diff --git a/bignum.h b/bignum.h
index 4ee0726ef3abc495723797294fe1c86f65d83fb9..5f80ad3af50aed1ece68fe262d6c3522f33875ca 100644
--- a/bignum.h
+++ b/bignum.h
@@ -78,6 +78,7 @@ nettle_mpz_random(mpz_t x,
 		  void *ctx, nettle_random_func random,
 		  const mpz_t n);
 
+/* sexp parsing */
 struct sexp_iterator;
 
 /* If LIMIT is non-zero, the number must be at most LIMIT bits.
@@ -85,4 +86,12 @@ struct sexp_iterator;
 int
 nettle_mpz_set_sexp(mpz_t x, unsigned limit, struct sexp_iterator *i);
 
+
+/* der parsing */
+struct asn1_der_iterator;
+
+int
+asn1_der_get_bignum(struct asn1_der_iterator *iterator,
+		    mpz_t x, unsigned limit);
+
 #endif /* NETTLE_BIGNUM_H_INCLUDED */