Ensure EC private keys retain leading zeros

RFC5915 requires the use of the I2OSP primitive as defined in RFC3447
for storing an EC Private Key. This converts the private key into an
OCTETSTRING and retains any leading zeros. This commit ensures that those
leading zeros are present if required.
Reviewed-by: NAndy Polyakov <appro@openssl.org>

crypto/ec/ec_asn1.c

@@ -1110,7 +1110,7 @@ int i2d_ECPrivateKey(EC_KEY *a, unsigned char **out)
 {
     int ret = 0, ok = 0;
     unsigned char *buffer = NULL;
-    size_t buf_len = 0, tmp_len;
+    size_t buf_len = 0, tmp_len, bn_len;
     EC_PRIVATEKEY *priv_key = NULL;
 
     if (a == NULL || a->group == NULL || a->priv_key == NULL ||
@@ -1126,18 +1126,32 @@ int i2d_ECPrivateKey(EC_KEY *a, unsigned char **out)
 
     priv_key->version = a->version;
 
-    buf_len = (size_t)BN_num_bytes(a->priv_key);
+    bn_len = (size_t)BN_num_bytes(a->priv_key);
+
+    /* Octetstring may need leading zeros if BN is to short */
+
+    buf_len = (EC_GROUP_get_degree(a->group) + 7) / 8;
+
+    if (bn_len > buf_len) {
+        ECerr(EC_F_I2D_ECPRIVATEKEY, EC_R_BUFFER_TOO_SMALL);
+        goto err;
+    }
+
     buffer = OPENSSL_malloc(buf_len);
     if (buffer == NULL) {
         ECerr(EC_F_I2D_ECPRIVATEKEY, ERR_R_MALLOC_FAILURE);
         goto err;
     }
 
-    if (!BN_bn2bin(a->priv_key, buffer)) {
+    if (!BN_bn2bin(a->priv_key, buffer + buf_len - bn_len)) {
         ECerr(EC_F_I2D_ECPRIVATEKEY, ERR_R_BN_LIB);
         goto err;
     }
 
+    if (buf_len - bn_len > 0) {
+        memset(buffer, 0, buf_len - bn_len);
+    }
+
     if (!ASN1_OCTET_STRING_set(priv_key->privateKey, buffer, buf_len)) {
         ECerr(EC_F_I2D_ECPRIVATEKEY, ERR_R_ASN1_LIB);
         goto err;