Once you have sshed into the system, use diskutil. If you are using a passphrase, you can append -stdinpasspharse for the tool to prompt the passphrase. Otherwise, you can include the passkey using just -passphrase.
diskutil corestorage unlockVolume $volumeIdentifier -passphrase /path/to/passkey
-OR-
diskutil corestorage unlockVolume $volumeIdentifier -stdinpassphrase
Here, $volumeIdentifier is the ID such as the one starting with 7EB3514D-. If you want to decrypt the drive (that is, making it no longer encrypted), you can use decryptVolume instead. The format is the same, as follows.
diskutil corestorage decryptVolume $volumeIdentifier -stdinpassphrase
-OR-
diskutil corestorage decryptVolume $volumeIdentifier -passphrase /path/to/passkey