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tor/src/or/onion_ntor.c

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/* Copyright (c) 2012-2017, The Tor Project, Inc. */
/* See LICENSE for licensing information */
/**
* \file onion_ntor.c
*
* \brief Implementation for the ntor handshake.
*
* The ntor circuit-extension handshake was developed as a replacement
* for the old TAP handshake. It uses Elliptic-curve Diffie-Hellman and
* a hash function in order to perform a one-way authenticated key
* exchange. The ntor handshake is meant to replace the old "TAP"
* handshake.
*
* We instantiate ntor with curve25519, HMAC-SHA256, and HKDF.
*
* This handshake, like the other circuit-extension handshakes, is
* invoked from onion.c.
*/
#include "orconfig.h"
#define ONION_NTOR_PRIVATE
#include "crypto.h"
#include "onion_ntor.h"
#include "torlog.h"
#include "util.h"
/** Free storage held in an ntor handshake state. */
void
ntor_handshake_state_free_(ntor_handshake_state_t *state)
{
if (!state)
return;
memwipe(state, 0, sizeof(*state));
tor_free(state);
}
/** Convenience function to represent HMAC_SHA256 as our instantiation of
* ntor's "tweaked hash'. Hash the <b>inp_len</b> bytes at <b>inp</b> into
* a DIGEST256_LEN-byte digest at <b>out</b>, with the hash changing
* depending on the value of <b>tweak</b>. */
static void
h_tweak(uint8_t *out,
const uint8_t *inp, size_t inp_len,
const char *tweak)
{
size_t tweak_len = strlen(tweak);
crypto_hmac_sha256((char*)out, tweak, tweak_len, (const char*)inp, inp_len);
}
/** Wrapper around a set of tweak-values for use with the ntor handshake. */
typedef struct tweakset_t {
const char *t_mac;
const char *t_key;
const char *t_verify;
const char *m_expand;
} tweakset_t;
/** The tweaks to be used with our handshake. */
static const tweakset_t proto1_tweaks = {
#define PROTOID "ntor-curve25519-sha256-1"
#define PROTOID_LEN 24
PROTOID ":mac",
PROTOID ":key_extract",
PROTOID ":verify",
PROTOID ":key_expand"
};
/** Convenience macro: copy <b>len</b> bytes from <b>inp</b> to <b>ptr</b>,
* and advance <b>ptr</b> by the number of bytes copied. */
#define APPEND(ptr, inp, len) \
STMT_BEGIN { \
memcpy(ptr, (inp), (len)); \
ptr += len; \
} STMT_END
/**
* Compute the first client-side step of the ntor handshake for communicating
* with a server whose DIGEST_LEN-byte server identity is <b>router_id</b>,
* and whose onion key is <b>router_key</b>. Store the NTOR_ONIONSKIN_LEN-byte
* message in <b>onion_skin_out</b>, and store the handshake state in
* *<b>handshake_state_out</b>. Return 0 on success, -1 on failure.
*/
int
onion_skin_ntor_create(const uint8_t *router_id,
const curve25519_public_key_t *router_key,
ntor_handshake_state_t **handshake_state_out,
uint8_t *onion_skin_out)
{
ntor_handshake_state_t *state;
uint8_t *op;
state = tor_malloc_zero(sizeof(ntor_handshake_state_t));
memcpy(state->router_id, router_id, DIGEST_LEN);
memcpy(&state->pubkey_B, router_key, sizeof(curve25519_public_key_t));
if (curve25519_secret_key_generate(&state->seckey_x, 0) < 0) {
/* LCOV_EXCL_START
* Secret key generation should be unable to fail when the key isn't
* marked as "extra-strong" */
tor_assert_nonfatal_unreached();
tor_free(state);
return -1;
/* LCOV_EXCL_STOP */
}
curve25519_public_key_generate(&state->pubkey_X, &state->seckey_x);
op = onion_skin_out;
APPEND(op, router_id, DIGEST_LEN);
APPEND(op, router_key->public_key, CURVE25519_PUBKEY_LEN);
APPEND(op, state->pubkey_X.public_key, CURVE25519_PUBKEY_LEN);
tor_assert(op == onion_skin_out + NTOR_ONIONSKIN_LEN);
*handshake_state_out = state;
return 0;
}
#define SERVER_STR "Server"
#define SERVER_STR_LEN 6
#define SECRET_INPUT_LEN (CURVE25519_PUBKEY_LEN * 3 + \
CURVE25519_OUTPUT_LEN * 2 + \
DIGEST_LEN + PROTOID_LEN)
#define AUTH_INPUT_LEN (DIGEST256_LEN + DIGEST_LEN + \
CURVE25519_PUBKEY_LEN*3 + \
PROTOID_LEN + SERVER_STR_LEN)
/**
* Perform the server side of an ntor handshake. Given an
* NTOR_ONIONSKIN_LEN-byte message in <b>onion_skin</b>, our own identity
* fingerprint as <b>my_node_id</b>, and an associative array mapping public
* onion keys to curve25519_keypair_t in <b>private_keys</b>, attempt to
* perform the handshake. Use <b>junk_keys</b> if present if the handshake
* indicates an unrecognized public key. Write an NTOR_REPLY_LEN-byte
* message to send back to the client into <b>handshake_reply_out</b>, and
* generate <b>key_out_len</b> bytes of key material in <b>key_out</b>. Return
* 0 on success, -1 on failure.
*/
int
onion_skin_ntor_server_handshake(const uint8_t *onion_skin,
const di_digest256_map_t *private_keys,
const curve25519_keypair_t *junk_keys,
const uint8_t *my_node_id,
uint8_t *handshake_reply_out,
uint8_t *key_out,
size_t key_out_len)
{
const tweakset_t *T = &proto1_tweaks;
/* Sensitive stack-allocated material. Kept in an anonymous struct to make
* it easy to wipe. */
struct {
uint8_t secret_input[SECRET_INPUT_LEN];
uint8_t auth_input[AUTH_INPUT_LEN];
curve25519_public_key_t pubkey_X;
curve25519_secret_key_t seckey_y;
curve25519_public_key_t pubkey_Y;
uint8_t verify[DIGEST256_LEN];
} s;
uint8_t *si = s.secret_input, *ai = s.auth_input;
const curve25519_keypair_t *keypair_bB;
int bad;
/* Decode the onion skin */
/* XXXX Does this possible early-return business threaten our security? */
if (tor_memneq(onion_skin, my_node_id, DIGEST_LEN))
return -1;
/* Note that on key-not-found, we go through with this operation anyway,
* using "junk_keys". This will result in failed authentication, but won't
* leak whether we recognized the key. */
keypair_bB = dimap_search(private_keys, onion_skin + DIGEST_LEN,
(void*)junk_keys);
if (!keypair_bB)
return -1;
memcpy(s.pubkey_X.public_key, onion_skin+DIGEST_LEN+DIGEST256_LEN,
CURVE25519_PUBKEY_LEN);
/* Make y, Y */
curve25519_secret_key_generate(&s.seckey_y, 0);
curve25519_public_key_generate(&s.pubkey_Y, &s.seckey_y);
/* NOTE: If we ever use a group other than curve25519, or a different
* representation for its points, we may need to perform different or
* additional checks on X here and on Y in the client handshake, or lose our
* security properties. What checks we need would depend on the properties
* of the group and its representation.
*
* In short: if you use anything other than curve25519, this aspect of the
* code will need to be reconsidered carefully. */
/* build secret_input */
curve25519_handshake(si, &s.seckey_y, &s.pubkey_X);
bad = safe_mem_is_zero(si, CURVE25519_OUTPUT_LEN);
si += CURVE25519_OUTPUT_LEN;
curve25519_handshake(si, &keypair_bB->seckey, &s.pubkey_X);
bad |= safe_mem_is_zero(si, CURVE25519_OUTPUT_LEN);
si += CURVE25519_OUTPUT_LEN;
APPEND(si, my_node_id, DIGEST_LEN);
APPEND(si, keypair_bB->pubkey.public_key, CURVE25519_PUBKEY_LEN);
APPEND(si, s.pubkey_X.public_key, CURVE25519_PUBKEY_LEN);
APPEND(si, s.pubkey_Y.public_key, CURVE25519_PUBKEY_LEN);
APPEND(si, PROTOID, PROTOID_LEN);
tor_assert(si == s.secret_input + sizeof(s.secret_input));
/* Compute hashes of secret_input */
h_tweak(s.verify, s.secret_input, sizeof(s.secret_input), T->t_verify);
/* Compute auth_input */
APPEND(ai, s.verify, DIGEST256_LEN);
APPEND(ai, my_node_id, DIGEST_LEN);
APPEND(ai, keypair_bB->pubkey.public_key, CURVE25519_PUBKEY_LEN);
APPEND(ai, s.pubkey_Y.public_key, CURVE25519_PUBKEY_LEN);
APPEND(ai, s.pubkey_X.public_key, CURVE25519_PUBKEY_LEN);
APPEND(ai, PROTOID, PROTOID_LEN);
APPEND(ai, SERVER_STR, SERVER_STR_LEN);
tor_assert(ai == s.auth_input + sizeof(s.auth_input));
/* Build the reply */
memcpy(handshake_reply_out, s.pubkey_Y.public_key, CURVE25519_PUBKEY_LEN);
h_tweak(handshake_reply_out+CURVE25519_PUBKEY_LEN,
s.auth_input, sizeof(s.auth_input),
T->t_mac);
/* Generate the key material */
crypto_expand_key_material_rfc5869_sha256(
s.secret_input, sizeof(s.secret_input),
(const uint8_t*)T->t_key, strlen(T->t_key),
(const uint8_t*)T->m_expand, strlen(T->m_expand),
key_out, key_out_len);
/* Wipe all of our local state */
memwipe(&s, 0, sizeof(s));
return bad ? -1 : 0;
}
/**
* Perform the final client side of the ntor handshake, using the state in
* <b>handshake_state</b> and the server's NTOR_REPLY_LEN-byte reply in
* <b>handshake_reply</b>. Generate <b>key_out_len</b> bytes of key material
* in <b>key_out</b>. Return 0 on success, -1 on failure.
