// Copyright © 2022 siddharth ravikumar <s@ricketyspace.net>
// SPDX-License-Identifier: ISC
package challenge
import (
"bufio"
"fmt"
"math/big"
"net"
"os"
"ricketyspace.net/cryptopals/lib"
)
func C37(args []string) {
usage := func() {
fmt.Println("Usage: cryptopals -c 37 [ client | server ] PORT [ PUBKEY ]")
fmt.Println("\n PUBKEY is required for the client. Valid values are")
fmt.Println(" 0, N1, N2, N3, N4, etc.")
}
if len(args) < 2 {
usage()
return
}
entity := args[0]
port, err := lib.StrToNum(args[1])
if err != nil {
fmt.Println("port invalid")
return
}
if port < 12000 {
fmt.Println("port number must be >= 12000")
return
}
// SRP params.
paramN := lib.StripSpaceChars(
`ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024
e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd
3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec
6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f
24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361
c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552
bb9ed529077096966d670c354e4abc9804f1746c08ca237327fff
fffffffffffff`)
paramG := "2"
paramK := "3"
// Process client's pub key if this is a client.
clientPubKey := []byte{}
if entity == "client" {
if len(args) != 3 {
usage()
return
}
pub := args[2]
// Validate pub key.
switch {
case pub[0] == '0':
clientPubKey = []byte{0}
case len(pub) > 1 && pub[0] == 'N':
pow, err := lib.StrToNum(pub[1:])
if err != nil {
fmt.Printf("error: public key invalid\n")
return
}
pubi, ok := new(big.Int).SetString(
lib.StripSpaceChars(paramN),
16,
)
if !ok {
fmt.Printf("unable to process pub key")
return
}
if pow == 1 {
clientPubKey = pubi.Bytes()
pubi = pubi.Exp(pubi, big.NewInt(int64(pow)), pubi)
} else {
pubi = pubi.Exp(pubi, big.NewInt(int64(pow)), pubi)
if pubi.Cmp(big.NewInt(0)) == 0 {
clientPubKey = []byte{0}
} else {
clientPubKey = pubi.Bytes()
}
}
default:
usage()
return
}
}
// Zero key.
zeroSessionKey := func() []byte {
// SHA256 of empty string.
sha256 := lib.Sha256{}
sha256.Init([]uint32{})
sha256.Message([]byte{})
return sha256.Hash()
}
// Register user on the server.
serverRegisterUser := func(server *lib.SRPServer, info []string) error {
if len(info) != 5 {
return fmt.Errorf("regiser user: info valid")
}
n := info[0]
g := info[1]
k := info[2]
ident := info[3]
pass := info[4]
user, err := lib.NewSRPUser(n, g, k, ident, pass)
if err != nil {
return fmt.Errorf("register user: %v", err)
}
if err = server.RegisterUser(user); err != nil {
return fmt.Errorf("register user: %v", err)
}
return nil
}
// Login user on the server.
serverLoginUser := func(server *lib.SRPServer, info []string,
conn net.Conn) error {
if len(info) != 2 {
return fmt.Errorf("login user: info invalid")
}
ident := info[0]
user, err := server.GetUser(ident)
if err != nil {
return fmt.Errorf("get user: %v", err)
}
if user.LoggedIn() {
return fmt.Errorf("user already has a session open")
}
clientPub := new(big.Int).SetBytes(lib.HexStrToBytes(info[1]))
user.EphemeralKeyGen() // Generate server pub key for user.
serverPub, err := user.EphemeralKeyPub()
if err != nil {
return fmt.Errorf("server pub key: %v", err)
}
// Make ACK packet
packet := fmt.Sprintf("%s+%s", lib.BytesToHexStr(user.Salt()),
lib.BytesToHexStr(serverPub.Bytes()))
// Send packet to client.
_, err = fmt.Fprintf(conn, "%s\n", packet)
if err != nil {
return fmt.Errorf("sending packet to client: %v", err)
}
// Compute session key.
err = user.SetScramblingParam(clientPub)
if err != nil {
return fmt.Errorf("setting scrambling param: %v", err)
}
err = user.ComputeSessionKey(clientPub)
if err != nil {
return fmt.Errorf("computing session key: %v", err)
}
// Wait and try to read hmac from client.
cpacket, err := bufio.NewReader(conn).ReadString('\n')
if err != nil {
return fmt.Errorf("hmac recv: %v", err)
}
hmac := []byte(cpacket[:len(cpacket)-1])
if !user.SessionKeyMacVerify(hmac) {
return fmt.Errorf("hmac verification failed")
}
// Login user.
user.LogIn()
return nil
}
// Logout user on the server.
serverLogoutUser := func(server *lib.SRPServer, ident string,
conn net.Conn) error {
user, err := server.GetUser(ident)
if err != nil {
return fmt.Errorf("get user: %v", err)
}
if !user.LoggedIn() {
return fmt.Errorf("user not logged in")
}
// Logout user.
user.LogOut()
return nil
}
// Handle connection from a client.
