Tell me more ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.
up vote 1 down vote favorite

I am wondering is there any encryption algorithm available which produces comparable string.

I am expecting the output as shown below.

Data        |       output
---------------------------------------------
testData    |  uTrildHcKpM
testData_1  |  uTrildHcKpMiopu

Please let me know if you happen to know such algorithms.

share|improve this question
Do you want order-preserving encryption, or prefix-preserving one, or both? – Paŭlo Ebermann♦ Oct 11 '12 at 17:48
I am not sure what is search order-preserving encryption and prefix-preserving. Could you plese let me know about it? – Narendra Oct 11 '12 at 21:13
2  
If I understand your desired property correctly, then the inherent weaknesses of that property lead to far too weak encryption. – CodesInChaos Oct 11 '12 at 21:46
In particular, I suspect it's possible to show that the only kind of cipher that offers this property is a substitution cipher that works on single characters. – CodesInChaos Oct 11 '12 at 21:46
@Narendra: order preserving means $a < b \Rightarrow E(a) < E(b)$, for some (natural) ordering on the plaintexts and ciphertexts. Prefix preserving means that $E(ab) = E(a)x$ for some $x$. – Paŭlo Ebermann♦ Oct 12 '12 at 8:03
show 1 more comment

2 Answers

up vote 2 down vote accepted

It is somewhat unclear what you mean by comparable strings. But from the example you give, one might construe that you want the same substrings in different plaintext messages to be encrypted in the same way. This property is obviously very bad for an encryption scheme (given a number of plaintext/ciphertext pairs it would be easy to derive a lot of information about the encryption scheme).

There are however early instances of encryption with such a bad property like the monoalphabetic substitutions (Caesar's cipher for example).

share|improve this answer
I think it's easy to show that monoalphabetic substitution is the only kind of cipher that has this property. – CodesInChaos Oct 11 '12 at 21:54
up vote 2 down vote

In cryptography terms, you are looking for an algorithm that has high confusion but no diffusion. Stream ciphers like RC4 satisfy this property when used with a fixed initialization vector, since they process the input stream byte by byte.

However, this is an absolutely dangerous way to use cryptography. The contents of the data will not be secure.

share|improve this answer
A stream cipher (with the same key and IV as correctly stated) will only provide the same ciphertext for two plaintext strings starting with the same substring, thus a very specific type of substrings (though the one given in the example above). Hence the lack of clarity about what is actually asked. – bob Oct 12 '12 at 17:27
Good point! Thanks for the clarification. – Stephen Touset Oct 12 '12 at 19:13

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.