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/* Copyright (c) 2001 Matej Pfajfar.
* Copyright (c) 2001-2004, Roger Dingledine.
* Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
* Copyright (c) 2007-2013, The Tor Project, Inc. */
/* See LICENSE for licensing information */
/**
* \file onion_tap.c
* \brief Functions to implement the original Tor circuit extension handshake
* (a.k.a TAP).
*
* We didn't call it "TAP" ourselves -- Ian Goldberg named it in "On the
* Security of the Tor Authentication Protocol". (Spoiler: it's secure, but
* its security is kind of fragile and implementation dependent. Never modify
* this implementation without reading and understanding that paper at least.)
**/
#include "or.h"
#include "config.h"
#include "onion_tap.h"
#include "rephist.h"
/*----------------------------------------------------------------------*/
/** Given a router's 128 byte public key,
* stores the following in onion_skin_out:
* - [42 bytes] OAEP padding
* - [16 bytes] Symmetric key for encrypting blob past RSA
* - [70 bytes] g^x part 1 (inside the RSA)
* - [58 bytes] g^x part 2 (symmetrically encrypted)
*
* Stores the DH private key into handshake_state_out for later completion
* of the handshake.
*
* The meeting point/cookies and auth are zeroed out for now.
*/
int
onion_skin_TAP_create(crypto_pk_t *dest_router_key,
crypto_dh_t **handshake_state_out,
char *onion_skin_out) /* TAP_ONIONSKIN_CHALLENGE_LEN bytes */
{
char challenge[DH_KEY_LEN];
crypto_dh_t *dh = NULL;
int dhbytes, pkbytes;
tor_assert(dest_router_key);
tor_assert(handshake_state_out);
tor_assert(onion_skin_out);
*handshake_state_out = NULL;
memset(onion_skin_out, 0, TAP_ONIONSKIN_CHALLENGE_LEN);
if (!(dh = crypto_dh_new(DH_TYPE_CIRCUIT)))
goto err;
dhbytes = crypto_dh_get_bytes(dh);
pkbytes = (int) crypto_pk_keysize(dest_router_key);
tor_assert(dhbytes == 128);
tor_assert(pkbytes == 128);
if (crypto_dh_get_public(dh, challenge, dhbytes))
goto err;
note_crypto_pk_op(ENC_ONIONSKIN);
/* set meeting point, meeting cookie, etc here. Leave zero for now. */
if (crypto_pk_public_hybrid_encrypt(dest_router_key, onion_skin_out,
TAP_ONIONSKIN_CHALLENGE_LEN,
challenge, DH_KEY_LEN,
PK_PKCS1_OAEP_PADDING, 1)<0)
goto err;
memwipe(challenge, 0, sizeof(challenge));
*handshake_state_out = dh;
return 0;
err:
memwipe(challenge, 0, sizeof(challenge));
if (dh) crypto_dh_free(dh);
return -1;
}
/** Given an encrypted DH public key as generated by onion_skin_create,
* and the private key for this onion router, generate the reply (128-byte
* DH plus the first 20 bytes of shared key material), and store the
* next key_out_len bytes of key material in key_out.
*/
int
onion_skin_TAP_server_handshake(
/*TAP_ONIONSKIN_CHALLENGE_LEN*/
const char *onion_skin,
crypto_pk_t *private_key,
crypto_pk_t *prev_private_key,
/*TAP_ONIONSKIN_REPLY_LEN*/
char *handshake_reply_out,
char *key_out,
size_t key_out_len)
{
char challenge[TAP_ONIONSKIN_CHALLENGE_LEN];
crypto_dh_t *dh = NULL;
ssize_t len;
char *key_material=NULL;
size_t key_material_len=0;
int i;
crypto_pk_t *k;
len = -1;
for (i=0;i<2;++i) {
k = i==0?private_key:prev_private_key;
if (!k)
break;
note_crypto_pk_op(DEC_ONIONSKIN);
len = crypto_pk_private_hybrid_decrypt(k, challenge,
TAP_ONIONSKIN_CHALLENGE_LEN,
onion_skin,
TAP_ONIONSKIN_CHALLENGE_LEN,
PK_PKCS1_OAEP_PADDING,0);
if (len>0)
break;
}
if (len<0) {
log_info(LD_PROTOCOL,
"Couldn't decrypt onionskin: client may be using old onion key");
goto err;
} else if (len != DH_KEY_LEN) {
log_warn(LD_PROTOCOL, "Unexpected onionskin length after decryption: %ld",
(long)len);
goto err;
}
dh = crypto_dh_new(DH_TYPE_CIRCUIT);
if (!dh) {
log_warn(LD_BUG, "Couldn't allocate DH key");
goto err;
}
if (crypto_dh_get_public(dh, handshake_reply_out, DH_KEY_LEN)) {
log_info(LD_GENERAL, "crypto_dh_get_public failed.");
goto err;
}
key_material_len = DIGEST_LEN+key_out_len;
key_material = tor_malloc(key_material_len);
len = crypto_dh_compute_secret(LOG_PROTOCOL_WARN, dh, challenge,
DH_KEY_LEN, key_material,
key_material_len);
if (len < 0) {
log_info(LD_GENERAL, "crypto_dh_compute_secret failed.");
goto err;
}
/* send back H(K|0) as proof that we learned K. */
memcpy(handshake_reply_out+DH_KEY_LEN, key_material, DIGEST_LEN);
/* use the rest of the key material for our shared keys, digests, etc */
memcpy(key_out, key_material+DIGEST_LEN, key_out_len);
memwipe(challenge, 0, sizeof(challenge));
memwipe(key_material, 0, key_material_len);
tor_free(key_material);
crypto_dh_free(dh);
return 0;
err:
memwipe(challenge, 0, sizeof(challenge));
if (key_material) {
memwipe(key_material, 0, key_material_len);
tor_free(key_material);
}
if (dh) crypto_dh_free(dh);
return -1;
}
/** Finish the client side of the DH handshake.
* Given the 128 byte DH reply + 20 byte hash as generated by
* onion_skin_server_handshake and the handshake state generated by
* onion_skin_create, verify H(K) with the first 20 bytes of shared
* key material, then generate key_out_len more bytes of shared key
* material and store them in key_out.
*
* After the invocation, call crypto_dh_free on handshake_state.
*/
int
onion_skin_TAP_client_handshake(crypto_dh_t *handshake_state,
const char *handshake_reply, /* TAP_ONIONSKIN_REPLY_LEN bytes */
char *key_out,
size_t key_out_len)
{
ssize_t len;
char *key_material=NULL;
size_t key_material_len;
tor_assert(crypto_dh_get_bytes(handshake_state) == DH_KEY_LEN);
key_material_len = DIGEST_LEN + key_out_len;
key_material = tor_malloc(key_material_len);
len = crypto_dh_compute_secret(LOG_PROTOCOL_WARN, handshake_state,
handshake_reply, DH_KEY_LEN, key_material,
key_material_len);
if (len < 0)
goto err;
if (tor_memneq(key_material, handshake_reply+DH_KEY_LEN, DIGEST_LEN)) {
/* H(K) does *not* match. Something fishy. */
log_warn(LD_PROTOCOL,"Digest DOES NOT MATCH on onion handshake. "
"Bug or attack.");
goto err;
}
/* use the rest of the key material for our shared keys, digests, etc */
memcpy(key_out, key_material+DIGEST_LEN, key_out_len);
memwipe(key_material, 0, key_material_len);
tor_free(key_material);
return 0;
err:
memwipe(key_material, 0, key_material_len);
tor_free(key_material);
return -1;
}
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