IDEA (International Data Encryption Algorithm) is the second version of a block cipher designed and presented by Lai and Massey. It is a 64-bit iterative block cipher with a 128-bit key and eight rounds. While the cipher is not Feistel, decryption is carried out in the same manner as encryption once the decryption subkeys have been calculated from the encryption subkeys. The cipher structure was designed to be easily implemented in both software and hardware, and the security of IDEA relies on the use of three incompatible types of arithmetic operations on 16-bit words. The speed of IDEA in software is similar to that of DES.