[New-bugs-announce] [issue40791] hmac.compare_digest could try harder to be constant-time.

[Devin Jeanpierre]

New submission from Devin Jeanpierre <jeanpierreda at gmail.com>:

`hmac.compare_digest` (via `_tscmp`) does not mark the accumulator variable `result` as volatile, which means that the compiler is allowed to short-circuit the comparison loop as long as it still reads from both strings.

In particular, when `result` is non-volatile, the compiler is allowed to change the loop from this:


```c
for (i=0; i < length; i++) {
    result |= *left++ ^ *right++;
}
return (result == 0);
```

into (the moral equivalent of) this:


```c
for (i=0; i < length; i++) {
    result |= *left++ ^ *right++;
    if (result) {
        for (; ++i < length;) {
            *left++; *right++;
        }
        return 1;
    }
}
return (result == 0);
```

(Code not tested.)

This might not seem like much, but it cuts out almost all of the data dependencies between `result`, `left`, and `right`, which in theory would free the CPU to race ahead using out of order execution -- it could execute code that depends on the result of `_tscmp`, even while `_tscmp` is still performing the volatile reads. (I have not actually benchmarked this. :)) In other words, this weird short circuiting could still actually improve performance. That, in turn, means that it would break constant-time guarantees.

(This is different from saying that it _would_ increase performance, but marking it volatile removes the worry.)

(Prior art/discussion: https://github.com/google/tink/commit/335291c42eecf29fca3d85fed6179d11287d253e )


I propose two changes, one trivial, and one that's more invasive:

1) Make `result` a `volatile unsigned char` instead of `unsigned char`. 

2) When SSL is available, instead use `CRYPTO_memcmp` from OpenSSL/BoringSSL. We are, in effect, "rolling our own crypto". The SSL libraries are more strictly audited for timing issues, down to actually checking the generated machine code. As tools improve, those libraries will grow to use those tools. If we use their functions, we get the benefit of those audits and improvements.

----------
components: Library (Lib)
messages: 370053
nosy: Devin Jeanpierre
priority: normal
severity: normal
status: open
title: hmac.compare_digest could try harder to be constant-time.
versions: Python 3.10, Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40791>
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