From 30013511504e925729ebc67772205a74c4b8aeea Mon Sep 17 00:00:00 2001 From: Guus Sliepen Date: Wed, 5 Oct 2011 22:00:51 +0200 Subject: [PATCH] Update SPTPS protocol. * Exchange nonce and ECDH public key first, calculate the ECDSA signature over the complete key exchange. * Make an explicit distinction between client and server in the signatures. * Add more comments and replace some magic numbers by #defines. Thanks to Erik Tews for very helpful hints and comments! --- THANKS | 1 + src/sptps.c | 192 ++++++++++++++++++++++++++++++++++++++-------------- src/sptps.h | 17 +++-- 3 files changed, 154 insertions(+), 56 deletions(-) diff --git a/THANKS b/THANKS index 4a6eae20..e27fa9b2 100644 --- a/THANKS +++ b/THANKS @@ -9,6 +9,7 @@ We would like to thank the following people for their contributions to tinc: * Delf Eldkraft * dnk * Enrique Zanardi +* Erik Tews * Flynn Marquardt * Grzegorz Dymarek * Hans Bayle diff --git a/src/sptps.c b/src/sptps.c index ac710f5d..dc602e3e 100644 --- a/src/sptps.c +++ b/src/sptps.c @@ -30,12 +30,34 @@ char *logfilename; #include "utils.c" +/* + Nonce MUST be exchanged first (done) + Signatures MUST be done over both nonces, to guarantee the signature is fresh + Otherwise: if ECDHE key of one side is compromised, it can be reused! + + Add explicit tag to beginning of structure to distinguish the client and server when signing. (done) + + Sign all handshake messages up to ECDHE kex with long-term public keys. (done) + + HMACed KEX finished message to prevent downgrade attacks and prove you have the right key material (done by virtue of ECDSA over the whole ECDHE exchange?) + + Explicit close message needs to be added. + + Maybe do add some alert messages to give helpful error messages? Not more than TLS sends. + + Use counter mode instead of OFB. + + Make sure ECC operations are fixed time (aka prevent side-channel attacks). +*/ + +// Log an error message. static bool error(sptps_t *s, int s_errno, const char *msg) { fprintf(stderr, "SPTPS error: %s\n", msg); errno = s_errno; return false; } +// Send a record (private version, accepts all record types, handles encryption and authentication). static bool send_record_priv(sptps_t *s, uint8_t type, const char *data, uint16_t len) { char plaintext[len + 23]; char ciphertext[len + 19]; @@ -51,7 +73,7 @@ static bool send_record_priv(sptps_t *s, uint8_t type, const char *data, uint16_ // Add plaintext (TODO: avoid unnecessary copy) memcpy(plaintext + 7, data, len); - if(s->state) { + if(s->outstate) { // If first handshake has finished, encrypt and HMAC if(!digest_create(&s->outdigest, plaintext, len + 7, plaintext + 7 + len)) return false; @@ -66,46 +88,66 @@ static bool send_record_priv(sptps_t *s, uint8_t type, const char *data, uint16_ } } +// Send an application record. bool send_record(sptps_t *s, uint8_t type, const char *data, uint16_t len) { // Sanity checks: application cannot send data before handshake is finished, // and only record types 0..127 are allowed. - if(!s->state) + if(!s->outstate) return error(s, EINVAL, "Handshake phase not finished yet"); - if(type & 128) + if(type >= SPTPS_HANDSHAKE) return error(s, EINVAL, "Invalid application record type"); return send_record_priv(s, type, data, len); } +// Send a Key EXchange record, containing a random nonce and an ECDHE public key. static bool send_kex(sptps_t *s) { size_t keylen = ECDH_SIZE; - size_t siglen = ecdsa_size(&s->mykey); - char data[32 + keylen + siglen]; - // Create a random nonce. - s->myrandom = realloc(s->myrandom, 32); - if(!s->myrandom) + // Make room for our KEX message, which we will keep around since send_sig() needs