Fix CompressionType for WinZip AES encrypted ZIP entries

[Copilot]

WinZip AES encrypted entries report CompressionType.Unknown instead of the actual compression method (Deflate, LZMA, etc.). This is because WinZip AES stores the compression method in extra data at offset 5, not in the standard header field (which contains marker 0x63).

Changes

• ZipEntry.cs: Added GetActualCompressionMethod() to extract compression method from WinZip AES extra data when present
• Test updates: Changed tests that previously expected Unknown to assert the actual compression type

Technical Details

WinZip AES extra data structure (7+ bytes):

• Bytes 0-1: Vendor version
• Bytes 2-3: Vendor ID
• Byte 4: AES strength
• Bytes 5-6: Actual compression method ← extracted here

For non-WinZip AES entries, behavior is unchanged.

Example

// Before: WinZip AES encrypted entry
entry.CompressionType  // Returns: Unknown

// After: WinZip AES encrypted entry  
entry.CompressionType  // Returns: Deflate (or LZMA, etc.)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Compressiontype per entry when encrypted</issue_title>
<issue_description>Hello,

I made a encrypted archive with password and Deflate.
When i open the archive i see each entry.CompressionType = Unknown.
I open the same zip in 7zip it shows me the Deflate value.

is there a way to get this?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Compressiontype per entry when encrypted #1212

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[kiloconnect]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (4 files)

• src/SharpCompress/Common/Zip/ZipEntry.cs
• tests/SharpCompress.Test/Zip/ZipArchiveTests.cs
• tests/SharpCompress.Test/Zip/ZipReaderAsyncTests.cs
• tests/SharpCompress.Test/Zip/ZipReaderTests.cs

[Copilot]

Pull request overview

Updates ZIP entry metadata handling so WinZip AES–encrypted ZIP entries report their actual compression type (e.g., Deflate/LZMA) based on AES extra data, aligning Entry.CompressionType with how extraction already determines the real method.

Changes:

• Add WinZip AES extra-field parsing in ZipEntry to derive the actual compression method for CompressionType.
• Update existing WinZip AES reader tests to assert the real compression type instead of Unknown.
• Add archive-level tests asserting CompressionType for WinZip AES and PKWARE encrypted ZIPs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/SharpCompress/Common/Zip/ZipEntry.cs	Derives CompressionType from WinZip AES extra data when the header method is WinzipAes (0x63).
tests/SharpCompress.Test/Zip/ZipReaderTests.cs	Updates WinZip AES reader test expectations from Unknown to LZMA/Deflate.
tests/SharpCompress.Test/Zip/ZipReaderAsyncTests.cs	Async equivalents of the WinZip AES test expectation updates.
tests/SharpCompress.Test/Zip/ZipArchiveTests.cs	Adds new tests validating CompressionType for WinZip AES and PKWARE encrypted entries.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

GetActualCompressionMethod() returns ZipCompressionMethod.None when _filePart is null, which changes the previous behavior of reporting CompressionType.Unknown in that case (old code switched on _filePart?.Header.CompressionMethod). If a ZipEntry can ever be constructed without a ZipFilePart, consider returning ZipCompressionMethod.WinzipAes/None only when you can prove the method, otherwise keep the prior Unknown behavior (e.g., return a sentinel that maps to CompressionType.Unknown).

[Copilot]

For WinZip AES entries, this reads the compression method from the extra field but does not validate the extra data (length == 7, vendor version, vendor ID) and it uses FirstOrDefault rather than SingleOrDefault. This can make Entry.CompressionType disagree with extraction behavior, since ZipFilePart.CreateDecompressionStream validates these fields and will throw on malformed or duplicate AES extra data. To keep behavior consistent, mirror the same validation and selection logic here and fall back to Unknown when the AES metadata is invalid.

Suggested change

	// For WinZip AES, the actual compression method is stored in the extra data
	var aesExtraData = _filePart.Header.Extra.FirstOrDefault(x =>
	x.Type == ExtraDataType.WinZipAes
	);
	if (aesExtraData is null || aesExtraData.DataBytes.Length < MinimumWinZipAesExtraDataLength)
	{
	return ZipCompressionMethod.WinzipAes;
	}
	// The compression method is at offset 5 in the extra data
	return (ZipCompressionMethod)
	System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(
	aesExtraData.DataBytes.AsSpan(WinZipAesCompressionMethodOffset)
	);
	// For WinZip AES, the actual compression method is stored in the extra data.
	// Mirror the validation logic used during extraction so that behavior is consistent.
	var aesExtraDataList = _filePart.Header.Extra
	.Where(x => x.Type == ExtraDataType.WinZipAes)
	.ToList();
	// If there is not exactly one AES extra data record, treat the metadata as invalid.
	if (aesExtraDataList.Count != 1)
	{
	// We do not know the actual compression method; keep WinzipAes so higher layers
	// can treat the compression type as unknown.
	return ZipCompressionMethod.WinzipAes;
	}
	var aesExtraData = aesExtraDataList[0];
	var data = aesExtraData.DataBytes;
	// WinZip AES extra data layout (7 bytes total):
	// 0-1: Vendor version (1 or 2)
	// 2-3: Vendor ID ("AE" -> 0x4541 little-endian)
	// 4 : AES strength (ignored here)
	// 5-6: Actual compression method
	if (data is null || data.Length != MinimumWinZipAesExtraDataLength)
	{
	return ZipCompressionMethod.WinzipAes;
	}
	var span = data.AsSpan();
	ushort vendorVersion =
	System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(span.Slice(0, 2));
	ushort vendorId =
	System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(span.Slice(2, 2));
	// Only accept known vendor versions and the WinZip AES vendor ID ("AE").
	bool isSupportedVendorVersion = vendorVersion == 1 || vendorVersion == 2;
	const ushort WinZipAesVendorId = 0x4541; // "AE" in little-endian
	if (!isSupportedVendorVersion || vendorId != WinZipAesVendorId)
	{
	return ZipCompressionMethod.WinzipAes;
	}
	// The actual compression method is at offset 5 in the extra data.
	ushort method =
	System.Buffers.Binary.BinaryPrimitives.ReadUInt16LittleEndian(
	span.Slice(WinZipAesCompressionMethodOffset, sizeof(ushort))
	);
	return (ZipCompressionMethod)method;