Cbc Mac Generation Requires Secret Keys
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  1. Cbc Mac Generation Requires Secret Keys 2016
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Cbc Mac Generation Requires Secret Keys 2016

ICSF provides support for both single-length and double-length MAC generation and MAC verification keys. With the ANSI X9.9-1 single key algorithm, use the single-length MAC and MACVER keys. ICSF provides support for the use of data-encrypting keys in the MAC generation and verification callable services, and also the use of a MAC generation key in the MAC verification callable service. Aug 29, 2016 If CBC-MAC with a fixed IV is great, surely CBC-MAC with a random IV must be super-great. But no, it isn’t. Using a random (or variable IV) is bad for the simple reason that verifying a CBC-MAC requires you to know the IV, and to know the IV you probably need to read it from somewhere. Typically this means the same untrusted place where you. Understanding CBC-MAC variable-length weakness Title says it all - I've started reading about CBC-MAC when this was posted. So essentially i know what the problem is but im having trouble understanding it (both visually and proving it mathematically). Cipher block chaining (CBC) is a widely used cipher mode that requires plaintext to be a multiple of the cipher's block size If shorter, we must add padding. A tweakable cipher includes a third input, a nonce-like value that modifies the encryption without the cost of changing the encryption key. You cannot 'decrypt' a CBC-MAC tag. A message authentication code (MAC) is a keyed integrity check. It means that the tag that is created from a MAC algorithm has always the same length regardless of the data length that you put in. In the case of CBC-MAC with Triple-DES that is the size of the block of 3DES which is 64-bit (or a shorter slice.

One-key MAC (OMAC) is a message authentication code constructed from a block cipher much like the CBC-MAC algorithm.

Officially there are two OMAC algorithms (OMAC1 and OMAC2) which are both essentially the same except for a small tweak. OMAC1 is equivalent to CMAC, which became an NIST recommendation in May 2005.

It is free for all uses: it is not covered by any patents.[citation needed]In cryptography, CMAC (Cipher-based Message Authentication Code)[1] is a block cipher-based message authentication code algorithm. It may be used to provide assurance of the authenticity and, hence, the integrity of binary data. This mode of operation fixes security deficiencies of CBC-MAC (CBC-MAC is secure only for fixed-length messages).

The core of the CMAC algorithm is a variation of CBC-MAC that Black and Rogaway proposed and analyzed under the name XCBC[2] and submitted to NIST.[3] The XCBC algorithm efficiently addresses the security deficiencies of CBC-MAC, but requires three keys. Iwata and Kurosawa proposed an improvement of XCBC and named the resulting algorithm One-Key CBC-MAC (OMAC) in their papers.[4] They later submitted OMAC1,[5] a refinement of OMAC, and additional security analysis.[6] The OMAC algorithm reduces the amount of key material required for XCBC. CMAC is equivalent to OMAC1.

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To generate an ℓ-bit CMAC tag (t) of a message (m) using a b-bit block cipher (E) and a secret key (k), one first generates two b-bit sub-keys (k1 and k2) using the following algorithm (this is equivalent to multiplication by x and x2 in a finite field GF(2b)). Let ≪ denote the standard left-shift operator and ⊕ denote bit-wise exclusive or:

  1. Calculate a temporary value k0 = Ek(0).
  2. If msb(k0) = 0, then k1 = k0 ≪ 1, else k1 = (k0 ≪ 1) ⊕ C; where C is a certain constant that depends only on b. (Specifically, C is the non-leading coefficients of the lexicographically first irreducible degree-b binary polynomial with the minimal number of ones: 0x1B for 64-bit, 0x87 for 128-bit, and 0x425 for 256-bit blocks.)
  3. If msb(k1) = 0, then k2 = k1 ≪ 1, else k2 = (k1 ≪ 1) ⊕ C.
  4. Return keys (k1, k2) for the MAC generation process.

As a small example, suppose b = 4, C = 00112, and k0 = Ek(0) = 01012. Then k1 = 10102 and k2 = 0100 ⊕ 0011 = 01112.

The CMAC tag generation process is as follows:

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  1. Divide message into b-bit blocks m = m1 ∥ .. ∥ mn−1 ∥ mn, where m1, .., mn−1 are complete blocks. (The empty message is treated as one incomplete block.)
  2. If mn is a complete block then mn′ = k1 ⊕ mn else mn′ = k2 ⊕ (mn ∥ 10..02).
  3. Let c0 = 00..02.
  4. For i = 1, .., n − 1, calculate ci = Ek(ci−1 ⊕ mi).
  5. cn = Ek(cn−1 ⊕ mn′)
  6. Output t = msbℓ(cn).

The verification process is as follows:

  1. Use the above algorithm to generate the tag.
  2. Check that the generated tag is equal to the received tag.

Implementations[edit]

  • Python implementation: see the usage of the AES_CMAC() function in 'impacket/blob/master/tests/misc/test_crypto.py', and its definition in 'impacket/blob/master/impacket/crypto.py' [7].
  • Ruby implementation [8]

References[edit]

  1. ^Dworkin, M J (2016). 'Recommendation for block cipher modes of operation'(PDF). doi:10.6028/nist.sp.800-38b.Cite journal requires journal= (help)
  2. ^Black, John; Rogaway, Phillip (2000-08-20). Advances in Cryptology – CRYPTO 2000. Springer, Berlin, Heidelberg. pp. 197–215. doi:10.1007/3-540-44598-6_12. ISBN978-3540445982.
  3. ^Black, J; Rogaway, P. 'A Suggestion for Handling Arbitrary-Length Messages with the CBC MAC'(PDF).Cite journal requires journal= (help)
  4. ^Iwata, Tetsu; Kurosawa, Kaoru (2003-02-24). 'OMAC: One-Key CBC MAC'. Fast Software Encryption. Lecture Notes in Computer Science. 2887. Springer, Berlin, Heidelberg. pp. 129–153. doi:10.1007/978-3-540-39887-5_11. ISBN978-3-540-20449-7.
  5. ^Iwata, Tetsu; Kurosawa, Kaoru (2003). 'OMAC: One-Key CBC MAC – Addendum'(PDF).Cite journal requires journal= (help)
  6. ^Iwata, Tetsu; Kurosawa, Kaoru (2003-12-08). 'Stronger Security Bounds for OMAC, TMAC, and XCBC'. In Johansson, Thomas; Maitra, Subhamoy (eds.). Progress in Cryptology – INDOCRYPT 2003. Lecture Notes in Computer Science. Springer Berlin Heidelberg. pp. 402–415. CiteSeerX10.1.1.13.8229. doi:10.1007/978-3-540-24582-7_30. ISBN9783540206095.
  7. ^'Impacket is a collection of Python classes for working with network protocols.: SecureAuthCorp/impacket'. 15 December 2018 – via GitHub.
  8. ^'Ruby C extension for the AES-CMAC keyed hash function (RFC 4493): louismullie/cmac-rb'. 4 May 2016 – via GitHub.

External links[edit]

Generation
  • RFC 4493 The AES-CMAC Algorithm
  • RFC 4494 The AES-CMAC-96 Algorithm and Its Use with IPsec
  • RFC 4615 The Advanced Encryption Standard-Cipher-based Message Authentication Code-Pseudo-Random Function-128 (AES-CMAC-PRF-128)
  • OMAC Online Test
Retrieved from 'https://en.wikipedia.org/w/index.php?title=One-key_MAC&oldid=950050016'
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