Possible to recreate GPG using pen and paper? exclusive)

Harry Rickards hrickards at l33tmyst.com
Sat Jun 6 18:12:03 CEST 2009

Hash: SHA1

On 06/06/09 09:31, gpg2.20.maniams at dfgh.net wrote:
> On Fri, Jun 5, 2009 at 11:10 PM, Harry Rickards - hrickards at l33tmyst.com
> <mailto:hrickards at l33tmyst.com>
> <+gpg2+maniams+68c803b295.hrickards#l33tmyst.com
> <http://l33tmyst.com>@spamgourmet.com <http://spamgourmet.com>> wrote:
>     Hash: SHA1
>     On 06/05/09 19:46, David Shaw wrote:
>     > On Jun 5, 2009, at 2:52 AM, Harry Rickards wrote:
>     >
>     >>
>     >> Would it be possible to do the same job that GPG does (using all the
>     >> same algorithms) simply using a pen and paper? You can do simple
>     >> public key crypto with RSA, by choosing two primes and doing a
>     >> multitude of stuff with them. I understand that it will take a while
>     >> to actually encrypt/decrypt something, and you'll need a
>     calculator as
>     >> well, but it would be fun to try all the same.
>     >
>     > It is definitely possible.  It might take a while and use a good
>     bit of
>     > paper, but it's possible.  You would need to understand the public key
>     > algorithm (RSA, for example) as well as the symmetric cipher
>     (3DES, AES,
>     > etc).  The actual bytes-in-a-row format is specified in RFC-4880
>     > (http://www.ietf.org/rfc/rfc4880.txt)
>     >
>     > David
>     Thanks for the link, I'll have a read through it (although it might take
>     a while - 28k words). When you say understand the algorithm, do you mean
>     understand that you take two prime numbers, and multiply them together
>     to get n, and then multiply them together using the totient function
>     etc, or understand *why* you take multiply them together using the
>     totient function etc?
>     - --
>     Many thanks
>     Harry Rickards (GPG Key ID:646ED06A)
> Hi Harry
> Great thought. If you try the above and succeed please share with us the
> tools used (what type of calculator ... what functions etc) and so the
> time taken
> But I'm unable to understand one thing. Having said that one may be able
> to create and also decrypt GPG compliant messages, _how_do_use_them   ?
> How do you send an encrypted file or message across to some one else
> ...etc...
> regards
> maniams

As to sending it, I suppose you could take pictures and type them in
(like they did in Little Brother by Cory Doctor - cc licensed), but it
would take forever. I've found a page at
http://sergematovic.tripod.com/rsa1.html that allows you to do RSA
encryption/decryption using a pencil/paper without using the Extended
Euclidean algorithm, something I just can't seem to get my head around.
For a calculator I'm actually using the python interpreter, it seems to
deal with big numbers pretty well. I don't know any python, but you can
mostly type in sums and it tells you the answers. To do powers, you have
to do pow(x,y) where x is the number and y is the power. For example
pow(5,2) squares 5. For anything more complicated I use Wolfram Alpha
(wolframalpha.com) which can deal with most big numbers, it can
certainly go a lot higher than Google.

- -- 
Many thanks
Harry Rickards (GPG Key ID:646ED06A)

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