Network Working Group K. Burgin Internet Draft National Security Agency Intended Status: Informational M. Peck Expires: October 21, 2013 The MITRE Corporation April 19, 2013 AES Encryption with HMAC-SHA2 for Kerberos 5 draft-ietf-kitten-aes-cts-hmac-sha2-00 Abstract This document specifies two encryption types and two corresponding checksum types for Kerberos 5. The new types use AES in CTS mode (CBC mode with ciphertext stealing) for confidentiality and HMAC with a SHA-2 hash for integrity. 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 October 21, 2013. Copyright and License Notice Copyright (c) 2013 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 Simplified BSD License. Burgin & Peck Expires October 21, 2013 [Page 1]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions used in this Document . . . . . . . . . . . . . . 3 3. Protocol Key Representation . . . . . . . . . . . . . . . . . 3 4. Key Generation from Pass Phrases . . . . . . . . . . . . . . . 3 5. Key Derivation Function . . . . . . . . . . . . . . . . . . . 4 6. Kerberos Algorithm Protocol Parameters . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 9.1. Normative References . . . . . . . . . . . . . . . . . . . 9 9.2. Informative References . . . . . . . . . . . . . . . . . . 9 Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Burgin & Peck Expires October 21, 2013 [Page 2]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 1. Introduction This document defines two encryption types and two corresponding checksum types for Kerberos 5 using AES with 128-bit or 256-bit keys. The new types conform to the framework specified in [RFC3961], but do not use the simplified profile. The new encryption types use AES in CTS mode (CBC mode with ciphertext stealing) similar to [RFC3962] but with several variations. The new types use the PBKDF2 algorithm for key generation from strings, with a modification to the use in [RFC3962] that the pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC-SHA-384 instead of HMAC-SHA-1. The new types use key derivation to produce keys for encryption, integrity protection, and checksum operations as in [RFC3962]. However, a key derivation function from [SP800-108] which uses the SHA-256 or SHA-384 hash algorithm is used in place of the DK key derivation function used in [RFC3961]. The new types use the HMAC algorithm with a hash from the SHA-2 family for integrity protection and checksum operations. 2. 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 RFC 2119 [RFC2119]. 3. Protocol Key Representation The AES key space is dense, so we can use random or pseudorandom octet strings directly as keys. The byte representation for the key is described in [FIPS197], where the first bit of the bit string is the high bit of the first byte of the byte string (octet string). 4. Key Generation from Pass Phrases We use a variation on the key generation algorithm specified in Section 4 of [RFC3962] with the following changes: * The pseudorandom function used by PBKDF2 will be the SHA-256 or SHA-384 HMAC of the passphrase and salt, instead of the SHA-1 HMAC of the passphrase and salt. If the enctype is "aes128-cts-hmac- sha256-128", then HMAC-SHA-256 is used as the PRF. If the enctype is "aes256-cts-hmac-sha384-192", then HMAC-SHA-384 is used as the Burgin & Peck Expires October 21, 2013 [Page 3]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 PRF. * The salt MUST contain at least 128 random bits as required in Section 5.1 of [SP800-132]. It MAY also contain other information such as the principal's realm and name components. * The final key derivation step uses the algorithm KDF-HMAC-SHA2 defined below in Section 5 instead of the DK function. * If no string-to-key parameters are specified, the default number of iterations is raised to 32,768. To ensure that different long-term keys are used with different enctypes, we prepend the enctype name to the salt string, separated by a null byte. The enctype name is "aes128-cts-hmac-sha256-128" or "aes256-cts-hmac-sha384-192" (without the quotes). The user's long- term key is derived as follows saltp = enctype-name | 0x00 | salt tkey = random-to-key(PBKDF2(passphrase, saltp, iter_count, keylength)) key = KDF-HMAC-SHA2(tkey, "kerberos") where "kerberos" is the byte string {0x6b65726265726f73}. where the pseudorandom function used by PBKDF2 is HMAC-SHA-256 when the enctype is "aes128-cts-hmac-sha256-128" and HMAC-SHA-384 when the enctype is "aes256-cts-hmac-sha384-192", the value for keylength is the AES key length, and the algorithm KDF-HMAC-SHA2 is defined in Section 5. 