Encrypt disk volume with LUKS, enable access without passphrase

Question

How can I set “no passphrase, and don't prompt for one” on a LUKS volume, leaving it encrypted as is?

Enabling LUKS encryption is a one-way, time-consuming operation. I need to divide the set-up of that encrypted volume into two steps:

• When setting up the machine: Encrypt the partition with LUKS, wait for it to complete, and have that be the last time the encryption or formatting is needed.

  This takes a long time, and it should be done when the other time-consuming activities (testing the hardware, installing the operating system, downloading software packages, etc.) are done, with no need for the eventual user to be involved.

• When configuring the OS later: Choose a passphrase for authenticating access to the partition.

  This needs to be done by the eventual user, and needs to be fast: choose a passphrase, and that is instantly the passphrase that protects the already-encrypted volume.

These two steps are done at different times in different locations by different people.

We can assume the person doing technical set-up has technical knowledge and professional care. We cannot assume this for the person setting a passphrase for the volume; it must be quick, simple, and uncomplicated: a single command that executes very quickly.

So I need a way to be able to choose no passphrase for the volume, and leave the option to simply defer until later the choice of passphrase — without reformatting the volume.