What does the 'key' field in a bitorrent query mean?

Question

I used utorrent 2.04(build 22450) and I curious about the key part in the HTTP GET query utorrent generated.I have read the specification for bitorrent protocol and still can't found the meaning for 'key' part in a bitorrent get query. After parse the query, it's kind of like that:

{
info_hash:'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
peer_id: '-UT2210-*b�A�\u000b7�\u001d\u0004�I',
port: '18584',
uploaded: '0',
downloaded: '0',
left: '0',
corrupt: '0',
key: '79E7F5D0',
event: 'started',
numwant: '200',
compact: '1',
no_peer_id: '1',
ipv6: '2004:150:20xx:xxxx:xxxx:xxxx:xxxx' }

The meaning of other entry is quite obvious, but I totally got no idea of what the key part mean. Can someone explain about the purpose of the key part? Thank you ~

Answer 1

To quote from the unofficial bittorrent spec:

key: Optional. An additional client identification mechanism that is not shared with any peers. It is intended to allow a client to prove their identity should their IP address change.