lib: flesh out SRPUser

1 files changed, 133 insertions, 0 deletions

diff --git a/lib/srp.go b/lib/srp.go
new file mode 100644
index 0000000..20208a6
--- /dev/null
+++ b/lib/srp.go
@@ -0,0 +1,133 @@
+// Copyright © 2022 siddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+import "math/big"
+
+// SRP - implementation.
+// Reference http://srp.stanford.edu/design.html
+
+// SRP Client UI - Spec
+//
+// > register s@ricketyspace.net
+// Enter password
+// > ******
+// Registering with server...registered!
+// > login s@ricketyspace.net
+// Enter password
+// > ******
+// Logging in...in!
+// s@ricketyspace.net>
+// s@ricketyspace.net> login s@ricketyspace.net
+// login command not allowed when logged in
+// s@ricketyspace.net> register rsd@gnu.org
+// register command not allowed when logged in
+// s@ricketyspace.net> logout
+// Logging out..out!
+// > register s@ricketyspace.net
+// Already registered!
+
+// SRP Server.
+type SRPServer struct {
+	users []SRPUser
+}
+
+// Registered user on the SRP server.
+type SRPUser struct {
+	// Large safe prime. Server and client agree upon the value of
+	// N.
+	n *big.Int
+	// Generator modulo N. Server and client agree upon the value
+	// of N.
+	g *big.Int
+	// Multipier parameter. Server and client agree upon the value
+	// of N.
+	k *big.Int
+	// Hashing object for H() function.
+	h Sha256
+	// User's email address
+	ident string
+	// Salt. Randomly generator by the server.
+	salt []byte
+	// Private key derived from salt and user's pass.
+	x *big.Int
+	// Scrambling parameter.
+	u *big.Int
+	// Secret ephemeral value.
+	b *big.Int
+	// Password verifier.
+	v *big.Int
+	// Session key.
+	sk []byte
+}
+
+// SRP client.
+type SRPClient struct {
+	session SRPClientSession
+}
+
+// User session on the SRP client.
+type SRPClientSession struct {
+	// Large safe prime. Client and server agree upon the value of
+	// N.
+	n *big.Int
+	// Generator modulo N. Client and server agree upon the value
+	// of N.
+	g *big.Int
+	// Multipier parameter. Client and server agree upon the value
+	// of N.
+	k *big.Int
+	// User's email address
+	ident string
+	// Scrambling parameter.
+	u *big.Int
+	// Secret ephemeral value.
+	a *big.Int
+	// Session key.
+	sk []byte
+}
+
+func NewSRPUser(n, g, k, ident, pass string) (*SRPUser, error) {
+	var err error
+	var ok bool
+
+	user := new(SRPUser)
+	user.n, ok = new(big.Int).SetString(StripSpaceChars(n), 16)
+	if !ok {
+		return nil, CPError{"n is invalid"}
+	}
+	user.g, ok = new(big.Int).SetString(StripSpaceChars(g), 16)
+	if !ok {
+		return nil, CPError{"g is invalid"}
+	}
+	user.k, ok = new(big.Int).SetString(StripSpaceChars(k), 16)
+	if !ok {
+		return nil, CPError{"k is invalid"}
+	}
+	user.ident = ident
+	user.x = big.NewInt(0)
+	user.v = big.NewInt(0)
+
+	// Initialize hashing object.
+	user.h = Sha256{}
+	user.h.Init([]uint32{})
+
+	// Generate salt.
+	user.salt, err = RandomBytes(8)
+	if err != nil {
+		return nil, err
+	}
+
+	// Generate private key `x` from salt+pass
+	m := make([]byte, 0)
+	copy(m, user.salt)
+	m = append(m, StrToBytes(pass)...)
+	user.h.Message(m)
+	user.x.SetBytes(user.h.Hash())
+
+	// Generate password verifier `v`
+	user.v.Exp(user.g, user.x, user.n)
+
+	return user, nil
+}