[CI test, dontmerge] PR #1579 with secp256k1_memclear using volatile_memset #1622
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This code is not supposed to handle secret data.
We rely on memset() and an __asm__ memory barrier where it's available or on SecureZeroMemory() on Windows. The fallback implementation uses a volatile function pointer to memset which the compiler is not clever enough to optimize.
There are two uses of the secp256k1_fe_clear() function that are now separated
into these two functions in order to reflect the intent:
1) initializing the memory prior to being used -> converted to fe_set_int( . , 0 )
2) zeroing the memory after being used such that no sensitive data remains. ->
remains as fe_clear()
In the latter case, 'magnitude' and 'normalized' need to be overwritten when
VERIFY is enabled.
Co-Authored-By: isle2983 <[email protected]>
Co-Authored-By: isle2983 <[email protected]> Co-Authored-By: Pieter Wuille <[email protected]>
All of the invocations of secp256k1_memclear() operate on stack memory and happen after the function is done with the memory object. This commit replaces existing memset() invocations and also adds secp256k1_memclear() to code locations where clearing was missing; there is no guarantee that this commit covers all code locations where clearing is necessary. Co-Authored-By: isle2983 <[email protected]>
This gives the caller more control about whether the state should be cleaned (= should be considered secret). Moreover, it gives the caller the possibility to clean a hash struct without finalizing it.
Quoting sipa (see bitcoin-core#1479 (comment)): "When performing an EC multiplication A = aG for secret a, the resulting _affine_ coordinates of A are presumed to not leak information about a (ECDLP), but the same is not necessarily true for the Jacobian coordinates that come out of our multiplication algorithm." For the ECDH point multiplication result, the result in Jacobi coordinates should be cleared not only to avoid leaking the scalar, but even more so as it's a representation of the resulting shared secret.
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Okay, awesome CI seems to like this. Let me close this PR, I think this is a sufficient test for this #if branch, even if further small changes will be made to #1579. |
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As suggested in #1579 (comment).