PGP Encryption

Developed by Phil Zimmerman in 1991. It combines symmetric, asymmetric (public key) encryption, hash, and digital signature all together providing confidentiality, integrity, and authentication. Good for email. The algorithm:

At the sender (Alice) side: 

1. Alice: Message M is hashed
2. Sign (encrypt) the hash value with her private key (EP)
3. Compress (zip) the message + signed hash value
4. Use a session key Ks to encrypt zipped output (ES)
5. Use Bob's public key to encrypt session key (EP)
6. Send encrypted message and encrypted session key 

At the recipient (Bob) side:

1. Use Bob's private key to decrypt session key (DP)
2. Use session key Ks to decrypt the zipped message
3. unzip, got message and signed hash value
4. Use Alice's public key to decrypt the hash value 
5. Computing hash value for the message received
6. Compare with hash value Alice computed.

Below are the PGP diagram:

Mathematically:

Note:

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