5. Key Derivation Function We use a key derivation function from Section 5.1 of [SP800-108] which uses the HMAC algorithm as the PRF. The counter i is expressed as four octets in big-endian order. The length of the output key in bits (denoted as k) is also represented as four octets in big-endian order. The "Label" input to the KDF is the usage constant supplied to the key derivation function, and the "Context" input is null. In the following summary, | indicates concatenation. The random-to-key function is the identity function, as defined in Section 6. The k- truncate function is defined in [RFC3961], Section 5.1. Burgin & Peck Expires October 21, 2013 [Page 4]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 When the encryption type is aes128-cts-hmac-sha256-128, the output key length k is 128 bits for all applications of KDF-HMAC-SHA2(key, constant) which is computed as follows: n = 1 K1 = HMAC-SHA-256(key, 00 00 00 01 | constant | 0x00 | 00 00 00 80) DR(key, constant) = k-truncate(K1) KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant)) When the encryption type is aes256-cts-hmac-sha384-192, the output key length k is 256 bits when computing the base-key and Ke, and the output key length k is 192 bits when deriving Kc and Ki. KDF-HMAC- SHA2(key, constant) is computed as follows: If deriving Kc or Ki (the constant ends with 0x99 or 0x55): k = 192 n = 1 K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 00 C0) DR(key, constant) = k-truncate(K1) KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant)) Otherwise (if deriving Ke or deriving the base-key from a passphrase as described in Section 4): k = 256 n = 1 K1 = HMAC-SHA-384(key, 00 00 00 01 | constant | 0x00 | 00 00 01 00) DR(key, constant) = k-truncate(K1) KDF-HMAC-SHA2(key, constant) = random-to-key(DR(key, constant)) The constants used for key derivation are the same as those used in the simplified profile. 6. Kerberos Algorithm Protocol Parameters The following parameters apply to the encryption types aes128-cts- hmac-sha256-128 and aes256-cts-hmac-sha384-192. The key-derivation function described in the previous section is used to produce the three intermediate keys. Typically, CBC mode [SP800- 38A] requires the input be padded to a multiple of the encryption algorithm block size, which is 128 bits for AES. However, to avoid ciphertext expansion, we use the CBC-CS3 variant to CBC mode defined in [SP800-38A+] (this mode is also referred to as CTS). Note that [SP800-38A+] requires the plaintext length to be greater than or equal to the block size. Each encryption will use a freshly generated 16-octet nonce generated at random by the message originator. The initialization vector (IV) Burgin & Peck Expires October 21, 2013 [Page 5]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 used by AES is obtained by xoring the random nonce with the cipherstate. The ciphertext is the concatenation of the random nonce, the output of AES in CBC-CS3 mode, and the HMAC of the nonce concatenated with the AES output. The HMAC is computed using either SHA-256 or SHA- 384. The output of SHA-256 is truncated to 128 bits and the output of SHA-384 is truncated to 192 bits. Sample test vectors are given in Appendix A. Decryption is performed by removing the HMAC, verifying the HMAC against the remainder, and then decrypting the remainder if the HMAC is correct. The encryption and checksum mechanisms below use the following notation from [RFC3961]. HMAC output size, h message block size, m encryption/decryption functions, E and D cipher block size, c Encryption Mechanism for AES-CTS-HMAC-SHA2 ------------------------------------------------------------------------ protocol key format 128- or 256-bit string specific key structure Three protocol-format keys: { Kc, Ke, Ki }. required checksum As defined below. mechanism key-generation seed key size (128 or 256 bits) length cipher state Random nonce of length c (128 bits) initial cipher state All bits zero encryption function N = random nonce of length c (128 bits) IV = N + cipherState (+ denotes XOR) C = E(Ke, plaintext, IV) using CBC-CS3-Encrypt defined in [SP800-38A+] H = HMAC(Ki, N | C) ciphertext = N | C | H[1..h] cipherState = N Burgin & Peck Expires October 21, 2013 [Page 6]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 decryption function (N, C, H) = ciphertext if (H != HMAC(Ki, N | C)[1..h]) stop, report error IV = N + cipherState (+ denotes XOR) P = D(Ke, C, IV) using CBC-CS3-Decrypt defined in [SP800-38A+] cipherState = N pseudo-random function Kp = KDF-HMAC-SHA2(protocol-key, "prf") PRF = HMAC(Kp, octet-string) key generation functions: string-to-key function tkey = random-to-key(PBKDF2(passphrase, saltp, iter_count, keylength)) base-key = KDF-HMAC-SHA2(tkey, "kerberos") where the pseudorandom function used by PBKDF2 is HMAC-SHA-256 or HMAC-SHA-384 as described in Section 4. default string-to-key 00 00 80 00 parameters random-to-key function identity function key-derivation function KDF-HMAC-SHA2 as defined in Section 5. The key usage number is expressed as four octets in big-endian order. Kc = KDF-HMAC-SHA2(base-key, usage | 0x99) Ke = KDF-HMAC-SHA2(base-key, usage | 0xAA) Ki = KDF-HMAC-SHA2(base-key, usage | 0x55); Checksum Mechanism for AES-CTS-HMAC-SHA2 ------------------------------------------------------------------------ associated cryptosystem AES-128-CTS or AES-256-CTS as appropriate get_mic HMAC(Kc, message)[1..h] verify_mic get_mic and compare Using this profile with each key size gives us two each of encryption and checksum algorithm definitions. Burgin & Peck Expires October 21, 2013 [Page 7]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 +--------------------------------------------------------------------+ | encryption types | +--------------------------------------------------------------------+ | type name etype value key size | +--------------------------------------------------------------------+ | aes128-cts-hmac-sha256-128 TBD1 128 | | aes256-cts-hmac-sha384-192 TBD2 256 | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | checksum types | +--------------------------------------------------------------------+ | type name sumtype value length | +--------------------------------------------------------------------+ | hmac-sha256-128-aes128 TBD3 128 | | hmac-sha384-192-aes256 TBD4 192 | +--------------------------------------------------------------------+ These checksum types will be used with the corresponding encryption types defined above. 7. IANA Considerations IANA is requested to assign: 1. Encryption type numbers for aes128-cts-hmac-sha256-128 and aes256-cts-hmac-sha384-192 in the Kerberos Encryption Type Numbers registry. Etype encryption type Reference ----- --------------- --------- TBD1 aes128-cts-hmac-sha256-128 [this document] TBD2 aes256-cts-hmac-sha384-192 [this document] 2. Checksum type numbers for hmac-sha256-128-aes128 and hmac-sha384-192-aes256 in the Kerberos Checksum Type Numbers registry. Sumtype Checksum type Size Reference ------- ------------- ---- --------- TBD3 hmac-sha256-128-aes128 16 [this document] TBD4 hmac-sha384-192-aes256 24 [this document] 8. Security Considerations This specification requires implementations to generate random values. The use of inadequate pseudo-random number generators Burgin & Peck Expires October 21, 2013 [Page 8]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 (PRNGs) can result in little or no security. The generation of quality random numbers is difficult. NIST Special Publication 800-90 [SP800-90] and [RFC4086] offer random number generation guidance. This document specifies a mechanism for generating keys from pass phrases or passwords. The salt and iteration count resist brute force and dictionary attacks, however, it is still important to choose or generate strong passphrases. 9. References 9.1. Normative References [SP800-38A+] National Institute of Standards and Technology, "Recommendation for Block Cipher Modes of Operation: Three Variants of Ciphertext Stealing for CBC Mode", Addendum to NIST Special Publication 800-38A, October 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) Encryption for Kerberos 5", RFC 3962, February 2005. [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [FIPS197] National Institute of Standards and Technology, "Advanced Encryption Standard (AES)", FIPS PUB 197, November 2001. 