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The Shœstring Foundation Weblog, Miscellaneous Byproducts

Matthias Bauer
bauerm (at) shoestringfoundation · org
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Sat, 14 Oct 2006

Poor Man's PGP Part 1: RC4 in a shell skript


With a shell account on an arbitrary POSIX semi-compliant system, one should have access to a Bourne-like Shell, awk, dc, sed and companions. Given a source of randomness this should be sufficient to code RSA + a symmetric cipher, kind of extremely poor man's PGP.

I had some problems finding ways to output binary stuff from ksh.
UPDATE: New version seems to work with bash.

Here is the first step towards it, RC4 in a shell skript. As expected, it's slow as mouldy molasses but it works and passes a test against OpenSSL's test vectors.

On Intel at 1.6 Ghz it encrypts/decrypts at 184 Bytes per second. One optimization could be to put the keystream generation entirely in a dc script, start that in a sub-process, and read single bytes from a fifo.
UPDATE: New version does this, 370 Bytes/sec now.

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