Air carrier flight crew inadvertently released brakes at the gate before aircraft was chocked.

Narrative

Flight to ZZZ was uneventful. Upon taxing into the gate the Marshaller gave the signal to stop (crossed wands). I did and immediately set the parking brake. I then began to shut the engines down and apply external power (when I noticed it was available on the overhead panel). During that process the Marshaller gave the signal that external power was available. I incorrectly interpreted this to mean the chocks were installed. I then released the parking brake. Immediately I sensed motion and rapidly applied foot brakes. I then reset the parking brake and waited for the ground crew to plug into the headset for further instructions. They told me to release the brakes in order to push the airplane back slightly. They then told me to release brakes; which I did. I completed my shutdown checklist. Upon exiting the airplane the ground crew notified me that the nose tether became taught and sheared a small bolt to a bracket holding the nose light wiring harness.The misinterpretation of ground crew hand signals. I interpreted the signal for 'external power' for the 'chocks are installed.'Firstly; I believe confirmation bias was a factor. I was expecting the 'chocks installed' signal and I 'saw what I wanted to see.' In hindsight; it's standard to be told the chocks are in verbally over the intercom. Occasionally the ground crew does not verbally notify us of that and a hand signal is then used. Secondly; I was not expecting a signal for external power is available; since I can independently verify that via the overhead panel. I had already applied external power when it became available; before the signal; which is independently verifiable by the ground crew; therefore the signal was unnecessary. Thirdly; I never took the time to asses whether it made sense to release the parking brake. Lastly; because we sometimes never get notified that the chocks are in; and are forced to ask the ground crew face-to-face upon opening the door; this signal was appreciated and confirmed what I wanted - alleviating that delay. Bottom line - don't rush the standard procedures; asses the validity of the signals; and when in doubt; ask.