serverHandleConn := func(server *lib.SRPServer, conn net.Conn) {
defer conn.Close()
fmt.Printf("Got connection from %v\n", conn.RemoteAddr())
// Read packet from client.
packet, err := bufio.NewReader(conn).ReadString('\n')
if err != nil {
fmt.Printf("Unable to read from client %v\n",
conn.RemoteAddr())
return
}
// Remove newline character from packet.
packet = packet[:len(packet)-1]
parts := lib.StrSplitAt('+', packet)
if len(parts) < 2 {
fmt.Fprintf(conn, "invalid request\n")
return
}
switch {
case parts[0] == "register":
err = serverRegisterUser(server, parts[1:])
if err != nil {
fmt.Fprintf(conn, "%v\n", err)
return
} else {
fmt.Fprintf(conn, "OK\n")
}
return
case parts[0] == "login":
err = serverLoginUser(server, parts[1:], conn)
if err != nil {
fmt.Fprintf(conn, "%v\n", err)
} else {
fmt.Fprintf(conn, "OK\n")
}
return
case parts[0] == "logout":
err = serverLogoutUser(server, parts[1], conn)
if err != nil {
fmt.Fprintf(conn, "%v\n", err)
} else {
fmt.Fprintf(conn, "OK\n")
}
return
default:
fmt.Fprintf(conn, "invalid action")
return
}
}
// Start SRP server.
serverSpawn := func() {
server := new(lib.SRPServer)
p := fmt.Sprintf(":%d", port)
ln, err := net.Listen("tcp", p)
if err != nil {
fmt.Printf("server listen error: %v\n", err)
return
}
for {
fmt.Println("Waiting for connection...")
conn, err := ln.Accept()
if err != nil {
fmt.Printf("server accept error: %v\n", err)
}
go serverHandleConn(server, conn)
}
}
// Register user with server.
clientRegisterUser := func(client *lib.SRPClient, ident string) error {
n := paramN
g := paramG
k := paramK
// Prompt for password.
fmt.Printf("password> ")
pass, err := bufio.NewReader(os.Stdin).ReadString('\n')
if err != nil {
return fmt.Errorf("unable to read password: %v", err)
}
// Create session for user.
client.Session, err = lib.NewSRPClientSession(n, g, k, ident)
if err != nil {
return fmt.Errorf("unable to create session: %v", err)
}
// Make SRP registration packet.
packet := fmt.Sprintf("%s+%s+%s+%s+%s+%s", "register",
n, g, k, ident, pass)
// Try to connect to server.
conn, err := net.Dial("tcp", fmt.Sprintf(":%d", port))
if err != nil {
return fmt.Errorf("unable connect to server: %v", err)
}
defer conn.Close()
// Send packet to server.
_, err = fmt.Fprintf(conn, "%s", packet)
if err != nil {
return fmt.Errorf("unable communicate with server: %v", err)
}
// Wait and try to get registration ACK from server.
spacket, err := bufio.NewReader(conn).ReadString('\n')
if err != nil {
return fmt.Errorf("server did not respond: %v", err)
}
// Remove newline character.
spacket = spacket[:len(spacket)-1]
if spacket != "OK" {
return fmt.Errorf("server registration failed: %s", spacket)
}
return nil
}
// Login user into the server.
clientLoginUser := func(client *lib.SRPClient, ident string) error {
n := paramN
g := paramG
k := paramK
// Prompt for password.
fmt.Printf("password> ")
pass, err := bufio.NewReader(os.Stdin).ReadString('\n')
if err != nil {
return fmt.Errorf("unable to read password: %v", err)
}
pass = pass[:len(pass)-1]
// Create session for user.
client.Session, err = lib.NewSRPClientSession(n, g, k, ident)
if err != nil {
return fmt.Errorf("unable to create session: %v", err)
}
// Set public key to N^t where t is 0, 1, 2, etc.
pub := clientPubKey
// Make SRP login packet.
packet := fmt.Sprintf("%s+%s+%s", "login",
ident, lib.BytesToHexStr(pub))
// Try to connect to server.
conn, err := net.Dial("tcp", fmt.Sprintf(":%d", port))
if err != nil {
return fmt.Errorf("unable connect to server: %v", err)
}
defer conn.Close()
// Send login packet to server.
_, err = fmt.Fprintf(conn, "%s\n", packet)
if err != nil {
return fmt.Errorf("unable communicate with server: %v", err)
}
// Wait and try to get registration ACK from server.
spacket, err := bufio.NewReader(conn).ReadString('\n')
if err != nil {
return fmt.Errorf("server did not respond: %v", err)
}
// Remove newline character.
spacket = spacket[:len(spacket)-1]
if !lib.StrHas(spacket, "+") {
return fmt.Errorf("pub exchange: %s", spacket)
}
parts := lib.StrSplitAt('+', spacket)
if len(parts) < 2 {
return fmt.Errorf("server login response invalid")
}
salt := lib.HexStrToBytes(parts[0])
serverPub := new(big.Int).SetBytes(lib.HexStrToBytes(parts[1]))
// Compute session key.
err = client.Session.SetScramblingParam(serverPub)
if err != nil {
return fmt.Errorf("setting scrambling param: %v", err)
}
err = client.Session.ComputeSessionKey(salt, pass, serverPub)
if err != nil {
return fmt.Errorf("computing session key: %v", err)
}
// Set to "zero" session key. Since the client's
// public key is a multiple of N, the session key will
// always be zero.
client.Session.SetSessionKey(zeroSessionKey())
// Compute session key hmac
hmac, err := client.Session.SessionKeyMac(salt)
if err != nil {
return fmt.Errorf("session key hmac: %v", err)
}
// Send hmac to server.
