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PGP provides data origin authentication and message integrity using digital signatures. A signature consists of an MD-5 hash of the message which is encrypted with the RSA private key of the originator. When the recipient wants to verify a message, he/she computes an MD5 hash of the message, decrypts the hash in the signature using the originators public key, and compares. If they are identical, then the originator of the message has been authenticated and the integrity of the message has been verified.
Signing a message is accomplished with the command:
pgp -s plain_text
Verification is accomplished with the command:
pgp plain_text