tinc/src/sptps.c

/*
    sptps.c -- Simple Peer-to-Peer Security
    Copyright (C) 2011-2012 Guus Sliepen <guus@tinc-vpn.org>,
                  2010      Brandon L. Black <blblack@gmail.com>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License along
    with this program; if not, write to the Free Software Foundation, Inc.,
    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/

#include "system.h"

#include "cipher.h"
#include "crypto.h"
#include "digest.h"
#include "ecdh.h"
#include "ecdsa.h"
#include "prf.h"
#include "sptps.h"

unsigned int sptps_replaywin = 16;

/*
   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. (done)

   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 (datagram version, accepts all record types, handles encryption and authentication).
static bool send_record_priv_datagram(sptps_t *s, uint8_t type, const char *data, uint16_t len) {
	char buffer[len + 23UL];

	// Create header with sequence number, length and record type
	uint32_t seqno = htonl(s->outseqno++);
	uint16_t netlen = htons(len);

	memcpy(buffer, &netlen, 2);
	memcpy(buffer + 2, &seqno, 4);
	buffer[6] = type;

	// Add plaintext (TODO: avoid unnecessary copy)
	memcpy(buffer + 7, data, len);

	if(s->outstate) {
		// If first handshake has finished, encrypt and HMAC
		cipher_set_counter(&s->outcipher, &seqno, sizeof seqno);
		if(!cipher_counter_xor(&s->outcipher, buffer + 6, len + 1UL, buffer + 6))
			return false;

		if(!digest_create(&s->outdigest, buffer, len + 7UL, buffer + 7UL + len))
			return false;

		return s->send_data(s->handle, type, buffer + 2, len + 21UL);
	} else {
		// Otherwise send as plaintext
		return s->send_data(s->handle, type, buffer + 2, len + 5UL);
	}
}
// 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) {
	if(s->datagram)
		return send_record_priv_datagram(s, type, data, len);

	char buffer[len + 23UL];

	// Create header with sequence number, length and record type
	uint32_t seqno = htonl(s->outseqno++);
	uint16_t netlen = htons(len);

	memcpy(buffer, &seqno, 4);
	memcpy(buffer + 4, &netlen, 2);
	buffer[6] = type;

	// Add plaintext (TODO: avoid unnecessary copy)
	memcpy(buffer + 7, data, len);

	if(s->outstate) {
		// If first handshake has finished, encrypt and HMAC
		if(!cipher_counter_xor(&s->outcipher, buffer + 4, len + 3UL, buffer + 4))
			return false;

		if(!digest_create(&s->outdigest, buffer, len + 7UL, buffer + 7UL + len))
			return false;

		return s->send_data(s->handle, type, buffer + 4, len + 19UL);
	} else {
		// Otherwise send as plaintext
		return s->send_data(s->handle, type, buffer + 4, len + 3UL);
	}
}

// Send an application record.
bool sptps_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->outstate)
		return error(s, EINVAL, "Handshake phase not finished yet");

	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;

	// Make room for our KEX message, which we will keep around since send_sig() needs it.
	if(s->mykex)
		abort();
	s->mykex = realloc(s->mykex, 1 + 32 + keylen);
	if(!s->mykex)
		return error(s, errno, strerror(errno));

	// 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, s->mykex + 1 + 32))
		return false;

	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, plus label
	char msg[(1 + 32 + keylen) * 2 + 1 + s->labellen];
	char sig[siglen];

	msg[0] = s->initiator;
	memcpy(msg + 1, s->mykex, 1 + 32 + keylen);
	memcpy(msg + 1 + 33 + keylen, s->hiskex, 1 + 32 + keylen);
	memcpy(msg + 1 + 2 * (33 + keylen), s->label, s->labellen);

	// Sign the result.
	if(!ecdsa_sign(&s->mykey, msg, sizeof msg, sig))
		return false;

	// Send the SIG exchange record.
	return send_record_priv(s, SPTPS_HANDSHAKE, sig, sizeof sig);
}

// 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->outstate) {
		bool result
			=  cipher_open_by_name(&s->incipher, "aes-256-ecb")
			&& cipher_open_by_name(&s->outcipher, "aes-256-ecb")
			&& digest_open_by_name(&s->indigest, "sha256", 16)
			&& digest_open_by_name(&s->outdigest, "sha256", 16);
		if(!result)
			return false;
	}

