at-container-registry/pkg/auth/appview_token.go

package auth

import (
	"crypto/rand"
	"encoding/base64"
	"fmt"
	"time"

	"github.com/bluesky-social/indigo/atproto/atcrypto"
	indigoauth "github.com/bluesky-social/indigo/atproto/auth"
	"github.com/bluesky-social/indigo/atproto/syntax"
	"github.com/golang-jwt/jwt/v5"
)

func randomNonce() string {
	buf := make([]byte, 16)
	_, _ = rand.Read(buf)
	return base64.RawURLEncoding.EncodeToString(buf)
}

// AppviewLxm is the lexicon method this token binds to. Hold doesn't enforce
// lxm today (we just trust the signature), but appview sends it for forward
// compatibility and so the JWT carries an unambiguous purpose.
//
// Kept as a typed NSID rather than a string so callers can't mistype it.
var AppviewLxm = syntax.NSID("io.atcr.hold.updateCrewTier")

// appviewServiceClaims extends RegisteredClaims with the `lxm` claim that
// indigo's SignServiceAuth uses. We define our own struct because we need to
// keep Subject (hold's verifier reads it as the acting user DID).
type appviewServiceClaims struct {
	jwt.RegisteredClaims
	LexMethod string `json:"lxm,omitempty"`
}

// CreateAppviewServiceToken creates a short-lived JWT for appview→hold
// communication, signed with the appview's private key.
//
// Supports both P-256 (ES256) and K-256/secp256k1 (ES256K) keys — the
// algorithm is selected from the concrete key type.
//
// Claims:
//   - iss: appview DID
//   - aud: hold DID
//   - sub: user DID being acted upon
//   - exp: now + 60s
//   - iat: now
//   - jti: random 16-byte nonce (replay prevention)
//   - lxm: lexicon method this token authorizes
//
// Signing uses indigo's atproto JWT signing method so the atcrypto key is
// used natively (no PKCS8 round-trip).
func CreateAppviewServiceToken(privateKey atcrypto.PrivateKey, appviewDID, holdDID, userDID string) (string, error) {
	var alg string
	switch privateKey.(type) {
	case *atcrypto.PrivateKeyP256:
		alg = "ES256"
	case *atcrypto.PrivateKeyK256:
		alg = "ES256K"
	default:
		return "", fmt.Errorf("unsupported appview key type %T (expected P-256 or K-256)", privateKey)
	}

	now := time.Now()
	claims := appviewServiceClaims{
		RegisteredClaims: jwt.RegisteredClaims{
			Issuer:    appviewDID,
			Audience:  jwt.ClaimStrings{holdDID},
			Subject:   userDID,
			ExpiresAt: jwt.NewNumericDate(now.Add(60 * time.Second)),
			IssuedAt:  jwt.NewNumericDate(now),
			ID:        randomNonce(),
		},
		LexMethod: AppviewLxm.String(),
	}

	sm := jwt.GetSigningMethod(alg)
	if sm == nil {
		return "", fmt.Errorf("%s signing method not registered", alg)
	}
	token := jwt.NewWithClaims(sm, claims)
	signed, err := token.SignedString(privateKey)
	if err != nil {
		return "", fmt.Errorf("failed to sign token: %w", err)
	}
	return signed, nil
}

// keep indigoauth referenced so its init() runs and overrides ES256/ES256K
// signing methods to accept atcrypto.PrivateKey directly.
var _ = indigoauth.SignServiceAuth