diff -ur a/src/cdecode.c b/src/cdecode.c
--- a/src/cdecode.c	2013-06-18 14:49:36.000000000 +0200
+++ b/src/cdecode.c	2020-06-18 10:10:50.033461596 +0200
@@ -10,10 +10,10 @@
 int base64_decode_value(char value_in)
 {
 	static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
-	static const char decoding_size = sizeof(decoding);
-	value_in -= 43;
-	if (value_in < 0 || value_in >= decoding_size) return -1;
-	return decoding[(int)value_in];
+	if (value_in < '+') return -1;
+	value_in -= '+';
+	if (value_in >= (char)(sizeof(decoding) / sizeof(*decoding)) ) return -1;
+	return decoding[value_in];
 }
 
 void base64_init_decodestate(base64_decodestate* state_in)