*/
int
onion_skin_ntor_client_handshake(
const ntor_handshake_state_t *handshake_state,
const uint8_t *handshake_reply,
uint8_t *key_out,
size_t key_out_len,
const char **msg_out)
{
const tweakset_t *T = &proto1_tweaks;
/* Sensitive stack-allocated material. Kept in an anonymous struct to make
* it easy to wipe. */
struct {
curve25519_public_key_t pubkey_Y;
uint8_t secret_input[SECRET_INPUT_LEN];
uint8_t verify[DIGEST256_LEN];
uint8_t auth_input[AUTH_INPUT_LEN];
uint8_t auth[DIGEST256_LEN];
} s;
uint8_t *ai = s.auth_input, *si = s.secret_input;
const uint8_t *auth_candidate;
int bad;
/* Decode input */
memcpy(s.pubkey_Y.public_key, handshake_reply, CURVE25519_PUBKEY_LEN);
auth_candidate = handshake_reply + CURVE25519_PUBKEY_LEN;
/* See note in server_handshake above about checking points. The
* circumstances under which we'd need to check Y for membership are
* different than those under which we'd be checking X. */
/* Compute secret_input */
curve25519_handshake(si, &handshake_state->seckey_x, &s.pubkey_Y);
bad = safe_mem_is_zero(si, CURVE25519_OUTPUT_LEN);
si += CURVE25519_OUTPUT_LEN;
curve25519_handshake(si, &handshake_state->seckey_x,
&handshake_state->pubkey_B);
bad |= (safe_mem_is_zero(si, CURVE25519_OUTPUT_LEN) << 1);
si += CURVE25519_OUTPUT_LEN;
APPEND(si, handshake_state->router_id, DIGEST_LEN);
APPEND(si, handshake_state->pubkey_B.public_key, CURVE25519_PUBKEY_LEN);
APPEND(si, handshake_state->pubkey_X.public_key, CURVE25519_PUBKEY_LEN);
APPEND(si, s.pubkey_Y.public_key, CURVE25519_PUBKEY_LEN);
APPEND(si, PROTOID, PROTOID_LEN);
tor_assert(si == s.secret_input + sizeof(s.secret_input));
/* Compute verify from secret_input */
h_tweak(s.verify, s.secret_input, sizeof(s.secret_input), T->t_verify);
/* Compute auth_input */
APPEND(ai, s.verify, DIGEST256_LEN);
APPEND(ai, handshake_state->router_id, DIGEST_LEN);
APPEND(ai, handshake_state->pubkey_B.public_key, CURVE25519_PUBKEY_LEN);
APPEND(ai, s.pubkey_Y.public_key, CURVE25519_PUBKEY_LEN);
APPEND(ai, handshake_state->pubkey_X.public_key, CURVE25519_PUBKEY_LEN);
APPEND(ai, PROTOID, PROTOID_LEN);
APPEND(ai, SERVER_STR, SERVER_STR_LEN);
tor_assert(ai == s.auth_input + sizeof(s.auth_input));
/* Compute auth */
h_tweak(s.auth, s.auth_input, sizeof(s.auth_input), T->t_mac);
bad |= (tor_memneq(s.auth, auth_candidate, DIGEST256_LEN) << 2);
crypto_expand_key_material_rfc5869_sha256(
s.secret_input, sizeof(s.secret_input),
(const uint8_t*)T->t_key, strlen(T->t_key),
(const uint8_t*)T->m_expand, strlen(T->m_expand),
key_out, key_out_len);
memwipe(&s, 0, sizeof(s));
if (bad) {
if (bad & 4) {
if (msg_out)
*msg_out = NULL; /* Don't report this one; we probably just had the
* wrong onion key.*/
log_fn(LOG_INFO, LD_PROTOCOL,
"Invalid result from curve25519 handshake: %d", bad);
}
if (bad & 3) {
if (msg_out)
*msg_out = "Zero output from curve25519 handshake";
log_fn(LOG_WARN, LD_PROTOCOL,
"Invalid result from curve25519 handshake: %d", bad);
}
}
return bad ? -1 : 0;
}
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