fuzz: Improve the fuzz target for CCrypter #34111

1. 
   Chand-ra commented at 8:53 am on December 19, 2025: none

   Add the following improvements to the fuzz target for CCrypter:

   • Add roundtrip assertions: Add verification logic by decrypting the resulting ciphertext from a successful call to Encrypt() or EncryptSecret() and verifying that the output matches the original plaintext.

   • Cover untested methods: Extend the target to explicitly test CCrypter::SetKey() and DecryptSecret().

2. fuzz: Add roundtrip assertions to `crypter`

   Add verification logic to the `crypter` fuzz target by decrypting
   the resulting ciphertext from a successful call to `Encrypt()` or
   `EncryptSecret()` and verifying that the output matches the
   original plaintext.
   
   This ensures that the encryption/decryption cycle preserves data
   correctness, rather than just checking for execution crashes.

   ef0031651a
3. fuzz: cover `SetKey()` and `DecryptSecret()` in `crypter`

   Extend the `crypter` fuzz target to cover `CCrypter::SetKey()` and
   `DecryptSecret()`.

   38c6ca7d75
4. DrahtBot added the label Fuzzing on Dec 19, 2025
5. 
   DrahtBot commented at 8:53 am on December 19, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34111.

   Reviews

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   Concept NACK	brunoerg

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6. 
   brunoerg commented at 6:08 pm on December 22, 2025: contributor

   Concept NACK

   We already have 100% coverage for wallet/crypter, see: https://maflcko.github.io/b-c-cov/fuzz.coverage/src/wallet/crypter.cpp.gcov.html.

   Also, we don’t need to cover DecryptSecret directly if this is already exercised by DecryptKey, same idea for SetKey. For reference, see #32118 which fixed this target and removed a direct call to DecryptSecret.

7. 
   Chand-ra commented at 6:13 am on December 23, 2025: none

   Concept NACK

   We already have 100% coverage for wallet/crypter, see: https://maflcko.github.io/b-c-cov/fuzz.coverage/src/wallet/crypter.cpp.gcov.html.

   Also, we don’t need to cover DecryptSecret directly if this is already exercised by DecryptKey, same idea for SetKey. For reference, see #32118 which fixed this target and removed a direct call to DecryptSecret.

   Makes sense. But shouldn’t making the encryption tests roundtrip better help in future regression catching?

   Closing the PR for now.

8. Chand-ra closed this on Dec 23, 2025

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Contributors
Chand-ra DrahtBot brunoerg

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Fuzzing