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Truncate the per-object crypt key to min(n/8+5, 16) bytes - #94
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PDF 32000-1:2008 Algorithm 1 step e takes the first n/8+5 bytes of the MD5 (at most 16) as the object key. cryptKey returned the whole digest, which is right only for a 128-bit key. Every 40-bit RC4 file therefore opened (the password check uses the document key, which was correct) and then panicked with "zlib: invalid header" on its first stream. Two fixtures, the same page encrypted by pdfcpu with RC4 at 40 and 128 bits and an empty user password: the 40-bit one failed before this change and both read after it.
…lter, which this reader does not accept
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Summary
cryptKeyreturned the whole 16-byte MD5 as the per-object key. PDF 32000-1:2008 Algorithm 1 step e takes the firstn/8+5bytes, at most 16. That is 16 for a 128-bit key, so those files read, and 10 for a 40-bit key, so every 40-bit RC4 file opened (the document key and the/Ucheck were right) and then panicked withzlib: invalid headeron its first stream./V 1 /R 2) and an empty user password, and a test that fails before the change and passes after it.TestCryptKeyDeterministicAndSaltAwareasserted a 16-byte key for a 6-byte document key, which encoded the defect; it now asserts 11, and 16 for a 16-byte key.Measured on 473 real ASX announcements produced by Adlib (40-bit RC4, empty user password): none read before this change, 20 of 20 sampled read in full after it, matching
pdftotext.Test plan
go test ./green on this branchTestReads40BitRC4WithEmptyUserPasswordfails on master, passes here