New ciphers are using authenticated encryption with associated data
(AEAD) and per-content key derived using HMAC-SHA256:
* AES256-GCM-HMAC-SHA256
* CHACHA20-POLY1305-HMAC-SHA256
They support content IDs of arbitrary length and are quite fast:
On my 2019 MBP:
- BLAKE2B-256 + AES256-GCM-HMAC-SHA256 - 648.7 MiB / second
- BLAKE2B-256 + CHACHA20-POLY1305-HMAC-SHA256 - 597.1 MiB / second
- HMAC-SHA256 + AES256-GCM-HMAC-SHA256 351 MiB / second
- HMAC-SHA256 + CHACHA20-POLY1305-HMAC-SHA256 316.2 MiB / second
Previous ciphers had several subtle issues:
* SALSA20 encryption, used weak nonce (64 bit prefix of content ID),
which means that for any two contents, whose IDs that have the same
64-bit prefix, their plaintext can be decoded from the ciphertext
alone.
* AES-{128,192,256}-CTR were not authenticated, so we were
required to hash plaintext after decryption to validate. This is not
recommended due to possibility of subtle timing attacks if an attacker
controls the ciphertext.
* SALSA20-HMAC was only validating checksum and not that the ciphertext
was for the correct content ID.
New repositories cannot be created using deprecated ciphers, but they
will still be supported for existing repositories, until at least 0.6.0.
The users are encouraged to migrate to one of new ciphers when 0.5.0 is
out.
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