diff --git a/ext/openssl/ossl.c b/ext/openssl/ossl.c
index a19ff23b10..5fd6bff98b 100644
--- a/ext/openssl/ossl.c
+++ b/ext/openssl/ossl.c
@@ -122,7 +122,7 @@ ossl_buf2str(char *buf, int len)
 }
 
 void
-ossl_bin2hex(unsigned char *in, char *out, size_t inlen)
+ossl_bin2hex(const unsigned char *in, char *out, size_t inlen)
 {
     const char *hex = "0123456789abcdef";
     size_t i;
diff --git a/ext/openssl/ossl.h b/ext/openssl/ossl.h
index 93deafb4b6..0b479a7200 100644
--- a/ext/openssl/ossl.h
+++ b/ext/openssl/ossl.h
@@ -131,7 +131,7 @@ do{\
  * Convert binary string to hex string. The caller is responsible for
  * ensuring out has (2 * len) bytes of capacity.
  */
-void ossl_bin2hex(unsigned char *in, char *out, size_t len);
+void ossl_bin2hex(const unsigned char *in, char *out, size_t len);
 
 /*
  * Our default PEM callback