LibCrypto: Support PEM files with a non-regular base64 line length

base64 contents' length is always a multiple of 4. But some PEM files,
like the one generated by OpenSSH, have the big base64 blob divided in
lines of 70 characters. So we can't feed the base64 decoder with whole
lines and instead only pass chunk of data of the correct length.

Userland/Libraries/LibCrypto/ASN1/PEM.cpp

@@ -15,6 +15,8 @@ ByteBuffer decode_pem(ReadonlyBytes data)
     GenericLexer lexer { data };
     ByteBuffer decoded;
 
+    Vector<u8, 4> buffer;
+
     // FIXME: Parse multiple.
     enum {
         PreStartData,
@@ -34,13 +36,33 @@ ByteBuffer decode_pem(ReadonlyBytes data)
                 lexer.consume_line();
                 break;
             }
-            auto b64decoded = decode_base64(lexer.consume_line().trim_whitespace(TrimMode::Right));
-            if (b64decoded.is_error()) {
-                dbgln("Failed to decode PEM: {}", b64decoded.error().string_literal());
+            auto maybe_error = [&]() -> ErrorOr<void> {
+                auto next_line = lexer.consume_line().trim_whitespace(TrimMode::Right);
+                if (!buffer.is_empty()) {
+                    while (buffer.size() < 4 && !next_line.is_empty()) {
+                        buffer.append(next_line[0]);
+                        next_line = next_line.bytes().slice(1);
+                    }
+                    if (buffer.size() != 4)
+                        return {};
+                    auto b64decoded = TRY(decode_base64(StringView { buffer }));
+                    TRY(decoded.try_append(b64decoded.data(), b64decoded.size()));
+                    buffer.clear();
+                }
+
+                auto remainder = next_line.length() % 4;
+                auto end = next_line.length() - remainder;
+
+                buffer.extend(next_line.bytes().slice_from_end(remainder));
+
+                auto b64decoded = TRY(decode_base64(next_line.bytes().trim(end)));
+                TRY(decoded.try_append(b64decoded.data(), b64decoded.size()));
+
                 return {};
-            }
-            if (decoded.try_append(b64decoded.value().data(), b64decoded.value().size()).is_error()) {
-                dbgln("Failed to decode PEM, likely OOM condition");
+            }();
+
+            if (maybe_error.is_error()) {
+                dbgln("Failed to decode PEM: {}", maybe_error.error());
                 return {};
             }
             break;
@@ -53,6 +75,11 @@ ByteBuffer decode_pem(ReadonlyBytes data)
         }
     }
 
+    if (!buffer.is_empty()) {
+        dbgln("Failed to decode PEM: Invalid length of base64 encoded string");
+        return {};
+    }
+
     return decoded;
 }