Re: Which encryption algorithm used?

[exon <exon () home se>]

John Borwick wrote:
> Pitts, Christopher C. wrote:
>
> > With a proper algorithm, it should be nearly impossible.  That being 
> > said, many implementations, stick a header or footer that can be used 
> > to identify the method used.  Take a look at mcrypt and it's bare 
> > function, you can use it to compare the stripped (--base IIRC) ouput 
> > of the different algorithms.
>
> [snip]
>
> What do you mean, "with a proper algorithm"?  Cryptographic algorithms 
> are supposed to be secure *even when the methods used are known*.  The 
> only thing that has to be secret is the key.

And that's about exactly what he means. You can't determine what cipher 
was used to encrypt a message by looking at the encrypted message if the 
output is secure when the methods used are known. That's because the 
encrypted message would consist of a bunch of pseudo-random characters 
with no appearent coherence what so ever.

In this sense, rotation-, simple-hash and XOR-ciphers don't use "proper 
algorithms" because it's fairly simple to conclude that one of the three 
was used to encrypt a message by looking at the encrypted text ("hello" 
would look something like "ifmmp" with a rotation cipher and a key of 1).

/exon