challenge.go

package acme

import (
	"context"
	"crypto"
	"crypto/ecdsa"
	"crypto/ed25519"
	"crypto/elliptic"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/subtle"
	"crypto/tls"
	"crypto/x509"
	"encoding/asn1"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"net/url"
	"reflect"
	"strconv"
	"strings"
	"time"

	"github.com/fxamacker/cbor/v2"
	"github.com/google/go-attestation/attest"
	"github.com/google/go-tpm/tpm2"
	"golang.org/x/exp/slices"

	"go.step.sm/crypto/jose"
	"go.step.sm/crypto/keyutil"
	"go.step.sm/crypto/pemutil"
	"go.step.sm/crypto/x509util"

	"github.com/smallstep/certificates/authority/provisioner"
)

type ChallengeType string

const (
	// HTTP01 is the http-01 ACME challenge type
	HTTP01 ChallengeType = "http-01"
	// DNS01 is the dns-01 ACME challenge type
	DNS01 ChallengeType = "dns-01"
	// TLSALPN01 is the tls-alpn-01 ACME challenge type
	TLSALPN01 ChallengeType = "tls-alpn-01"
	// DEVICEATTEST01 is the device-attest-01 ACME challenge type
	DEVICEATTEST01 ChallengeType = "device-attest-01"
)

var (
	// InsecurePortHTTP01 is the port used to verify http-01 challenges. If not set it
	// defaults to 80.
	InsecurePortHTTP01 int

	// InsecurePortTLSALPN01 is the port used to verify tls-alpn-01 challenges. If not
	// set it defaults to 443.
	//
	// This variable can be used for testing purposes.
	InsecurePortTLSALPN01 int
)

// Challenge represents an ACME response Challenge type.
type Challenge struct {
	ID              string        `json:"-"`
	AccountID       string        `json:"-"`
	AuthorizationID string        `json:"-"`
	Value           string        `json:"-"`
	Type            ChallengeType `json:"type"`
	Status          Status        `json:"status"`
	Token           string        `json:"token"`
	ValidatedAt     string        `json:"validated,omitempty"`
	URL             string        `json:"url"`
	Error           *Error        `json:"error,omitempty"`
}

// ToLog enables response logging.
func (ch *Challenge) ToLog() (interface{}, error) {
	b, err := json.Marshal(ch)
	if err != nil {
		return nil, WrapErrorISE(err, "error marshaling challenge for logging")
	}
	return string(b), nil
}

// Validate attempts to validate the Challenge. Stores changes to the Challenge
// type using the DB interface. If the Challenge is validated, the 'status' and
// 'validated' attributes are updated.
func (ch *Challenge) Validate(ctx context.Context, db DB, jwk *jose.JSONWebKey, payload []byte) error {
	// If already valid or invalid then return without performing validation.
	if ch.Status != StatusPending {
		return nil
	}
	switch ch.Type {
	case HTTP01:
		return http01Validate(ctx, ch, db, jwk)
	case DNS01:
		return dns01Validate(ctx, ch, db, jwk)
	case TLSALPN01:
		return tlsalpn01Validate(ctx, ch, db, jwk)
	case DEVICEATTEST01:
		return deviceAttest01Validate(ctx, ch, db, jwk, payload)
	default:
		return NewErrorISE("unexpected challenge type '%s'", ch.Type)
	}
}

func http01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error {
	u := &url.URL{Scheme: "http", Host: http01ChallengeHost(ch.Value), Path: fmt.Sprintf("/.well-known/acme-challenge/%s", ch.Token)}

	// Append insecure port if set.
	// Only used for testing purposes.
	if InsecurePortHTTP01 != 0 {
		u.Host += ":" + strconv.Itoa(InsecurePortHTTP01)
	}

	vc := MustClientFromContext(ctx)
	resp, err := vc.Get(u.String())
	if err != nil {
		return storeError(ctx, db, ch, false, WrapError(ErrorConnectionType, err,
			"error doing http GET for url %s", u))
	}
	defer resp.Body.Close()
	if resp.StatusCode >= 400 {
		return storeError(ctx, db, ch, false, NewError(ErrorConnectionType,
			"error doing http GET for url %s with status code %d", u, resp.StatusCode))
	}

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return WrapErrorISE(err, "error reading "+
			"response body for url %s", u)
	}
	keyAuth := strings.TrimSpace(string(body))

	expected, err := KeyAuthorization(ch.Token, jwk)
	if err != nil {
		return err
	}
	if keyAuth != expected {
		return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
			"keyAuthorization does not match; expected %s, but got %s", expected, keyAuth))
	}

	// Update and store the challenge.
	ch.Status = StatusValid
	ch.Error = nil
	ch.ValidatedAt = clock.Now().Format(time.RFC3339)

	if err = db.UpdateChallenge(ctx, ch); err != nil {
		return WrapErrorISE(err, "error updating challenge")
	}
	return nil
}

