DES encryption and cipher modes

Question

I need to encrypt an ISO 8583 message... the problem here is that the message is longer than the key. I need some one help me how to encrypt this string.

For example: I have 300 chars in my string; should I encrypt each 16 chars alone then concat them, since my master key length is 16 bytes?

I appreciate your help...

---

ISO 8583-1:2003 Financial transaction card originated messages -- Interchange message specifications -- Part 1: Messages, data elements and code values.

Answer 1

DES is a block cipher, and block ciphers have different modes of operation.

The mode you mentioned is known as ECB (Electronic Codebook), and is not very secure (actually, neither is DES, but more on that later).

I'd suggest you use CBC or some other mode.

You can read about block cipher modes of operation here: Block cipher modes of operation

As for the cipher itself, I'd suggest you avoid using DES if this is at all possible. DES is extremely easy to crack nowadays. Please use AES, or at least 3DES if AES is not available.

EDIT: In response to the updated question, yes, you would need to pad the last block if the plaintext size is not a multiple of the block size.

Answer 2

There are many different modes of operation for a block cipher. If you just need to applay ECB to your plain text, just split the plain text into equally sized blocks of size 8 bytes (DES block size) and encrypt each separately.
Depending on what you want to achieve, you could also use

• ECB which encrypts block wise with each block being independent from all other (previous) blocks. Drawback: known plain text attacks can reveal patterns in your cipher text because the same plain text will always be encrypted to the same cipher text
• CBC here you have an initialization vector and each blocks depends on all previously encrypted blocks. This is why you need an IV for the first block.
• CFB this is an interesting one because it allows you to turn your block cipher into a stream cipher, which might be useful if you want to encrypt a video stream or whatever (similarly for OFB which basically generates a key stream)
• CTS cipher text stealing might be of use if you want to encrypt data but want to avoid padding. A usage example might be to encrypt a blob in your database, which you cannot resize after it has been written to the DB.

There are still many more modes, but these are the most commonly used ones (imho).
As others have pointed out visit Wikipedia for all the details.

Update:
As for the padding, you have different possibilities. I'd recommend to use the ANSI X.923 standard which basically requires you to pad the last buffer with zeroes and append a counter in the last byte which gives you the number of valid bytes in the last block. The same idea is used in ISO10126 but this time padding is done with random bytes. Note that you can avoid padding at all when using CTS.

Maybe ask yourself if it's actually easier to use a crypto library to do the job for you.
If you're using C++ go for Crypto++ (not so straightforward, but consistent c++ style), Java and .NET have built in crypto providers. If you want to use plain C i can recommend libTomCrypt (very easy to use).