Abstract
The author examines the application of linear cryptanalysis to the RC5 private-key ciphers and show that there are expected to be weak keys for which the attack is applicable to many rounds. It is demonstrated that, for the 12-round nominal RC5 version with a 64 bit block size and a 128 bit key, there are 228 weak keys for which only ~217 known plaintexts are required to break the cipher. There are 268 keys for which the cipher is theoretically breakable, requiring ~257 known plaintexts. The analysis highlights the sensitivity of RC5 security to its key scheduling algorithm.

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