INTERNET DRAFT R. Housley
Intended Status: Informational Vigil Security
M. Dworkin
NIST
Expires: 29 July 2009 29 January 2009
Advanced Encryption Standard (AES) Key Wrap Algorithm with Padding
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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 key to be wrapped is a multiple of 64 bits,
allowing a key of any practical length to be wrapped.
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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
bits.
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 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
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
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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 2.2.3.1, 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
algorithm.
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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
AVI.
Inputs: Ciphertext, (n+1) 64-bit values {C0, C1, ..., Cn}, and
Key, K (the KEK).
Outputs: Plaintext, m octets {Q1, Q2, ..., Qm}.
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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.
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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
hexadecimal.
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.
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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/encryption/kms/key-wrap.pdf]
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
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
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