using perl-rsa

17 Dec 2003

      [managed to type kent instead of keng this time, touch typist and not
watching the screen either].

Kent Crispin <kent@songbird.com> writes:
...
[...]
...
print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<>
)]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`
BTW, how do you use this code?  I'm not a big "dc" user, I must admit.
You first of all extract your a PGP key with pgpacket.  (You'll need
to change the password to be no password (press enter when prompted
for password), so that PGP doesn't encrypt the private key.

Next, you can do:

	% perl rsa.pl [e] [n] < plaintext > ciphertext
	% perl rsa.pl [d] [n] < plaintext > ciphertext

to encrypt and decrypt.

Here's a small example (Fred is a notional eternity user who's key,
both public and private is included in the eternity distribution):

Fred's public key:

Fred's secret key (no password):

-----BEGIN PGP SECRET KEY BLOCK-----
Version: 2.6.3i

lQD4AzPfUPsAAAECAMn6jtL3ZNjmBrkDLIktHNmBGc+a59BSvFPN8howj5zAj1U8
5QOfVtV4kOliK2AkCcL2yyz3Wz+AHiYdVyuQJF0ABREAAfsEPkWwiGybJRn00/dB
RLZjOZHAmMRVDH6UC9si+GpHDsOs5/eLbK6GUqmbWXG4I4LadxRQz/nplQ+lCVYP
ecrbAQDixLjtXlj9ks1e9ZcroU71Pvk84oQjpUhiU1rAIkKh9wEA5APIvVJETEjm
DxEpdE6hJkty+nb6v8VNOYlslo8Ol0sA/iwRycjjcNs4A2c9Owt+xLCiMp2JIb0z
yf7d4xEcLI1vUXG0FEZyZWQgPGZyZWRAZXRlcm5pdHk+
=CHwS
-----END PGP SECRET KEY BLOCK-----

(to extract public key do:

% pgp -kxa fred fredpub ~/.pgp/pubring.pgp 

and to get private key do:

% pgp -kxa fred fredpri ~/.pgp/secring.pgp 

)

Next get pgpacket.pl, see ftp://ftp.ox.ac.uk/pub/crypto/pgp/utils/ or
something like that.

Then run pgpacket on the fredpri.asc (it contains all the info that
you want, without looking at fredpub.asc, ie the values for e, d and
n):

% pgpacket fredpri.asc

The values for e, d, and n are:

e = 11
d = 043E45B0886C9B2519F4D3F74144B6633991C098C4550C7E940BDB22F86A470E
    C3ACE7F78B6CAE8652A99B5971B82382DA771450CFF9E9950FA509560F79CADB
n = C9FA8ED2F764D8E606B9032C892D1CD98119CF9AE7D052BC53CDF21A308F9CC0
    8F553CE5039F56D57890E9622B602409C2F6CB2CF75B3F801E261D572B90245D

Now you can encrypt for fred like so:

% echo hello world | perl rsa.pl 11 C9FA8ED2F764D8E606B9032C892D1CD98119CF9AE7D052BC53CDF21A308F9CC08F553CE5039F56D57890E9622B602409C2F6CB2CF75B3F801E261D572B90245D > hello.rsa
%

and to decrypt:

% perl rsa.pl 043E45B0886C9B2519F4D3F74144B6633991C098C4550C7E940BDB22F86A470EC3ACE7F78B6CAE8652A99B5971B82382DA771450CFF9E9950FA509560F79CADB C9FA8ED2F764D8E606B9032C892D1CD98119CF9AE7D052BC53CDF21A308F9CC08F553CE5039F56D57890E9622B602409C2F6CB2CF75B3F801E261D572B90245D < hello.rsa
hello world
%

Now dc can do modular exponentiation like so (reverse polish notation):

dc> m e ^ n %

but that's going to take a long time because it will evaluate m ^ e
and then do mod n.

So the clever thing that the program does is to implement Knuth's
modexp algorithm which reduces the work factor immensly. 

Actually GNU dc 1.1 (distributed with GNU bc-1.04) now includes "|" as
a modular exponentiation operator, so you can do: "m e n |" and it'll
do it directly.  This feature was a special, just for perl rsa because
Ken Pizzini, author of GNU dc, happens to be one of the major
contributors of hacks to shorten the perl program, and so have a
vested interest.

With the dc 1.1 | operator, Ken got the script down to:

$/=$z;exec"dc -e'16i0[lN*lMlKlN|+lMlN/dsM0<J]dsJ\U@{[
pop,pop,unpack'H*',<>]}\EsMsKsNx[II*~aSad0<ZLaP]dsZx'"

a considerable improvement over:

print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<>
)]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`
...
I dug out a midi interpreter I wrote a number of years ago, 
and it is indeed trivial to modify it to read any text as input.  
Unfortunately, I wrote that long before the midi file spec was 
finalized, and the hardware I wrote it for is also long gone.  But 
it's probably not much work to get file output working again...and 
the thought of a general text-to-midi translator is rather 
entertaining -- I could play this entire mail message through it, for 
example... It would definitely make better music if some rhythmic 
variation was part of the coding, but that would make it a little 
harder to make an automatic decoder...
I'd find it most cool to hear an audio file of the above.

Adam
-- 
Have *you* exported RSA today? --> http://www.dcs.ex.ac.uk/~aba/rsa/

print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<>
)]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`

using perl-rsa

Adam Back