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FlateStream: Verify Adler-32 checksum

Jason Crain requested to merge jcrain/poppler:adler into master

A variant of the PDFex CBC gadget attack modifies Deflate compressed data to assist with the creation of CBC gadgets. The attacker can use CBC gadgets to prefix a URL to an encrypted plaintext, to create a hyperlink which will leak decrypted data when clicked. However, modifying the deflate data results in several random bytes in the decompressed data, so we can limit this attack by verifying the Deflate data's Adler-32 checksum.

See https://pdf-insecurity.org/encryption/encryption.html

Issue #814 (closed)

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