diff --git a/sys/base64/base64.c b/sys/base64/base64.c
index 048fc99f48..e6f82424dc 100644
--- a/sys/base64/base64.c
+++ b/sys/base64/base64.c
@@ -56,7 +56,7 @@ static char getsymbol(unsigned char code)
     return (char)BASE64_NOT_DEFINED;
 }
 
-int base64_encode(unsigned char *data_in, size_t data_in_size, \
+int base64_encode(const unsigned char *data_in, size_t data_in_size,
                   unsigned char *base64_out, size_t *base64_out_size)
 {
     size_t required_size = 4 * ((data_in_size + 2) / 3);
@@ -159,7 +159,7 @@ static int getcode(char symbol)
     return BASE64_NOT_DEFINED;
 }
 
-int base64_decode(unsigned char *base64_in, size_t base64_in_size, \
+int base64_decode(const unsigned char *base64_in, size_t base64_in_size,
                   unsigned char *data_out, size_t *data_out_size)
 {
     size_t required_size = ((base64_in_size / 4) * 3);
diff --git a/sys/include/base64.h b/sys/include/base64.h
index 46f332a321..0a7117223f 100644
--- a/sys/include/base64.h
+++ b/sys/include/base64.h
@@ -50,7 +50,7 @@ extern "C" {
             BASE64_ERROR_DATA_IN if `data_in` equals NULL,
             BASE64_ERROR_DATA_IN_SIZE if `data_in_size` is less then 1.
  */
-int base64_encode(unsigned char *data_in, size_t data_in_size, \
+int base64_encode(const unsigned char *data_in, size_t data_in_size,
                   unsigned char *base64_out, size_t *base64_out_size);
 
 /**
@@ -71,7 +71,7 @@ int base64_encode(unsigned char *data_in, size_t data_in_size, \
             BASE64_ERROR_DATA_IN if `base64_in` equals NULL,
             BASE64_ERROR_DATA_IN_SIZE if `base64_in_size` is less then 4.
  */
-int base64_decode(unsigned char *base64_in, size_t base64_in_size, \
+int base64_decode(const unsigned char *base64_in, size_t base64_in_size,
                   unsigned char *data_out, size_t *data_out_size);
 
 #ifdef __cplusplus