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Fix PDF decryption of ancient 40-bit RC4 with R=2

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NoDRM 2023-08-02 16:55:41 +02:00
parent b9bad26d4b
commit 7f6dd84389
2 changed files with 17 additions and 6 deletions
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2
CHANGELOG.md
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@ -99,4 +99,4 @@ This is v10.0.9, a release candidate for v10.1.0. I don't expect there to be maj
## Fixes on master (not yet released):
- (none)
- Fix a bug where decrypting a 40-bit RC4 pdf with R=2 didn't work.

21
DeDRM_plugin/ineptpdf.py
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@ -1369,11 +1369,12 @@ class PDFDocument(object):
keylen = len(key)
iv = bytes([0x00]*keylen)
aes = AES.new(key, AES.MODE_CBC, iv)
if not encrypt:
plaintext = AES.new(key,AES.MODE_CBC,iv, True).decrypt(data)
plaintext = aes.decrypt(data)
return plaintext
else:
aes = AES.new(key, AES.MODE_CBC, iv, False)
new_data = bytes(data * repetitions)
crypt = aes.encrypt(new_data)
return crypt
@ -1478,14 +1479,23 @@ class PDFDocument(object):
EncMetadata = b'True'
if (EncMetadata == ('False' or 'false') or V < 4) and R >= 4:
hash.update(codecs.decode(b'ffffffff','hex'))
# Finish hash:
hash = hash.digest()
if R >= 3:
# 8
for _ in range(50):
hash = hashlib.md5(hash.digest()[:length//8])
key = hash.digest()[:length//8]
hash = hashlib.md5(hash[:length//8]).digest()
if R == 2:
# R=2 only uses first five bytes.
key = hash[:5]
else:
key = hash[:length//8]
if R == 2:
# Algorithm 3.4
u1 = ARC4.new(key).decrypt(password)
u1 = ARC4.new(key).decrypt(self.PASSWORD_PADDING)
elif R >= 3:
# Algorithm 3.5
hash = hashlib.md5(self.PASSWORD_PADDING) # 2
@ -1498,6 +1508,7 @@ class PDFDocument(object):
k = b''.join(bytes([c ^ i]) for c in key )
x = ARC4.new(k).decrypt(x)
u1 = x+x # 32bytes total
if R == 2:
is_authenticated = (u1 == U)
else: