Randomized forward-secure Curve25519-AES256-CTR-HMAC-SHA512 for Pythonista
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README.markdown

Curve25519-AES256-CTR-HMAC-SHA512

Howto

An authenticated encryption mode. Works on Pythonista without installing any additional modules. (Pythonista includes, but does not document, the PyCrypto module.) To use with Python, do

pip install -r requirements.txt

To generate a keypair for yourself, do

write_keypair()

Send k_curve25519.pub to whomever you want to message you.

To encrypt a message to a recipient, do

encrypt(recipient_public_key, 'message')

To decrypt a message encrypted under your public key, using a key saved in a keyfile, do

decrypt_withkeyfile(encrypted_message)

The public keys are just plain base64 files.

Encryption mode summary

Uses elliptic-curve Diffie-Hellman key exchange on djb's Curve25519, AES-256 in CTR mode, and HMAC-SHA512 over the encrypted message. The encryption and authentication keys are derived using a random nonce and the ECDH shared-secret. HMAC-SHA512 is used as the key-derivation-function.

This provides nonceless deniable encryption. Note that it leaks message length. This will be changed in a future version; a padding byte is included, so messages using this version are future-compatible.

The Curve25519 implementation in pure Python is directly adapted from djb's naclcrypto-20090310 spec.

(The Montgomery ladder routine has been converted to imperative form to avoid running out of stack-space on some Pythonista builds.)

encrypt generates an ephemeral keypair, derives an AES key and an HMAC key, and encrypts and authenticates a message.