Merge bitcoin/bitcoin#29815: crypto: chacha20: always use our fallback timingsafe_bcmp rather than libc's

2d18194 crypto: chacha20: always use our fallback timingsafe_bcmp rather than libc's (Cory Fields)

Pull request description:

  Looking at libc sources, apple and openbsd implementations match our naive fallback. Only FreeBSD (and only x86_64) seems to [implement an optimized version](https://github.com/freebsd/freebsd-src/blob/main/lib/libc/amd64/string/timingsafe_bcmp.S).

  It's not worth the hassle of using a platform-specific function for such little gain.

  Additionally, as mentioned below, this is the only case outside of sha2 that requires an autoconf check, and I have upcoming PRs to remove the sha2 ones.

  Apple's [impl is unoptimized](https://opensource.apple.com/source/Libc/Libc-1244.1.7/string/FreeBSD/timingsafe_bcmp.c.auto.html).

  As-is [OpenBSD's impl](https://github.com/openbsd/src/blob/master/lib/libc/string/timingsafe_bcmp.c).

  Relevant IRC conversation with sipa:

  > \<cfields\> sipa: chacha20poly1305.cpp uses libc's timingsafe_bcmp when possible. But looking around at apple/freebsd/openbsd, I don't see any impl that doesn't use the naive implementation that matches our fallback...
  > \<cfields\> is there any reason to belive there's an optimized impl somewhere that we're actually hitting?
  > \<cfields\> asking because after cleaning up sha2, timingsafe_bcmp is the last autoconf check that remains in all of crypto. It'd make life easy if we could just always use our internal one.
  > \<cfields\> *all of crypto/
  > \<sipa\> cfields: let's get rid of the dependency then
  > \<sipa\> it's a trivial function
  > \<sipa\> and if we need it for some platforms, no real reason not to use it on all

  After the above discusstion, I did end up finding the x86_64-optimized FreeBSD impl, but I don't think that's all that significant.

ACKs for top commit:
  sipa:
    utACK 2d18194
  fanquake:
    ACK 2d18194
  TheCharlatan:
    ACK 2d18194
  theStack:
    ACK 2d18194

Tree-SHA512: b9583e19ac2f77c5d572aa5b95bc4b53669d5717e5708babef930644980de7c5d06a9c7decd5c2b559d70b8597328ecfe513375e3d8c3ef523db80012dfe9266

Author: fanquake

configure.ac

@@ -968,8 +968,6 @@ AC_CHECK_DECLS([setsid])
 
 AC_CHECK_DECLS([pipe2])
 
-AC_CHECK_FUNCS([timingsafe_bcmp])
-
 AC_MSG_CHECKING([for __builtin_clzl])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
  (void) __builtin_clzl(0);

src/crypto/chacha20poly1305.cpp

@@ -2,10 +2,6 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
-
 #include <crypto/chacha20poly1305.h>
 
 #include <crypto/common.h>
@@ -30,10 +26,7 @@ void AEADChaCha20Poly1305::SetKey(Span<const std::byte> key) noexcept
 
 namespace {
 
-#ifndef HAVE_TIMINGSAFE_BCMP
-#define HAVE_TIMINGSAFE_BCMP
-
-int timingsafe_bcmp(const unsigned char* b1, const unsigned char* b2, size_t n) noexcept
+int timingsafe_bcmp_internal(const unsigned char* b1, const unsigned char* b2, size_t n) noexcept
 {
     const unsigned char *p1 = b1, *p2 = b2;
     int ret = 0;
@@ -42,8 +35,6 @@ int timingsafe_bcmp(const unsigned char* b1, const unsigned char* b2, size_t n)
     return (ret != 0);
 }
 
-#endif
-
 /** Compute poly1305 tag. chacha20 must be set to the right nonce, block 0. Will be at block 1 after. */
 void ComputeTag(ChaCha20& chacha20, Span<const std::byte> aad, Span<const std::byte> cipher, Span<std::byte> tag) noexcept
 {
@@ -97,7 +88,7 @@ bool AEADChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std:
     m_chacha20.Seek(nonce, 0);
     std::byte expected_tag[EXPANSION];
     ComputeTag(m_chacha20, aad, cipher.first(cipher.size() - EXPANSION), expected_tag);
-    if (timingsafe_bcmp(UCharCast(expected_tag), UCharCast(cipher.last(EXPANSION).data()), EXPANSION)) return false;
+    if (timingsafe_bcmp_internal(UCharCast(expected_tag), UCharCast(cipher.last(EXPANSION).data()), EXPANSION)) return false;
 
     // Decrypt (starting at block 1).
     m_chacha20.Crypt(cipher.first(plain1.size()), plain1);