// Command webauthn-sign is a Phase-1 smoke-test helper. It performs the
// two WebAuthn crypto operations the shell driver cannot easily do with
// openssl: generate an Ed25519 keypair (so the approval-service can
// pre-register a mock credential) and produce a WebAuthn PublicKey
// assertion over a server-supplied challenge (so the smoke test can
// drive /api/v1/approvals/confirm without a browser).
//
// Two subcommands:
//
//   gen-key                      → {"priv_hex": "...", "pub_hex": "..."}
//   sign-assertion [flags]       → {"credential_id": "...", "client_data_json": "...",
//                                   "authenticator_data": "...", "signature": "..."}
//
// Everything is base64url-encoded without padding, matching the wire
// format the approval-service's WebAuthn shim accepts.
package main

import (
	"crypto/ed25519"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"flag"
	"fmt"
	"os"
	"strings"
)

func main() {
	if len(os.Args) < 2 {
		die("usage: webauthn-sign {gen-key|sign-assertion}")
	}
	switch os.Args[1] {
	case "gen-key":
		runGenKey()
	case "sign-assertion":
		runSignAssertion(os.Args[2:])
	default:
		die("unknown subcommand %q", os.Args[1])
	}
}

func runGenKey() {
	pub, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		die("gen key: %v", err)
	}
	out := map[string]string{
		"priv_hex":    hex.EncodeToString(priv),
		"pub_hex":     hex.EncodeToString(pub),
		"pub_b64url":  base64.RawURLEncoding.EncodeToString(pub),
	}
	writeJSON(out)
}

func runSignAssertion(argv []string) {
	fs := flag.NewFlagSet("sign-assertion", flag.ExitOnError)
	var (
		privHex       = fs.String("priv-hex", "", "hex-encoded 64-byte Ed25519 private key")
		challengeB64u = fs.String("challenge-b64url", "", "base64url (no pad) WebAuthn challenge bytes")
		rpID          = fs.String("rp-id", "", "WebAuthn relying party id (hostname)")
		origin        = fs.String("origin", "", "browser origin (e.g. https://trust.basicstack.de)")
		credentialID  = fs.String("credential-id", "", "credential_id to emit on the wire; defaults to b64url(pub)")
	)
	if err := fs.Parse(argv); err != nil {
		die("parse flags: %v", err)
	}
	if *privHex == "" || *challengeB64u == "" || *rpID == "" || *origin == "" {
		die("sign-assertion requires -priv-hex, -challenge-b64url, -rp-id, -origin")
	}

	privRaw, err := hex.DecodeString(*privHex)
	if err != nil {
		die("decode priv-hex: %v", err)
	}
	if len(privRaw) != ed25519.PrivateKeySize {
		die("priv-hex must decode to %d bytes, got %d", ed25519.PrivateKeySize, len(privRaw))
	}
	priv := ed25519.PrivateKey(privRaw)
	pub := priv.Public().(ed25519.PublicKey)
	credID := *credentialID
	if credID == "" {
		credID = base64.RawURLEncoding.EncodeToString(pub)
	}

	// Build clientDataJSON per the WebAuthn spec.
	clientData := map[string]any{
		"type":      "webauthn.get",
		"challenge": strings.TrimRight(*challengeB64u, "="),
		"origin":    *origin,
	}
	clientDataJSON, err := json.Marshal(clientData)
	if err != nil {
		die("marshal clientDataJSON: %v", err)
	}

	// authenticator_data = sha256(rpID) || flags (0x01=UP) || signCount (4 bytes, zero).
	rpHash := sha256.Sum256([]byte(*rpID))
	authData := make([]byte, 0, 37)
	authData = append(authData, rpHash[:]...)
	authData = append(authData, 0x01)
	authData = append(authData, 0, 0, 0, 0)

	// Signed bytes = authenticator_data || sha256(clientDataJSON).
	cdHash := sha256.Sum256(clientDataJSON)
	signed := make([]byte, 0, len(authData)+len(cdHash))
	signed = append(signed, authData...)
	signed = append(signed, cdHash[:]...)
	sig := ed25519.Sign(priv, signed)

	out := map[string]string{
		"credential_id":       credID,
		"client_data_json":    base64.RawURLEncoding.EncodeToString(clientDataJSON),
		"authenticator_data":  base64.RawURLEncoding.EncodeToString(authData),
		"signature":           base64.RawURLEncoding.EncodeToString(sig),
	}
	writeJSON(out)
}

func writeJSON(v any) {
	enc := json.NewEncoder(os.Stdout)
	enc.SetIndent("", "  ")
	if err := enc.Encode(v); err != nil {
		die("encode output: %v", err)
	}
}

func die(format string, args ...any) {
	fmt.Fprintf(os.Stderr, "webauthn-sign: "+format+"\n", args...)
	os.Exit(1)
}