p = prime number (public)
g = modulus (public)
a = Alice private key (private)
b = Bob private key (private)
A = Alice public key (public)
B = Bob public ket = (public)
Sx = Shared key (public)
eA = Eve private key for Alice (private)
eB = Eve private key for Bob (private)
Ea = Spoofed Alice public key will be sent to Bob (public)
Eb = Spoofed Bob public key will be sent to Alice (public)
Diffie Hellman Algorithm
p = 23, g = 5
| a = 4 | b = 3 |
|--------------------------------------------|-----------------------------------------------|
| A = g ^ a mod p = 5 ^ 4 mod 23 = 4 | B = g ^ b mod p = 5 ^ 3 mod 23 = 10 |
| Send Alice public key (A) to Bob | Send Bob public key (B) to Alice |
| Bob public key (B) is 10 | Alice public key (A) is 4 |
| Compute shared key from Bob pubic key (B) | Compute shared key from Alice public key (A) |
| Sa = B ^ a mod p = 10 ^ 4 mod 23 = 18 | Sb = A ^ b mod p = 4 ^ 3 mod 23 = 18 |
Sa == Sb
Discrete logarithm problem (A = g ^ ? mod p && B ^ ? mod p)
MITM Attack
p = 23, g = 5
| a = 4 | eA = 6, eB = 9 | b = 3 |
|--------------------------------------------------|-----------------------------------------------|----------------------------------------------------|
| A = g ^ a mod p = 5 ^ 4 mod 23 = 4 | Eb = g ^ eB mod p = 5 ^ 9 mod 23 = 11 | - |
| Send Alice public key (A) to Bob (Eve) | Send Spoofed Bob public key (Ea) to Alice | - |
| Bob (Eve) public key (Eb) is 11 | Alice public key (A) is 4 | - |
| Compute shared key from Bob (Eve) pubic key (Eb) | Compute shared key from Alice public key (A) | - |
| Sae = Eb ^ a mod p = 11 ^ 3 mod 23 = 13 | Sa = A ^ eB mod p = 4 ^ 9 mod 23 = 13 | - |
| - | Ea = g ^ eA mod p = 5 ^ 6 mod 23 = 8 | B = g ^ b mod p = 5 ^ 3 mod 23 = 10 |
| - | Send Spoofed Alice public key (Eb) to Bob | Send Bob public key (B) to Alice (Eve) |
| - | Bob public key (B) is 10 | Alice (Eve) public key (Ea) is 8 |
| - | Compute shared key from Bob public key (B) | Compute shared key from Alice (Eve) pubic key (Ea) |
| - | Sb = B ^ eA mod p = 10 ^ 6 mod 23 = 6 | Sbe = Ea ^ b mod p = 8 ^ 3 mod 23 = 6 |
(Now Eve can intercept connection between Alice and Bob)