HS Presentation 11/29/12

gistfile1.md

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Encoding Constrained Byte Arrays

(or Fun With Lisp)

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The BitTorrent Handshake

5 components:

• a single byte protocol length
• a variable byte protocol string
• 8 reserved bytes (to indicate protocol changes)
• a 20-byte info hash
• a 20-byte peer id

Exactly what these are aren't important -- that they are constrained is.

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Somewhere in your code, you need to check that this is valid

If you don't, you run the risk of sending garbage.

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So, you could do this check with a bunch of if statements...

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Or, you could build a system for constraining byte arrays!

(bit-struct handshake
  [len :byte]
  [protocol :len/bytes]
  [reserved :8/bytes]
  [info-hash :20/bytes]
  [peer-id :20/bytes])

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And a function that encodes based on that spec...

(encode handshake 19 "BitTorrent protocol" (reserved-bytes) info-hash
peer-id)
;; => Returns a ByteArray if the data meets the constraints

(encode handshake 5 "BitTorrent protocol" (reserved-bytes) info-hash
peer-id)
;; => Or throws an exception if it doesn't!

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Oh, and encode knows how to convert all of these things, including strings, to bytes. :-)

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