I can't authenticate MD5 for RIPv2 in packet tracer 7.3, the subcommand "rip" is not shown, I'm wondering if it is available in this version? please look at the following lines of command for your further reference:
Router(config)#key chain KEY
Router(config-keychain)#key 1
Router(config-keychain-key)#key-string password1
Router(config-keychain-key)#
Router(config-keychain-key)#interface fa0/0
Router(config-if)#
Router(config-if)#ip ?
access-group Specify access control for packets
address Set the IP address of an interface
authentication authentication subcommands
flow NetFlow Related commands
hello-interval Configures IP-EIGRP hello interval
helper-address Specify a destination address for UDP broadcasts
inspect Apply inspect name
ips Create IPS rule
mtu Set IP Maximum Transmission Unit
nat NAT interface commands
ospf OSPF interface commands
proxy-arp Enable proxy ARP
split-horizon Perform split horizon
summary-address Perform address summarization
virtual-reassembly Virtual Reassembly
