Document Base64Unescape() padding

PiperOrigin-RevId: 482905442
Change-Id: Ic3a1e62f8a17efee5508d94835635aa489940f2d

absl/strings/escaping.h

@@ -122,6 +122,8 @@ std::string Utf8SafeCHexEscape(absl::string_view src);
 // Converts a `src` string encoded in Base64 to its binary equivalent, writing
 // it to a `dest` buffer, returning `true` on success. If `src` contains invalid
 // characters, `dest` is cleared and returns `false`.
+// Padding is optional. If padding is included, it must be correct. In the
+// padding, '=' and '.' are treated identically.
 bool Base64Unescape(absl::string_view src, std::string* dest);
 
 // WebSafeBase64Unescape()
@@ -129,6 +131,8 @@ bool Base64Unescape(absl::string_view src, std::string* dest);
 // Converts a `src` string encoded in Base64 to its binary equivalent, writing
 // it to a `dest` buffer, but using '-' instead of '+', and '_' instead of '/'.
 // If `src` contains invalid characters, `dest` is cleared and returns `false`.
+// Padding is optional. If padding is included, it must be correct. In the
+// padding, '=' and '.' are treated identically.
 bool WebSafeBase64Unescape(absl::string_view src, std::string* dest);
 
 // Base64Escape()

absl/strings/escaping_test.cc

@@ -617,6 +617,48 @@ TEST(Base64, EscapeAndUnescape) {
   TestEscapeAndUnescape<std::string>();
 }
 
+TEST(Base64, Padding) {
+  // Padding is optional.
+  // '.' is an acceptable padding character, just like '='.
+  std::initializer_list<absl::string_view> good_padding = {
+    "YQ",
+    "YQ==",
+    "YQ=.",
+    "YQ.=",
+    "YQ..",
+  };
+  for (absl::string_view b64 : good_padding) {
+    std::string decoded;
+    EXPECT_TRUE(absl::Base64Unescape(b64, &decoded));
+    EXPECT_EQ(decoded, "a");
+    std::string websafe_decoded;
+    EXPECT_TRUE(absl::WebSafeBase64Unescape(b64, &websafe_decoded));
+    EXPECT_EQ(websafe_decoded, "a");
+  }
+  std::initializer_list<absl::string_view> bad_padding = {
+    "YQ=",
+    "YQ.",
+    "YQ===",
+    "YQ==.",
+    "YQ=.=",
+    "YQ=..",
+    "YQ.==",
+    "YQ.=.",
+    "YQ..=",
+    "YQ...",
+    "YQ====",
+    "YQ....",
+    "YQ=====",
+    "YQ.....",
+  };
+  for (absl::string_view b64 : bad_padding) {
+    std::string decoded;
+    EXPECT_FALSE(absl::Base64Unescape(b64, &decoded));
+    std::string websafe_decoded;
+    EXPECT_FALSE(absl::WebSafeBase64Unescape(b64, &websafe_decoded));
+  }
+}
+
 TEST(Base64, DISABLED_HugeData) {
   const size_t kSize = size_t(3) * 1000 * 1000 * 1000;
   static_assert(kSize % 3 == 0, "kSize must be divisible by 3");