I'm taking up a cryptography class and this is for an assignment. We're asked to find the differences between AES/RSA in the different modes of operation vs the one timepad method.

  1. From wiki, I can see that in all the different modes of operation(Eg CBC), the ek(encryption) is done. However, not much is said about what kind of encryption is used. What difference will it make if I use AES or DES in the encryption part in the process? I get that RSA is better than DES/AES dues to assymetricity. Apart from this difference, I can't figure out how the using just AES instead of DES would make AES with CBC would make it better than DES with CBC.

  2. So, I read up and found this which says that the main reason why AES is better that DES is because of the key size. Is this the only difference?


1 Answer 1

  1. Modes of operation are generally supposed to be independent of the underlying block cipher. They generally have a proof of security showing that the security of a system using said mode reduces to the security of the block cipher. However some modes, such as CTR, don't work well with block ciphers of short length (aka, old ciphers) and can leak information. Popular modes of operation should work equally well with all modern block ciphers.

    You wouldn't use RSA in a mode of operation. Typically, RSA might be used to encrypt an AES key and AES would then be used in a mode of operation on the data.

  2. AES is more secure than DES. There are attacks that leverage weaknesses DES's inner workings, while AES stands strong. AES also uses a larger block size (128 vs 64 bits), and small block sizes themselves have inherent weaknesses. Also, DES should not be used by itself, if used it should be used as TDES (triple-iterated DES) and TDES is very slow in software. (That's not a security feature, but it is a pragmatic issue.)

    The provided link does mention the block size and speed issues. (And I think it was implied that DES was inherently weaker than AES, given that the question also asked about TDES (that is, DES iterated 3 times, which doesn't have the same weaknesses as DES).)

Briefly on RSA vs AES: They fulfill very different roles. RSA isn't "better" than AES, just useful in areas AES isn't (aka, asymmetric encryption). Conversely, RSA is much slower than AES and has many more caveats in implementation than AES, meaning it's easier to screw-up.

  • $\begingroup$ Other than its atrociously low key size, DES is still pretty strong... the only non-brute-force attacks on it require an insane amount of known plaintext. There are weaknesses there, sure, but other than the key length issue, I trust DES. (And indeed do trust 3DES.) Of course, I agree entirely that AES is stronger. $\endgroup$
    – Reid
    Sep 14, 2013 at 20:39
  • $\begingroup$ @Reid One thing I don't understand is why use 3DES? Assuming it's a Encrypt(key k1), Decrypt(key k2), Encrypt(key k1). Is that a correct example of 3DES? IF yes, Why decrypt with K2 when it is not even encrypted with k2? $\endgroup$
    – P R
    Sep 14, 2013 at 20:41
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    $\begingroup$ @PR: See this answer; it covers it in detail. $\endgroup$
    – Reid
    Sep 14, 2013 at 20:44
  • $\begingroup$ @Reid: I agree. The original wording against DES was a bit strong. $\endgroup$
    – B-Con
    Sep 14, 2013 at 20:49
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    $\begingroup$ 3DES is also more cumbersome in it's use. The keys use parity bits and - a bit less of an issue - there are known weak keys. There is also little difference between 2 key and 3 key triple DES, which can lead to confusion. And a 3DES 128 bit key contains 112 bits of actual key data (-parity) and about 63 to 80 bits of security. $\endgroup$
    – Maarten Bodewes
    Sep 16, 2013 at 23:14

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