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Date: Thu, 2 Sep 2021 16:25:59 -0400
Message-ID: <CAKws9z3V+NU5gNV_Ecmn37y92O2C_LrwehVwUFTHFrqkN9aAtQ@mail.gmail.com>
To: PHP Internals <internals@lists.php.net>
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Subject: Sodium - Stream Counter API
From: security@paragonie.com (Paragon Initiative Enterprises Security Team)

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The ext-sodium API exposes the following functions:

* sodium_crypto_stream(int $len, string $nonce, string $key): string
* sodium_crypto_stream_keygen(): string
* sodium_crypto_stream_xor(string $message,  string $nonce, string $key):
string

(And the new XChaCha variant landing in PHP 8.1, of course.)

This is good for, like, 99% of the use cases for the crypto_stream API.
However, there is a missed opportunity: Being able to seek arbitrary
starting points in the XSalsa20 or XChaCha20 keystream when
encrypting/decrypting data.

The underlying libsodium API has a distinct feature for this: Functions
with the _ic suffix
<https://github.com/jedisct1/libsodium/blob/6d566070b48efd2fa099bbe9822914455150aba9/src/libsodium/crypto_stream/salsa20/ref/salsa20_ref.c#L64-L66>
.

Let's add an optional integer argument to the following PHP functions:

- sodium_crypto_stream_xor
- sodium_crypto_stream_xchacha20_xor
The only question I have is: Should this land in 8.1 (it's small and
self-contained) or 8.2?

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