[patch 1/2] use chacha20 from openssl (1.1.0+) when possible

[Jakub Jelen]

On Fri, 2019-07-12 at 15:54 +1000, Damien Miller wrote:
> On Thu, 17 Jan 2019, Yuriy M. Kaminskiy wrote:
> 
> > On some cpu's optimized chacha implementation in openssl (1.1.0+)
> > is
> > notably faster (and on others it is just faster) than generic C
> > implementation in openssh.
> > 
> > Sadly, openssl's chacha20-poly1305 (EVP_chacha20_poly1305) uses
> > different scheme (with padding/etc - see rfc8439) and it looks it
> > is not
> > possible to use in openssh.
> > 
> > OpenSSL 1.1.1+ also exports "raw" poly1305 primitive, but I
> > have not tried it yet (it was not in 1.1.0).
> > 
> > Trivial benchmark:
> > time ssh -c chacha20-poly1305 at openssh.com -S none -o Compression=no
> > \
> > localhost 'dd if=/dev/zero bs=100000 count=10000' >/dev/null
> > (comparing "user time" only)
> > 
> > openssh: 7.9p1, self-compiled, based on upstream package from
> > debian/unstable,
> > hostkey - ecdsa/p256, pubkey auth key - ecdh/p256
> > 
> > Machine: pretty old amd k8 (w/ SSE2, but no SSSE3/AVX/AESNI)
> > OS: linux/debian/stretch, openssl 1.1.0j-1deb9u1
> > i386: speed: +8%
> > amd64: speed: +10%
> > 
> > Machine: raspberry pi 3b+ (BCM2837B0, 4-core Cortex-A53 @1.4GHz)
> > OS: raspbian/stretch
> > 
> > baseline: armhf/raspbian: unpatched ssh-7.9p1: 30.8s
> > 
> > with openssl 1.1.0j-1deb9u1 from raspbian (compiled for armv6
> > without neon):
> > 
> > armhf/raspbian: 24.7 seconds, speed: +25%
> > 
> > with openssl 1.1.0j-1deb9u1 from debian/stretch/armhf (compiled for
> > armv7 with neon autodetection):
> > armhf: 22.2 seconds, speed: +39%
> > 
> > Patches against 7.9p1 (tested) and git master (untested, only
> > resolved
> > configure.ac conflict) attached.
> 
> Thanks for this - it seems to work okay with OpenSSL when patched to
> -current, but when I adapt it for OpenBSD/LibreSSL the encryption is
> broken and the connection fails right after KEX.
> 
> I expect that there is some difference between OpenSSL and LibreSSL
> wrt
> IV lengths or something. OpenSSH does need to support both, so this
> will
> take a little figuring out.
> 
> One comment on the patch itself: it passes do_encrypt though in a
> bunch
> of places and I'm not sure the usage is correct in all of them. In
> fact
> I don't think it can even be made consistent for decryption, as the
> ctx->main_evp has to be used in encryption mode (not decryption) to
> generate the poly1305 key.
> 
> Given this is a stream cipher and there is AFAIK no difference
> between
> encryption and decryption, I think it would be better just fix
> do_encrypt
> to 1 to avoid inconsistency.

Hi Damien,
do you have any update on this?

Indeed, it looks like LibreSSL has the IV of 96 b [1], while OpenSSL
uses 128 bits (including the 32b counter) [2]. Otherwise, I did not
notice any differences.

I have really no experience with OpenBSD so I do not have simple way to
test my changes, but I believe something like this should address the
difference:

diff --git a/cipher-chachapoly.c b/cipher-chachapoly.c
index a58616fb..7e6995f6 100644
--- a/cipher-chachapoly.c
+++ b/cipher-chachapoly.c
@@ -109,7 +109,14 @@ chachapoly_crypt(struct chachapoly_ctx *ctx, u_int
seqnr, u_char *dest,
     const u_char *src, u_int len, u_int aadlen, u_int authlen, int
do_encrypt)
 {
 #if defined(WITH_OPENSSL) && defined(HAVE_EVP_CHACHA20)
+#if defined(LIBRESSL_VERSION_NUMBER)
+#define CHACHA_IV_OFFSET 4
+	u_char seqbuf[12];
+#else
+#define CHACHA_IV_OFFSET 8
+	/* OpenSSL IV contains also the counter in the first 4 bytes */
 	u_char seqbuf[16];
+#endif
 	int r = SSH_ERR_LIBCRYPTO_ERROR;
 #else
 	u_char seqbuf[8];
@@ -125,7 +132,7 @@ chachapoly_crypt(struct chachapoly_ctx *ctx, u_int
seqnr, u_char *dest,
 	memset(poly_key, 0, sizeof(poly_key));
 #if defined(WITH_OPENSSL) && defined(HAVE_EVP_CHACHA20)
 	memset(seqbuf + 0, 0, 8);
-	POKE_U64(seqbuf + 8, seqnr);
+	POKE_U64(seqbuf + CHACHA_IV_OFFSET, seqnr);
 	if (!EVP_CipherInit(ctx->main_evp, NULL, NULL, seqbuf,
do_encrypt))
 		goto out;
 	if (EVP_Cipher(ctx->main_evp, poly_key, (u_char *)poly_key,
sizeof(poly_key)) < 0)

For the do_encrypt, you are right. Chacha20 is stream cipher so there
is no difference between decryption and encryption but the EVP API
requires this argument. For consistency, I would be for using 1 in all
the cases.

If you have some wip branch you used for porting to openbsd or
something I can test, I guess I can try that.

[1] https://man.openbsd.org/man3/EVP_EncryptInit.3
[2] 
https://www.openssl.org/docs/man1.1.1/man3/EVP_chacha20_poly1305.html

Regards,
-- 
Jakub Jelen
Senior Software Engineer
Security Technologies
Red Hat, Inc.