TxtCipher code review: RC4 keystream reuse and a versioned AES format

AI;DR – The security review described here was done by Claude Code with the Fable model, and the resulting changes and this blog post were developed with significant help from it. If you don’t want to read “AI slop”, stop reading now. TxtCipher is a small web page that encrypts and decrypts text with a password. I wrote it to exchange text with TxtCrypt, a macOS program that uses RC4, and later added AES-256-GCM as the default mode. The whole thing is one HTML file with no dependencies, served straight from the SourceForge SVN repository, and it works in any current browser on Windows, Linux and Android. I recently had Claude Code, using the Fable model, review it with a focus on security. Two of the findings are worth writing down, because they affect how the tool should be used. RC4 mode: one password, one keystream The TxtCrypt format is simple: the UTF-8 bytes of the password are the RC4 key, there is no salt and no IV, and the result is Base64 encoded. That is what makes the two…