Work with -pedantic!
diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c
index 00693bc..f866126 100644
--- a/crypto/bn/bn_lib.c
+++ b/crypto/bn/bn_lib.c
@@ -571,7 +571,7 @@
 	}
 
 /* ignore negative */
-BIGNUM *BN_bin2bn(unsigned char *s, int len, BIGNUM *ret)
+BIGNUM *BN_bin2bn(const unsigned char *s, int len, BIGNUM *ret)
 	{
 	unsigned int i,m;
 	unsigned int n;