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Ensure that all data of StaticSecret is cleared on drop #41

merged 1 commit into from
Apr 23, 2019


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@untoldwind untoldwind commented Apr 12, 2019

The clone inside clamp_scalar seems to to have no use and might lead to a situation where the StaticSecret remains in memory after dropping it since the original byte-array is not cleared.

@hdevalence hdevalence changed the base branch from master to develop April 23, 2019 17:06
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The modified version of the code is certainly cleaner, but I don't think it makes any difference regarding memory clearing: in either case, the array is kept only on the stack, and we don't explicitly wipe the stack, so while this prevents a redundant copy, it doesn't affect memory wiping.

@hdevalence hdevalence merged commit 1cc37a1 into dalek-cryptography:develop Apr 23, 2019
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Merged, thanks for submitting this patch!

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untoldwind commented May 3, 2019

Actually (and unluckily) you are right. I've taken a look at generate llvm code of both versions and while they are marginally different they both have have an @llvm.memcpy.p0i8.p0i8.i64 of the original array to somewhere further up the stack.
And on top of that the Scalar::from_bits is doing another @llvm.memcpy.p0i8.p0i8.i64 (though from the previously memcpyed version down the stack)

I know this is kind of academic (since the stack is overwritten all the time), but I think the only way to ensure that there are no remains anywhere-ever would be to either

  • replace the [u8;32]-parameters with &mut [u8;32], doing the memcpy manually (e.g. with
    copy_from_slice or so) and then zero out the original ( is doing it this way)
  • Wrap [u8;32] in a struct with a clear-on-drop just like the StaticSecret itself.

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