eexec: PostScript charstring encryption and decryption routines

PostScript Type 1 fonts make use of two types of encryption: charstring encryption and eexec encryption. Charstring encryption is used for the charstrings themselves, while eexec is used to encrypt larger sections of the font program, such as the Private and CharStrings dictionaries. Despite the different names, the algorithm is the same, although eexec encryption uses a fixed initial key R=55665.

The algorithm uses cipher feedback, meaning that the ciphertext is used to modify the key. Because of this, the routines in this module return the new key at the end of the operation.

fontTools.misc.eexec.deHexString(h)

fontTools.misc.eexec.decrypt(cipherstring, R)

Decrypts a string using the Type 1 encryption algorithm.

Parameters:

• cipherstring – String of ciphertext.

• R – Initial key.

Returns:

Plaintext string. R: Output key for subsequent decryptions.

Return type:

decryptedStr

Examples:

>>> testStr = b"\0\0asdadads asds\265"
>>> decryptedStr, R = decrypt(testStr, 12321)
>>> decryptedStr == b'0d\nh\x15\xe8\xc4\xb2\x15\x1d\x108\x1a<6\xa1'
True
>>> R == 36142
True

fontTools.misc.eexec.encrypt(plainstring, R)

Encrypts a string using the Type 1 encryption algorithm.

Note that the algorithm as described in the Type 1 specification requires the plaintext to be prefixed with a number of random bytes. (For eexec the number of random bytes is set to 4.) This routine does not add the random prefix to its input.

Parameters:

• plainstring – String of plaintext.

• R – Initial key.

Returns:

Ciphertext string. R: Output key for subsequent encryptions.

Return type:

cipherstring

Examples:

>>> testStr = b"\0\0asdadads asds\265"
>>> decryptedStr, R = decrypt(testStr, 12321)
>>> decryptedStr == b'0d\nh\x15\xe8\xc4\xb2\x15\x1d\x108\x1a<6\xa1'
True
>>> R == 36142
True

>>> testStr = b'0d\nh\x15\xe8\xc4\xb2\x15\x1d\x108\x1a<6\xa1'
>>> encryptedStr, R = encrypt(testStr, 12321)
>>> encryptedStr == b"\0\0asdadads asds\265"
True
>>> R == 36142
True

fontTools.misc.eexec.hexString(s)