Source file src/crypto/x509/x509.go

Documentation: crypto/x509

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Package x509 parses X.509-encoded keys and certificates.
  //
  // On UNIX systems the environment variables SSL_CERT_FILE and SSL_CERT_DIR
  // can be used to override the system default locations for the SSL certificate
  // file and SSL certificate files directory, respectively.
  package x509
  
  import (
  	"bytes"
  	"crypto"
  	"crypto/dsa"
  	"crypto/ecdsa"
  	"crypto/elliptic"
  	"crypto/rsa"
  	_ "crypto/sha1"
  	_ "crypto/sha256"
  	_ "crypto/sha512"
  	"crypto/x509/pkix"
  	"encoding/asn1"
  	"encoding/pem"
  	"errors"
  	"fmt"
  	"io"
  	"math/big"
  	"net"
  	"net/url"
  	"strconv"
  	"strings"
  	"time"
  	"unicode/utf8"
  
  	"golang_org/x/crypto/cryptobyte"
  	cryptobyte_asn1 "golang_org/x/crypto/cryptobyte/asn1"
  )
  
  // pkixPublicKey reflects a PKIX public key structure. See SubjectPublicKeyInfo
  // in RFC 3280.
  type pkixPublicKey struct {
  	Algo      pkix.AlgorithmIdentifier
  	BitString asn1.BitString
  }
  
  // ParsePKIXPublicKey parses a DER encoded public key. These values are
  // typically found in PEM blocks with "BEGIN PUBLIC KEY".
  //
  // Supported key types include RSA, DSA, and ECDSA. Unknown key
  // types result in an error.
  //
  // On success, pub will be of type *rsa.PublicKey, *dsa.PublicKey,
  // or *ecdsa.PublicKey.
  func ParsePKIXPublicKey(derBytes []byte) (pub interface{}, err error) {
  	var pki publicKeyInfo
  	if rest, err := asn1.Unmarshal(derBytes, &pki); err != nil {
  		return nil, err
  	} else if len(rest) != 0 {
  		return nil, errors.New("x509: trailing data after ASN.1 of public-key")
  	}
  	algo := getPublicKeyAlgorithmFromOID(pki.Algorithm.Algorithm)
  	if algo == UnknownPublicKeyAlgorithm {
  		return nil, errors.New("x509: unknown public key algorithm")
  	}
  	return parsePublicKey(algo, &pki)
  }
  
  func marshalPublicKey(pub interface{}) (publicKeyBytes []byte, publicKeyAlgorithm pkix.AlgorithmIdentifier, err error) {
  	switch pub := pub.(type) {
  	case *rsa.PublicKey:
  		publicKeyBytes, err = asn1.Marshal(pkcs1PublicKey{
  			N: pub.N,
  			E: pub.E,
  		})
  		if err != nil {
  			return nil, pkix.AlgorithmIdentifier{}, err
  		}
  		publicKeyAlgorithm.Algorithm = oidPublicKeyRSA
  		// This is a NULL parameters value which is required by
  		// https://tools.ietf.org/html/rfc3279#section-2.3.1.
  		publicKeyAlgorithm.Parameters = asn1.NullRawValue
  	case *ecdsa.PublicKey:
  		publicKeyBytes = elliptic.Marshal(pub.Curve, pub.X, pub.Y)
  		oid, ok := oidFromNamedCurve(pub.Curve)
  		if !ok {
  			return nil, pkix.AlgorithmIdentifier{}, errors.New("x509: unsupported elliptic curve")
  		}
  		publicKeyAlgorithm.Algorithm = oidPublicKeyECDSA
  		var paramBytes []byte
  		paramBytes, err = asn1.Marshal(oid)
  		if err != nil {
  			return
  		}
  		publicKeyAlgorithm.Parameters.FullBytes = paramBytes
  	default:
  		return nil, pkix.AlgorithmIdentifier{}, errors.New("x509: only RSA and ECDSA public keys supported")
  	}
  