// http01ChallengeHost checks if a Challenge value is an IPv6 address
// and adds square brackets if that's the case, so that it can be used
// as a hostname. Returns the original Challenge value as the host to
// use in other cases.
func http01ChallengeHost(value string) string {
	if ip := net.ParseIP(value); ip != nil && ip.To4() == nil {
		value = "[" + value + "]"
	}
	return value
}

func tlsAlert(err error) uint8 {
	var opErr *net.OpError
	if errors.As(err, &opErr) {
		v := reflect.ValueOf(opErr.Err)
		if v.Kind() == reflect.Uint8 {
			return uint8(v.Uint())
		}
	}
	return 0
}

func tlsalpn01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error {
	config := &tls.Config{
		NextProtos: []string{"acme-tls/1"},
		// https://tools.ietf.org/html/rfc8737#section-4
		// ACME servers that implement "acme-tls/1" MUST only negotiate TLS 1.2
		// [RFC5246] or higher when connecting to clients for validation.
		MinVersion:         tls.VersionTLS12,
		ServerName:         serverName(ch),
		InsecureSkipVerify: true, //nolint:gosec // we expect a self-signed challenge certificate
	}

	var hostPort string

	// Allow to change TLS port for testing purposes.
	if port := InsecurePortTLSALPN01; port == 0 {
		hostPort = net.JoinHostPort(ch.Value, "443")
	} else {
		hostPort = net.JoinHostPort(ch.Value, strconv.Itoa(port))
	}

	vc := MustClientFromContext(ctx)
	conn, err := vc.TLSDial("tcp", hostPort, config)
	if err != nil {
		// With Go 1.17+ tls.Dial fails if there's no overlap between configured
		// client and server protocols. When this happens the connection is
		// closed with the error no_application_protocol(120) as required by
		// RFC7301. See https://golang.org/doc/go1.17#ALPN
		if tlsAlert(err) == 120 {
			return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
				"cannot negotiate ALPN acme-tls/1 protocol for tls-alpn-01 challenge"))
		}
		return storeError(ctx, db, ch, false, WrapError(ErrorConnectionType, err,
			"error doing TLS dial for %s", hostPort))
	}
	defer conn.Close()

	cs := conn.ConnectionState()
	certs := cs.PeerCertificates

	if len(certs) == 0 {
		return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
			"%s challenge for %s resulted in no certificates", ch.Type, ch.Value))
	}

	if cs.NegotiatedProtocol != "acme-tls/1" {
		return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
			"cannot negotiate ALPN acme-tls/1 protocol for tls-alpn-01 challenge"))
	}

	leafCert := certs[0]

	// if no DNS names present, look for IP address and verify that exactly one exists
	if len(leafCert.DNSNames) == 0 {
		if len(leafCert.IPAddresses) != 1 || !leafCert.IPAddresses[0].Equal(net.ParseIP(ch.Value)) {
			return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
				"incorrect certificate for tls-alpn-01 challenge: leaf certificate must contain a single IP address or DNS name, %v", ch.Value))
		}
	} else {
		if len(leafCert.DNSNames) != 1 || !strings.EqualFold(leafCert.DNSNames[0], ch.Value) {
			return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
				"incorrect certificate for tls-alpn-01 challenge: leaf certificate must contain a single IP address or DNS name, %v", ch.Value))
		}
	}

	idPeAcmeIdentifier := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 31}
	idPeAcmeIdentifierV1Obsolete := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 30, 1}
	foundIDPeAcmeIdentifierV1Obsolete := false

	keyAuth, err := KeyAuthorization(ch.Token, jwk)
	if err != nil {
		return err
	}
	hashedKeyAuth := sha256.Sum256([]byte(keyAuth))

	for _, ext := range leafCert.Extensions {
		if idPeAcmeIdentifier.Equal(ext.Id) {
			if !ext.Critical {
				return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
					"incorrect certificate for tls-alpn-01 challenge: acmeValidationV1 extension not critical"))
			}

			var extValue []byte
			rest, err := asn1.Unmarshal(ext.Value, &extValue)

			if err != nil || len(rest) > 0 || len(hashedKeyAuth) != len(extValue) {
				return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
					"incorrect certificate for tls-alpn-01 challenge: malformed acmeValidationV1 extension value"))
			}

			if subtle.ConstantTimeCompare(hashedKeyAuth[:], extValue) != 1 {
				return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
					"incorrect certificate for tls-alpn-01 challenge: "+
						"expected acmeValidationV1 extension value %s for this challenge but got %s",
					hex.EncodeToString(hashedKeyAuth[:]), hex.EncodeToString(extValue)))
			}

			ch.Status = StatusValid
			ch.Error = nil
			ch.ValidatedAt = clock.Now().Format(time.RFC3339)

