Class PKESessionKeyV3

pgpy/packet/packets.py:94–284

5.1. 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 Public-Key Encrypted Session Key packets and/or Symmetric-Key Encrypted Session Key packets may precede a Symmetricall

class PKESessionKeyV3(PKESessionKey):
    """
    5.1.  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 Public-Key Encrypted Session Key
    packets and/or Symmetric-Key Encrypted Session Key packets 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:

     - A one-octet number giving the version number of the packet type.
       The currently defined value for packet version is 3.

     - An eight-octet number that gives the Key ID of the public key to
       which the session key is encrypted.  If the session key is
       encrypted to a subkey, then the Key ID of this subkey is used
       here instead of the Key ID of the primary key.

     - A one-octet number giving the public-key algorithm used.

     - A string of octets that is the encrypted session key.  This
       string takes up the remainder of the packet, and its contents are
       dependent on the public-key algorithm used.

    Algorithm Specific Fields for RSA encryption

     - multiprecision integer (MPI) of RSA encrypted value m**e mod n.

    Algorithm Specific Fields for Elgamal encryption:

     - MPI of Elgamal (Diffie-Hellman) value g**k mod p.

     - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.

    The value "m" in the above formulas is derived from the session key
    as follows.  First, the session key is prefixed with a one-octet
    algorithm identifier that specifies the symmetric encryption
    algorithm used to encrypt the following Symmetrically Encrypted Data
    Packet.  Then a two-octet checksum is appended, which is equal to the
    sum of the preceding session key octets, not including the algorithm
    identifier, modulo 65536.  This value is then encoded as described in
    PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
    form the "m" value used in the formulas above.  See Section 13.1 of
    this document for notes on OpenPGP's use of PKCS#1.

    Note that when an implementation forms several PKESKs with one
    session key, forming a message that can be decrypted by several keys,
    the implementation MUST make a new PKCS#1 encoding for each key.