Three-pass protocol

In cryptography, a three-pass protocol for sending messages is a framework which allows one party to securely send a message to a second party without the need to exchange or distribute encryption keys. Such message protocols should not be confused with various other algorithms which use 3 passes for authentication.

It is called a three-pass protocol because the sender and the receiver exchange three encrypted messages. The first three-pass protocol was developed by Adi Shamir circa 1980, and is described in more detail in a later section. The basic concept of the three-pass protocol is that each party has a private encryption key and a private decryption key. The two parties use their keys independently, first to encrypt the message, and then to decrypt the message.

The protocol uses an encryption function E and a decryption function D. The encryption function uses an encryption key e to change a plaintext message m into an encrypted message, or ciphertext, ⁠

E

(

e

,

m

)

{\displaystyle E(e,m)}

⁠. Corresponding to each encryption key e there is a decryption key d which allows the message to be recovered using the decryption function, ⁠

D

(

d

,

E

(

e

,

m

)

)

=

m

{\displaystyle D(d,E(e,m))=m}

⁠. Sometimes the encryption function and decryption function are the same.

In order for the encryption function and decryption function to be suitable for the three-pass protocol they must have the property that for any message m, any encryption key e with corresponding decryption key d and any independent encryption key k, ⁠

D

(

d

,

E

(

k

,

E

(

e

,

m

)

)

)

=

E

(

k

,

m

)

{\displaystyle D(d,E(k,E(e,m)))=E(k,m)}

⁠.