			if err = db.UpdateChallenge(ctx, ch); err != nil {
				return WrapErrorISE(err, "tlsalpn01ValidateChallenge - error updating challenge")
			}
			return nil
		}

		if idPeAcmeIdentifierV1Obsolete.Equal(ext.Id) {
			foundIDPeAcmeIdentifierV1Obsolete = true
		}
	}

	if foundIDPeAcmeIdentifierV1Obsolete {
		return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
			"incorrect certificate for tls-alpn-01 challenge: obsolete id-pe-acmeIdentifier in acmeValidationV1 extension"))
	}

	return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
		"incorrect certificate for tls-alpn-01 challenge: missing acmeValidationV1 extension"))
}

func dns01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error {
	// Normalize domain for wildcard DNS names
	// This is done to avoid making TXT lookups for domains like
	// _acme-challenge.*.example.com
	// Instead perform txt lookup for _acme-challenge.example.com
	domain := strings.TrimPrefix(ch.Value, "*.")

	vc := MustClientFromContext(ctx)
	txtRecords, err := vc.LookupTxt("_acme-challenge." + domain)
	if err != nil {
		return storeError(ctx, db, ch, false, WrapError(ErrorDNSType, err,
			"error looking up TXT records for domain %s", domain))
	}

	expectedKeyAuth, err := KeyAuthorization(ch.Token, jwk)
	if err != nil {
		return err
	}
	h := sha256.Sum256([]byte(expectedKeyAuth))
	expected := base64.RawURLEncoding.EncodeToString(h[:])
	var found bool
	for _, r := range txtRecords {
		if r == expected {
			found = true
			break
		}
	}
	if !found {
		return storeError(ctx, db, ch, false, NewError(ErrorRejectedIdentifierType,
			"keyAuthorization does not match; expected %s, but got %s", expectedKeyAuth, txtRecords))
	}

	// Update and store the challenge.
	ch.Status = StatusValid
	ch.Error = nil
	ch.ValidatedAt = clock.Now().Format(time.RFC3339)

	if err = db.UpdateChallenge(ctx, ch); err != nil {
		return WrapErrorISE(err, "error updating challenge")
	}
	return nil
}

type payloadType struct {
	AttObj string `json:"attObj"`
	Error  string `json:"error"`
}

type attestationObject struct {
	Format       string                 `json:"fmt"`
	AttStatement map[string]interface{} `json:"attStmt,omitempty"`
}

// TODO(bweeks): move attestation verification to a shared package.
func deviceAttest01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey, payload []byte) error {
	// Load authorization to store the key fingerprint.
	az, err := db.GetAuthorization(ctx, ch.AuthorizationID)
	if err != nil {
		return WrapErrorISE(err, "error loading authorization")
	}

	// Parse payload.
	var p payloadType
	if err := json.Unmarshal(payload, &p); err != nil {
		return WrapErrorISE(err, "error unmarshalling JSON")
	}
	if p.Error != "" {
		return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
			"payload contained error: %v", p.Error))
	}

	attObj, err := base64.RawURLEncoding.DecodeString(p.AttObj)
	if err != nil {
		return WrapErrorISE(err, "error base64 decoding attObj")
	}

	att := attestationObject{}
	if err := cbor.Unmarshal(attObj, &att); err != nil {
		return WrapErrorISE(err, "error unmarshalling CBOR")
	}

	prov := MustProvisionerFromContext(ctx)
	if !prov.IsAttestationFormatEnabled(ctx, provisioner.ACMEAttestationFormat(att.Format)) {
		return storeError(ctx, db, ch, true,
			NewError(ErrorBadAttestationStatementType, "attestation format %q is not enabled", att.Format))
	}

	switch att.Format {
	case "apple":
		data, err := doAppleAttestationFormat(ctx, prov, ch, &att)
		if err != nil {
			var acmeError *Error
			if errors.As(err, &acmeError) {
				if acmeError.Status == 500 {
					return acmeError
				}
				return storeError(ctx, db, ch, true, acmeError)
			}
			return WrapErrorISE(err, "error validating attestation")
		}
		// Validate nonce with SHA-256 of the token.
		if len(data.Nonce) != 0 {
			sum := sha256.Sum256([]byte(ch.Token))
			if subtle.ConstantTimeCompare(data.Nonce, sum[:]) != 1 {
				return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatementType, "challenge token does not match"))
			}
		}

		// Validate Apple's ClientIdentifier (Identifier.Value) with device
		// identifiers.
		//
		// Note: We might want to use an external service for this.
		if data.UDID != ch.Value && data.SerialNumber != ch.Value {
			return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatementType, "permanent identifier does not match"))
		}

