Correctly implement fuzzers for new encryption streams

[alinpahontu2912]

Correctly implement fuzzers fro the new encryption streams: zipcryptostream and winzipaesstream

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-meta
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR updates the fuzzers for the new ZIP encryption streams to exercise the actual ZipArchive encryption/decryption paths (ZipCrypto and WinZip AES) rather than constructing the internal crypto streams via reflection.

Changes:

• Replace reflection-based stream construction with ZipArchive.CreateEntry(..., password, ZipEncryptionMethod) + Open/OpenAsync(password) round-trip workflows.
• Add validation that the entry is encrypted and the expected encryption method is recorded, and verify plaintext round-trips correctly (sync + async).
• Add a “wrong password” scenario intended to ensure failures are handled as InvalidDataException (but it currently doesn’t assert failure if no exception is thrown).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/ZipCryptoStreamFuzzer.cs	Moves from reflected ZipCryptoStream.Create to ZipArchive-based encrypt/decrypt round-trip for ZipCrypto (sync + async).
src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/WinZipAesStreamFuzzer.cs	Moves from reflected WinZipAesStream.Create to ZipArchive-based encrypt/decrypt round-trip for AES128/192/256 (sync + async).

Comments suppressed due to low confidence (2)

src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/ZipCryptoStreamFuzzer.cs:101

• The “wrong password must fail” check currently passes even if decryption succeeds without throwing. Use Assert.Throws so the fuzzer reliably flags regressions where a wrong password is accepted.

        // Decrypting with a wrong password must fail cleanly with InvalidDataException, never crash.
        try
        {
            using Stream stream = readEntry.Open("wrong-password".AsSpan());
            stream.CopyTo(Stream.Null);
        }
        catch (InvalidDataException)
        {
            // Expected: the header password verifier rejects the wrong key.
        }

src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/WinZipAesStreamFuzzer.cs:113

• The “wrong password must fail” check currently passes even if decryption succeeds without throwing. Use Assert.Throws so the fuzzer reliably flags regressions where a wrong password is accepted.

        // Decrypting with a wrong password must fail cleanly with InvalidDataException, never crash.
        try
        {
            using Stream stream = readEntry.Open("wrong-password".AsSpan());
            stream.CopyTo(Stream.Null);
        }
        catch (InvalidDataException)
        {
            // Expected: the AES password verifier / HMAC rejects the wrong key.
        }

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[alinpahontu2912]

@MihuBot fuzz WinZipAesStreamFuzzer

[MihuBot]

Ran the fuzzer(s) successfully. Code coverage reports:

• WinZipAesStreamFuzzer

[alinpahontu2912]

@MihuBot fuzz ZipCryptoStreamFuzzer

[MihuBot]

Ran the fuzzer(s) successfully. Code coverage reports:

• ZipCryptoStreamFuzzer

[alinpahontu2912]

@MihuBot fuzz ZipCryptoStreamFuzzer

[alinpahontu2912]

@MihuBot fuzz WinZipAesStreamFuzzer

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[alinpahontu2912]

@MihuBot fuzz ZipCryptoStreamFuzzer

[MihuBot]

Ran the fuzzer(s) successfully. Code coverage reports:

• ZipCryptoStreamFuzzer

[MihaZupan]

Thanks, this looks a lot better!

[alinpahontu2912]

@MihuBot fuzz ZipEncryptionStreamFuzzer

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[MihuBot]

Ran the fuzzer(s) successfully. Code coverage reports:

• ZipEncryptionStreamFuzzer

[MihaZupan]

Suggested change

	Stream wrongStream = async
	? await readEntry.OpenAsync(wrongPassword.AsSpan())
	: readEntry.Open(wrongPassword.AsSpan());
	try
	{
	if (async)
	{
	await wrongStream.CopyToAsync(Stream.Null);
	}
	else
	{
	wrongStream.CopyTo(Stream.Null);
	}
	}
	finally
	{
	if (async)
	{
	await wrongStream.DisposeAsync();
	}
	else
	{
	wrongStream.Dispose();
	}
	}
	if (async)
	{
	await using Stream wrongStream = await readEntry.OpenAsync(wrongPassword.AsSpan());
	await wrongStream.CopyToAsync(Stream.Null);
	}
	else
	{
	using Stream wrongStream = readEntry.Open(wrongPassword.AsSpan());
	wrongStream.CopyTo(Stream.Null);
	}

[MihaZupan]

Suggested change

	// AES is authenticated: the password verifier and HMAC make accepting a wrong key
	// cryptographically infeasible, so a wrong password must fail with InvalidDataException.
	// The AES variant of ZIP encryption uses a form of HMAC that makes accidentally accepting the wrong key
	// statistically unlikely, so a wrong password must fail with InvalidDataException.

"AES is authenticated" and "cryptographically infeasible" are too strong statements here :)

[MihaZupan]

Just making the note that you may see less edge-case coverage because your decryption logic is only exercising well-formed inputs produced by our own library.

[MihaZupan]

Suggested change

	if (bytes.IsEmpty)
	{
	return;
	}
	// Use the first byte to select the encryption method so all variants get exercised.
	ZipEncryptionMethod method = s_methods[bytes[0] % s_methods.Length];
	ReadOnlySpan<byte> payload = bytes.Slice(1);
	if (bytes.Length < 2)
	{
	return;
	}
	// Use the first byte to select the encryption method so all variants get exercised.
	ZipEncryptionMethod method = s_methods[bytes[0] % s_methods.Length];
	ReadOnlySpan<byte> payload = bytes.Slice(2); // Slice 2 to keep MemoryMarshal.Cast<byte, char> below aligned