DES encryption/decryption

Question

I'm using a DES algorithm I found on the web. It works fine but I have a problem. As you know, DES encrypts/decrypts blocks of 64 bytes each. But what happens if in a big file the last block doesn't end at a 64 byte block boundary? I know, there would be errors.

I'm using the following code:

Des d1;
char *plaintext=new char[64];
char *chyphertext=new char[64];
h.open("requisiti.txt",ios::in|ios::binary);.
k.open("requisiti2.txt",ios::out|ios::binary);
while(!h.eof())
{
    h.read(plaintext,64);
    chyphertext=d1.Encrypt(plaintext);
    //decryption is the same.just change Encrypt to Decrypt
    k.write(chyphertext,64);
}
h.close();
k.close();
remove("requisiti.txt");
rename("requisiti2.txt","requisiti.txt");

So I need a solution like "padding", but I don't know a simple algorithm for it. Please help me to encrypt/decrypt file in a good way.

Answer 1

First, I'd like to point that DES works on 64bits chunks (making it 8bytes, not 64), as you can see in http://en.wikipedia.org/wiki/Data_Encryption_Standard (check data block size).

Now you're looking for some padding (and unpadding when deciphering). You can look at http://en.wikipedia.org/wiki/Padding_(cryptography)

I personnally like PKCS#7 because it's easy and usually adds a little overhead compared to standard size.

For encryption:

• check the size of the chunk you just read from file
• if it's 64bits, add a new chunk [8,8,8,...8], otherwise, pad it with the number of missing bytes (see example below)
• encrypt
• note that LAST packet is always containing padding with that algorithm (worst case is 8 bytes of padding)

Example:

• read 0a 0b 0c, missing 5 bytes to fit in 8 bytes
• padded packet :0a 0b 0c 05 05 05 05 05

For decryption :

• read packet
• decrypt
• if it's the last packet, check value of the last byte (say it's n)
• remove n bytes at the end of your packet

Hope this makes it more clear and helps you

EDIT

If your input file is pure text, you can pad with 0, if it's binary (and it must be since you're opening it as binary), PKCS#7 is better

Think about a file created like that : dd if=/dev/zero of=temp.zero count=100 a few of hundred bytes of zeros, what is padding and what ain't ?

Implementation is really easy :

• think memset
• don't forget to add last chunk if ile is a multiple of 8

By the way, DES is nowadays seriously broken, you should think about using a decent cipher if concerned with security (thinking AES at least, check http://en.wikipedia.org/wiki/Data_Encryption_Standard#Replacement_algorithms )

Answer 2

First of all: never use eof() to check whether the end of file is reached since it doesn't predict the end of file.

while(h.read(plaintext,64))
{
    if (std::h.gcount() < 64)   // gcount returns the number of characters extracted by the last unformatted input operation.
        for (int i=std::h.gcount(); i<64; i++)
            paintext[i] = 0; // pad the last block         
    chyphertext=d1.Encrypt(plaintext);
    //decryption is the same.just change Encrypt to Decrypt
    k.write(chyphertext,64);
}

Answer 3

I'm not sure what you are using DES for, but you should really use something else if you are actually trying to protect the data you are encrypting. DES is not secure anymore.

Also, I would imagine that a good library would do the padding for you.

Answer 4

First of all do not use DES! DES is broken and can be brute forced quite fast. Secondly you are using ECB mode you can read on wiki why you should avoid this one. Your data can be tampered with and you will not know about this - use AE mode like GCM. Like someone mentioned earlier DES have 64 bits not bytes block size which is 8 bytes.