		// Update attestation key fingerprint to compare against the CSR
		az.Fingerprint = data.Fingerprint
	case "step":
		data, err := doStepAttestationFormat(ctx, prov, ch, jwk, &att)
		if err != nil {
			var acmeError *Error
			if errors.As(err, &acmeError) {
				if acmeError.Status == 500 {
					return acmeError
				}
				return storeError(ctx, db, ch, true, acmeError)
			}
			return WrapErrorISE(err, "error validating attestation")
		}

		// Validate the YubiKey serial number from the attestation
		// certificate with the challenged Order value.
		//
		// Note: We might want to use an external service for this.
		if data.SerialNumber != ch.Value {
			subproblem := NewSubproblemWithIdentifier(
				ErrorMalformedType,
				Identifier{Type: "permanent-identifier", Value: ch.Value},
				"challenge identifier %q doesn't match the attested hardware identifier %q", ch.Value, data.SerialNumber,
			)
			return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatementType, "permanent identifier does not match").AddSubproblems(subproblem))
		}

		// Update attestation key fingerprint to compare against the CSR
		az.Fingerprint = data.Fingerprint

	case "tpm":
		data, err := doTPMAttestationFormat(ctx, prov, ch, jwk, &att)
		if err != nil {
			// TODO(hs): we should provide more details in the error reported to the client;
			// "Attestation statement cannot be verified" is VERY generic. Also holds true for the other formats.
			var acmeError *Error
			if errors.As(err, &acmeError) {
				if acmeError.Status == 500 {
					return acmeError
				}
				return storeError(ctx, db, ch, true, acmeError)
			}
			return WrapErrorISE(err, "error validating attestation")
		}

		// TODO(hs): currently this will allow a request for which no PermanentIdentifiers have been
		// extracted from the AK certificate. This is currently the case for AK certs from the CLI, as we
		// haven't implemented a way for AK certs requested by the CLI to always contain the requested
		// PermanentIdentifier. Omitting the check below doesn't allow just any request, as the Order can
		// still fail if the challenge value isn't equal to the CSR subject.
		if len(data.PermanentIdentifiers) > 0 && !slices.Contains(data.PermanentIdentifiers, ch.Value) { // TODO(hs): add support for HardwareModuleName
			subproblem := NewSubproblemWithIdentifier(
				ErrorMalformedType,
				Identifier{Type: "permanent-identifier", Value: ch.Value},
				"challenge identifier %q doesn't match any of the attested hardware identifiers %q", ch.Value, data.PermanentIdentifiers,
			)
			return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "permanent identifier does not match").AddSubproblems(subproblem))
		}

		// Update attestation key fingerprint to compare against the CSR
		az.Fingerprint = data.Fingerprint
	default:
		return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatementType, "unexpected attestation object format"))
	}

	// Update and store the challenge.
	ch.Status = StatusValid
	ch.Error = nil
	ch.ValidatedAt = clock.Now().Format(time.RFC3339)

	// Store the fingerprint in the authorization.
	//
	// TODO: add method to update authorization and challenge atomically.
	if az.Fingerprint != "" {
		if err := db.UpdateAuthorization(ctx, az); err != nil {
			return WrapErrorISE(err, "error updating authorization")
		}
	}

	if err := db.UpdateChallenge(ctx, ch); err != nil {
		return WrapErrorISE(err, "error updating challenge")
	}
	return nil
}

var (
	oidSubjectAlternativeName = asn1.ObjectIdentifier{2, 5, 29, 17}
)

type tpmAttestationData struct {
	Certificate          *x509.Certificate
	VerifiedChains       [][]*x509.Certificate
	PermanentIdentifiers []string
	Fingerprint          string
}

// coseAlgorithmIdentifier models a COSEAlgorithmIdentifier.
// Also see https://www.w3.org/TR/webauthn-2/#sctn-alg-identifier.
type coseAlgorithmIdentifier int32

const (
	coseAlgES256 coseAlgorithmIdentifier = -7
	coseAlgRS256 coseAlgorithmIdentifier = -257
)

func doTPMAttestationFormat(ctx context.Context, prov Provisioner, ch *Challenge, jwk *jose.JSONWebKey, att *attestationObject) (*tpmAttestationData, error) {
	ver, ok := att.AttStatement["ver"].(string)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "ver not present")
	}
	if ver != "2.0" {
		return nil, NewError(ErrorBadAttestationStatementType, "version %q is not supported", ver)
	}

	x5c, ok := att.AttStatement["x5c"].([]interface{})
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c not present")
	}
	if len(x5c) == 0 {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c is empty")
	}

	akCertBytes, ok := x5c[0].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
	}
	akCert, err := x509.ParseCertificate(akCertBytes)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
	}

	intermediates := x509.NewCertPool()
	for _, v := range x5c[1:] {
		intCertBytes, vok := v.([]byte)
		if !vok {
			return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
		}
		intCert, err := x509.ParseCertificate(intCertBytes)
		if err != nil {
			return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
		}
		intermediates.AddCert(intCert)
	}

