CTP Issue 22: Authorization Token is obtained by truncating the results of the HMAC_SHA1 computation to retain only the leading 32 bits

Issue 22 suggests that 32 bits isn't enough for the authorization token.

See http://danforsberg.info:8080/draft-ietf-seamoby-ctp/issue22 for details.

Possible resolutions:

1) Make the authorization token longer, say 64 bits.
2) Keep the authorization token at 32 bits.

Suggested resolution: Make the authorization token longer, say 64 bits.
Justification: The probability of a birthday attack is proportional to
2**(sqrt(n)), where n is the number of bits in the identifier. If n is 32,
then sqrt(n) is 16, which is not enough protection (65536). Making n equal
64 means sqrt(n) is 32 (over 4 billion) which should be enough.