Constant-time HMAC comparison

[0xd6cb6d73]

Use a constant-time function to compare HMAC values.

The current code uses the language's basic == operator to compare the expected value to the one obtained from the network message. This may enable HMAC forging through a timing attack.

Although I believe my fix is sound, I have not been able to test whether it truly is constant time.

[0xd6cb6d73]

The short-circuit when lhs and rhs aren't the same length matches what the standard library's blessed implementation CryptographicOperations.FixedTimeEquals does: https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.cryptographicoperations.fixedtimeequals?view=net-10.0#remarks. Unfortunately, that does not seem to be available in any version of .NET framework.