readme.txt

这个程序是VB编写的加密解密程序,希望对大家有所帮助.

Class to encrypt and decrypt a clear text with variable character length encoding. There is no relationship between byte boundaries and encrypted characters, ie an encrypted character may theoretically be anything from 1 to 255 bits long; however this depends on the 'quality' of the key. I am not a cryptoanalyst, but I have a hunch that the variable character length makes this code rather difficult if not impossible to break. One approach may be to scan the encryption for repeating bit-groups and guess the meaning of short groups occuring more frequently than others (like "the", "a", "an" and so forth), or for patterns occuring with a language-specific frequency of vowels and consonants. To forestall that, the encrypting engine uses an initial hash (also derived from the key) and XORs every character with that before encrypting, keeping the result of the XOR for the next character. The decrypting engine of course reverses that process.

Another approach to break this code could be an attempt to reconstruct the code-tree from the encrypted bit patterns. Even if that succeeds (and I don't know how), there is still the hash, although that could be broken with medium effort. The initial hash is anything from 0 to 255 and the seed for the randomizer is in the range of 20,000 thru 60,000 giving an average of about 5,000,000 and a maximum of roughly 15,000,000 initial states for hash and randomizer combined.

A third possibility is a brute force key permutation, decrypting with every conceivable key and checking the decrypted result against a dictionary for the presence of meaningful words.

And finally there is always the possibility to guess your password, so use passwords of several words and numbers (not first names and birthdays of course). 

The encryption engine now features an MD5 message digester which produces a key- and message-dependent binary signature. This signature may be included in the encrypted text to verify it's authenticity. Another advantage of prefixing a message with a signature is the additional scrambling of the encryted text through the previous encrypting of the unpredictable signature (see 'hash' above). This makes the encryption key- and text-dependent, that is, the encryption for a particular character is different when the text is different (apart from being different through hashing).

Speed (without debugging) is in excess of 40,000 bytes/sec for encrypting and more than 130,000 bps for decrypting (depending of course on your PC's clock frequency).