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wired guide

X25519 key exchange

X25519 is the key exchange a TLS 1.3 handshake runs. This page uses the Alice and Bob private keys from RFC 7748 6.1; every printed value matches that section.

Code

main.c
#define WIRED_MAIN
#include "wired.h"


/* The private keys from RFC 7748 6.1. */
static const u8 alice_priv[32] = {
    0x77, 0x07, 0x6d, 0x0a, 0x73, 0x18, 0xa5, 0x7d, 0x3c, 0x16, 0xc1,
    0x72, 0x51, 0xb2, 0x66, 0x45, 0xdf, 0x4c, 0x2f, 0x87, 0xeb, 0xc0,
    0x99, 0x2a, 0xb1, 0x77, 0xfb, 0xa5, 0x1d, 0xb9, 0x2c, 0x2a};
static const u8 bob_priv[32] = {0x5d, 0xab, 0x08, 0x7e, 0x62, 0x4a, 0x8a, 0x4b,
                                0x79, 0xe1, 0x7f, 0x8b, 0x83, 0x80, 0x0e, 0xe6,
                                0x6f, 0x3b, 0xb1, 0x29, 0x26, 0x18, 0xb6, 0xfd,
                                0x1c, 0x2f, 0x8b, 0x27, 0xff, 0x88, 0xe0, 0xeb};


int wired_main(int argc, char** argv) {
  (void)argc;
  (void)argv;
  u8 alice_pub[32], bob_pub[32], alice_shared[32], bob_shared[32];
  wired_x25519_base(alice_pub, alice_priv);
  wired_x25519_base(bob_pub, bob_priv);
  /* Each side combines its own private key with the other's public key.
   * A 0 return means a low-order peer key: reject the handshake. */
  if (!wired_x25519(alice_shared, alice_priv, bob_pub)) return 1;
  if (!wired_x25519(bob_shared, bob_priv, alice_pub)) return 1;
  wired_dprintf(2, "alice public: ");
  wired_dump_hex(2, wired_span_of(alice_pub, 32));
  wired_dprintf(2, "\nbob public:   ");
  wired_dump_hex(2, wired_span_of(bob_pub, 32));
  wired_dprintf(2, "\nshared:       ");
  wired_dump_hex(2, wired_span_of(alice_shared, 32));
  int eq = wired_span_eq(
      wired_span_of(alice_shared, 32), wired_span_of(bob_shared, 32));
  wired_dprintf(2, "\nshared equal: %s\n", eq ? "yes" : "no");
  return 0;
}

Output

Terminal window
$ ./tls-x25519
alice public: 8520f0098930a754748b7ddcb43ef75a0dbf3a0d26381af4eba4a98eaa9b4e6a
bob public:   de9edb7d7b7dc1b4d35b61c2ece435373f8343c85b78674dadfc7e146f882b4f
shared:       4a5d9d5ba4ce2de1728e3bf480350f25e07e21c947d19e3376f09b3c1e161742
shared equal: yes

How it works

Section titled “How it works”
  • wired_x25519_base(pub, priv) computes a public key from a 32-byte private key.
  • wired_x25519(shared, priv, peer_pub) combines your private key with the peer’s public key. Both sides get the same secret.
  • wired_x25519 returns 0 for a low-order peer key (an all-zero result). You must reject that return (RFC 7748 6.1).