	// Allocate memory for key material
	size_t keylen = digest_keylength(&s->indigest) + digest_keylength(&s->outdigest) + cipher_keylength(&s->incipher) + cipher_keylength(&s->outcipher);

	s->key = realloc(s->key, keylen);
	if(!s->key)
		return error(s, errno, strerror(errno));

	// Create the HMAC seed, which is "key expansion" + session label + server nonce + client nonce
	char seed[s->labellen + 64 + 13];
	strcpy(seed, "key expansion");
	if(s->initiator) {
		memcpy(seed + 13, s->mykex + 1, 32);
		memcpy(seed + 45, s->hiskex + 1, 32);
	} else {
		memcpy(seed + 13, s->hiskex + 1, 32);
		memcpy(seed + 45, s->mykex + 1, 32);
	}
	memcpy(seed + 78, s->label, s->labellen);

	// Use PRF to generate the key material
	if(!prf(shared, len, seed, s->labellen + 64 + 13, s->key, keylen))
		return false;

	return true;
}

// Send an ACKnowledgement record.
static bool send_ack(sptps_t *s) {
	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 error(s, EIO, "Invalid ACK record length");

	if(s->initiator) {
		bool result
			= cipher_set_counter_key(&s->incipher, s->key)
			&& digest_set_key(&s->indigest, s->key + cipher_keylength(&s->incipher), digest_keylength(&s->indigest));
		if(!result)
			return false;
	} else {
		bool result
			= cipher_set_counter_key(&s->incipher, s->key + cipher_keylength(&s->outcipher) + digest_keylength(&s->outdigest))
			&& digest_set_key(&s->indigest, s->key + cipher_keylength(&s->outcipher) + digest_keylength(&s->outdigest) + cipher_keylength(&s->incipher), digest_keylength(&s->indigest));
		if(!result)
			return false;
	}

	free(s->key);
	s->key = NULL;
	s->instate = true;

	return true;
}

// 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
	if(s->hiskex)
		abort();
	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 != 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 + s->labellen];

	msg[0] = !s->initiator;
	memcpy(msg + 1, s->hiskex, 1 + 32 + keylen);
	memcpy(msg + 1 + 33 + keylen, s->mykex, 1 + 32 + keylen);
	memcpy(msg + 1 + 2 * (33 + keylen), s->label, s->labellen);

	// Verify signature.
	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, s->hiskex + 1 + 32, shared))
		return false;

	// Generate key material from shared secret.
	if(!generate_key_material(s, shared, sizeof shared))
		return false;

	free(s->mykex);
	free(s->hiskex);

	s->mykex = NULL;
	s->hiskex = NULL;

	// Send cipher change record
	if(s->outstate && !send_ack(s))
		return false;

	// TODO: only set new keys after ACK has been set/received
	if(s->initiator) {
		bool result
			= cipher_set_counter_key(&s->outcipher, s->key + cipher_keylength(&s->incipher) + digest_keylength(&s->indigest))
			&& digest_set_key(&s->outdigest, s->key + cipher_keylength(&s->incipher) + digest_keylength(&s->indigest) + cipher_keylength(&s->outcipher), digest_keylength(&s->outdigest));
		if(!result)
			return false;
	} else {
		bool result
			=  cipher_set_counter_key(&s->outcipher, s->key)
			&& digest_set_key(&s->outdigest, s->key + cipher_keylength(&s->outcipher), digest_keylength(&s->outdigest));
		if(!result)
			return false;
	}

	return true;
}

// Force another Key EXchange (for testing purposes).
bool sptps_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 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 = SPTPS_SIG;
			return true;
		case SPTPS_SIG:
			// If we already sent our secondary public ECDH key, we expect the peer to send his.
			if(!receive_sig(s, data, len))
				return false;
			if(s->outstate)
				s->state = SPTPS_ACK;
			else {
				s->outstate = true;
				if(!receive_ack(s, NULL, 0))
					return false;
				s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
				s->state = SPTPS_SECONDARY_KEX;
			}

			return true;
		case SPTPS_ACK:
			// We expect a handshake message to indicate transition to the new keys.
			if(!receive_ack(s, data, len))
				return false;
			s->receive_record(s->handle, SPTPS_HANDSHAKE, NULL, 0);
			s->state = SPTPS_SECONDARY_KEX;
			return true;
		// TODO: split ACK into a VERify and ACK?
		default:
			return error(s, EIO, "Invalid session state");
	}
}

