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Zarmack Tanen 2015-09-07 16:01:38 +02:00
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12
SRP Design.md
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@ -11,6 +11,7 @@ their shortcomings.
The following is a description of SRP-6 and 6a, the latest versions of SRP:
```
N A large safe prime (N = 2q+1, where q is prime)
All arithmetic is done modulo N.
g A generator modulo N
@ -25,17 +26,20 @@ The following is a description of SRP-6 and 6a, the latest versions of SRP:
A,B Public ephemeral values
x Private key (derived from p and s)
v Password verifier
```
The host stores passwords using the following formula:
```
x = H(s, p) (s is chosen randomly)
v = g^x (computes password verifier)
```
The host then keeps {I, s, v} in its password database.
The authentication protocol itself goes as follows:
```
User -> Host: I, A = g^a (identifies self, a = random number)
Host -> User: s, B = kv + g^b (sends salt, b = random number)
@ -47,16 +51,16 @@ Host -> User: s, B = kv + g^b (sends salt, b = random number)
Host: S = (Av^u) ^ b (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: M = H(H(N) xor H(g), H(I), s, A, B, K)
Host -> User: H(A, M, K)
```
The two parties also employ the following safeguards: