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Hi all,
I was informed that the hmac.secure_compare() function added in 14532 is not time-independent when processing unicode values:
"The function as given is probably not timing independent if the attacker can provide unicode values. This is because (in CPython at least) all integer values in the range [-5, 256] inclusive are made singletons to avoid the performance hit of integer object creation, meaning that as long as (x ^ y) < 257, no integer object is created and the function appears constant time. When that assumption is violated, you get a timing delta that is actually fairly large compared to the delta for a single character compare."
One way to work around this issue is to perform strict 8-bit byte comparisons by converting any string type parameters to bytes via a utf-8 encode. The attached patch does this.
Regards,
Jon Oberheide