	// TODO(hs): this can be removed when permanent-identifier/hardware-module-name are handled correctly in
	// the stdlib in https://cs.opensource.google/go/go/+/refs/tags/go1.19:src/crypto/x509/parser.go;drc=b5b2cf519fe332891c165077f3723ee74932a647;l=362,
	// but I doubt that will happen.
	if len(akCert.UnhandledCriticalExtensions) > 0 {
		unhandledCriticalExtensions := akCert.UnhandledCriticalExtensions[:0]
		for _, extOID := range akCert.UnhandledCriticalExtensions {
			if !extOID.Equal(oidSubjectAlternativeName) {
				// critical extensions other than the Subject Alternative Name remain unhandled
				unhandledCriticalExtensions = append(unhandledCriticalExtensions, extOID)
			}
		}
		akCert.UnhandledCriticalExtensions = unhandledCriticalExtensions
	}

	roots, ok := prov.GetAttestationRoots()
	if !ok {
		return nil, NewErrorISE("no root CA bundle available to verify the attestation certificate")
	}

	// verify that the AK certificate was signed by a trusted root,
	// chained to by the intermediates provided by the client. As part
	// of building the verified certificate chain, the signature over the
	// AK certificate is checked to be a valid signature of one of the
	// provided intermediates. Signatures over the intermediates are in
	// turn also verified to be valid signatures from one of the trusted
	// roots.
	verifiedChains, err := akCert.Verify(x509.VerifyOptions{
		Roots:         roots,
		Intermediates: intermediates,
		CurrentTime:   time.Now().Truncate(time.Second),
		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageAny},
	})
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is not valid")
	}

	// validate additional AK certificate requirements
	if err := validateAKCertificate(akCert); err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "AK certificate is not valid")
	}

	// TODO(hs): implement revocation check; Verify() doesn't perform CRL check nor OCSP lookup.

	sans, err := x509util.ParseSubjectAlternativeNames(akCert)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "failed parsing AK certificate Subject Alternative Names")
	}

	permanentIdentifiers := make([]string, len(sans.PermanentIdentifiers))
	for i, pi := range sans.PermanentIdentifiers {
		permanentIdentifiers[i] = pi.Identifier
	}

	// extract and validate pubArea, sig, certInfo and alg properties from the request body
	pubArea, ok := att.AttStatement["pubArea"].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "invalid pubArea in attestation statement")
	}
	if len(pubArea) == 0 {
		return nil, NewError(ErrorBadAttestationStatementType, "pubArea is empty")
	}

	sig, ok := att.AttStatement["sig"].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "invalid sig in attestation statement")
	}
	if len(sig) == 0 {
		return nil, NewError(ErrorBadAttestationStatementType, "sig is empty")
	}

	certInfo, ok := att.AttStatement["certInfo"].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "invalid certInfo in attestation statement")
	}
	if len(certInfo) == 0 {
		return nil, NewError(ErrorBadAttestationStatementType, "certInfo is empty")
	}

	alg, ok := att.AttStatement["alg"].(int64)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "invalid alg in attestation statement")
	}

	// only RS256 and ES256 are allowed
	coseAlg := coseAlgorithmIdentifier(alg)
	if coseAlg != coseAlgRS256 && coseAlg != coseAlgES256 {
		return nil, NewError(ErrorBadAttestationStatementType, "invalid alg %d in attestation statement", alg)
	}

	// set the hash algorithm to use to SHA256
	hash := crypto.SHA256

	// recreate the generated key certification parameter values and verify
	// the attested key using the public key of the AK.
	certificationParameters := &attest.CertificationParameters{
		Public:            pubArea,  // the public key that was attested
		CreateAttestation: certInfo, // the attested properties of the key
		CreateSignature:   sig,      // signature over the attested properties
	}
	verifyOpts := attest.VerifyOpts{
		Public: akCert.PublicKey, // public key of the AK that attested the key
		Hash:   hash,
	}
	if err = certificationParameters.Verify(verifyOpts); err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "invalid certification parameters")
	}

	// decode the "certInfo" data. This won't fail, as it's also done as part of Verify().
	tpmCertInfo, err := tpm2.DecodeAttestationData(certInfo)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "failed decoding attestation data")
	}

	keyAuth, err := KeyAuthorization(ch.Token, jwk)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "failed creating key auth digest")
	}
	hashedKeyAuth := sha256.Sum256([]byte(keyAuth))

	// verify the WebAuthn object contains the expect key authorization digest, which is carried
	// within the encoded `certInfo` property of the attestation statement.
	if subtle.ConstantTimeCompare(hashedKeyAuth[:], []byte(tpmCertInfo.ExtraData)) == 0 {
		return nil, NewError(ErrorBadAttestationStatementType, "key authorization does not match")
	}

