Improve the security and performance of syncookies:

Security improvements:
- Increase the size of each syncookie secret from 32 to 128 bits
  in order to make brute force attacks on the secrets much more
  difficult.
- Always return the lowest order dword from the MD5 hash; this
  allows us to expose 2 more bits of the cookie and makes ACK
  floods which seek to guess the cookie value more difficult.

Performance improvements:
- Increase the lifetime of each syncookie from 4 seconds to 16
  seconds.  This increases the usefulness of syncookies during
  an attack.
- From Yahoo!: Reduce the number of calls to MD5Update; this
  results in a ~17% increase in cookie generation time here.

Reviewed by:	hsu, jayanth, jlemon, nectar
MFC After:	15 seconds

0 files changed, 0 insertions, 0 deletions