Draft: Post-Quantum Decentralized Group Chat

[tbraun96]

Post-Quantum Decentralized Group Chat (PQGC)

• PQGC will first generate a cryptographically-secure random symmetric key of 32 bytes in length (as well as a nonce)
• PQGC will then use a symmetric block cipher to encrypt the input plaintext using this random symmetric key and random nonce
• PQGC will then asymmetrically encrypt the random symmetric key using the public key of each group member to generate n encrypted symmetric keys
• Finally, PQGC will return a GroupMessage containing the ciphertext, n encrypted symmetric keys, and the relevant nonces.

The message size in bytes is 32n + ciphertext_len(message). The 32n part is figured because there will be n 32-byte kyber ciphertexts for the encrypted symmetric key, and ciphertext_len(message) is the output length of the symmetric block cipher against the input message. The message size is kept relatively small, and even with 1000 participants, the message is ~32KB which is highly acceptable for modern networks.

TODO:

• Authenticity via signing
• Ratcheting and forward secrecy
• Move from options to errors
• Fix invalid nonce sized inputs into kyber-pke (causes panics)