/lib/partman/active_partition/45cipher/choices is in ubiquity 2.18.7.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | #!/bin/sh
. /usr/share/debconf/confmodule
set -e
dev=$1
id=$2
part=$dev/$id
cd $dev
choice_cipher () {
	if [ -f $part/cipher ]; then
		cipher=$(cat $part/cipher)
	else
		db_metaget partman-basicfilesystems/text/no_mountpoint description
		cipher="$RET" # "none"
	fi
	db_metaget partman-crypto/text/specify_cipher description
	printf "encryption\t%s\${!TAB}%s\n" "$RET" "$cipher"
}
[ -f $part/method ] || exit 0
method=$(cat $part/method)
if [ $method = crypto ]; then
	choice_cipher
fi
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