	// decode the (attested) public key and determine its fingerprint.  This won't fail, as it's also done as part of Verify().
	pub, err := tpm2.DecodePublic(pubArea)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "failed decoding pubArea")
	}

	publicKey, err := pub.Key()
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "failed getting public key")
	}

	data := &tpmAttestationData{
		Certificate:          akCert,
		VerifiedChains:       verifiedChains,
		PermanentIdentifiers: permanentIdentifiers,
	}

	if data.Fingerprint, err = keyutil.Fingerprint(publicKey); err != nil {
		return nil, WrapErrorISE(err, "error calculating key fingerprint")
	}

	// TODO(hs): pass more attestation data, so that that can be used/recorded too?
	return data, nil
}

var (
	oidExtensionExtendedKeyUsage = asn1.ObjectIdentifier{2, 5, 29, 37}
	oidTCGKpAIKCertificate       = asn1.ObjectIdentifier{2, 23, 133, 8, 3}
)

// validateAKCertifiate validates the X.509 AK certificate to be
// in accordance with the required properties. The requirements come from:
// https://www.w3.org/TR/webauthn-2/#sctn-tpm-cert-requirements.
//
//   - Version MUST be set to 3.
//   - Subject field MUST be set to empty.
//   - The Subject Alternative Name extension MUST be set as defined
//     in [TPMv2-EK-Profile] section 3.2.9.
//   - The Extended Key Usage extension MUST contain the OID 2.23.133.8.3
//     ("joint-iso-itu-t(2) internationalorganizations(23) 133 tcg-kp(8) tcg-kp-AIKCertificate(3)").
//   - The Basic Constraints extension MUST have the CA component set to false.
//   - An Authority Information Access (AIA) extension with entry id-ad-ocsp
//     and a CRL Distribution Point extension [RFC5280] are both OPTIONAL as
//     the status of many attestation certificates is available through metadata
//     services. See, for example, the FIDO Metadata Service.
func validateAKCertificate(c *x509.Certificate) error {
	if c.Version != 3 {
		return fmt.Errorf("AK certificate has invalid version %d; only version 3 is allowed", c.Version)
	}
	if c.Subject.String() != "" {
		return fmt.Errorf("AK certificate subject must be empty; got %q", c.Subject)
	}
	if c.IsCA {
		return errors.New("AK certificate must not be a CA")
	}
	if err := validateAKCertificateExtendedKeyUsage(c); err != nil {
		return err
	}
	if err := validateAKCertificateSubjectAlternativeNames(c); err != nil {
		return err
	}

	return nil
}

// validateAKCertificateSubjectAlternativeNames checks if the AK certificate
// has TPM hardware details set.
func validateAKCertificateSubjectAlternativeNames(c *x509.Certificate) error {
	sans, err := x509util.ParseSubjectAlternativeNames(c)
	if err != nil {
		return fmt.Errorf("failed parsing AK certificate Subject Alternative Names: %w", err)
	}

	details := sans.TPMHardwareDetails
	manufacturer, model, version := details.Manufacturer, details.Model, details.Version

	switch {
	case manufacturer == "":
		return errors.New("missing TPM manufacturer")
	case model == "":
		return errors.New("missing TPM model")
	case version == "":
		return errors.New("missing TPM version")
	}

	return nil
}

// validateAKCertificateExtendedKeyUsage checks if the AK certificate
// has the "tcg-kp-AIKCertificate" Extended Key Usage set.
func validateAKCertificateExtendedKeyUsage(c *x509.Certificate) error {
	var (
		valid = false
		ekus  []asn1.ObjectIdentifier
	)
	for _, ext := range c.Extensions {
		if ext.Id.Equal(oidExtensionExtendedKeyUsage) {
			if _, err := asn1.Unmarshal(ext.Value, &ekus); err != nil || !ekus[0].Equal(oidTCGKpAIKCertificate) {
				return errors.New("AK certificate is missing Extended Key Usage value tcg-kp-AIKCertificate (2.23.133.8.3)")
			}
			valid = true
		}
	}

	if !valid {
		return errors.New("AK certificate is missing Extended Key Usage extension")
	}

	return nil
}

// Apple Enterprise Attestation Root CA from
// https://www.apple.com/certificateauthority/private/
const appleEnterpriseAttestationRootCA = `-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`

var (
	oidAppleSerialNumber                    = asn1.ObjectIdentifier{1, 2, 840, 113635, 100, 8, 9, 1}
	oidAppleUniqueDeviceIdentifier          = asn1.ObjectIdentifier{1, 2, 840, 113635, 100, 8, 9, 2}
	oidAppleSecureEnclaveProcessorOSVersion = asn1.ObjectIdentifier{1, 2, 840, 113635, 100, 8, 10, 2}
	oidAppleNonce                           = asn1.ObjectIdentifier{1, 2, 840, 113635, 100, 8, 11, 1}
)

