The International Data Encryption Algorithm (IDEA) cipher is a secure, secret, key block encryption algorithm that was developed by James Massey and Xuejia Lai (X. Lai, On the Design and Security of Block Ciphers,Ž ETH Series on Information Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992). It evolved in 1992 from earlier algorithms called the Proposed Encryption Standard and the Improved Proposed Encryption Standard. IDEA operates on 64-bit Plaintext blocks and uses a 128-bit key. It applies both confusion and diffusion.

The IDEA algorithm performs eight rounds and operates on 16-bit subblocks by using algebraic calculations that are amenable to hardware implementation.

These operations are modulo 216 addition, modulo 216 + 1 multiplication, and the Exclusive Or. With its 128-bit key, an IDEA cipher is much more difficult to crack than DES. IDEA operates in the modes described for DES and is applied in the Pretty Good Privacy (PGP) e-mail encryption system that was developed by Phil Zimmerman.

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