Fix decryption of literal strings

[onyck]

What?

This PR fixes the decryption of literal strings that contain escape sequences.

Fixes #122.

Why?

The current code decrypts literal strings without processing escape sequences. If an encrypted literal string does contain escape sequences, the decrypted value is nonsense and can cause issues such as #122.

Encrypted literal strings may contain escape sequences according to section 7.3.4.2 of the PDF 1.7 specification, so escape sequences should be resolved prior to decryption.

How?

This PR uses the existing PDFString.asBytes() method to convert the literal string to a byte array while handling escape sequences. The byte array is then decrypted using the cipher transformer.

Testing?

Added and ran unit tests.

New Dependencies?

No

Screenshots

N/A

Suggested Reading?

Yes

Anything Else?

N/A

Checklist

• I read CONTRIBUTING.md.
• I read MAINTAINERSHIP.md#pull-requests.
• I added/updated unit tests for my changes.
• I added/updated integration tests for my changes.
• I ran the integration tests.
• I tested my changes in Node, Deno, and the browser.
• I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• I added/updated doc comments for any new/modified public APIs.
• My changes work for both new and existing PDF files.
• I ran the linter on my changes.

[d1c0n]

@Sharcoux Do you think it's possible to merge this? I've encountered the problem mentioned in the issue #122 plenty of times. I've tested this PR and it this seems to work ok. I'm not really familiar with the PDF spec or the encryption flow though, I have no idea if this can introduce any problems.

[Sharcoux]

Perfect @onyck
Thanks a lot!

[Sharcoux]

Released in v2.6.0