Add LUKS2 fixture builder script using real cryptsetup

tests/fixtures/luks/README.md

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+# LUKS2 fixtures from real cryptsetup
+
+Other LUKS tests use images built by `tests/imagebuild_luks.py`, so reader and
+builder share assumptions. These images are meant to be made by the real
+`cryptsetup`.
+
+**Status:** `make_fixtures.sh` has been written but not yet run (no
+cryptsetup in the authoring sandbox), so the `.img.xz` files and
+`expected.json` are not committed yet. Run the script and commit its output.
+
+## Regenerate
+
+    sh tests/fixtures/luks/make_fixtures.sh [output-dir]
+
+Needs `cryptsetup` (2.4+), `xz` and `python3`. It works on plain files with
+`--disable-locks --force-offline-reencrypt`, so root should not be needed.
+If rootless fails on your system, run the same script as root
+(`sudo sh ...`); no loop devices are used either way. The cryptsetup version
+used is recorded in `expected.json` (`cryptsetup` field).
+
+## Contents
+
+Plaintext: a 768 KiB file where each 512-byte sector reads
+`STRATA-FIXTURE sector N` (NUL-padded). The first 640 KiB are replaced by
+the LUKS header (`--reduce-device-size`), leaving 128 KiB of payload.
+
+| Image | KDF | Sector | Keyslots |
+|---|---|---|---|
+| `luks2-argon2id-4k.img.xz` | argon2id | 4096 | 0: passphrase 1 |
+| `luks2-argon2i-512.img.xz` | argon2i | 512 | 0: passphrase 1 (argon2i), 1: passphrase 2 (pbkdf2, 1000 iterations) |
+
+Common parameters: `aes-xts-plain64`, 512-bit key, sha256, metadata 16k,
+keyslots area 512k, `--pbkdf-force-iterations 4 --pbkdf-memory 64
+--pbkdf-parallel 1`.
+
+Passphrases (public, no trailing newline): `strata-fixture-1`,
+`strata-fixture-2`.
+
+`expected.json` holds, per image, the `luksDump --dump-json-metadata` output
+and the volume key (hex) from `--dump-volume-key`.
+
+## License
+
+MIT, same as the repository. The contents are public test data.

tests/fixtures/luks/make_fixtures.sh

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+#!/bin/sh
+# Build the real-cryptsetup LUKS2 fixtures. See README.md.
+# Usage: sh make_fixtures.sh [output-dir]   (default: this script's directory)
+# Needs cryptsetup >= 2.4, xz, python3. Tries rootless first.
+set -eu
+
+here=$(cd "$(dirname "$0")" && pwd)
+out=${1:-$here}
+work=$(mktemp -d)
+trap 'rm -rf "$work"' EXIT INT TERM
+
+# Public passphrases (no trailing newline: --key-file reads raw bytes).
+printf '%s' 'strata-fixture-1' > "$work/pw1"
+printf '%s' 'strata-fixture-2' > "$work/pw2"
+
+IMG_SIZE=786432      # 768 KiB plain file = 1536 sectors of 512 bytes
+HDR_SIZE=655360      # 640 KiB reserved for header + keyslots area
+HDR_SECTORS=$((HDR_SIZE / 512))
+COMMON="--type luks2 --disable-locks --batch-mode"
+PBKDF="--pbkdf-force-iterations 4 --pbkdf-memory 64 --pbkdf-parallel 1"
+
+plain() {  # plain <file>: every 512-byte sector reads "STRATA-FIXTURE sector N"
+    python3 -I - "$1" "$IMG_SIZE" <<'PY'
+import sys
+path, size = sys.argv[1], int(sys.argv[2])
+with open(path, "wb") as f:
+    for n in range(size // 512):
+        f.write(("STRATA-FIXTURE sector %d" % n).encode().ljust(512, b"\0"))
+PY
+}
+
+encrypt() {  # encrypt <file> <sector-size> <pbkdf>
+    # shellcheck disable=SC2086
+    cryptsetup reencrypt --encrypt $COMMON --force-offline-reencrypt \
+        --reduce-device-size "$HDR_SIZE" --offset "$HDR_SECTORS" \
+        --luks2-metadata-size 16k --luks2-keyslots-size 512k \
+        --cipher aes-xts-plain64 --key-size 512 --hash sha256 \
+        --sector-size "$2" --pbkdf "$3" $PBKDF --key-file "$work/pw1" "$1"
+}
+
+# A: argon2id, 4096-byte sectors
+plain "$work/luks2-argon2id-4k.img"
+encrypt "$work/luks2-argon2id-4k.img" 4096 argon2id
+
+# B: argon2i, 512-byte sectors, plus a pbkdf2 keyslot (slot 1, passphrase 2)
+plain "$work/luks2-argon2i-512.img"
+encrypt "$work/luks2-argon2i-512.img" 512 argon2i
+# shellcheck disable=SC2086
+cryptsetup luksAddKey $COMMON --pbkdf pbkdf2 --pbkdf-force-iterations 1000 \
+    --key-file "$work/pw1" "$work/luks2-argon2i-512.img" "$work/pw2"
+
+# expected.json: header metadata + volume key per image, tool version
+for img in luks2-argon2id-4k luks2-argon2i-512; do
+    f=$work/$img.img
+    cryptsetup luksDump --disable-locks --dump-json-metadata "$f" > "$work/$img.json"
+    cryptsetup luksDump --disable-locks --dump-volume-key --batch-mode \
+        --key-file "$work/pw1" "$f" > "$work/$img.vk"
+done
+cryptsetup --version > "$work/version"
+python3 -I - "$work" "$out/expected.json" <<'PY'
+import json, re, sys
+work, dest = sys.argv[1], sys.argv[2]
+res = {"cryptsetup": open(work + "/version").read().strip(),
+       "passphrases": {"1": "strata-fixture-1", "2": "strata-fixture-2"},
+       "plaintext": "every 512-byte sector N = 'STRATA-FIXTURE sector N' NUL-padded",
+       "images": {}}
+for name in ("luks2-argon2id-4k", "luks2-argon2i-512"):
+    meta = json.load(open("%s/%s.json" % (work, name)))
+    vk = open("%s/%s.vk" % (work, name)).read()
+    m = re.search(r"([0-9a-fA-F]{2}(?:\s+[0-9a-fA-F]{2})*)\s*$", vk)
+    hexkey = re.sub(r"\s+", "", m.group(1)).lower() if m else None
+    res["images"][name + ".img.xz"] = {"volume_key_hex": hexkey, "metadata": meta}
+with open(dest, "w") as f:
+    json.dump(res, f, indent=2, sort_keys=True)
+    f.write("\n")
+PY
+
+for img in luks2-argon2id-4k luks2-argon2i-512; do
+    xz -9e -c "$work/$img.img" > "$out/$img.img.xz"
+done
+echo "wrote fixtures to $out"