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In [[cryptography]], '''MDC-2 (Modification Detection Code 2, sometimes called Meyer-Schilling)''' is a [[cryptographic hash function]]. MDC-2 is a [[One-way compression function|hash function based on a block cipher]] with a proof of security in the ideal-cipher model. The length of the output hash depends on the underlying block cipher used.

== Algorithm ==
For a given message <math>M</math> to hash and a given block cipher encryption function <math>E</math>, the MDC-2 algorithm proceeds as follows. Let <math>n</math> be the block length, <math>A_1, B_1</math> two different constants of size <math>n</math>. If <math>M = M_1||..||M_m</math> where each <math>M_i</math> has size <math>n</math>, then the hash <math>V_m||W_m</math> of the message is given by:
*for <math>i = 1</math> to <math>m</math>:
**<math>V_i = M_i \oplus E(M_i,A_i)</math>
**<math>W_i = M_i \oplus E(M_i,B_i)</math>
**<math>V_i^L || V_i^R = V_i</math>
**<math>W_i^L || W_i^R = W_i</math>
**<math>A_{i+1} = V_i^R||W_i^L</math>
**<math>B_{i+1} = W_i^R||V_i^L</math>
*return <math>A_{m+1}||B_{m+1}</math>

==MDC-2DES hashes==
When MDC-2 uses the [[Data Encryption Standard|DES]] block cipher, the 128-bit (16-byte) MDC-2 hashes are typically represented as 32-digit [[hexadecimal]] numbers. The following demonstrates a 43-byte [[ASCII]] input and the corresponding MDC-2 hash:

MDC2("The quick brown fox jumps over the lazy og")
= 000ed54e093d61679aefbeae05bfe33a

Even a small change in the message will (with probability) result in a completely different hash, e.g. changing <tt>d</tt> to <tt>c</tt>:

MDC2("The quick brown fox jumps over the lazy og")
= 775f59f8e51aec29c57ac6ab850d58e8

The hash of the zero-length string is:

MDC2("")
= 52525252525252522525252525252525

==Patent issues==

MDC-2 was covered by , issued on March 13, 1990 but filed by [[IBM]] on August 28, 1987.<br />
For this reason, support for MDC-2 has been disabled in [[OpenSSL]] on most [[Linux]] distributions and is not implemented by many other cryptographic libraries.

But the maximum lifespan of 20 years from filing date means that the could not have lasted beyond August 28, 2007 ; in fact it has expired in 2002, because IBM has not paid the renewal fee. The same goes for the Canadian patent. There is no patent for Europe. This means that MDC-2 can be freely used.

== See also ==
* [[One-way compression function]]

==Notes==
<references/>

==Source==

[http://wikipedia.org/ http://wikipedia.org/]

[[Category:Cryptography]]

MDC-2 - Revision history

Admin: Created page with "In cryptography, '''MDC-2 (Modification Detection Code 2, sometimes called Meyer-Schilling)''' is a cryptographic hash function. MDC-2 is a One-way compression funct..."