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From: mark on 26 Feb 2010 17:42
I'm not sure where newbies can ask questions about cryptography, so
I'm posting here. I'm just reviewing some materials and don't quit
know the answer to this.


If we are using a synchronous stream cipher where k is of length m
bits, producing a keystream of length n bits, how large is the
keyspace?
From: WTShaw on 26 Feb 2010 17:52
On Feb 26, 4:42 pm, mark <cheesemonke...(a)gmail.com> wrote:
> I'm not sure where newbies can ask questions about cryptography, so
> I'm posting here.  I'm just reviewing some materials and don't quit
> know the answer to this.
>
> If we are using a synchronous stream cipher where k is of length m
> bits, producing a keystream of length n bits, how large is the
> keyspace?

Since binary loops tend to be rather short as compared other
information units and bits come in only two kinds, representing useful
key sizes is awkward. This is why there is so much noise about such
things being so easy to solve because relatively they are.
From: J.D. on 26 Feb 2010 18:05
On Feb 26, 5:42 pm, mark <cheesemonke...(a)gmail.com> wrote:
> I'm not sure where newbies can ask questions about cryptography, so
> I'm posting here.  I'm just reviewing some materials and don't quit
> know the answer to this.
>
> If we are using a synchronous stream cipher where k is of length m
> bits, producing a keystream of length n bits, how large is the
> keyspace?

If the key is m bits long then the key space is 2^m -- that's how many
possible keys there are that are m bits long. Note that "the key" is
not the same thing as "the key-stream" of a stream cipher: the key (as
the secret input to the stream cipher algorithm) is used to _generate_
the key-stream.
From: unruh on 26 Feb 2010 20:22
On 2010-02-26, mark <cheesemonkey22(a)gmail.com> wrote:
> I'm not sure where newbies can ask questions about cryptography, so
> I'm posting here. I'm just reviewing some materials and don't quit
> know the answer to this.
>
>
> If we are using a synchronous stream cipher where k is of length m
> bits, producing a keystream of length n bits, how large is the
> keyspace?

I assume by k you mean the key. The key space is anything from 0 to 2^m
where it is up to you to decide how you are going to generate the keys.
If you always use your dogs name as the key then the size of the key
space is 1. If you randomly choose m bits, then the keyspace is 2^m
Other choices lie somewhere betwen these two.

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