// Check datagram for valid HMAC
bool sptps_verify_datagram(sptps_t *s, const char *data, size_t len) {
	if(!s->instate || len < 21)
		return false;

	char buffer[len + 23];
	uint16_t netlen = htons(len - 21);

	memcpy(buffer, &netlen, 2);
	memcpy(buffer + 2, data, len);

	return digest_verify(&s->indigest, buffer, len - 14, buffer + len - 14);
}

// Receive incoming data, datagram version.
static bool sptps_receive_data_datagram(sptps_t *s, const char *data, size_t len) {
	if(len < (s->instate ? 21 : 5))
		return error(s, EIO, "Received short packet");

	uint32_t seqno;
	memcpy(&seqno, data, 4);
	seqno = ntohl(seqno);

	if(!s->instate) {
		if(seqno != s->inseqno) {
			fprintf(stderr, "Received invalid packet seqno: %d != %d\n", seqno, s->inseqno);
			return error(s, EIO, "Invalid packet seqno");
		}

		s->inseqno = seqno + 1;

		uint8_t type = data[4];

		if(type != SPTPS_HANDSHAKE)
			return error(s, EIO, "Application record received before handshake finished");

		return receive_handshake(s, data + 5, len - 5);
	}

	// Replay protection using a sliding window of configurable size.
	// s->inseqno is expected sequence number
	// seqno is received sequence number
	// s->late[] is a circular buffer, a 1 bit means a packet has not been received yet
	// The circular buffer contains bits for sequence numbers from s->inseqno - s->replaywin * 8 to (but excluding) s->inseqno.
	if(s->replaywin) {
		if(seqno != s->inseqno) {
			if(seqno >= s->inseqno + s->replaywin * 8) {
				// Prevent packets that jump far ahead of the queue from causing many others to be dropped.
				if(s->farfuture++ < s->replaywin >> 2) {
					fprintf(stderr, "Packet is %d seqs in the future, dropped (%u)\n", seqno - s->inseqno, s->farfuture);
					return false;
				}
				// Unless we have seen lots of them, in which case we consider the others lost.
				fprintf(stderr, "Lost %d packets\n", seqno - s->inseqno);
				memset(s->late, 0, s->replaywin);
			} else if (seqno < s->inseqno) {
				// If the sequence number is farther in the past than the bitmap goes, or if the packet was already received, drop it.
				if((s->inseqno >= s->replaywin * 8 && seqno < s->inseqno - s->replaywin * 8) || !(s->late[(seqno / 8) % s->replaywin] & (1 << seqno % 8))) {
					fprintf(stderr, "Received late or replayed packet, seqno %d, last received %d", seqno, s->inseqno);
					return false;
				}
			} else {
				// We missed some packets. Mark them in the bitmap as being late.
				for(int i = s->inseqno; i < seqno; i++)
					s->late[(i / 8) % s->replaywin] |= 1 << i % 8;
			}
		}

		// Mark the current packet as not being late.
		s->late[(seqno / 8) % s->replaywin] &= ~(1 << seqno % 8);
		s->farfuture = 0;
	}

	if(seqno > s->inseqno)
		s->inseqno = seqno + 1;

	uint16_t netlen = htons(len - 21);

	char buffer[len + 23];

	memcpy(buffer, &netlen, 2);
	memcpy(buffer + 2, data, len);

	memcpy(&seqno, buffer + 2, 4);

	// Check HMAC and decrypt.
	if(!digest_verify(&s->indigest, buffer, len - 14, buffer + len - 14))
		return error(s, EIO, "Invalid HMAC");

	cipher_set_counter(&s->incipher, &seqno, sizeof seqno);
	if(!cipher_counter_xor(&s->incipher, buffer + 6, len - 4, buffer + 6))
		return false;

	// Append a NULL byte for safety.
	buffer[len - 14] = 0;

	uint8_t type = buffer[6];

	if(type < SPTPS_HANDSHAKE) {
		if(!s->instate)
			return error(s, EIO, "Application record received before handshake finished");
		if(!s->receive_record(s->handle, type, buffer + 7, len - 21))
			return false;
	} else if(type == SPTPS_HANDSHAKE) {
		if(!receive_handshake(s, buffer + 7, len - 21))
			return false;
	} else {
		return error(s, EIO, "Invalid record type");
	}

	return true;
}

// Receive incoming data. Check if it contains a complete record, if so, handle it.
bool sptps_receive_data(sptps_t *s, const char *data, size_t len) {
	if(s->datagram)
		return sptps_receive_data_datagram(s, data, len);

	while(len) {
		// First read the 2 length bytes.
		if(s->buflen < 6) {
			size_t toread = 6 - s->buflen;
			if(toread > len)
				toread = len;

			memcpy(s->inbuf + s->buflen, data, toread);

			s->buflen += toread;
			len -= toread;
			data += toread;
		
