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Copilot review overview
🟡 Changes recommended
Crypt-filter selection remains incorrect for omitted or non-StdCF stream filters.
Review effort: Balanced
Findings: 2
Open (2)
What changed in this PR
Adds crypt-filter key-length fallback for V4 PDFs missing top-level /Length.
Changes:
- Derives bit length from crypt-filter
/Length. - Applies fallback in Python and Perl extractors.
| File | Description |
|---|---|
run/pdf2john.py |
Adds crypt-filter length resolution. |
run/lib/PDF.pm |
Propagates StdCF length into hash generation. |
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Address Copilot review comments on PR openwall#6043: - The PDF spec says /StmF and /StrF default to /Identity (no encryption), not /StdCF. The previous code assumed /StdCF when /StmF was absent, which could read an unrelated filter entry and emit a wrong key length. - The Perl path hard-coded 'StdCF' instead of resolving the actual filter name from $$encrypt{StmF}, making the two extractors inconsistent. Both pdf2john.py and PDF.pm now iterate over StmF then StrF, skip any /Identity entry, and use the first filter that carries a /Length value. The CF /Length (bytes) → bits multiplication is unchanged. Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
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PDF 1.4+ (V4) allows the encryption dictionary to omit the top-level /Length entry when the key length is specified only inside the applicable crypt filter (CF → StmF filter → /Length). The PDF spec (§3.5) states that the top-level /Length is optional for V2 and V3; in practice some generators (e.g. PDFsharp 6.2.0 / Aspose) also omit it for V4 and rely only on the CF /Length.
Both extractors incorrectly defaulted to 40 bits in that case, causing JtR to treat the document as 40-bit RC4 and fail to crack even when the true algorithm is AES-128.
Fix: when /Length is absent at the top level and V==4, read the key length from the crypt filter identified by /StmF (defaulting to /StdCF). The CF /Length is in bytes; multiply by 8 to get bits.
pdf2john.py: derived via encrypt_dict → /CF → /StmF filter → /Length*8. PDF.pm (pdf2john.pl): fill in $$encrypt{Length} at the point where the CF is already validated, so all downstream uses (hash printf and RC4 key derivation) automatically pick up the correct value.
Fixes #6033