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using System;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Math;
using Org.BouncyCastle.Security;
namespace Org.BouncyCastle.Crypto.Agreement.Srp
{
/**
* Implements the client side SRP-6a protocol. Note that this class is stateful, and therefore NOT threadsafe.
* This implementation of SRP is based on the optimized message sequence put forth by Thomas Wu in the paper
* "SRP-6: Improvements and Refinements to the Secure Remote Password Protocol, 2002"
*/
public class Srp6Client
{
protected BigInteger N;
protected BigInteger g;
protected BigInteger privA;
protected BigInteger pubA;
protected BigInteger B;
protected BigInteger x;
protected BigInteger u;
protected BigInteger S;
protected BigInteger M1;
protected BigInteger M2;
protected BigInteger Key;
protected IDigest digest;
protected SecureRandom random;
public Srp6Client()
{
}
/**
* Initialises the client to begin new authentication attempt
* @param N The safe prime associated with the client's verifier
* @param g The group parameter associated with the client's verifier
* @param digest The digest algorithm associated with the client's verifier
* @param random For key generation
*/
public virtual void Init(BigInteger N, BigInteger g, IDigest digest, SecureRandom random)
{
this.N = N;
this.g = g;
this.digest = digest;
this.random = random;
}
public virtual void Init(Srp6GroupParameters group, IDigest digest, SecureRandom random)
{
Init(group.N, group.G, digest, random);
}
/**
* Generates client's credentials given the client's salt, identity and password
* @param salt The salt used in the client's verifier.
* @param identity The user's identity (eg. username)
* @param password The user's password
* @return Client's public value to send to server
*/
public virtual BigInteger GenerateClientCredentials(byte[] salt, byte[] identity, byte[] password)
{
this.x = Srp6Utilities.CalculateX(digest, N, salt, identity, password);
this.privA = SelectPrivateValue();
this.pubA = g.ModPow(privA, N);
return pubA;
}
/**
* Generates client's verification message given the server's credentials
* @param serverB The server's credentials
* @return Client's verification message for the server
* @throws CryptoException If server's credentials are invalid
*/
public virtual BigInteger CalculateSecret(BigInteger serverB)
{
this.B = Srp6Utilities.ValidatePublicValue(N, serverB);
this.u = Srp6Utilities.CalculateU(digest, N, pubA, B);
this.S = CalculateS();
return S;
}
protected virtual BigInteger SelectPrivateValue()
{
return Srp6Utilities.GeneratePrivateValue(digest, N, g, random);
}
private BigInteger CalculateS()
{
BigInteger k = Srp6Utilities.CalculateK(digest, N, g);
BigInteger exp = u.Multiply(x).Add(privA);
BigInteger tmp = g.ModPow(x, N).Multiply(k).Mod(N);
return B.Subtract(tmp).Mod(N).ModPow(exp, N);
}
/**
* Computes the client evidence message M1 using the previously received values.
* To be called after calculating the secret S.
* @return M1: the client side generated evidence message
* @throws CryptoException
*/
public virtual BigInteger CalculateClientEvidenceMessage()
{
// Verify pre-requirements
if (this.pubA == null || this.B == null || this.S == null)
{
throw new CryptoException("Impossible to compute M1: " +
"some data are missing from the previous operations (A,B,S)");
}
// compute the client evidence message 'M1'
this.M1 = Srp6Utilities.CalculateM1(digest, N, pubA, B, S);
return M1;
}
/** Authenticates the server evidence message M2 received and saves it only if correct.
* @param M2: the server side generated evidence message
* @return A boolean indicating if the server message M2 was the expected one.
* @throws CryptoException
*/
public virtual bool VerifyServerEvidenceMessage(BigInteger serverM2)
{
// Verify pre-requirements
if (this.pubA == null || this.M1 == null || this.S == null)
{
throw new CryptoException("Impossible to compute and verify M2: " +
"some data are missing from the previous operations (A,M1,S)");
}
// Compute the own server evidence message 'M2'
BigInteger computedM2 = Srp6Utilities.CalculateM2(digest, N, pubA, M1, S);
if (computedM2.Equals(serverM2))
{
this.M2 = serverM2;
return true;
}
return false;
}
/**
* Computes the final session key as a result of the SRP successful mutual authentication
* To be called after verifying the server evidence message M2.
* @return Key: the mutually authenticated symmetric session key
* @throws CryptoException
*/
public virtual BigInteger CalculateSessionKey()
{
// Verify pre-requirements (here we enforce a previous calculation of M1 and M2)
if (this.S == null || this.M1 == null || this.M2 == null)
{
throw new CryptoException("Impossible to compute Key: " +
"some data are missing from the previous operations (S,M1,M2)");
}
this.Key = Srp6Utilities.CalculateKey(digest, N, S);
return Key;
}
}
}
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