it. + s->mykex = realloc(s->mykex, 1 + 32 + keylen); + if(!s->mykex) return error(s, errno, strerror(errno)); - randomize(s->myrandom, 32); - memcpy(data, s->myrandom, 32); + // Set version byte to zero. + s->mykex[0] = SPTPS_VERSION; + + // Create a random nonce. + randomize(s->mykex + 1, 32); // Create a new ECDH public key. - if(!ecdh_generate_public(&s->ecdh, data + 32)) + if(!ecdh_generate_public(&s->ecdh, s->mykex + 1 + 32)) return false; - // Sign the former. - if(!ecdsa_sign(&s->mykey, data, 32 + keylen, data + 32 + keylen)) + return send_record_priv(s, SPTPS_HANDSHAKE, s->mykex, 1 + 32 + keylen); +} + +// Send a SIGnature record, containing an ECDSA signature over both KEX records. +static bool send_sig(sptps_t *s) { + size_t keylen = ECDH_SIZE; + size_t siglen = ecdsa_size(&s->mykey); + + // Concatenate both KEX messages, plus tag indicating if it is from the connection originator + char msg[(1 + 32 + keylen) * 2 + 1]; + char sig[siglen]; + + msg[0] = s->initiator; + memcpy(msg + 1, s->mykex, 1 + 32 + keylen); + memcpy(msg + 2 + 32 + keylen, s->hiskex, 1 + 32 + keylen); + + // Sign the result. + if(!ecdsa_sign(&s->mykey, msg, sizeof msg, sig)) return false; - // Send the handshake record. - return send_record_priv(s, 128, data, sizeof data); + // Send the SIG exchange record. + return send_record_priv(s, SPTPS_HANDSHAKE, sig, sizeof sig); } -static bool generate_key_material(sptps_t *s, const char *shared, size_t len, const char *hisrandom) { +// Generate key material from the shared secret created from the ECDHE key exchange. +static bool generate_key_material(sptps_t *s, const char *shared, size_t len) { // Initialise cipher and digest structures if necessary - if(!s->state) { + if(!s->outstate) { bool result = cipher_open_by_name(&s->incipher, "aes-256-ofb") && cipher_open_by_name(&s->outcipher, "aes-256-ofb") @@ -126,11 +168,11 @@ static bool generate_key_material(sptps_t *s, const char *shared, size_t len, co char seed[s->labellen + 64 + 13]; strcpy(seed, "key expansion"); if(s->initiator) { - memcpy(seed + 13, hisrandom, 32); - memcpy(seed + 45, s->myrandom, 32); + memcpy(seed + 13, s->mykex + 1, 32); + memcpy(seed + 45, s->hiskex + 1, 32); } else { - memcpy(seed + 13, s->myrandom, 32); - memcpy(seed + 45, hisrandom, 32); + memcpy(seed + 13, s->hiskex + 1, 32); + memcpy(seed + 45, s->mykex + 1, 32); } memcpy(seed + 78, s->label, s->labellen); @@ -141,10 +183,12 @@ static bool generate_key_material(sptps_t *s, const char *shared, size_t len, co return true; } +// Send an ACKnowledgement record. static bool send_ack(sptps_t *s) { - return send_record_priv(s, 128, "", 0); + return send_record_priv(s, SPTPS_HANDSHAKE, "", 0); } +// Receive an ACKnowledgement record. static bool receive_ack(sptps_t *s, const char *data, uint16_t len) { if(len) return false; @@ -153,33 +197,58 @@ static bool receive_ack(sptps_t *s, const char *data, uint16_t len) { return error(s, ENOSYS, "receive_ack() not completely implemented yet"); } +// Receive a Key EXchange record, respond by sending a SIG record. static bool receive_kex(sptps_t *s, const char *data, uint16_t len) { + // Verify length of the HELLO record + if(len != 1 + 32 + ECDH_SIZE) + return error(s, EIO, "Invalid KEX record length"); + + // Ignore version number for now. + + // Make a copy of the KEX message, send_sig() and receive_sig() need it + s->hiskex = realloc(s->hiskex, len); + if(!s->hiskex) + return error(s, errno, strerror(errno)); + + memcpy(s->hiskex, data, len); + + return send_sig(s); +} + +// Receive a SIGnature record, verify it, if it passed, compute the shared secret and calculate the session keys. +static bool