fix: SecretKey.fromBytes perform constant time comparison to zero bytes

[has5aan]

Summary

Replaces the call to isZeroBytes with cypto.timingSafeEqual to achieve constant time comparison to empty buffer.

[dapplion]

Hey @has5aan :wave: thanks for your contribution! Could you detail what attack is this fix guarding against? If you can call fromBytes function you know the secret key, so there's no more information you can gain from timing attacks

[has5aan]

Hey @has5aan 👋 thanks for your contribution! Could you detail what attack is this fix guarding against? If you can call fromBytes function you know the secret key, so there's no more information you can gain from timing attacks

Hi @dapplion; Valid point, the function fromBytes receives the secret key and doesn't generate any, the idea was to be thorough when performing empty buffer check.

[shuse2]

@dapplion Hello! fromBytes is used in signing function as well to convert Buffer to secretKey. If we have blsSign(message, signerID) function which does not accept the private key, attacker can still measure the time of this blsSign and get the information of amount of leading zeros on the private key

[CLAassistant]

All committers have signed the CLA.