How to capture an auto-generated passphrase into a shell variable?

[nickpiaddo]

Hi,

I am currently working on a project, nixos-fde-config, for which I need to capture a passphrase
auto-generated by age.

  # Compress and encrypt header and key files
  PASSPHRASE=$(tar -C /root/secrets -cz luks-keyfile luks-header -f - \
    | age --encrypt --passphrase --output /mnt/etc/nixos/initrd/secrets/secboot 2>&1)

  # Auto-generated passphrase
  if [[ -n "${PASSPHRASE}" ]]; then
    cat <<-END

  ------------------------------------------------------------------------------
  Write down your autogenerated passphrase, and keep it in a secure location.

  $(echo -n "${PASSPHRASE#Using the autogenerated passphrase}" | tr -d '".')


  Hit Enter to continue.
END
    read -r -n 1
  fi

Context

The script uses a pinned version of age-v1.0.0 with which the code snippet above works.

However, when I tried to update it to the latest version (i.e. v1.2.1) I noticed that
PASSPHRASE is always empty. Even when the end-user chooses to auto-generate one while running
nixos-fde-config.

I do not know go (yet!), but I went through the source code of age, and spotted the lines
below in version v1.0.0 of the function passphrasePromptForEncryption.

age/cmd/age/age.go

Lines 241 to 242 in 552aa0a

	// TODO: consider printing this to the terminal, instead of stderr.
	fmt.Fprintf(os.Stderr, "Using the autogenerated passphrase %q.\n", p)

As far as I can tell, passphrasePromptForEncryption is in charge of generating, then printing
passphrases.

Since v1.1.0, the same function switched to printing the auto-generated passphrase through
printToTerminal, which calls withTerminal who, I assume, writes the output to
/dev/tty.

age/cmd/age/age.go

Line 301 in 7354aa0

err := printfToTerminal("using autogenerated passphrase %q", p)

age/cmd/age/tui.go

Lines 115 to 120 in 7354aa0

	func printfToTerminal(format string, v ...interface{}) error {
	return withTerminal(func(_, out *os.File) error {
	_, err := fmt.Fprintf(out, "age: "+format+"\n", v...)
	return err
	})
	}

age/cmd/age/tui.go

Lines 105 to 108 in 7354aa0

	} else if tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0); err == nil {
	defer tty.Close()
	return f(tty, tty)
	} else if term.IsTerminal(int(os.Stdin.Fd())) {

I know it is a bit cliché, but how can I get newer versions of age to output auto-generated
passphrases to stderr?

Thank you!