New SP 800-56A compliant version of DH_compute_key().

crypto/dh/dh.h

@@ -202,6 +202,7 @@ int	DH_check(const DH *dh,int *codes);
 int	DH_check_pub_key(const DH *dh,const BIGNUM *pub_key, int *codes);
 int	DH_generate_key(DH *dh);
 int	DH_compute_key(unsigned char *key,const BIGNUM *pub_key,DH *dh);
+int	DH_compute_key_padded(unsigned char *key,const BIGNUM *pub_key,DH *dh);
 DH *	d2i_DHparams(DH **a,const unsigned char **pp, long length);
 int	i2d_DHparams(const DH *a,unsigned char **pp);
 #ifndef OPENSSL_NO_FP_API

crypto/dh/dh_key.c

@@ -86,6 +86,21 @@ int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
 	return dh->meth->compute_key(key, pub_key, dh);
 	}
 
+int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh)
+	{
+	int rv, pad;
+	rv = dh->meth->compute_key(key, pub_key, dh);
+	if (rv <= 0)
+		return rv;
+	pad = BN_num_bytes(dh->p) - rv;
+	if (pad > 0)
+		{
+		memmove(key + pad, key, rv);
+		memset(key, 0, pad);
+		}
+	return rv + pad;
+	}
+
 static DH_METHOD dh_ossl = {
 "OpenSSL DH Method",
 generate_key,