2.3: Public Key Encryption and Decryption

Key Pair

Consists of:

• Encryption Public Key (e_a)

• Decryption Private Key (d_a)

Public Key

Can be publicized as belonging to the owner

Private Key

Should be kept secret and shared with no one

Sending a Message Process (Public Key Encryption)

1. Obtain the receiver’s public key e_r

2. Parameterize the Public Key Encryption Algorithm E with e_r

3. Encrypt m to get c, Send c

4. Recover m using corresponding Public Key Decryption Algorithm D parameterized by private key d_r

What is required for Message Encryption?

Receiver’s public key is needed to encrypt the message

Ciphertext Decryption

Receiver uses their private key to decrypt the ciphertext to receive message

Public Key Integrity

Critical because if A could replace B’s public key with their own, anyone who thought they were encrypting plaintext for B with B’s public key would instead be making the plaintext recoverable by A

Symmetric Key Distribution

For group of n users, each pair should use a different symmetric key. About (n choose 2) keys, ~O(n²) keys

Public Key Distribution

For group of n users, each user only needs one key pair to allow others to encrypt for them. Requires only n keys total.

Hybrid Encryption

Combines the strengths of asymmetric (public key) and symmetric encryption to provide both security and efficiency

Symmetric is typically faster but Public is convenient for establishing shared secret keys between endpoints

Hybrid Encryption Process

1. Random symmetric key (k) is used to encrypt the message

2. k is then encrypted using the recipient public key

RSA Encryption

Primarily used to encrypt relatively short data keys or session keys for key management rather than the message itself

Form of public key encryption/decryption