			// Exit early if we don't have the full length.
			if(s->buflen < 6)
				return true;

			// Decrypt the length bytes

			if(s->instate) {
				if(!cipher_counter_xor(&s->incipher, s->inbuf + 4, 2, &s->reclen))
					return false;
			} else {
				memcpy(&s->reclen, s->inbuf + 4, 2);
			}

			s->reclen = ntohs(s->reclen);

			// If we have the length bytes, ensure our buffer can hold the whole request.
			s->inbuf = realloc(s->inbuf, s->reclen + 23UL);
			if(!s->inbuf)
				return error(s, errno, strerror(errno));

			// Add sequence number.
			uint32_t seqno = htonl(s->inseqno++);
			memcpy(s->inbuf, &seqno, 4);

			// Exit early if we have no more data to process.
			if(!len)
				return true;
		}

		// Read up to the end of the record.
		size_t toread = s->reclen + (s->instate ? 23UL : 7UL) - s->buflen;
		if(toread > len)
			toread = len;

		memcpy(s->inbuf + s->buflen, data, toread);
		s->buflen += toread;
		len -= toread;
		data += toread;

		// If we don't have a whole record, exit.
		if(s->buflen < s->reclen + (s->instate ? 23UL : 7UL))
			return true;

		// Check HMAC and decrypt.
		if(s->instate) {
			if(!digest_verify(&s->indigest, s->inbuf, s->reclen + 7UL, s->inbuf + s->reclen + 7UL))
				return error(s, EIO, "Invalid HMAC");

			if(!cipher_counter_xor(&s->incipher, s->inbuf + 6UL, s->reclen + 1UL, s->inbuf + 6UL))
				return false;
		}

		// Append a NULL byte for safety.
		s->inbuf[s->reclen + 7UL] = 0;

		uint8_t type = s->inbuf[6];

		if(type < SPTPS_HANDSHAKE) {
			if(!s->instate)
				return error(s, EIO, "Application record received before handshake finished");
			if(!s->receive_record(s->handle, type, s->inbuf + 7, s->reclen))
				return false;
		} else if(type == SPTPS_HANDSHAKE) {
			if(!receive_handshake(s, s->inbuf + 7, s->reclen))
				return false;
		} else {
			return error(s, EIO, "Invalid record type");
		}

		s->buflen = 4;
	}

	return true;
}

// Start a SPTPS session.
bool sptps_start(sptps_t *s, void *handle, bool initiator, bool datagram, 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);

	s->handle = handle;
	s->initiator = initiator;
	s->datagram = datagram;
	s->mykey = mykey;
	s->hiskey = hiskey;
	s->replaywin = sptps_replaywin;
	if(s->replaywin) {
		s->late = malloc(s->replaywin);
		if(!s->late)
			return error(s, errno, strerror(errno));
	}

	s->label = malloc(labellen);
	if(!s->label)
		return error(s, errno, strerror(errno));

	if(!datagram) {
		s->inbuf = malloc(7);
		if(!s->inbuf)
			return error(s, errno, strerror(errno));
		s->buflen = 4;
		memset(s->inbuf, 0, 4);
	}

	memcpy(s->label, label, labellen);
	s->labellen = labellen;

	s->send_data = send_data;
	s->receive_record = receive_record;

	// Do first KEX immediately
	s->state = SPTPS_KEX;
	return send_kex(s);
}

// Stop a SPTPS session.
bool sptps_stop(sptps_t *s) {
	// Clean up any resources.
	ecdh_free(&s->ecdh);
	free(s->inbuf);
	s->inbuf = NULL;
	free(s->mykex);
	s->mykex = NULL;
	free(s->hiskex);
	s->hiskex = NULL;
	free(s->key);
	s->key = NULL;
	free(s->label);
	s->label = NULL;
	free(s->late);
	s->late = NULL;
	return true;
}