International Data Encryption Algorithm

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In cryptography, the International Data Encryption Algorithm (IDEA) is a block cipher designed by James Massey of ETH Zurich and Xuejia Lai and was first described in 1991. As a block cipher, it is also symmetric. The algorithm was intended as a replacement for the Data Encryption Standard. IDEA is a minor revision of an earlier cipher, PES (Proposed Encryption Standard); IDEA was originally called IPES (Improved PES).

The cipher was designed under a research contract with the Hasler Foundation, which became part of Ascom-Tech AG. The cipher is patented in a number of countries but is freely available for non-commercial use. The name "IDEA" is also a trademark. The patents will expire in 2011–2012. Today, IDEA is licensed in all the countries where it is patented by MediaCrypt.

IDEA was used in Pretty Good Privacy (PGP) v2.0, and was incorporated after the original cipher used in v1.0, BassOmatic, was found to be insecure.[2] IDEA is an optional algorithm in the OpenPGP standard.



IDEA operates on 64-bit blocks using a 128-bit key, and consists of a series of eight identical transformations (a round, see the illustration) and an output transformation (the half-round). The processes for encryption and decryption are similar. IDEA derives much of its security by interleaving operations from different groupsmodular addition and multiplication, and bitwise eXclusive OR (XOR) — which are algebraically "incompatible" in some sense. In more detail, these operators, which all deal with 16-bit quantities, are:

  • Bitwise eXclusive OR (denoted with a blue circled plus ).
  • Addition modulo 216 (denoted with a green boxed plus ).
  • Multiplication modulo 216+1, where the all-zero word (0x0000) is interpreted as 216 (denoted by a red circled dot ).

After the eight rounds comes a final "half round", the output transformation illustrated below:

International Data Encryption Algorithm InfoBox Diagram Output Trans.png

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