diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index c98e0bd..d3b6644 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -306,6 +306,11 @@
  * will be returned. */
 OPENSSL_EXPORT BN_ULONG BN_get_word(const BIGNUM *bn);
 
+/* BN_get_u64 sets |*out| to the absolute value of |bn| as a |uint64_t| and
+ * returns one. If |bn| is too large to be represented as a |uint64_t|, it
+ * returns zero. */
+OPENSSL_EXPORT int BN_get_u64(const BIGNUM *bn, uint64_t *out);
+
 
 /* ASN.1 functions. */