Re: Blowfish Encryption

On Tue, Jun 08, 2010 at 09:48:43AM -0400, Adam Richardson wrote: > Hi Paul, > > > > If one has multiple samples of encrypted emails, it's likely that the > several > > of the samples will end using the same cipher text, as many people end > their > > emails with a consistent signature. This repeated cipher text improves > the > > ability of those trying to attack (decrypt the message.) Hence, most > > professionals recommend avoiding ECB mode. > > Well, bcrypt mentions CBC in the (brief) documentation, but the C code > contains a couple of tables, one containing probably 1024 32-bit long > integer values. Based on your description, that sounds like ECB, right? > > > I provided the description of ECB merely to provide a general theoretical > understanding of ECB (i.e., there is a one-to-one correspondence between any > chunk of plaintext and any resulting ciphertext that is always the same.) > However, in practical terms, tables you see in code usually have to do with > S-boxes, or some other type of the implementation. So, seeing tables in the > implementation code gives you no more info as to the mode. Makes sense. In the code, they're labeled that way (e.g. S[4][256]). > > > > Bcrypt doesn't all the specification of what mode enc/dec is done in. > That is, I can't specify to the program ECB, CBC or other. > > > Sounds likely, given the goal for portability. > > > > Also, according to the docs for bcrypt, it hashes your password out to > he maximum size for the cipher (448 bytes?). This sounds like an > implementation-specific decision which may not be echoed by PHP's mcrypt > functions. Does that sound reasonable? > > > > You'll have to match the hashing process to generate your key. > > > > > > Now, looking at your PHP code, I see that it appears your mixing and > matching > > some of the families of calls in ways that might lead to unexpected > results. > > Try the below: > > > > $ciphertext = mcrypt_encrypt( > > $cipher = MCRYPT_BLOWFISH, > > $key, > > $plaintext, > > $mode = 'cbc', // I just tossed this in as an example, but you should > match > > the mode bcrypt is using > > $iv = 'use only once, sometimes a count, or a date' // needed for > > decryption, too, although it doesn't have to remain a secret. > > ); > > Another point: my code above is actually from a post by someone else on > this list. Now, the iv above is based on a random number. If I encrypt > the file on Monday, and then attempt to decrypt it on Tuesday using a > different (random-number-based) iv, will the file decrypt properly? > > > NO, if you're using a mode other than ECB. If you're using CBC or some other > mode that utilizes the IV, the same IV must be used for encryption AND > decryption. However, when using ECB, the IV isn't used, so it wouldn't matter > (if you pass in an IV, it's just ignored.) > > The IV is used to make sure no two plaintexts will be represented by the same > cipher texts, and must be shared between those wishing to encrypt and decrypt > the message. However, it doesn't have to kept secret. > > > > Paul > > -- > Paul M. Foster > > > Sounds like you're making progress :) I'm busy today (off to the doctor for a > bum knee), but I'll probably look through bcrypt later this week just to better > understand its implementation (that is to say, sorry I don't have more > implementation details of that particular encryption scheme right now, but > maybe later.) Yeah, this is great. Thanks so much for your help. I really know very little about encryption. If you think of something else, feel free to comment. Paul -- Paul M. Foster