type appleAttestationData struct {
	Nonce        []byte
	SerialNumber string
	UDID         string
	SEPVersion   string
	Certificate  *x509.Certificate
	Fingerprint  string
}

func doAppleAttestationFormat(ctx context.Context, prov Provisioner, ch *Challenge, att *attestationObject) (*appleAttestationData, error) {
	// Use configured or default attestation roots if none is configured.
	roots, ok := prov.GetAttestationRoots()
	if !ok {
		root, err := pemutil.ParseCertificate([]byte(appleEnterpriseAttestationRootCA))
		if err != nil {
			return nil, WrapErrorISE(err, "error parsing apple enterprise ca")
		}
		roots = x509.NewCertPool()
		roots.AddCert(root)
	}

	x5c, ok := att.AttStatement["x5c"].([]interface{})
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c not present")
	}
	if len(x5c) == 0 {
		return nil, NewError(ErrorRejectedIdentifierType, "x5c is empty")
	}

	der, ok := x5c[0].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
	}
	leaf, err := x509.ParseCertificate(der)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
	}

	intermediates := x509.NewCertPool()
	for _, v := range x5c[1:] {
		der, ok = v.([]byte)
		if !ok {
			return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
		}
		cert, err := x509.ParseCertificate(der)
		if err != nil {
			return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
		}
		intermediates.AddCert(cert)
	}

	if _, err := leaf.Verify(x509.VerifyOptions{
		Intermediates: intermediates,
		Roots:         roots,
		CurrentTime:   time.Now().Truncate(time.Second),
		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageAny},
	}); err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is not valid")
	}

	data := &appleAttestationData{
		Certificate: leaf,
	}
	if data.Fingerprint, err = keyutil.Fingerprint(leaf.PublicKey); err != nil {
		return nil, WrapErrorISE(err, "error calculating key fingerprint")
	}
	for _, ext := range leaf.Extensions {
		switch {
		case ext.Id.Equal(oidAppleSerialNumber):
			data.SerialNumber = string(ext.Value)
		case ext.Id.Equal(oidAppleUniqueDeviceIdentifier):
			data.UDID = string(ext.Value)
		case ext.Id.Equal(oidAppleSecureEnclaveProcessorOSVersion):
			data.SEPVersion = string(ext.Value)
		case ext.Id.Equal(oidAppleNonce):
			data.Nonce = ext.Value
		}
	}

	return data, nil
}

// Yubico PIV Root CA Serial 263751
// https://developers.yubico.com/PIV/Introduction/piv-attestation-ca.pem
const yubicoPIVRootCA = `-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`

// Serial number of the YubiKey, encoded as an integer.
// https://developers.yubico.com/PIV/Introduction/PIV_attestation.html
var oidYubicoSerialNumber = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 41482, 3, 7}

type stepAttestationData struct {
	Certificate  *x509.Certificate
	SerialNumber string
	Fingerprint  string
}

func doStepAttestationFormat(ctx context.Context, prov Provisioner, ch *Challenge, jwk *jose.JSONWebKey, att *attestationObject) (*stepAttestationData, error) {
	// Use configured or default attestation roots if none is configured.
	roots, ok := prov.GetAttestationRoots()
	if !ok {
		root, err := pemutil.ParseCertificate([]byte(yubicoPIVRootCA))
		if err != nil {
			return nil, WrapErrorISE(err, "error parsing root ca")
		}
		roots = x509.NewCertPool()
		roots.AddCert(root)
	}

	// Extract x5c and verify certificate
	x5c, ok := att.AttStatement["x5c"].([]interface{})
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c not present")
	}
	if len(x5c) == 0 {
		return nil, NewError(ErrorRejectedIdentifierType, "x5c is empty")
	}
	der, ok := x5c[0].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
	}
	leaf, err := x509.ParseCertificate(der)
	if err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
	}
	intermediates := x509.NewCertPool()
	for _, v := range x5c[1:] {
		der, ok = v.([]byte)
		if !ok {
			return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
		}
		cert, err := x509.ParseCertificate(der)
		if err != nil {
			return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
		}
		intermediates.AddCert(cert)
	}
	if _, err := leaf.Verify(x509.VerifyOptions{
		Intermediates: intermediates,
		Roots:         roots,
		CurrentTime:   time.Now().Truncate(time.Second),
		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageAny},
	}); err != nil {
		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is not valid")
	}

