Steganographically encode messages with LLMs and Arithmetic Coding
Good morning HN! For a while now I have been toying with this idea and now finally have a working prototype. This project allows you to encode secret messages into ordinary looking text by using arithmetic coding with a probability model derived from an LLM. By encrypting the message and then decompressing the encrypted message using the arithmetic coder, you get output which looks just like randomly sampled output from the LLM. Except, it actually encodes your secret messages in the specific choices of tokens. By using authenticated encryption, only a user who knows the key can know that a…
Steganographically encode messages with LLMs and Arithmetic Coding launched on February 12, 2025 with 24 votes, #670 of 5,999 launches that month and more than 90% of that year's launches. The site was still online in September 2026, 1 year on.
In plain words
This project enables users to hide encrypted messages within seemingly normal text generated by language models through arithmetic coding. The technique combines LLM probability models with encryption to embed secret data into token sequences, making hidden messages appear as ordinary AI-generated text to observers. Only recipients with the correct key can detect and decrypt the messages. This approach allows covert communication through public channels without revealing that a secret conversation is occurring.
written from the facts on this page · September 2026
From the sources
Steganographic message encoder using LLMs. Contribute to shawnz/textcoder development by creating an account on GitHub.
Textcoder is a proof-of-concept tool for steganographically encoding secret messages such that they appear as ordinary, unrelated text. It works by taking the secret message, encrypting it to produce a pseudorandom bit stream, and then using arithmetic coding to decompress that bit stream based on a statistical model derived from an LLM. This produces text which appears to be sampled randomly from the LLM, while actually encoding the secret message in the specific token choices. For example, the secret message hello, world! could be encoded into the following text: % echo 'hello, world!' | textcoder -p 'foobar' > encoded.txt % cat encoded.txt "Goodbye 2024! Can't wait to start fresh with a…from github.com
In the maker’s words, at launch
Good morning HN! For a while now I have been toying with this idea and now finally have a working prototype. This project allows you to encode secret messages into ordinary looking text by using arithmetic coding with a probability model derived from an LLM. By encrypting the message and then decompressing the…
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