From 8e1700059a73f7090528194fab0b36751d6d1693 Mon Sep 17 00:00:00 2001
From: siddharth <s@ricketyspace.net>
Date: Mon, 11 Apr 2022 20:03:08 -0400
Subject: lib: add srp session key functions

---
 lib/srp.go      | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++
 lib/srp_test.go | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 164 insertions(+)

diff --git a/lib/srp.go b/lib/srp.go
index 2303793..a7f14e1 100644
--- a/lib/srp.go
+++ b/lib/srp.go
@@ -199,6 +199,30 @@ func (u *SRPUser) SetScramblingParam(a *big.Int) error {
 	return nil
 }
 
+func (u *SRPUser) ComputeSessionKey(a *big.Int) error {
+	if a.Cmp(big.NewInt(0)) != 1 {
+		return CPError{"a is invalid"}
+	}
+
+	// v^u
+	vu := new(big.Int)
+	vu.Exp(u.v, u.u, u.n)
+
+	// S = (A * v^u)  ^ b
+	s := new(big.Int)
+	s.Mul(a, vu)
+	s.Exp(s, u.b, u.n)
+	sb := s.Bytes()
+
+	// K = H(S)
+	m := make([]byte, 0)
+	m = append(m, sb...)
+	u.h.Message(m)
+	u.sk = u.h.Hash()
+
+	return nil
+}
+
 func NewSRPClientSession(n, g, k, ident string) (*SRPClientSession, error) {
 	var ok bool
 
@@ -270,3 +294,56 @@ func (s *SRPClientSession) SetScramblingParam(b *big.Int) error {
 	}
 	return nil
 }
+
+func (s *SRPClientSession) ComputeSessionKey(salt []byte,
+	pass string, b *big.Int) error {
+	if len(salt) < 1 {
+		return CPError{"salt invalid"}
+	}
+	if len(pass) < 1 {
+		return CPError{"pass invalid"}
+	}
+
+	// salt+pass
+	sp := make([]byte, 0)
+	copy(sp, salt)
+	sp = append(sp, StrToBytes(pass)...)
+
+	// x = H(salt+pass)
+	x := new(big.Int)
+	s.h.Message(sp)
+	x.SetBytes(s.h.Hash())
+
+	// g^x
+	gx := new(big.Int)
+	gx.Exp(s.g, x, s.n)
+
+	// k * g^x
+	kgx := new(big.Int)
+	kgx.Mul(s.k, gx)
+
+	// B - (k * g^x)
+	bkgx := new(big.Int)
+	bkgx.Sub(b, kgx)
+
+	// u * x
+	ux := new(big.Int)
+	ux.Mul(s.u, x)
+
+	// a + u*x
+	aux := new(big.Int)
+	aux.Add(s.a, ux)
+
+	// S = (B - (k * g^x)) ^ (a + u*x)
+	sec := new(big.Int)
+	sec.Exp(bkgx, aux, s.n)
+	sb := sec.Bytes()
+
+	// K = H(S)
+	m := make([]byte, 0)
+	m = append(m, sb...)
+	s.h.Message(m)
+	s.sk = s.h.Hash()
+
+	return nil
+}
diff --git a/lib/srp_test.go b/lib/srp_test.go
index a819b15..1445e01 100644
--- a/lib/srp_test.go
+++ b/lib/srp_test.go
@@ -248,3 +248,90 @@ func TestSRPScramblingParamter(t *testing.T) {
 		return
 	}
 }
+
+func TestSRPSessionKey(t *testing.T) {
+	n := StripSpaceChars(
+		`ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024
+                 e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd
+                 3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec
+                 6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f
+                 24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361
+                 c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552
+                 bb9ed529077096966d670c354e4abc9804f1746c08ca237327fff
+                 fffffffffffff`)
+	g := "2"
+	k := "3"
+	ident := "s@ricketyspace.net"
+	pass := "d59d6c93af0f37f272d924979"
+
+	// Init srp server user.
+	user, err := NewSRPUser(n, g, k, ident, pass)
+	if err != nil {
+		t.Errorf("unable to create user on server: %v\n", err)
+		return
+	}
+
+	// Get server's pub for user.
+	user.EphemeralKeyGen()
+	pubB, err := user.EphemeralKeyPub()
+	if err != nil {
+		t.Errorf("server ephemeral pub error: %v\n", err)
+		return
+	}
+
+	// Init srp client session.
+	session, err := NewSRPClientSession(n, g, k, ident)
+	if err != nil {
+		t.Errorf("unable to create client session: %v\n", err)
+		return
+	}
+
+	// Get client's pub for user.
+	pubA, err := session.EphemeralKeyPub()
+	if err != nil {
+		t.Errorf("client ephemeral pub error: %v\n", err)
+		return
+	}
+
+	// Compute server's scrambling parameter for user.
+	err = user.SetScramblingParam(pubA)
+	if err != nil {
+		t.Errorf("unable generate server scrambling parameter: %v\n", err)
+		return
+	}
+
+	// Compute client's scrambling paramter for user.
+	err = session.SetScramblingParam(pubB)
+	if err != nil {
+		t.Errorf("unable generate client scrambling parameter: %v\n", err)
+		return
+	}
+
+	// The server's and client's scrambling parameter must be the
+	// same.
+	if user.u.Cmp(session.u) != 0 {
+		t.Error("Error: scrambling parameter of server != client\n")
+		return
+	}
+
+	// Compute server's session key for user.
+	err = user.ComputeSessionKey(pubA)
+	if err != nil {
+		t.Errorf("unable to compute server's session key: %v", err)
+		return
+	}
+
+	// Compute client's session key for for user.
+	err = session.ComputeSessionKey(user.salt, pass, pubB)
+	if err != nil {
+		t.Errorf("unable to compute client's session key: %v", err)
+		return
+	}
+
+	// Verify that the session key is the same.
+	if !BytesEqual(user.sk, session.sk) {
+		t.Errorf("server's and client's session key not equal:"+
+			" server_sk(%v): client_sk(%v)", user.sk, session.sk)
+		return
+	}
+}
-- 
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