  	return publicKeyBytes, publicKeyAlgorithm, nil
  }
  
  // MarshalPKIXPublicKey serialises a public key to DER-encoded PKIX format.
  func MarshalPKIXPublicKey(pub interface{}) ([]byte, error) {
  	var publicKeyBytes []byte
  	var publicKeyAlgorithm pkix.AlgorithmIdentifier
  	var err error
  
  	if publicKeyBytes, publicKeyAlgorithm, err = marshalPublicKey(pub); err != nil {
  		return nil, err
  	}
  
  	pkix := pkixPublicKey{
  		Algo: publicKeyAlgorithm,
  		BitString: asn1.BitString{
  			Bytes:     publicKeyBytes,
  			BitLength: 8 * len(publicKeyBytes),
  		},
  	}
  
  	ret, _ := asn1.Marshal(pkix)
  	return ret, nil
  }
  
  // These structures reflect the ASN.1 structure of X.509 certificates.:
  
  type certificate struct {
  	Raw                asn1.RawContent
  	TBSCertificate     tbsCertificate
  	SignatureAlgorithm pkix.AlgorithmIdentifier
  	SignatureValue     asn1.BitString
  }
  
  type tbsCertificate struct {
  	Raw                asn1.RawContent
  	Version            int `asn1:"optional,explicit,default:0,tag:0"`
  	SerialNumber       *big.Int
  	SignatureAlgorithm pkix.AlgorithmIdentifier
  	Issuer             asn1.RawValue
  	Validity           validity
  	Subject            asn1.RawValue
  	PublicKey          publicKeyInfo
  	UniqueId           asn1.BitString   `asn1:"optional,tag:1"`
  	SubjectUniqueId    asn1.BitString   `asn1:"optional,tag:2"`
  	Extensions         []pkix.Extension `asn1:"optional,explicit,tag:3"`
  }
  
  type dsaAlgorithmParameters struct {
  	P, Q, G *big.Int
  }
  
  type dsaSignature struct {
  	R, S *big.Int
  }
  
  type ecdsaSignature dsaSignature
  
  type validity struct {
  	NotBefore, NotAfter time.Time
  }
  
  type publicKeyInfo struct {
  	Raw       asn1.RawContent
  	Algorithm pkix.AlgorithmIdentifier
  	PublicKey asn1.BitString
  }
  
  // RFC 5280,  4.2.1.1
  type authKeyId struct {
  	Id []byte `asn1:"optional,tag:0"`
  }
  
  type SignatureAlgorithm int
  
  const (
  	UnknownSignatureAlgorithm SignatureAlgorithm = iota
  	MD2WithRSA
  	MD5WithRSA
  	SHA1WithRSA
  	SHA256WithRSA
  	SHA384WithRSA
  	SHA512WithRSA
  	DSAWithSHA1
  	DSAWithSHA256
  	ECDSAWithSHA1
  	ECDSAWithSHA256
  	ECDSAWithSHA384
  	ECDSAWithSHA512
  	SHA256WithRSAPSS
  	SHA384WithRSAPSS
  	SHA512WithRSAPSS
  )
  
  func (algo SignatureAlgorithm) isRSAPSS() bool {
  	switch algo {
  	case SHA256WithRSAPSS, SHA384WithRSAPSS, SHA512WithRSAPSS:
  		return true
  	default:
  		return false
  	}
  }
  
  func (algo SignatureAlgorithm) String() string {
  	for _, details := range signatureAlgorithmDetails {
  		if details.algo == algo {
  			return details.name
  		}
  	}
  	return strconv.Itoa(int(algo))
  }
  
  type PublicKeyAlgorithm int
  
  const (
  	UnknownPublicKeyAlgorithm PublicKeyAlgorithm = iota
  	RSA
  	DSA
  	ECDSA
  )
  
  var publicKeyAlgoName = [...]string{
  	RSA:   "RSA",
  	DSA:   "DSA",
  	ECDSA: "ECDSA",
  }
  
  func (algo PublicKeyAlgorithm) String() string {
  	if 0 < algo && int(algo) < len(publicKeyAlgoName) {
  		return publicKeyAlgoName[algo]
  	}
  	return strconv.Itoa(int(algo))
  }
  
