#!/usr/bin/env bash
# dm-sign.sh — produce a signed DM for the fleet DM system.
#
# Signs a message with an SSH key (namespace "dm", Ed25519) and prints the
# signed wire format that `dm.py verify-sig` checks:
#
#   [from:<identity>] [id:<id>]
#
#   <message body>
#
#   -----BEGIN SSH SIGNATURE-----
#   ...
#   -----END SSH SIGNATURE-----
#
# The signed payload is exactly "[from:X] [id:Y]\n\n<body>" (no trailing
# newline) — verify-sig reconstructs it by stripping everything from the
# signature block onward, so the two must match byte-for-byte.
#
# Usage:
#   dm-sign.sh --from <identity> [--key <privkey>] [--id <id>] <message>
#
# Defaults: --key ~/.ssh/id_frontdoor, --id = 8 random hex chars.
# The private key is only ever read locally; it is never moved or copied.
# Pipe the output straight into `dm.py send --raw` (never truncate it).
#
# Example:
#   dm.py send --agent opm --target main --raw \
#     "$(dm-sign.sh --from operator-main 'hello from the operator')"
set -euo pipefail

FROM=""
KEY="$HOME/.ssh/id_frontdoor"
ID="$(head -c4 /dev/urandom | od -An -tx1 | tr -d ' \n')"

usage() {
    sed -n '2,/^set -euo/p' "$0" | sed 's/^# \?//'
}

while [[ $# -gt 0 ]]; do
    case "$1" in
        --from) FROM="${2:?--from needs a value}"; shift 2 ;;
        --key)  KEY="${2:?--key needs a value}"; shift 2 ;;
        --id)   ID="${2:?--id needs a value}"; shift 2 ;;
        -h|--help) usage; exit 0 ;;
        --) shift; break ;;
        -*) echo "error: unknown option: $1" >&2; exit 1 ;;
        *) break ;;
    esac
done

if [[ $# -eq 0 ]]; then
    echo "error: no message given" >&2
    echo "usage: dm-sign.sh --from <identity> [--key <privkey>] [--id <id>] <message>" >&2
    exit 1
fi
MESSAGE="$*"

[[ -n "$FROM" ]] || { echo "error: --from <identity> is required" >&2; exit 1; }
[[ -f "$KEY" ]]  || { echo "error: private key not found: $KEY" >&2; exit 1; }

TD="$(mktemp -d)"
trap 'rm -rf "$TD"' EXIT
PAYLOAD="$TD/payload"

# Payload: header, blank line, body — NO trailing newline (verify-sig strips).
printf '[from:%s] [id:%s]\n\n%s' "$FROM" "$ID" "$MESSAGE" > "$PAYLOAD"

# Never reuse a stale signature: a leftover .sig from an earlier run would
# silently sign the wrong payload (burned 20 minutes on the board, 2026-10-03).
rm -f "$PAYLOAD.sig"
ssh-keygen -Y sign -f "$KEY" -n dm "$PAYLOAD" >/dev/null

printf '[from:%s] [id:%s]\n\n%s\n\n' "$FROM" "$ID" "$MESSAGE"
cat "$PAYLOAD.sig"