Question

I have a program that regularly appends small pieces (say 8 bytes) of sensitive data to a number of logfiles. I would like this data to be encrypted. I want the program to start automatically at boot time, so I don’t want to type a password at program start. I also don’t want it to store a password somewhere, since that would almost defeat the purpose of encryption.

For these reasons, it seems to me that public key encryption would be a good choice. The program knows my public key, but my private key is password protected somewhere else.

So far, so good. But when I try to use PyCrypto to RSA (or ElGamal)-encrypt a small 5-byte string, the output explodes to 128 bytes. My logfiles are large enough as it is… On the other hand, when I try a symmetric crypto, like Blowfish, the output string is just as large as the input string.

So, my question is: Is there a reasonably secure public key encryption algorithm where I can encrypt data 8 bytes at a time and don’t have it blow up? (I guess a factor of 2 would be OK). I think what I want is a public key stream cipher.

If there is not such a thing, I think I will just give up and use a symmetric crypto and give the password manually on startup.

Answer 1

Typically this is solved in the way that the program creates some (real) random numbers which are used as a secret key to a symmetric encryption algorithm.

In you program you have to do something like:

1. Generate some real random data (maybe use /dev/random) as a secret key.
2. Encrypt the secret key with the public key algorithm.
3. Use the secret key for some other symmetric algorithm.

To decrypt this,

1. Use the private key to decrypt the secret key.
2. Use the secret key and the symmetric algorithm to decrypt the data.

You might want to get some random data (e.g. >=256bit) for a ‘good’ key.