receive_sig(sptps_t *s, const char *data, uint16_t len) { size_t keylen = ECDH_SIZE; size_t siglen = ecdsa_size(&s->hiskey); // Verify length of KEX record. - if(len != 32 + keylen + siglen) + if(len != siglen) return error(s, EIO, "Invalid KEX record length"); + // Concatenate both KEX messages, plus tag indicating if it is from the connection originator + char msg[(1 + 32 + keylen) * 2 + 1]; + + msg[0] = !s->initiator; + memcpy(msg + 1, s->hiskex, 1 + 32 + keylen); + memcpy(msg + 2 + 32 + keylen, s->mykex, 1 + 32 + keylen); + // Verify signature. - if(!ecdsa_verify(&s->hiskey, data, 32 + keylen, data + 32 + keylen)) + if(!ecdsa_verify(&s->hiskey, msg, sizeof msg, data)) return false; // Compute shared secret. char shared[ECDH_SHARED_SIZE]; - if(!ecdh_compute_shared(&s->ecdh, data + 32, shared)) + if(!ecdh_compute_shared(&s->ecdh, s->hiskex + 1 + 32, shared)) return false; // Generate key material from shared secret. - if(!generate_key_material(s, shared, sizeof shared, data)) + if(!generate_key_material(s, shared, sizeof shared)) return false; // Send cipher change record if necessary - if(s->state) - if(!send_ack(s)) - return false; + //if(s->outstate && !send_ack(s)) + // return false; - // TODO: set cipher/digest keys + // TODO: only set new keys after ACK has been set/received if(s->initiator) { bool result = cipher_set_key(&s->incipher, s->key, false) @@ -198,39 +267,56 @@ static bool receive_kex(sptps_t *s, const char *data, uint16_t len) { return false; } + s->outstate = true; + s->instate = true; + return true; } +// Force another Key EXchange (for testing purposes). +bool force_kex(sptps_t *s) { + if(!s->outstate || s->state != SPTPS_SECONDARY_KEX) + return error(s, EINVAL, "Cannot force KEX in current state"); + + s->state = SPTPS_KEX; + return send_kex(s); +} + +// Receive a handshake record. static bool receive_handshake(sptps_t *s, const char *data, uint16_t len) { // Only a few states to deal with handshaking. + fprintf(stderr, "Received handshake message, current state %d\n", s->state); switch(s->state) { - case 0: - // We have sent our public ECDH key, we expect our peer to sent one as well. + case SPTPS_SECONDARY_KEX: + // We receive a secondary KEX request, first respond by sending our own. + if(!send_kex(s)) + return false; + case SPTPS_KEX: + // We have sent our KEX request, we expect our peer to sent one as well. if(!receive_kex(s, data, len)) return false; - s->state = 1; + s->state = SPTPS_SIG; return true; - case 1: - // We receive a secondary key exchange request, first respond by sending our own public ECDH key. - if(!send_kex(s)) - return false; - case 2: + case SPTPS_SIG: // If we already sent our secondary public ECDH key, we expect the peer to send his. - if(!receive_kex(s, data, len)) + if(!receive_sig(s, data, len)) return false; - s->state = 3; + // s->state = SPTPS_ACK; + s->state = SPTPS_SECONDARY_KEX; return true; - case 3: - // We expect an empty handshake message to indicate transition to the new keys. + case SPTPS_ACK: + // We expect a handshake message to indicate transition to the new keys. if(!receive_ack(s, data, len)) return false; - s->state = 1; + s->state = SPTPS_SECONDARY_KEX; return true; + // TODO: split ACK into a VERify and ACK? default: return error(s, EIO, "Invalid session state"); } } +// Receive incoming data. Check if it contains a complete record, if so, handle it. bool receive_data(sptps_t *s, const char *data, size_t len) { while(len) { // First read the 2 length bytes. @@ -239,7 +325,7 @@ bool receive_data(sptps_t *s, const char *data, size_t len) { if(toread > len) toread = len; - if(s->state) { + if(s->instate) { if(!cipher_decrypt(&s->incipher, data, toread, s->inbuf + s->buflen, NULL, false)) return false; } else { @@ -275,11 +361,11 @@ bool receive_data(sptps_t *s, const char *data, size_t len) { uint16_t reclen; memcpy(&reclen, s->inbuf + 4, 2); reclen = htons(reclen); - size_t toread = reclen + (s->state ? 