Akelarre is a block cipher

Block cipher

In cryptography, a block cipher is a symmetric key cipher operating on fixed-length groups of bits, called blocks, with an unvarying transformation. A block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext...

 proposed in 1996, combining the basic design of IDEA with ideas from RC5

RC5

In cryptography, RC5 is a block cipher notable for its simplicity. Designed by Ronald Rivest in 1994, RC stands for "Rivest Cipher", or alternatively, "Ron's Code"...

. It was shown to be susceptible to a ciphertext-only attack

Ciphertext-only attack

In cryptography, a ciphertext-only attack or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts....

 in 1997.

Akelarre is a 128-bit block cipher with a variable key-length which must be some multiple of 64 bits. The number of rounds is variable, but four are suggested. The round function of Akelarre is similar to IDEA in structure.

After the successful cryptanalysis of Akelarre, its designers responded with an updated variant called Ake98. This cipher differs from the original Akelarre in the new AR-box (Addition-Rotation box), the swapping of words at the end of a round, and the addition of subkeys at the beginning of each round. In 2004, Jorge Nakahara, Jr. and Daniel Santana de Freitas found large classes of weak key

Weak key

In cryptography, a weak key is a key, which, used with a specific cipher, makes the cipher behave in some undesirable way. Weak keys usually represent a very small fraction of the overall keyspace, which usually means that, if one generates a random key to encrypt a message, weak keys are very...

s for Ake98. These weak keys allow a cryptanalysis faster than exhaustive search using only 71 known plaintexts, for up to 11.5 rounds of Ake98.