8000 Implement Type-1 decryption · matplotlib/matplotlib@604c8a4 · GitHub
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jkseppanjkseppan
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Implement Type-1 decryption
This is a prerequisite of subsetting.
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‎doc/users/next_whats_new/type1decrypt.rst

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Type1Font objects now decrypt the encrypted part
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------------------------------------------------
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Type 1 fonts have a large part of their code encrypted as an obsolete
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copy-protection measure. This part is now available decrypted as the
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``decrypted`` attribute of :class:`~matplotlib.type1font.Type1Font`.
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This decrypted data is not yet parsed, but this is a prerequisite for
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implementing subsetting.

‎lib/matplotlib/tests/test_type1font.py

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@@ -15,6 +15,8 @@ def test_Type1Font():
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assert font.parts[2] == rawdata[0x8985:0x8ba6]
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assert font.parts[1:] == slanted.parts[1:]
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assert font.parts[1:] == condensed.parts[1:]
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assert font.decrypted.startswith(b'dup\n< 8000 /span>/Private 18 dict dup begin')
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assert font.decrypted.endswith(b'mark currentfile closefile\n')
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differ = difflib.Differ()
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diff = list(differ.compare(
@@ -67,3 +69,11 @@ def test_overprecision():
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assert matrix == '0.001 0 0.000167 0.001 0 0'
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# and here we had -9.48090361795083
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assert angle == '-9.4809'
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def test_encrypt_decrypt_roundtrip():
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data = b'this is my plaintext \0\1\2\3'
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encrypted = t1f.Type1Font._encrypt(data, 'eexec')
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decrypted = t1f.Type1Font._decrypt(encrypted, 'eexec')
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assert encrypted != decrypted
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assert data == decrypted

‎lib/matplotlib/type1font.py

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@@ -46,10 +46,12 @@ class Type1Font:
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parts : tuple
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A 3-tuple of the cleartext part, the encrypted part, and the finale of
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zeros.
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decrypted : bytes
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The decrypted form of parts[1].
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prop : dict[str, Any]
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A dictionary of font properties.
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"""
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__slots__ = ('parts', 'prop')
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__slots__ = ('parts', 'decrypted', 'prop')
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def __init__(self, input):
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"""
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data = self._read(file)
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self.parts = self._split(data)
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self.decrypted = self._decrypt(self.parts[1], 'eexec')
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self._parse()
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def _read(self, file):
@@ -139,6 +142,57 @@ def _split(self, data):
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_token_re = re.compile(br'/{0,2}[^]\0\t\r\v\n ()<>{}/%[]+')
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_instring_re = re.compile(br'[()\\]')
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@staticmethod
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def _decrypt(ciphertext, key, ndiscard=4):
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"""
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Decrypt ciphertext using the Type-1 font algorithm
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The key argument can be an integer, or one of the strings
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'eexec' and 'charstring', which map to the key specified for the
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corresponding part of Type-1 fonts.
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The ndiscard argument should be an integer, usually 4.
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That number of bytes is discarded from the beginning of plaintext.
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"""
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key = {'eexec': 55665, 'charstring': 4330}.get(key, key)
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if not isinstance(key, int):
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raise ValueError(f'Invalid decryption key {key!r}')
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plaintext = bytearray(len(ciphertext))
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for i, byte in enumerate(ciphertext):
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plaintext[i] = byte ^ (key >> 8)
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key = ((key+byte) * 52845 + 22719) & 0xffff
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return bytes(plaintext[ndiscard:])
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@staticmethod
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def _encrypt(plaintext, key, ndiscard=4):
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"""
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Encrypt plaintext using the Type-1 font algorithm
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The key argument can be an integer, or one of the strings
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'eexec' and 'charstring', which map to the key specified for the
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corresponding part of Type-1 fonts.
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The ndiscard argument should be an integer, usually 4. That
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number of bytes is prepended to the plaintext before encryption.
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This function prepends NUL bytes for reproducibility, even though
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the original algorithm uses random bytes, presumably to avoid
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cryptanalysis.
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"""
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key = {'eexec': 55665, 'charstring': 4330}.get(key, key)
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if not isinstance(key, int):
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raise ValueError(f'Invalid encryption key {key!r}')
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ciphertext = bytearray(len(plaintext) + ndiscard)
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for i, byte in enumerate(b'\0' * ndiscard + plaintext):
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ciphertext[i] = byte ^ (key >> 8)
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key = ((key + ciphertext[i]) * 52845 + 22719) & 0xffff
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return bytes(ciphertext)
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@classmethod
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def _tokens(cls, text):
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"""

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