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Re: Private Data Communication Protocol Encryption

by fizbin (Chaplain)
on Jun 20, 2011 at 09:20 UTC ( #910532=note: print w/replies, xml ) Need Help??


in reply to Private Data Communication Protocol Encryption

Okay, I've figured out what was going on there. Sheesh, that was confusing.

First off, the easy (one line!) fix:

$plaintext = substr($input . ("\x00" x $ciphertextsize), 0, $ciphertex +tsize);

The private data communication protocol is designed to send one 64-bit number, not a variable-length string.

That being said, the code above and the web page it links to both have a subtle error in the protocol. Well, "error" is maybe too strong a word - a subtle quirk, maybe? Both the prefilled values on the webpage and the constants in the code look like byte strings encoded in hex.

That is, it's natural to think that $encryptionkey = 'c488fdd0d81f'; defines a six byte encryption key.

It doesn't. It defines a twelve byte encryption key, where all the bytes used happen to be in the range 48-57 or 96-101.

If you're trying to develop perl code to create test AdX RTB responses (that is the point of this, right?), you'll need to do a few things differently:

  • You'll need to interpret both keys as hex strings; this is pretty easy - $encryptionkey = pack("H*", 'c488fdd0d81f'); does what you want.
  • You'll need to send the winning price not as a string "1.50" but as a number of micros (millionths of a dollar) packed into a 64-bit number. Try this:
    print "Enter price to encrypt:"; chomp (my $input = <>); my $micros = int($input * 1_000_000); $plaintext = pack("N", $micros >> 32) . pack("N", $micros);
  • Depending on your decoding library, this might not be necessary, but you should still consider it anyway: the first eight bytes of your initialization vector should hold the time. Also, the random values should be drawn from the whole byte range, not just 48-57. (which is what bytes '0' through '9' give) Try this:
    use Time::HiRes; $initvector = join("", map {pack("N", $_)} Time::HiRes::gettimeofday() +, rand(2**32), rand(2**32));

Hope this helps. I do wish they'd add perl and python implementations to the sample site; it could have saved you some unneeded frustration.

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/; map{y/X_/\n /;print}map{pop@$_}@/for@/

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