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fix: correct error encoding for 4th byte in Base64 decode validation #6124

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fix: correct error encoding for 4th byte in Base64 decode validation #6124

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@dizer-ti dizer-ti commented Nov 17, 2025

In Base64 decode validation, bytes 'a', 'b', and 'c' properly shift the error parameter with shl(248, ...) before storing, but byte 'd' was missing this operation. This caused incorrect ABI encoding when the 4th byte in a chunk was invalid.

Changed line 211 from:
mstore(4, add(d, 43))
to:
mstore(4, shl(248, add(d, 43)))

@dizer-ti dizer-ti requested a review from a team as a code owner November 17, 2025 17:51
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Latest commit: ff972aa

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coderabbitai bot commented Nov 17, 2025

Walkthrough

The change modifies the Base64 decoding logic in contracts/utils/Base64.sol specifically for 4-byte input chunk processing. The fourth decoded byte is now left-shifted by 248 bits before being stored in the 32-byte memory word at offset 4. This repositions the decoded value from the least significant portion to the most significant byte of the word. The surrounding control flow and error handling logic remain unaffected.

Possibly related PRs

  • Base64.decode #5765: Directly modifies the same Base64 decode assembly logic for 4-byte chunk handling.

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✅ Passed checks (3 passed)
Check name Status Explanation
Title check ✅ Passed The title accurately describes the main change: correcting error encoding for the 4th byte in Base64 decode validation, which is the primary focus of this bugfix.
Description check ✅ Passed The description is directly related to the changeset, explaining the bug (missing shl(248, ...) on byte 'd'), the specific line changed, and the correct fix applied.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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🔇 Additional comments (1)
contracts/utils/Base64.sol (1)

207-213: LGTM! Critical bug fix for correct ABI encoding.

This change correctly applies shl(248, ...) to byte d, making it consistent with the error encoding for bytes a, b, and c (lines 190, 197, 204). The shift positions the invalid character in the most significant byte of the 32-byte word, which is required for proper ABI encoding of the bytes1 parameter in the InvalidBase64Char(bytes1) error. Without this shift, the error data would be malformed when the 4th byte of a chunk was invalid.

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