Paul Sheer optimised the OpenSSL to/from libGMP conversions for the case

where they both use the same limb size. I've tweaked his patch slightly, so
blame me if it breaks.

Submitted by: Paul Sheer
Reviewed by: Geoff Thorpe

CHANGES

@@ -681,6 +681,11 @@
 
  Changes between 0.9.8g and 0.9.8h  [xx XXX xxxx]
 
+  *) Update the GMP engine glue to do direct copies between BIGNUM and
+     mpz_t when openssl and GMP use the same limb size. Otherwise the
+     existing "conversion via a text string export" trick is still used.
+     [Paul Sheer <paulsheer@gmail.com>, Geoff Thorpe]
+
   *) Zlib compression BIO. This is a filter BIO which compressed and
      uncompresses any data passed through it.
      [Steve Henson]

engines/e_gmp.c

@@ -85,6 +85,8 @@
 #include <openssl/crypto.h>
 #include <openssl/buffer.h>
 #include <openssl/engine.h>
+#include <openssl/rsa.h>
+#include <openssl/bn.h>
 
 #ifndef OPENSSL_NO_HW
 #ifndef OPENSSL_NO_GMP
@@ -251,9 +253,25 @@ static int e_gmp_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void))
 	return to_return;
 	}
 
-/* HACK - use text I/O functions in openssl and GMP to handle conversions. This
- * is vile. */
+
+/* Most often limb sizes will be the same. If not, we use hex conversion
+ * which is neat, but extremely inefficient. */
 static int bn2gmp(const BIGNUM *bn, mpz_t g)
+	{
+	bn_check_top(bn);
+	if(((sizeof(bn->d[0]) * 8) == GMP_NUMB_BITS) &&
+			(BN_BITS2 == GMP_NUMB_BITS)) 
+		{
+		/* The common case */
+		if(!_mpz_realloc (g, bn->top))
+			return 0;
+		memcpy(&g->_mp_d[0], &bn->d[0], bn->top * sizeof(bn->d[0]));
+		g->_mp_size = bn->top;
+		if(bn->neg)
+			g->_mp_size = -g->_mp_size;
+		return 1;
+		}
+	else
 		{
 		int toret;
 		char *tmpchar = BN_bn2hex(bn);
@@ -262,8 +280,25 @@ static int bn2gmp(const BIGNUM *bn, mpz_t g)
 		OPENSSL_free(tmpchar);
 		return toret;
 		}
+	}
 
 static int gmp2bn(mpz_t g, BIGNUM *bn)
+	{
+	if(((sizeof(bn->d[0]) * 8) == GMP_NUMB_BITS) &&
+			(BN_BITS2 == GMP_NUMB_BITS))
+		{
+		/* The common case */
+		int s = (g->_mp_size >= 0) ? g->_mp_size : -g->_mp_size;
+		BN_zero(bn);
+		if(bn_expand2 (bn, s) == NULL)
+			return 0;
+		bn->top = s;
+		memcpy(&bn->d[0], &g->_mp_d[0], s * sizeof(bn->d[0]));
+		bn_correct_top(bn);
+		bn->neg = g->_mp_size >= 0 ? 0 : 1;
+		return 1;
+		}
+	else
 		{
 		int toret;
 		char *tmpchar = OPENSSL_malloc(mpz_sizeinbase(g, 16) + 10);
@@ -273,6 +308,7 @@ static int gmp2bn(mpz_t g, BIGNUM *bn)
 		OPENSSL_free(tmpchar);
 		return toret;
 		}
+	}
 
 #ifndef OPENSSL_NO_RSA 
 typedef struct st_e_gmp_rsa_ctx