The IDEA algorithm (used in PGP) is _______ bits long.

56 bits is the key length for the DES (Data Encryption Standard) algorithm, which is significantly weaker and different from IDEA.

158 bits is not a standard key length associated with the IDEA algorithm or common symmetric encryption standards.

The IDEA (International Data Encryption Algorithm) is a symmetric-key block cipher that uses a 128-bit key to encrypt and decrypt data in 64-bit blocks, making it a strong encryption standard for its time, notably integrated into early versions of Pretty Good Privacy (PGP).

168 bits is the effective key length for Triple DES (3DES), which applies DES three times with different keys, and is distinct from the IDEA algorithm.