Module: kdf

class doubleratchet.kdf.KDF

Bases: ABC

A KDF is defined as a cryptographic function that takes a secret and random KDF key and some input data and returns output data. The output data is indistinguishable from random provided the key isn’t known (i.e. a KDF satisfies the requirements of a cryptographic “PRF”). If the key is not secret and random, the KDF should still provide a secure cryptographic hash of its key and input data.

https://signal.org/docs/specifications/doubleratchet/#kdf-chains

abstract async static derive(key, data, length)

Parameters

• key (bytes) – The KDF key.

• data (bytes) – The input data.

• length (int) – The desired size of the output data, in bytes.

Return type

bytes

Returns

length bytes of output data, derived from the KDF key and the input data.