Public-Key Encrypted Session Key Packets (Tag 1)
A Public-Key Encrypted Session Key packet holds the session key used to encrypt a message. Zero or more Encrypted Session Key packets (either Public-Key or Symmetric-Key) may precede a Symmetrically Encrypted Data Packet, which holds an encrypted message. The message is encrypted with the session key, and the session key is itself encrypted and stored in the Encrypted Session Key packet(s). The Symmetrically Encrypted Data Packet is preceded by one Public-Key Encrypted Session Key packet for each OpenPGP key to which the message is encrypted. The recipient of the message finds a session key that is encrypted to their public key, decrypts the session key, and then uses the session key to decrypt the message.
The body of this packet consists of:
Algorithm Specific Fields for RSA encryption
Algorithm Specific Fields for Elgamal encryption: