From ec6b754871b25b2e1e04b295bb57ff838f1bbf33 Mon Sep 17 00:00:00 2001
From: Alex Thiessen <alex.thiessen@siemens.com>
Date: Sun, 6 Sep 2026 16:59:26 +0200
Subject: [PATCH] Fix Base64 padding separated by whitespace

Track the previous non-whitespace Base64 character when deciding whether
padding terminates a decoded byte. This preserves YAML 1.1 !!binary
decoding when whitespace appears between padding characters.

Cover whitespace-separated padding with a focused test that preserves
the single-byte result for the canonical padded form.

Reference verification:
- PyYAML 6.0.3 (YAML 1.1 !!binary, native value): matched 4d.
- libyaml 0.2.5 (parser/event layer): matched the yaml-cpp event stream.
---
 src/binary.cpp       | 4 +++-
 test/binary_test.cpp | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/binary.cpp b/src/binary.cpp
index 6c7e687d7..aed36b692 100644
--- a/src/binary.cpp
+++ b/src/binary.cpp
@@ -75,6 +75,7 @@ std::vector<unsigned char> DecodeBase64(const std::string &input) {
 
   unsigned value = 0;
   std::size_t cnt = 0;
+  bool previous_was_padding = false;
   for (std::size_t i = 0; i < input.size(); i++) {
     if (std::isspace(static_cast<unsigned char>(input[i]))) {
       // skip newlines
@@ -87,7 +88,7 @@ std::vector<unsigned char> DecodeBase64(const std::string &input) {
     value = (value << 6) | d;
     if (cnt == 3) {
       *out++ = value >> 16;
-      if (i > 0 && input[i - 1] != '=')
+      if (!previous_was_padding)
         *out++ = value >> 8;
       if (input[i] != '=')
         *out++ = value;
@@ -95,6 +96,7 @@ std::vector<unsigned char> DecodeBase64(const std::string &input) {
     } else {
       ++cnt;
     }
+    previous_was_padding = input[i] == '=';
   }
   if (cnt != 0) {
     // An invalid number of characters were encountered.
diff --git a/test/binary_test.cpp b/test/binary_test.cpp
index 30e0e46b0..93c999bb0 100644
--- a/test/binary_test.cpp
+++ b/test/binary_test.cpp
@@ -19,6 +19,11 @@ TEST(BinaryTest, DecodingTooShort) {
   EXPECT_TRUE(result.empty());
 }
 
+TEST(BinaryTest, DecodingWhitespaceBetweenPadding) {
+  const std::vector<unsigned char> &result = YAML::DecodeBase64("TQ= =");
+  EXPECT_EQ(std::vector<unsigned char>{'M'}, result);
+}
+
 TEST(BinaryTest, EmptyBinary) {
     YAML::Binary b;
     EXPECT_TRUE(b.size() == 0);