It seems that the easiest would be to use AWS SDK with a tJava. The first operation would be to list the key (AWSKMSClient.listKeyResult) then use directly the SDK again with the decrypt function.
Java doc is here.
For the initial connection, both AWS KMS Client and any of the txxxxConnection implement AWSClientBuilder so with a bit of manipulation you can probably reuse the connection. Or you can get one of the component and change the code directly / create a custom component.
In the Amazon EC2 console, select the instance, and then choose Connect LiteBlue
In the Connect To Your Instance dialog box, choose Get Password (it will take a few minutes after the instance is launched before the password is available).
Choose Browse and navigate to the private key file you created when you launched the instance. Select the file and choose Open to copy the entire contents of the file into the Contents field.
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