tree 21de4b8e982c666a7dad6e458136e5f4053d8420
parent 833633c61ee3ab86cfbdfdbe2df71c26304eaa75
author Atul Luykx <aluykx@google.com> 1682975779 +0000
committer CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com> 1682975779 +0000

Fix implementation of RFC6979 private key derivation

RFC6979 private key derivation allows one to derive an elliptic curve
private key using HMAC-DRBG. It is defined in such a way that you can
use any hash function, assuming its output is sufficiently long to
guarantee the required security level.

The function derivePrivateKey was incorrectly using the private_key
buffer as an intermediate value, resulting in problems when the buffer
is not sufficiently large to store SHA512-HMAC output.

We fix the problem by
1. having hmac check its output buffer size, and
2. remove the private_key buffer as an input to derivePrivateKey

Change-Id: Idf6fe3115e022df485ab24916547e25a09999458
Reviewed-on: https://pigweed-review.googlesource.com/c/open-dice/+/144130
Reviewed-by: Andrew Scull <ascull@google.com>
Commit-Queue: Atul Luykx <aluykx@google.com>