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I am in attempt to understand relative insecurity of certain encryption schemes. Particularly of interest is DES and RC2. I know AES is better and should be used to encrypt. But practically, if something is encrypted with (1) DES or (2) RC2 (3) RC4 what level security is achieved.

In affect I am asking:

If a adversery is given a {DES/RC2/RC4} ciphertext, is it possible that they can decrypt or can a determined adversary definitely decrypt?

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For DES the answer is that it can definitely be decrypted using brute-force. A 56 bit key is far too small. – CodesInChaos Jun 3 '13 at 5:09
It depends on the type of attack which is possible. If your plaintext is random data without any structure, every encryption scheme (with a key size of more than zero) is "secure" against "definitely retrieving the plaintext". – Paŭlo Ebermann Jun 3 '13 at 7:25

Yes, an adversary can definitely decrypt a DES message, given sufficient funding. Fifteen years ago, in 1998, the EFF built a DES cracker (nicknamed Deep Crack) that can recover a DES key in a day. Today, anyone with the money can purchase a commercially available DES cracker named COPACOBANA.

For RC2, I'm not aware of any practical attacks. (You still shouldn't use it.)

For RC4, the answer is complicated. It depends. It depends upon how you use RC4. There are lots of attacks known, and whether they will be applicable or not to your particular situation will depend upon many fine details of your particular situation.

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Thanks for the great edits, @JohnDeters! – D.W. Jun 3 '13 at 19:03

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