IDEA, also known as the International Data Encryption Algorithm, is a cipher first expressed in 1991 by Xuejia Lai and James Massey. It is a symmetric-key block cipher, which means that the same key is used to encrypt the plaintext and decrypt the ciphertext. IDEA requires both the encrypting and decrypting parties to have the confidential key that is different from public-key or other asymmetric encryption schemes. The secret key is known only to the encrypting party.
IDEA was used in PGP beginning with version 2.0. To date, no cryptographic weaknesses have been found in the algorithm.