_, err = fmt.Fprintf(conn, "%s\n", hmac)
if err != nil {
return fmt.Errorf("sending hmac: %v", err)
}
// Wait and try to get registration ACK from server.
spacket, err = bufio.NewReader(conn).ReadString('\n')
if err != nil {
return fmt.Errorf("server did not respond: %v", err)
}
// Remove newline character.
spacket = spacket[:len(spacket)-1]
if spacket != "OK" {
return fmt.Errorf("login failed: %s", spacket)
}
// Login user.
client.LogIn()
return nil
}
// Logout user.
clientLogoutUser := func(client *lib.SRPClient) error {
// Make logout packet.
packet := fmt.Sprintf("%s+%s", "logout", client.Ident())
// Try to connect to server.
conn, err := net.Dial("tcp", fmt.Sprintf(":%d", port))
if err != nil {
return fmt.Errorf("unable connect to server: %v", err)
}
defer conn.Close()
// Send login packet to server.
_, err = fmt.Fprintf(conn, "%s\n", packet)
if err != nil {
return fmt.Errorf("logout send: %v", err)
}
// Wait and try to get logout ACK from server.
spacket, err := bufio.NewReader(conn).ReadString('\n')
if err != nil {
return fmt.Errorf("logout recv: %v", err)
}
// Remove newline character.
spacket = spacket[:len(spacket)-1]
if spacket != "OK" {
return fmt.Errorf("logout ack: %s", spacket)
}
// Logout user.
client.Session = nil
return nil
}
// Start SRP client.
clientSpawn := func() {
client := new(lib.SRPClient)
// Enter repl.
for {
// Read message from stdin.
fmt.Printf("%s> ", client.Ident())
msg, err := bufio.NewReader(os.Stdin).ReadString('\n')
if err != nil {
fmt.Printf("read error: %v\n", err)
return
}
// Remove newline character.
msg = msg[:len(msg)-1]
msg_parts := lib.StrSplitAt(' ', msg)
switch {
case !client.LoggedIn() && msg_parts[0] == "register" &&
len(msg_parts) == 2:
err := clientRegisterUser(client, msg_parts[1])
if err != nil {
fmt.Printf("Registration failed: %v\n", err)
} else {
fmt.Printf("Registered!\n")
}
case !client.LoggedIn() && msg_parts[0] == "login" &&
len(msg_parts) == 2:
err := clientLoginUser(client, msg_parts[1])
if err != nil {
fmt.Printf("Login failed: %v\n", err)
} else {
fmt.Printf("Logged in!\n")
}
case client.LoggedIn() && msg_parts[0] == "logout":
err := clientLogoutUser(client)
if err != nil {
fmt.Printf("Logout failed: %v\n", err)
} else {
fmt.Printf("Logged out!\n")
}
}
}
}
// Take action based on entity.
switch {
case entity == "server":
serverSpawn()
case entity == "client":
clientSpawn()
default:
fmt.Println("uknown entity")
}
}
// Output:
//
// https://ricketyspace.net/cryptopals/c37.webm
//
// $ ./cryptopals -c 37 server 12000
// Waiting for connection...
// Waiting for connection...
// Got connection from 127.0.0.1:25923
// Waiting for connection...
// Got connection from 127.0.0.1:47437
// Waiting for connection...
// Got connection from 127.0.0.1:47489
// Waiting for connection...
// Got connection from 127.0.0.1:17179
// Waiting for connection...
// Got connection from 127.0.0.1:24965
// Waiting for connection...
// Got connection from 127.0.0.1:26381
// Waiting for connection...
// Got connection from 127.0.0.1:4572
// Waiting for connection...
// Got connection from 127.0.0.1:47202
// Waiting for connection...
// Got connection from 127.0.0.1:42154
//
// $ ./cryptopals -c 37 client 12000 0
// > register bob
// password> theonjoy
// Registered!
// > login bob
// password>
// Logged in!
// bob> logout
// Logged out!
// > ^C
// ada$ ./cryptopals -c 37 client 12000 N1
// > login bob
// password>
// Logged in!
// bob> logout
// Logged out!
// > ^C
// ada$ ./cryptopals -c 37 client 12000 N2
// > login bob
// password>
// Logged in!
// bob> logout
// Logged out!
// > ^C
// ada$ ./cryptopals -c 37 client 12000 N9
// > login bob
// password>
// Logged in!
// bob> logout
// Logged out!
// > ^C
