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Question 75. What is RC2?

RC2 is a variable key-size block cipher designed by Rivest for RSA Data Security. "RC" stands for "Ron's Code" or "Rivest's Cipher." It is faster than DES and is designed as a "drop-in" replacement for DES (see Question 64). It can be made more secure or less secure than DES against exhaustive key search by using appropriate key sizes. It has a block size of 64 bits and is about two to three times faster than DES in software. The algorithm is confidential and proprietary to RSA Data Security. RC2 can be used in the same modes as DES (see Question 66).

An agreement between the Software Publishers Association (SPA) and the United States government gives RC2 and RC4 (see Question 87) special status by means of which the export approval process is simpler and quicker than the usual cryptographic export process. However, to qualify for quick export approval a product must limit the RC2 and RC4 key sizes to 40 bits; 56 bits is allowed for foreign subsidiaries and overseas offices of United States companies. An additional string (40 to 88 bits long) called a salt can be used to thwart attackers who try to precompute a large look-up table of possible encryptions. The salt is appended to the encryption key, and this lengthened key is used to encrypt the message; the salt is then sent, unencrypted, with the message. RC2 and RC4 have been widely used by developers who want to export their products; DES is almost never approved for export.