Add Chacha20-Poly1305, AES128-GCM and AES256-GCM support (#93)

* Add Chacha20-Poly1305 authenticated encryption

* Add general AEAD approach.
* Add chacha20-poly1305@openssh.com algo using LibTomCrypt chacha and
  poly1305 routines.

Chacha20-Poly1305 is generally faster than AES256 on CPU w/o dedicated
AES instructions, having the same key size.
Compiling in will add ~5,5kB to binary size on x86-64.

function                                             old     new   delta
chacha_crypt                                           -    1397   +1397
_poly1305_block                                        -     608    +608
poly1305_done                                          -     595    +595
dropbear_chachapoly_crypt                              -     457    +457
.rodata                                            26976   27392    +416
poly1305_process                                       -     290    +290
poly1305_init                                          -     221    +221
chacha_setup                                           -     218    +218
encrypt_packet                                      1068    1270    +202
dropbear_chachapoly_getlength                          -     147    +147
decrypt_packet                                       756     897    +141
chacha_ivctr64                                         -     137    +137
read_packet                                          543     637     +94
dropbear_chachapoly_start                              -      94     +94
read_kex_algos                                       792     880     +88
chacha_keystream                                       -      69     +69
dropbear_mode_chachapoly                               -      48     +48
sshciphers                                           280     320     +40
dropbear_mode_none                                    24      48     +24
dropbear_mode_ctr                                     24      48     +24
dropbear_mode_cbc                                     24      48     +24
dropbear_chachapoly_mac                                -      24     +24
dropbear_chachapoly                                    -      24     +24
gen_new_keys                                         848     854      +6
------------------------------------------------------------------------------
(add/remove: 14/0 grow/shrink: 10/0 up/down: 5388/0)         Total: 5388 bytes

* Add AES128-GCM and AES256-GCM authenticated encryption

* Add general AES-GCM mode.
* Add aes128-gcm@openssh.com and aes256-gcm@openssh.com algo using
  LibTomCrypt gcm routines.

AES-GCM is combination of AES CTR mode and GHASH, slower than AES-CTR on
CPU w/o dedicated AES/GHASH instructions therefore disabled by default.
Compiling in will add ~6kB to binary size on x86-64.

function                                             old     new   delta
gcm_process                                            -    1060   +1060
.rodata                                            26976   27808    +832
gcm_gf_mult                                            -     820    +820
gcm_add_aad                                            -     660    +660
gcm_shift_table                                        -     512    +512
gcm_done                                               -     471    +471
gcm_add_iv                                             -     384    +384
gcm_init                                               -     347    +347
dropbear_gcm_crypt                                     -     309    +309
encrypt_packet                                      1068    1270    +202
decrypt_packet                                       756     897    +141
gcm_reset                                              -     118    +118
read_packet                                          543     637     +94
read_kex_algos                                       792     880     +88
sshciphers                                           280     360     +80
gcm_mult_h                                             -      80     +80
dropbear_gcm_start                                     -      62     +62
dropbear_mode_gcm                                      -      48     +48
dropbear_mode_none                                    24      48     +24
dropbear_mode_ctr                                     24      48     +24
dropbear_mode_cbc                                     24      48     +24
dropbear_ghash                                         -      24     +24
dropbear_gcm_getlength                                 -      24     +24
gen_new_keys                                         848     854      +6
------------------------------------------------------------------------------
(add/remove: 14/0 grow/shrink: 10/0 up/down: 6434/0)         Total: 6434 bytes

1 files changed, 36 insertions, 8 deletions

diff --git a/common-kex.c b/common-kex.c
index 16b7e27..5e2923f 100644
--- a/common-kex.c
+++ b/common-kex.c
@@ -329,9 +329,12 @@ static void gen_new_keys() {
 	hashkeys(S2C_key, sizeof(S2C_key), &hs, 'D');
 
 	if (ses.newkeys->recv.algo_crypt->cipherdesc != NULL) {
-		int recv_cipher = find_cipher(ses.newkeys->recv.algo_crypt->cipherdesc->name);
-		if (recv_cipher < 0)
-			dropbear_exit("Crypto error");
+		int recv_cipher = -1;
+		if (ses.newkeys->recv.algo_crypt->cipherdesc->name != NULL) {
+			recv_cipher = find_cipher(ses.newkeys->recv.algo_crypt->cipherdesc->name);
+			if (recv_cipher < 0)
+				dropbear_exit("Crypto error");
+		}
 		if (ses.newkeys->recv.crypt_mode->start(recv_cipher, 
 				recv_IV, recv_key, 
 				ses.newkeys->recv.algo_crypt->keysize, 0, 
@@ -341,9 +344,12 @@ static void gen_new_keys() {
 	}
 
