492584340 bed1220780894115 5068f738

      Wrap  :  138bdeaa9b8fa7fc 61f97742e72248ee 5ae6ae5360d1ae6a
            :  5f54f373fa543b6a

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 key.

   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 as strong or stronger than other cryptographic
   algorithms employed in an overall system.

   The use of different constants in the A value ensures that a padded
   key will no be confused with an unpadded key.  In addition, the two
   algorithms provide roughly the same amount of integrity protection.

   A previous padding technique was specified for wrapping HMAC keys
   with AES [OLD-KW].  The technique in this document is preferred, and
   the technique in this document is not limited to wrapping HMAC keys.

Housley & Dworkin                                               [Page 6]

INTERNET DRAFT                                              January 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.

   AES-KW2   J. Schaad and R. Housley, "Advanced Encryption Standard
             (AES) Key Wrap Algorithm", RFC 3394, September 2002.

   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

   OLD-KW    J. Schaad 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.

9. Acknowledgments

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

Authors Addresses

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   EMail: housley@vigilsec.com

   Morris Dworkin
   National Institute of Standards and Technology
   100 Bureau Drive, Mail Stop 8930
   Gaithersburg, MD 20899-8930
   EMail: dworkin@nist.gov

Housley & Dworkin                                               [Page 7]

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