International Data Encryption Algorithm (IDEA) is a once-proprietary free and open block cipher that was once intended to replace Data Encryption Standard (DES). Once called Improved Proposed Encryption Standard (IPES)I, DEA is a minor revision to the Proposed Encryption Standard (PES).
IDEA uses similar processes for encryption and decryption, with some inverted ordering of round keys. It consists of a series of 8 rounds and operates on 64-bit blocks using a 128-bit key. IDEA suffered from weak keys until its key schedule was revised, and it may call for further revision in the future.
IDEA has been and is optionally available for use with Pretty Good Privacy (PGP). IDEA has been succeeded by the IDEA NXT algorithm, itself once known as FOX.
“Once brute-force techniques became stronger, the US government and encryption community looked to a replacement for the NIST-approved DES that was widely in use. IDEA was proposed as a replacement algorithm for DES, but it failed to do so and today, in the era of AES (Rijndael) which replaced DES, IDEA is obsolete.”