crypto: BIP324 ciphersuite follow-up

follow-up to #28008.
* move `dummy_tag` variable in FSChaCha20Poly1305 crypto_tests
outside of the loop to be reused every time
* use easy to read `cipher.last()` in `AEADChaCha20Poly1305::Decrypt()`
* comment for initiator in `BIP324Cipher::Initialize()`
* systematically damage ciphertext with bit positions in bip324_tests
* use 4095 max bytes for aad in bip324 fuzz test

1 files changed, 1 insertions, 1 deletions

diff --git a/src/crypto/chacha20poly1305.cpp b/src/crypto/chacha20poly1305.cpp
index c936dd2265..2636ebe2b2 100644
--- a/src/crypto/chacha20poly1305.cpp
+++ b/src/crypto/chacha20poly1305.cpp
@@ -95,7 +95,7 @@ bool AEADChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std:
     m_chacha20.Seek64(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.data() + cipher.size() - EXPANSION), EXPANSION)) return false;
+    if (timingsafe_bcmp(UCharCast(expected_tag), UCharCast(cipher.last(EXPANSION).data()), EXPANSION)) return false;
 
     // Decrypt (starting at block 1).
     m_chacha20.Crypt(UCharCast(cipher.data()), UCharCast(plain1.data()), plain1.size());