IDEA in 448 bytes of 80x86

IDEA also has the distinction of being the conventional cypher used in PGP in conjuction with the RSA public key algorithm.

There is one other feature of tiny IDEA. Because of the way CFB works, it doesn't need a decryption key (technically, tiny IDEA doesn't have an IDEAinvertkey function). This means whichever option you choose (+ or -) it will be encrypting your file; it's just that you need to choose the other option to reverse the operation.

There are a couple of problems with the tiny IDEA program:

The key you enter becomes the IDEA encryption key. Because some characters cannot be typed at the keyboard (NUL, etc) it reduces the keyspace required for a brute force attack. It is still far stronger than PKZip encryption/DES/Norton Diskreet etc.

The length of the encrypted file is the same as the plaintext. This feature can be used for traffic analysis. PGP compresses the plaintext first for additional efficiency.

Technical details about the tiny IDEA program