How does rosmsg md5 calculate the md5sum?

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Hello!

I need to communicate through ros topics between two different ros versions. One of the msgs depends on trajectory_msgs which has changed between versions so it gives a md5sum error. To overcome this I tried to add the old version of trajectory_msgs to my catkin workspace and recompile. The output of rosmsg shows that after I source my workspace, the msg changes but the md5 does not, any ideas?

$~/indigo_ws$ rosmsg show trajectory_msgs/JointTrajectoryPoint 
    float64[] positions
    float64[] velocities
    float64[] accelerations
    float64[] effort
    duration time_from_start

$~/indigo_ws$ rosmsg md5 trajectory_msgs/JointTrajectoryPoint 
f3cd1e1c4d320c79d6985c904ae5dcd3
$~/indigo_ws$ source devel/setup.bash 
$~/indigo_ws$ rosmsg md5 trajectory_msgs/JointTrajectoryPoint 
f3cd1e1c4d320c79d6985c904ae5dcd3
$~/indigo_ws$ rosmsg show trajectory_msgs/JointTrajectoryPoint 
    float64[] positions
    float64[] velocities
    float64[] accelerations
    duration time_from_start

$~/indigo_ws$ rosmsg md5 trajectory_msgs/JointTrajectoryPoint 
f3cd1e1c4d320c79d6985c904ae5dcd3