	// Verify proof of possession of private key validating the key
	// authorization. Per recommendation at
	// https://w3c.github.io/webauthn/#sctn-signature-attestation-types the
	// signature is CBOR-encoded.
	var sig []byte
	csig, ok := att.AttStatement["sig"].([]byte)
	if !ok {
		return nil, NewError(ErrorBadAttestationStatementType, "sig not present")
	}
	if err := cbor.Unmarshal(csig, &sig); err != nil {
		return nil, NewError(ErrorBadAttestationStatementType, "sig is malformed")
	}
	keyAuth, err := KeyAuthorization(ch.Token, jwk)
	if err != nil {
		return nil, err
	}

	switch pub := leaf.PublicKey.(type) {
	case *ecdsa.PublicKey:
		if pub.Curve != elliptic.P256() {
			return nil, WrapError(ErrorBadAttestationStatementType, err, "unsupported elliptic curve %s", pub.Curve)
		}
		sum := sha256.Sum256([]byte(keyAuth))
		if !ecdsa.VerifyASN1(pub, sum[:], sig) {
			return nil, NewError(ErrorBadAttestationStatementType, "failed to validate signature")
		}
	case *rsa.PublicKey:
		sum := sha256.Sum256([]byte(keyAuth))
		if err := rsa.VerifyPKCS1v15(pub, crypto.SHA256, sum[:], sig); err != nil {
			return nil, NewError(ErrorBadAttestationStatementType, "failed to validate signature")
		}
	case ed25519.PublicKey:
		if !ed25519.Verify(pub, []byte(keyAuth), sig) {
			return nil, NewError(ErrorBadAttestationStatementType, "failed to validate signature")
		}
	default:
		return nil, NewError(ErrorBadAttestationStatementType, "unsupported public key type %T", pub)
	}

	// Parse attestation data:
	// TODO(mariano): add support for other extensions.
	data := &stepAttestationData{
		Certificate: leaf,
	}
	if data.Fingerprint, err = keyutil.Fingerprint(leaf.PublicKey); err != nil {
		return nil, WrapErrorISE(err, "error calculating key fingerprint")
	}
	for _, ext := range leaf.Extensions {
		if !ext.Id.Equal(oidYubicoSerialNumber) {
			continue
		}
		var serialNumber int
		rest, err := asn1.Unmarshal(ext.Value, &serialNumber)
		if err != nil || len(rest) > 0 {
			return nil, WrapError(ErrorBadAttestationStatementType, err, "error parsing serial number")
		}
		data.SerialNumber = strconv.Itoa(serialNumber)
		break
	}

	return data, nil
}

// serverName determines the SNI HostName to set based on an acme.Challenge
// for TLS-ALPN-01 challenges RFC8738 states that, if HostName is an IP, it
// should be the ARPA address https://datatracker.ietf.org/doc/html/rfc8738#section-6.
// It also references TLS Extensions [RFC6066].
func serverName(ch *Challenge) string {
	var serverName string
	ip := net.ParseIP(ch.Value)
	if ip != nil {
		serverName = reverseAddr(ip)
	} else {
		serverName = ch.Value
	}
	return serverName
}

// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
// address addr suitable for rDNS (PTR) record lookup or an error if it fails
// to parse the IP address.
// Implementation taken and adapted from https://golang.org/src/net/dnsclient.go?s=780:834#L20
func reverseAddr(ip net.IP) (arpa string) {
	if ip.To4() != nil {
		return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa."
	}
	// Must be IPv6
	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
	// Add it, in reverse, to the buffer
	for i := len(ip) - 1; i >= 0; i-- {
		v := ip[i]
		buf = append(buf, hexit[v&0xF],
			'.',
			hexit[v>>4],
			'.')
	}
	// Append "ip6.arpa." and return (buf already has the final .)
	buf = append(buf, "ip6.arpa."...)
	return string(buf)
}

// Convert unsigned integer to decimal string.
// Implementation taken from https://golang.org/src/net/parse.go
func uitoa(val uint) string {
	if val == 0 { // avoid string allocation
		return "0"
	}
	var buf [20]byte // big enough for 64bit value base 10
	i := len(buf) - 1
	for val >= 10 {
		v := val / 10
		buf[i] = byte('0' + val - v*10)
		i--
		val = v
	}
	// val < 10
	buf[i] = byte('0' + val)
	return string(buf[i:])
}

const hexit = "0123456789abcdef"

// KeyAuthorization creates the ACME key authorization value from a token
// and a jwk.
func KeyAuthorization(token string, jwk *jose.JSONWebKey) (string, error) {
	thumbprint, err := jwk.Thumbprint(crypto.SHA256)
	if err != nil {
		return "", WrapErrorISE(err, "error generating JWK thumbprint")
	}
	encPrint := base64.RawURLEncoding.EncodeToString(thumbprint)
	return fmt.Sprintf("%s.%s", token, encPrint), nil
}

// storeError the given error to an ACME error and saves using the DB interface.
func storeError(ctx context.Context, db DB, ch *Challenge, markInvalid bool, err *Error) error {
	ch.Error = err
	if markInvalid {
		ch.Status = StatusInvalid
	}
	if err := db.UpdateChallenge(ctx, ch); err != nil {
		return WrapErrorISE(err, "failure saving error to acme challenge")
	}
	return nil
}