23UL : 7UL) - s->buflen; + size_t toread = reclen + (s->instate ? 23UL : 7UL) - s->buflen; if(toread > len) toread = len; - if(s->state) { + if(s->instate) { if(!cipher_decrypt(&s->incipher, data, toread, s->inbuf + s->buflen, NULL, false)) return false; } else { @@ -291,21 +377,21 @@ bool receive_data(sptps_t *s, const char *data, size_t len) { data += toread; // If we don't have a whole record, exit. - if(s->buflen < reclen + (s->state ? 23UL : 7UL)) + if(s->buflen < reclen + (s->instate ? 23UL : 7UL)) return true; // Check HMAC. - if(s->state) + if(s->instate) if(!digest_verify(&s->indigest, s->inbuf, reclen + 7UL, s->inbuf + reclen + 7UL)) error(s, EIO, "Invalid HMAC"); uint8_t type = s->inbuf[6]; // Handle record. - if(type < 128) { + if(type < SPTPS_HANDSHAKE) { if(!s->receive_record(s->handle, type, s->inbuf + 7, reclen)) return false; - } else if(type == 128) { + } else if(type == SPTPS_HANDSHAKE) { if(!receive_handshake(s, s->inbuf + 7, reclen)) return false; } else { @@ -318,6 +404,7 @@ bool receive_data(sptps_t *s, const char *data, size_t len) { return true; } +// Start a SPTPS session. bool start_sptps(sptps_t *s, void *handle, bool initiator, ecdsa_t mykey, ecdsa_t hiskey, const char *label, size_t labellen, send_data_t send_data, receive_record_t receive_record) { // Initialise struct sptps memset(s, 0, sizeof *s); @@ -344,14 +431,17 @@ bool start_sptps(sptps_t *s, void *handle, bool initiator, ecdsa_t mykey, ecdsa_ s->receive_record = receive_record; // Do first KEX immediately + s->state = SPTPS_KEX; return send_kex(s); } +// Stop a SPTPS session. bool stop_sptps(sptps_t *s) { // Clean up any resources. ecdh_free(&s->ecdh); free(s->inbuf); - free(s->myrandom); + free(s->mykex); + free(s->hiskex); free(s->key); free(s->label); return true; diff --git a/src/sptps.h b/src/sptps.h index b1026d53..6eb933c4 100644 --- a/src/sptps.h +++ b/src/sptps.h @@ -24,10 +24,13 @@ #include "ecdh.h" #include "ecdsa.h" -#define STATE_FIRST_KEX 0 // Waiting for peer's ECDHE pubkey -#define STATE_NORMAL 1 -#define STATE_WAIT_KEX 2 // Waiting for peer's ECDHE pubkey -#define STATE_WAIT_ACK 3 // Waiting for peer's acknowledgement of pubkey reception +#define SPTPS_KEX 0 +#define SPTPS_SECONDARY_KEX 1 // Waiting for peer's ECDHE pubkey +#define SPTPS_SIG 2 // Waiting for peer's ECDHE pubkey +#define SPTPS_ACK 3 // Waiting for peer's acknowledgement of pubkey reception + +#define SPTPS_HANDSHAKE 128 +#define SPTPS_VERSION 128 typedef bool (*send_data_t)(void *handle, const char *data, size_t len); typedef bool (*receive_record_t)(void *handle, uint8_t type, const char *data, uint16_t len); @@ -39,10 +42,12 @@ typedef struct sptps { char *inbuf; size_t buflen; + bool instate; cipher_t incipher; digest_t indigest; uint32_t inseqno; + bool outstate; cipher_t outcipher; digest_t outdigest; uint32_t outseqno; @@ -51,7 +56,8 @@ typedef struct sptps { ecdsa_t hiskey; ecdh_t ecdh; - char *myrandom; + char *mykex; + char *hiskex; char *key; char *label; size_t labellen; @@ -65,3 +71,4 @@ extern bool start_sptps(sptps_t *s, void *handle, bool initiator, ecdsa_t mykey, extern bool stop_sptps(sptps_t *s); extern bool send_record(sptps_t *s, uint8_t type, const char *data, uint16_t len); extern bool receive_data(sptps_t *s, const char *data, size_t len); +extern bool force_kex(sptps_t *s); -- 2.20.1