Question

The iPhone supports the following encryption algorithms

enum {
    kCCAlgorithmAES128 = 0,
    kCCAlgorithmDES,            
    kCCAlgorithm3DES,           
    kCCAlgorithmCAST,           
    kCCAlgorithmRC4,
    kCCAlgorithmRC2 
};

I want to use only symmetric algorithm since asymmetric encryptions requires more computation overhead.

So I want to know which of the ones listed is the best algorithm and also what is the key-length in order to avoid excessive computation overhead.

Answer 1

Key length

Bruce Schneier wrote back in 1999:

Longer key lengths are better, but
only up to a point. AES will have
128-bit, 192-bit, and 256-bit key
lengths. This is far longer than
needed for the foreseeable future. In
fact, we cannot even imagine a world
where 256-bit brute force searches are
possible. It requires some fundamental
breakthroughs in physics and our
understanding of the universe. For
public-key cryptography, 2048-bit keys
have same sort of property; longer is
meaningless.

Block ciphers

AES

It’s the current standard encryption algorithm. It’s considered to be safe by most people. That’s what you should be using if you haven’t got a very deep knowledge in cryptography.

DES

DES is the predecessor of AES and is considered broken because of its short key length.

3DES

Is a variation of DES with a longer key length. It’s still in use but there are some known attacks. Still it’s not yet broken.

RC2

It’s considered to be weak.

Stream ciphers

RC4

It has some known vulnerabilities but is still used today, for example in SSL. I recommend not to use it in new products.

Conclusion

Use either RC4 or AES, depending if you need a stream or a block cipher.