9.2. Informative References [SP800-38A] National Institute of Standards and Technology, "Recommendation for Block Cipher Modes of Operation - Methods and Techniques", NIST Special Publication 800- 38A, February 2001. [SP800-90] National Institute of Standards and Technology, Recommendation for Random Number Generation Using Deterministic Random Bit Generators (Revised), NIST Special Publication 800-90, March 2007. Burgin & Peck Expires October 21, 2013 [Page 9]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 [SP800-108] National Institute of Standards and Technology, "Recommendation for Key Derivation Using Pseudorandom Functions", NIST Special Publication 800-108, October 2009. [SP800-132] National Institute of Standards and Technology, "Recommendation for Password-Based Key Derivation, Part 1: Storage Applications", NIST Special Publication 800- 132, June 2010. Appendix A. Test Vectors Sample results for string-to-key conversion: Iteration count = 32768 Pass phrase = "password" Saltp for creating 128-bit master key: 61 65 73 31 32 38 2D 63 74 73 2D 68 6D 61 63 2D 73 68 61 32 35 36 2D 31 32 38 00 F3 60 61 DC E2 E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E 41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E (The saltp is "aes128-cts-hmac-sha256-128" | 0x00 | 16 random bytes | "ATHENA.MIT.EDUraeburn") 128-bit master key: 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C Saltp for creating 256-bit master key: 61 65 73 32 35 36 2D 63 74 73 2D 68 6D 61 63 2D 73 68 61 33 38 34 2D 31 39 32 00 F3 60 61 DC E2 E1 B3 59 00 83 87 46 B8 78 2F 1D 41 54 48 45 4E 41 2E 4D 49 54 2E 45 44 55 72 61 65 62 75 72 6E (The saltp is "aes256-cts-hmac-sha384-192" | 0x00 | 16 random bytes | "ATHENA.MIT.EDUraeburn") 256-bit master key: 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 Sample results for key derivation: enctype aes128-cts-hmac-sha256-128: 128-bit master key: 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C Kc value for key usage 2 (constant = 0x0000000299): B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3 Ke value for key usage 2 (constant = 0x00000002AA): 9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E Ki value for key usage 2 (constant = 0x0000000255): 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C Burgin & Peck Expires October 21, 2013 [Page 10]

Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013 enctype aes256-cts-hmac-sha384-192: 256-bit master key: 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 Kc value for key usage 2 (constant = 0x0000000299): EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4 BA 41 F2 8F AF 69 E7 3D Ke value for key usage 2 (constant = 0x00000002AA): 56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7 A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49 Ki value for key usage 2 (constant = 0x0000000255): 69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6 22 C4 D0 0F FC 23 ED 1F Sample encryptions (using the default cipher state): 128-bit master key: 37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C 128-bit AES key (Ke, key usage 2): 9B 19 7D D1 E8 C5 60 9D 6E 67 C3 E3 7C 62 C7 2E 128-bit HMAC key (Ki, key usage 2): 9F DA 0E 56 AB 2D 85 E1 56 9A 68 86 96 C2 6A 6C Plaintext: 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 IV | Ciphertext | Authentication Tag: 8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17 13 64 FB 39 DC C0 E3 D9 83 A7 DB 5B 4B 9F FB CA 42 F6 65 88 29 F2 1F C8 95 75 AE 93 C7 57 18 AB 3C 7C FB 28 E1 256-bit master key: 6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98 00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52 256-bit AES key (Ke, key usage 2): 56 AB 22 BE E6 3D 82 D7 BC 52 27 F6 77 3F 8E A7 A5 EB 1C 82 51 60 C3 83 12 98 0C 44 2E 5C 7E 49 192-bit HMAC key (Ki, key usage 2): 69 B1 65 14 E3 CD 8E 56 B8 20 10 D5 C7 30 12 B6 22 C4 D0 0F FC 23 ED 1F Plaintext: 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 IV | Ciphertext | Authentication Tag: 8D 32 50 F6 36 AB 81 02 BE 6F AB 1E 57 D8 F8 17 50 CB FF DC DF 38 69 D7 0B EA FF C3 2C 47 0B C6 5B 72 C3 37 2D 6E D7 B3 47 E9 0B BD 8F 31 F5 79 58 F9 69 50 BA A1 41 64 6E 65 6C F6 7C Burgin & Peck Expires October 21, 2013 [Page 11]

```
Internet-Draft AES-CTS HMAC-SHA2 For Kerberos 5 April 19, 2013
Sample checksums:
Checksum type: hmac-sha256-128-aes128
128-bit master key:
37 05 D9 60 80 C1 77 28 A0 E8 00 EA B6 E0 D2 3C
128-bit HMAC key (Kc, key usage 2):
B3 1A 01 8A 48 F5 47 76 F4 03 E9 A3 96 32 5D C3
Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14
Checksum:
D7 83 67 18 66 43 D6 7B 41 1C BA 91 39 FC 1D EE
Checksum type: hmac-sha384-192-aes256
256-bit master key:
6D 40 4D 37 FA F7 9F 9D F0 D3 35 68 D3 20 66 98
00 EB 48 36 47 2E A8 A0 26 D1 6B 71 82 46 0C 52
192-bit HMAC key (Kc, key usage 2):
EF 57 18 BE 86 CC 84 96 3D 8B BB 50 31 E9 F5 C4
BA 41 F2 8F AF 69 E7 3D
Plaintext:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14
Checksum:
45 EE 79 15 67 EE FC A3 7F 4A C1 E0 22 2D E8 0D
43 C3 BF A0 66 99 67 2A
Authors' Addresses
Kelley W. Burgin
National Security Agency
EMail: kwburgi@tycho.ncsc.mil
Michael A. Peck
The MITRE Corporation
EMail: mpeck@mitre.org
Burgin & Peck Expires October 21, 2013 [Page 12]
```