// Copyright © 2022 siddharth // SPDX-License-Identifier: ISC package challenge import ( "bufio" "fmt" "math/big" "net" "os" "ricketyspace.net/cryptopals/lib" ) func C35(args []string) { if len(args) < 2 { fmt.Println("Usage: cryptopals -c 35 ENTITY PORT") return } entity := args[0] port, err := lib.StrToNum(args[1]) if err != nil { fmt.Println("Port invalid") return } if port < 12000 { fmt.Println("Error: port number must be >= 12000") return } // Cipher functions. // // Pack a cipher and it's iv into packet. cipherPacketEncode := func(cipher, iv []byte) string { packet := append(cipher, iv...) return lib.BytesToHexStr(packet) } // Unpack packet into cipher and iv. cipherPacketDecode := func(packet string) (cipher, iv []byte) { stream := lib.HexStrToBytes(packet) cipher = stream[0 : len(stream)-16] iv = stream[len(stream)-16:] return cipher, iv } // Encipher using AES-CBC. encipher := func(skey *big.Int, msg string) (string, error) { // Make key out of DH sesssion key. sha1 := lib.Sha1{} sha1.Init([]uint32{}) sha1.Message(skey.Bytes()) skeyb := sha1.Hash() if len(skeyb) < 16 { // Pad it to 16 bytes. skeyb = append(skeyb, make([]byte, 16-len(skeyb))...) } k := skeyb[0:16] // Make initialization vector. iv, err := lib.RandomBytes(16) if err != nil { return "", err } // Encrypt msg using AES-CBC cipher := lib.AESEncryptCBC([]byte(msg), k, iv) // Pack encrypted message in a format that can be sent // over the wire. packet := cipherPacketEncode(cipher, iv) // Return packed encrypted message. return packet, nil } // Decipher using AES-CBC. decipher := func(skey *big.Int, packet string) (string, error) { // Make key out of DH sesssion key. sha1 := lib.Sha1{} sha1.Init([]uint32{}) sha1.Message(skey.Bytes()) skeyb := sha1.Hash() if len(skeyb) < 16 { // Pad it to 16 bytes. skeyb = append(skeyb, make([]byte, 16-len(skeyb))...) } k := skeyb[0:16] // Decode packet cipher, iv := cipherPacketDecode(packet) // Decrypt message. msg, err := lib.AESDecryptCBC(cipher, k, iv) if err != nil { return "", err } return string(msg), nil } // Server handling. // // Do DH handshake with client and return DH session key. serverDHHandshake := func(conn net.Conn) (*big.Int, error) { // Read p+g packet from client. packet, err := bufio.NewReader(conn).ReadString('\n') if err != nil { return nil, err } // Remove newline character from packet. packet = packet[:len(packet)-1] // Try to read DH paramters from packet. params := lib.StrSplitAt('+', packet) if len(params) != 2 { return nil, lib.CPError{"DH paramters invalid"} } // Try make DH for this client connection. dh, ok := lib.NewDH(params[0], params[1]) if !ok { return nil, lib.CPError{"DH initialization failed"} } // Send ACK to client. fmt.Fprintf(conn, "ACK\n") // Wait and get client's DH public key packet, err = bufio.NewReader(conn).ReadString('\n') if err != nil { return nil, err } // Remove newline character. packet = packet[:len(packet)-1] // Parse client's DH public key. cPub, ok := new(big.Int).SetString(lib.StripSpaceChars(packet), 10) if !ok { return nil, lib.CPError{"DH public key invalid"} } // Send server's DH public key for this connection. fmt.Fprintf(conn, "%v\n", dh.Pub()) // Return DH session key. return dh.SharedSecret(cPub), nil } // Handle connection from a client. serverHandleConn := func(conn net.Conn) { defer conn.Close() // Do DH handshake. skey, err := serverDHHandshake(conn) if err != nil { fmt.Printf("DH Handshake failed [%v]: %v\n", conn.RemoteAddr(), err) return } fmt.Printf("Made secure connection with %v\n", conn.RemoteAddr()) // Enter read-echo loop. for { packet, err := bufio.NewReader(conn).ReadString('\n') if err != nil { fmt.Printf("Closed connection to [%v]\n", conn.RemoteAddr()) return } // Remove newline character. packet = packet[:len(packet)-1] // Decipher packet. msg, err := decipher(skey, packet) if err != nil { fmt.Printf("Message decryption failed for [%v]: %v\n", conn.RemoteAddr(), err) return } fmt.Printf("Received from [%v]: '%v'\n", conn.RemoteAddr(), msg) // Make msg to client. msg = fmt.Sprintf("-> %s", msg) // Encrypt and echo back message. cpacket, err := encipher(skey, msg) if err != nil { fmt.Printf("Message encryption failed for [%v]: %v\n", conn.RemoteAddr(), err) return } // Send message to client. fmt.Fprintf(conn, "%s\n", cpacket) } } // Start "secure" echo server. serverSpawn := func() { 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", err) } go serverHandleConn(conn) } } // Client handling. // // Try to make secure connection with server using DH and // return server's DH public key. clientSecureConnToServer := func() (net.Conn, *big.Int, error) { // Try to connect to server. addr := fmt.Sprintf(":%d", port) conn, err := net.Dial("tcp", addr) if err != nil { return conn, nil, err } // Initialize DH. p := lib.StripSpaceChars( `ffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024 e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd 3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec 6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f 24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361 c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552 bb9ed529077096966d670c354e4abc9804f1746c08ca237327fff fffffffffffff`) g := "2" dh, ok := lib.NewDH(p, g) if !ok { return conn, nil, lib.CPError{"DH initialization failed"} } // Make DH packet: p+g packet := fmt.Sprintf("%v+%v", p, g) // Sent DH packet to server. fmt.Fprintf(conn, "%s\n", packet) // Wait and try to get ACK from server. spacket, err := bufio.NewReader(conn).ReadString('\n') if err != nil { return conn, nil, err } // Remove newline character. spacket = spacket[:len(spacket)-1] if spacket != "ACK" { return conn, nil, lib.CPError{"ACK failed"} } // Send DH public key to server. fmt.Fprintf(conn, "%v\n", dh.Pub()) // Get server's DH public key. spacket, err = bufio.NewReader(conn).ReadString('\n') if err != nil { return conn, nil, err } // Remove newline character. spacket = spacket[:len(spacket)-1] // Parse server's DH public key. sPub, ok := new(big.Int).SetString(lib.StripSpaceChars(spacket), 10) if !ok { return conn, nil, lib.CPError{"Server's DH key invalid"} } // Return server connection and DH session key. return conn, dh.SharedSecret(sPub), nil } // Start the client. clientSpawn := func() { // Make a secure connection to server. conn, skey, err := clientSecureConnToServer() if err != nil { fmt.Printf("Unable to establish secure connection: %v\n", err) return } defer conn.Close() fmt.Printf("Made secure connection with %v\n", conn.RemoteAddr()) // Enter write loop. for { // Read message from stdin. fmt.Printf("> ") 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] // Encrypt message. packet, err := encipher(skey, msg) if err != nil { fmt.Printf("Encryption error: %v\n", err) return } // Send message to server. fmt.Fprintf(conn, "%s\n", packet) // Read response from server. spacket, err := bufio.NewReader(conn).ReadString('\n') if err != nil { fmt.Printf("Server closed connection\n") return } // Remove newline character. spacket = spacket[:len(spacket)-1] // Decipher packet from server. smsg, err := decipher(skey, spacket) if err != nil { fmt.Printf("Message decryption failed: %v\n", err) return } // Barf server's response. fmt.Printf("%s\n", smsg) } } // Take action based on entity. switch { case entity == "server": serverSpawn() case entity == "client": clientSpawn() default: fmt.Println("Error: Uknown entity") } } // Output: // // Part I: // // $ ./cryptopals -c 35 server 12000 // Waiting for connection... // Waiting for connection... // Made secure connection with 127.0.0.1:40557 // Received from [127.0.0.1:40557]: 'Put this together while listening to' // Received from [127.0.0.1:40557]: 'Kara Swisher's Sway.' // // $ ./cryptopals -c 35 client 12000 // Made secure connection with 127.0.0.1:12000 // > Put this together while listening to // -> Put this together while listening to // > Kara Swisher's Sway. // -> Kara Swisher's Sway. // > ^C //