 	if (ses.newkeys->trans.algo_crypt->cipherdesc != NULL) {
-		int trans_cipher = find_cipher(ses.newkeys->trans.algo_crypt->cipherdesc->name);
-		if (trans_cipher < 0)
-			dropbear_exit("Crypto error");
+		int trans_cipher = -1;
+		if (ses.newkeys->trans.algo_crypt->cipherdesc->name != NULL) {
+			trans_cipher = find_cipher(ses.newkeys->trans.algo_crypt->cipherdesc->name);
+			if (trans_cipher < 0)
+				dropbear_exit("Crypto error");
+		}
 		if (ses.newkeys->trans.crypt_mode->start(trans_cipher, 
 				trans_IV, trans_key, 
 				ses.newkeys->trans.algo_crypt->keysize, 0, 
@@ -868,19 +874,29 @@ static void read_kex_algos() {
 
 	/* mac_algorithms_client_to_server */
 	c2s_hash_algo = buf_match_algo(ses.payload, sshhashes, NULL, NULL);
+#if DROPBEAR_AEAD_MODE
+	if (((struct dropbear_cipher_mode*)c2s_cipher_algo->mode)->aead_crypt != NULL) {
+		c2s_hash_algo = NULL;
+	} else
+#endif
 	if (c2s_hash_algo == NULL) {
 		erralgo = "mac c->s";
 		goto error;
 	}
-	TRACE(("hash c2s is  %s", c2s_hash_algo->name))
+	TRACE(("hash c2s is  %s", c2s_hash_algo ? c2s_hash_algo->name : "<implicit>"))
 
 	/* mac_algorithms_server_to_client */
 	s2c_hash_algo = buf_match_algo(ses.payload, sshhashes, NULL, NULL);
+#if DROPBEAR_AEAD_MODE
+	if (((struct dropbear_cipher_mode*)s2c_cipher_algo->mode)->aead_crypt != NULL) {
+		s2c_hash_algo = NULL;
+	} else
+#endif
 	if (s2c_hash_algo == NULL) {
 		erralgo = "mac s->c";
 		goto error;
 	}
-	TRACE(("hash s2c is  %s", s2c_hash_algo->name))
+	TRACE(("hash s2c is  %s", s2c_hash_algo ? s2c_hash_algo->name : "<implicit>"))
 
 	/* compression_algorithms_client_to_server */
 	c2s_comp_algo = buf_match_algo(ses.payload, ses.compress_algos, NULL, NULL);
@@ -925,8 +941,14 @@ static void read_kex_algos() {
 		ses.newkeys->trans.crypt_mode =
 			(struct dropbear_cipher_mode*)c2s_cipher_algo->mode;
 		ses.newkeys->recv.algo_mac = 
+#if DROPBEAR_AEAD_MODE
+			s2c_hash_algo == NULL ? ses.newkeys->recv.crypt_mode->aead_mac :
+#endif
 			(struct dropbear_hash*)s2c_hash_algo->data;
 		ses.newkeys->trans.algo_mac = 
+#if DROPBEAR_AEAD_MODE
+			c2s_hash_algo == NULL ? ses.newkeys->trans.crypt_mode->aead_mac :
+#endif
 			(struct dropbear_hash*)c2s_hash_algo->data;
 		ses.newkeys->recv.algo_comp = s2c_comp_algo->val;
 		ses.newkeys->trans.algo_comp = c2s_comp_algo->val;
@@ -941,8 +963,14 @@ static void read_kex_algos() {
 		ses.newkeys->trans.crypt_mode =
 			(struct dropbear_cipher_mode*)s2c_cipher_algo->mode;
 		ses.newkeys->recv.algo_mac = 
+#if DROPBEAR_AEAD_MODE
+			c2s_hash_algo == NULL ? ses.newkeys->recv.crypt_mode->aead_mac :
+#endif
 			(struct dropbear_hash*)c2s_hash_algo->data;
 		ses.newkeys->trans.algo_mac = 
+#if DROPBEAR_AEAD_MODE
+			s2c_hash_algo == NULL ? ses.newkeys->trans.crypt_mode->aead_mac :
+#endif
 			(struct dropbear_hash*)s2c_hash_algo->data;
 		ses.newkeys->recv.algo_comp = c2s_comp_algo->val;
 		ses.newkeys->trans.algo_comp = s2c_comp_algo->val;