From 8f6adc7e54648a4a9f85e38fa7a6a69a0ef673aa Mon Sep 17 00:00:00 2001
From: GregMcFarland <39377085+GregMcFarland@users.noreply.github.com>
Date: Thu, 17 May 2018 15:44:14 -0500
Subject: [PATCH] Fix decode issue

In base64_decode_block(), step_b & step_c have a bug in that they will cause bits of the next byte to be written even though there are no more bytes in the encoded data. This adds an additional check for more encoded data before writing the additional bits to the next byte.
---
 src/cdecode.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/cdecode.c b/src/cdecode.c
index a6c0a42..82fbf9b 100644
--- a/src/cdecode.c
+++ b/src/cdecode.c
@@ -22,6 +22,16 @@ void base64_init_decodestate(base64_decodestate* state_in)
 	state_in->plainchar = 0;
 }
 
+int is_next_codechar_anequal(const char* code_in_next, const int length_in_remaining)
+{
+	// Do we have any characters remaining after this one?
+	if (length_in_remaining) {
+		if (*code_in_next == '=')
+			return (1); // We have an equal
+	}
+	return (0); // No more charcters available
+}
+
 int base64_decode_block(const char* code_in, const int length_in, char* plaintext_out, base64_decodestate* state_in)
 {
 	const char* codechar = code_in;
@@ -56,7 +66,8 @@ int base64_decode_block(const char* code_in, const int length_in, char* plaintex
 				fragment = (char)base64_decode_value(*codechar++);
 			} while (fragment < 0);
 			*plainchar++ |= (fragment & 0x030) >> 4;
-			*plainchar    = (fragment & 0x00f) << 4;
+			if (!is_next_codechar_anequal(codechar, length_in - (codechar - code_in)))
+				*plainchar    = (fragment & 0x00f) << 4;
 	case step_c:
 			do {
 				if (codechar == code_in+length_in)
@@ -68,7 +79,8 @@ int base64_decode_block(const char* code_in, const int length_in, char* plaintex
 				fragment = (char)base64_decode_value(*codechar++);
 			} while (fragment < 0);
 			*plainchar++ |= (fragment & 0x03c) >> 2;
-			*plainchar    = (fragment & 0x003) << 6;
+			if (!is_next_codechar_anequal(codechar, length_in - (codechar - code_in)))
+				*plainchar    = (fragment & 0x003) << 6;
 	case step_d:
 			do {
 				if (codechar == code_in+length_in)