The message is encrypted with the AES algorithm using a random session key.
The public key is used by the RSA/Elgamal algorithm to encrypt the session key.
Use the PGP public key extraction page to extract the public key from a PGP
public key block and paste it below.
(The key shown on start of this page is my public key.)
The Key ID is added to the encrypted message to identify the public key used in encryption.
GnuPG tries all local avaliable secret keys matching the public key algorithm for decryption,
if the key ID is all 8 bytes zero.
Other software may need the key ID for decryption.