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 <draft-housley-aes-key-wrap-with-pad-00.txt> Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright 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. 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. 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 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 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 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. 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 AVI. 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 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. 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. [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 Housley & Dworkin [Page 7]