SRP-2 Protocol Design

This is a description of SRP-2, which resists the attack recently discovered against SRP-1. Although there are no known weaknesses in SRP-2, its use of a composite modulus with unknown factorization makes parameter certification a serious issue. The SRP-3 protocol avoids these issues by using a safe prime modulus and fixes the weakness in SRP-1 using a different technique.
```  N    The product of two safe primes p and q
(p = 2j+1, q = 2k+1, j, k, p, q all prime)
All arithmetic is done modulo N.
g    A generator of the group G((p-1)(q-1)/2) mod N
s    User's salt
H()  One-way hash function
^    (Modular) Exponentiation
Ws,Ys   Secret values
Wp,Yp   Public values
```
The host stores passwords using the following formula:
```  Xs = s * H(P)              (s is chosen randomly)
Xp = g ^ Xs                (computes password verifier)
```
The host then keeps {u, s, Xp} in its password database. The authentication protocol itself goes as follows:
```User -> Host:  u, Wp = g ^ Ws          (identifies self, Ws = random number)
Host:  Yp = g ^ Ys             (Ys = random number)
Host -> User:  s, Z = Xp + Yp          (sends salt, adds verifier to residue)

User:  Xs = s * H(P)           (user enters password)
User:  Yp = Z - (g ^ Xs)       (recovers Yp from message)
User:  S = Yp ^ (2*Ws + Xs)    (computes session key)
User:  K = H(S)

Host:  S = (Wp^2 * Xp) ^ Ys    (computes session key)
Host:  K = H(S)
```
Now the two parties have a shared, strong session key K. To complete authentication, they need to prove to each other that their keys match. One possible way:
```User -> Host:  H(Z, K)                 (hash with host's value)
Host -> User:  H(Wp, K)                (hash with user's value)
```
The two parties also employ the following safeguards:
1. The user will abort if he receives Z == 0 (mod N).
2. The host will abort if it detects that Wp == 0 (mod N).
3. The user must show his proof of K first. If the server detects that the user's proof is incorrect, it must abort without showing its own proof of K.
A paper describing this protocol is also available.

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