Broken Protocol Example 
RFC 2082: RIP-2 MD5 Authentication

There are a number of other problems with this RFC, some of which are
documented in J. Etienne's "Flaws in RIPv2 packet's authentication"

    (http://off.net/~jme/ietf/draft-etienne-ripv2-auth-flaws-00.txt)

There is also update:  RFC4822 (January 2007)
"RIPv2 Cryptographic Authentication"
Basically a FIPS update to SHA1-HMAC
Solves some of the problems (doesn't use time anymore)
Other things remain broken