Add EC_GROUP_order_bits for OpenSSL compatibility

Change-Id: I37149fa4274357d84befff85728ce2337131afa7
Reviewed-on: https://boringssl-review.googlesource.com/c/33804
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/ec/ec.c

diff --git a/crypto/fipsmodule/ec/ec.c b/crypto/fipsmodule/ec/ec.c
index bd0662a..90b9d71 100644
--- a/crypto/fipsmodule/ec/ec.c
+++ b/crypto/fipsmodule/ec/ec.c
@@ -625,6 +625,10 @@
   return 1;
 }
 
+int EC_GROUP_order_bits(const EC_GROUP *group) {
+  return BN_num_bits(&group->order);
+}
+
 int EC_GROUP_get_cofactor(const EC_GROUP *group, BIGNUM *cofactor,
                           BN_CTX *ctx) {
   // All |EC_GROUP|s have cofactor 1.

include/openssl/ec.h

diff --git a/include/openssl/ec.h b/include/openssl/ec.h
index 966393e..c65a1a7 100644
--- a/include/openssl/ec.h
+++ b/include/openssl/ec.h
@@ -133,6 +133,9 @@
 // |group| that specifies the order of the group.
 OPENSSL_EXPORT const BIGNUM *EC_GROUP_get0_order(const EC_GROUP *group);
 
+// EC_GROUP_order_bits returns the number of bits of the order of |group|.
+OPENSSL_EXPORT int EC_GROUP_order_bits(const EC_GROUP *group);
+
 // EC_GROUP_get_cofactor sets |*cofactor| to the cofactor of |group| using
 // |ctx|, if it's not NULL. It returns one on success and zero otherwise.
 OPENSSL_EXPORT int EC_GROUP_get_cofactor(const EC_GROUP *group,