Validate ciphertext length in AES._CBC.decrypt#438
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Nadav0077 wants to merge 1 commit intoapple:mainfrom
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Validate ciphertext length in AES._CBC.decrypt#438Nadav0077 wants to merge 1 commit intoapple:mainfrom
Nadav0077 wants to merge 1 commit intoapple:mainfrom
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The CBC decrypt path silently accepted ciphertext whose length was not a multiple of the 16-byte AES block size. The trailing partial block was passed to Block.init(blockBytes:), which PKCS#7-pads short inputs internally. The pad bytes were then treated as ciphertext, producing plaintext bytes that have no relationship to the caller's input. The encrypt path already rejects mis-aligned input in noPadding mode; decrypt now performs the same length check unconditionally, since any valid CBC ciphertext is a multiple of the block size.
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Validate ciphertext length in
AES._CBC.decryptChecklist
If you've made changes to
gybfiles./scripts/generate_boilerplate_files_with_gyb.shand included updated generated files in a commit of this pull requestMotivation:
I noticed
AES._CBC.decrypthappily accepts ciphertext whose length isn't a multiple of the 16-byte block size, which it really shouldn't, any valid CBC ciphertext is block-aligned by construction.What happens today is: the decrypt loop slices the ciphertext into 16-byte chunks, but the trailing short chunk gets passed to
Block.init(blockBytes:), which silently PKCS#7-pads short inputs to a full block. Those pad bytes then get treated as if they were ciphertext, decrypted, and returned as plaintext. So you don't get an error, you just get garbage bytes appended to the result. The encrypt side already rejects this innoPaddingmode (incorrectParameterSize); the decrypt side was inconsistent.Modifications:
Added a single guard at the top of
AES._CBC.decryptthat throwsCryptoKitError.incorrectParameterSizeifciphertext.count % blockSize != 0. Same error the encrypt path already uses for the symmetric case.Also added a regression test (
testDecryptRejectsNonBlockAlignedCiphertext) that feeds a 17-byte ciphertext in bothnoPadding=falseandnoPadding=truemodes and asserts the right error is thrown.Result:
Mis-aligned ciphertext now throws
CryptoKitError.incorrectParameterSizeinstead of silently returning garbage. Behavior on valid (block-aligned) ciphertext is unchanged, verified by running the existing Wycheproof CBC vectors plus the rest ofCBCTestson Swift 6.3.1 (Windows, x86_64-unknown-windows-msvc), all 7 tests pass.