From: Shanti Braford <shanti@...>
Date: 2007-10-31T10:05:29+09:00
Subject: (BitTorrent-related) binary "string" to hexadecimal?

Hello,

Let's say one were "hypothetically", and that's a big hypothetical
there, writing a BitTorrent tracker in ruby...


Quick Synopsis: I've got the SHA1 hexdigest of a .torrent metainfo file
saved on the server.

Clients connect to the tracker and send the 'info_hash' param; this is
the raw SHA1 digest (not stored in hex).

I'm trying to lookup the hexdigest (of known torrents) with the incoming
'info_hash' param, which is a binary "String" (ruby class wise).

So..

For our example below ('foo' is not a valid bencoded string but it makes
it easy to demonstrate in this example):
digest_str = Digest::SHA1.hexdigest('foo')
# => "0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33"

Next up we're trying to write the /announce portion of the BitTorrent
tracker.

The 'info_hash' param comes through as an escaped binary representation
of the info_hash (not hex).

To replicate what's going on from the Client end, I've done the
following:

digest = Digest::SHA1.digest('foo')
# => "\v?ǵ????]\r?<[?uڊ3"

The torrent client then escapes the digest for transmission over http
wire:

digest_escaped = CGI::escape(digest)
# => "%0B%EE%C7%B5%EA%3F%0F%DB%C9%5D%0D%D4%7F%3C%5B%C2u%DA%8A3"

Back on my end, of course I can first CGI unescape this:

digest = CGI::unescape(params[:info_hash)
# => "\v?ǵ????]\r?<[?uڊ3"

So my $1,000,000 question :)  How in ruby do we get from
"\v?ǵ????]\r?<[?uڊ3" to "0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33" ?

I am aware of a workaround: store the initial SHA1 digest of the torrent
in binary from in MySQL.  I'm more comfortable working with strings, and
haven't dealt with storing binary data in MySQL... but I'll take a look
at that option if there's no easy way to go from ruby binary
representation to hex.

Anyone have any ideas or pointers?

- Shanti

ps. this is written in rails but the question takes place more at the
ruby level, so I figured this forum would be more fitting to ask in!
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