When using Pretty Good Privacy (PGP) to digitally sign a message, the signature is created in a two- step process. First, the message to be signed is submitted to PGP's cryptographic hash algorithm…

Blowfish is a symmetric block cipher used in PGP for encrypting message content, not for generating the cryptographic hash digest required for signing.

DES (Data Encryption Standard) is a symmetric block cipher used for bulk data encryption, not a cryptographic hash function, and is not used for message digesting in PGP.

SHA-1 (Secure Hash Algorithm 1) is a cryptographic hash function supported by the OpenPGP standard (RFC 4880) for computing the message digest in the digital signature process. PGP hashes the plaintext message with SHA-1 to produce a fixed-length digest, which is then encrypted with the sender's private key to form the actual digital signature.

CAST (e.g., CAST-128) is a symmetric encryption algorithm used in PGP for encrypting session keys and message data, not for the hashing step of the signature process.