# [RC5] 40 bit encryption

Don Cohen cohen at maine.rr.com
Fri Mar 15 22:12:07 EST 2002

```On Friday 15 March 2002 07:51 pm, John Campbell wrote:
> On Fri, 15 Mar 2002, Don Cohen wrote:
> > Not quit,  you chose 2^40 as the number of keys in forty bit encription
> > (which would be a 1 followed by 39 0's).
>
> 	No... 2^40 is a 1 followed by 40 0s (assuming you're talking binary
> representation).
>
> > The correct number of keys is
> > 2^41 - 1
>
> 	How do you figure?
>

I was thinking that the largest value of an n bit number is 2^n - 1
forgetting that 0, although being a poor choice, can be a valid key.

> 	One-bit encryption has 2^1 (2) keys: 0 or 1.
>
> 	Two-bit encryption has 2^2 (4) keys: 00, 01, 10, or 11.
>
> 	Three-bit encryption has 2^3 (8) keys: 000, 001, 010, 011, 100, 101,
> 110, or 111.
>
> 	Four-bit encryption has 2^4 (16) keys: 0000, 0001, 0010, 0011, 0100,
> 0101, 0110, 0111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, or 1111.
>
> 	And so it goes, up to 40-bit encryption, which has 2^40 keys, which
> I am not going to list all of, because 1,099,511,627,776 keys would make
> this email far too large.
>
> ---
> John Campbell
> jcampbel at lynn.ci-n.com
>
> QotD:  The chances of getting eaten up by a lion on Main Street aren't
> one in a million, but once would be enough.

--
Don Cohen
cohen at maine.rr.com
http://usmcug.usm.maine.edu/~cohen
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