[alinpahontu2912]

Perform a double pass of the encrypted content of a zip entry created with WinZip AES Encryption. The first pass should only calculate the HMAC and validate it to ensure the content hasn't been tampered with while the second pass should perform the decryption and decompression of the content.

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[Copilot]

🟡 Changes recommended

Ciphertext modified between verification and decryption can produce altered plaintext without an authentication failure.

1 open finding

• Decrypts mutable ciphertext after discarding its HMAC · New

What changed in this PR

Moves WinZip AES authentication ahead of decryption and decompression in ZIP entry reads.

Changes:

• Adds an upfront HMAC verification pass followed by a rewind.
• Defers update-mode state changes until content loads successfully.
• Adds authentication-failure, truncation, and rewind tests.

File	Description
src/​libraries/​System.IO.Compression/​tests/​WinZipAesStreamConformanceTests.cs	Tests verification, rewind, and truncated input.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipCryptoStream.cs	Uses the shared truncated-header resource.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipArchiveEntry.cs	Defers update-mode state and encryption setup.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipArchiveEntry.Async.cs	Adjusts asynchronous loading and key assignment.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​WinZipAesStream.cs	Authenticates ciphertext before returning a decryption stream.
src/​libraries/​System.IO.Compression/​src/​Resources/​Strings.resx	Generalizes the truncated-header resource name.
src/​libraries/​System.IO.Compression.ZipFile/​tests/​ZipFile.Encryption.cs	Tests authentication failures when opening entries.

🧠 Review effort: Balanced

[Copilot]

🟡 Changes recommended

Failed write-only opens can discard existing encrypted content, and the empty-payload test does not exercise zero ciphertext.

2 open findings

• Write-only Browser opens discard content before key setup succeeds · New
• Decrypts mutable ciphertext after discarding its HMAC

Previously missed (1)

In code that hasn't changed since last review

Empty-entry fixture writes a UTF-8 BOM and misses zero-ciphertext validation

src/​libraries/​System.IO.Compression.ZipFile/​tests/​ZipFile.Encryption.cs:382

This fixture is not empty: CreateArchiveWithEntries uses StreamWriter with Encoding.UTF8, which writes a three-byte byte-order mark even for an empty string. The test therefore misses authentication failure with zero ciphertext, including the remaining == 0 verification path. Create and close the entry stream without a StreamWriter so that path is covered in both synchronous and asynchronous cases.

🧠 Review effort: Balanced

[GrabYourPitchforks]

Placing hold pending offline discussion.