diff --git a/util.c b/util.c
index 3b539b1..2788d7c 100644
--- a/util.c
+++ b/util.c
@@ -144,7 +144,7 @@ init_yubikey(YK_KEY **yk)
 int challenge_response(YK_KEY *yk, int slot,
 		       char *challenge, unsigned int len,
 		       bool hmac, unsigned int flags, bool verbose,
-		       char *response, int res_size, int *res_len)
+		       char *response, int res_size, unsigned int *res_len)
 {
 	int yk_cmd;
 	unsigned int response_len = 0;
diff --git a/util.h b/util.h
index aa1d2ec..e076169 100644
--- a/util.h
+++ b/util.h
@@ -67,16 +67,16 @@ int get_user_cfgfile_path(const char *common_path, const char *filename, const c
 #define CR_RESPONSE_SIZE	20
 
 struct chalresp_state {
-  unsigned char challenge[CR_CHALLENGE_SIZE];
+  char challenge[CR_CHALLENGE_SIZE];
   uint8_t challenge_len;
-  unsigned char response[CR_RESPONSE_SIZE];
+  char response[CR_RESPONSE_SIZE];
   uint8_t response_len;
   uint8_t slot;
 };
 
 typedef struct chalresp_state CR_STATE;
 
-int generate_random(char *buf, int len);
+int generate_random(void *buf, int len);
 
 int get_user_challenge_file(YK_KEY *yk, const char *chalresp_path, const char *username, char **fn);
 
@@ -86,9 +86,9 @@ int write_chalresp_state(FILE *f, CR_STATE *state);
 int init_yubikey(YK_KEY **yk);
 int check_firmware_version(YK_KEY *yk, bool verbose, bool quiet);
 int challenge_response(YK_KEY *yk, int slot,
-		       unsigned char *challenge, unsigned int len,
+		       char *challenge, unsigned int len,
 		       bool hmac, unsigned int flags, bool verbose,
-		       unsigned char *response, int res_size, int *res_len);
+		       char *response, int res_size, unsigned int *res_len);
 
 #endif /* HAVE_CR */
 
diff --git a/ykpamcfg.c b/ykpamcfg.c
index 27e5ef5..f0946d0 100644
--- a/ykpamcfg.c
+++ b/ykpamcfg.c
@@ -118,7 +118,7 @@ parse_args(int argc, char **argv,
 int
 do_add_hmac_chalresp(YK_KEY *yk, uint8_t slot, bool verbose, char *output_dir, int *exit_code)
 {
-  unsigned char buf[CR_RESPONSE_SIZE + 16];
+  char buf[CR_RESPONSE_SIZE + 16];
   CR_STATE state;
   unsigned int flags = 0;
   int ret = 0;