Base64Decoder does not detect aligned internal padding

var decoder = new DotNext.Buffers.Text.Base64Decoder();
decoder.DecodeFromUtf16("AA==");
decoder.DecodeFromUtf16("AA==");

Expected behavior: an exception is thrown

Actual behavior: no exception is thrown

The best solution is probably to create member bool gotPadding or similar, set it if buffer[^1] == '=', and throw exception if gotPadding is set when starting decoding. Also related, it’s possible to save some memory using forbidden states in reservedBuffer, examples:

1. find first null byte/char instead of storing buffer size separately (null bytes are illegal in base64 input anyways)
2. use -1 to mean gotPadding = true (end of input stream)
3. create separate Utf8Base64Decoder and Utf16Base64Decoder structs holding 4 bytes and 8 bytes buffer

[alxu-avpt]

Thanks!

[sakno]

Fixed in 4.12.2 and published.