  // OIDs for signature algorithms
  //
  // pkcs-1 OBJECT IDENTIFIER ::= {
  //    iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 1 }
  //
  //
  // RFC 3279 2.2.1 RSA Signature Algorithms
  //
  // md2WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 2 }
  //
  // md5WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 4 }
  //
  // sha-1WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 5 }
  //
  // dsaWithSha1 OBJECT IDENTIFIER ::= {
  //    iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 3 }
  //
  // RFC 3279 2.2.3 ECDSA Signature Algorithm
  //
  // ecdsa-with-SHA1 OBJECT IDENTIFIER ::= {
  // 	  iso(1) member-body(2) us(840) ansi-x962(10045)
  //    signatures(4) ecdsa-with-SHA1(1)}
  //
  //
  // RFC 4055 5 PKCS #1 Version 1.5
  //
  // sha256WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 11 }
  //
  // sha384WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 12 }
  //
  // sha512WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 13 }
  //
  //
  // RFC 5758 3.1 DSA Signature Algorithms
  //
  // dsaWithSha256 OBJECT IDENTIFIER ::= {
  //    joint-iso-ccitt(2) country(16) us(840) organization(1) gov(101)
  //    csor(3) algorithms(4) id-dsa-with-sha2(3) 2}
  //
  // RFC 5758 3.2 ECDSA Signature Algorithm
  //
  // ecdsa-with-SHA256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
  //    us(840) ansi-X9-62(10045) signatures(4) ecdsa-with-SHA2(3) 2 }
  //
  // ecdsa-with-SHA384 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
  //    us(840) ansi-X9-62(10045) signatures(4) ecdsa-with-SHA2(3) 3 }
  //
  // ecdsa-with-SHA512 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
  //    us(840) ansi-X9-62(10045) signatures(4) ecdsa-with-SHA2(3) 4 }
  
  var (
  	oidSignatureMD2WithRSA      = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 2}
  	oidSignatureMD5WithRSA      = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 4}
  	oidSignatureSHA1WithRSA     = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 5}
  	oidSignatureSHA256WithRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 11}
  	oidSignatureSHA384WithRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 12}
  	oidSignatureSHA512WithRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 13}
  	oidSignatureRSAPSS          = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 10}
  	oidSignatureDSAWithSHA1     = asn1.ObjectIdentifier{1, 2, 840, 10040, 4, 3}
  	oidSignatureDSAWithSHA256   = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 3, 2}
  	oidSignatureECDSAWithSHA1   = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 1}
  	oidSignatureECDSAWithSHA256 = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 3, 2}
  	oidSignatureECDSAWithSHA384 = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 3, 3}
  	oidSignatureECDSAWithSHA512 = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 3, 4}
  
  	oidSHA256 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 1}
  	oidSHA384 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 2}
  	oidSHA512 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 3}
  
  	oidMGF1 = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 8}
  
  	// oidISOSignatureSHA1WithRSA means the same as oidSignatureSHA1WithRSA
  	// but it's specified by ISO. Microsoft's makecert.exe has been known
  	// to produce certificates with this OID.
  	oidISOSignatureSHA1WithRSA = asn1.ObjectIdentifier{1, 3, 14, 3, 2, 29}
  )
  
  var signatureAlgorithmDetails = []struct {
  	algo       SignatureAlgorithm
  	name       string
  	oid        asn1.ObjectIdentifier
  	pubKeyAlgo PublicKeyAlgorithm
  	hash       crypto.Hash
  }{
  	{MD2WithRSA, "MD2-RSA", oidSignatureMD2WithRSA, RSA, crypto.Hash(0) /* no value for MD2 */},
  	{MD5WithRSA, "MD5-RSA", oidSignatureMD5WithRSA, RSA, crypto.MD5},
  	{SHA1WithRSA, "SHA1-RSA", oidSignatureSHA1WithRSA, RSA, crypto.SHA1},
  	{SHA1WithRSA, "SHA1-RSA", oidISOSignatureSHA1WithRSA, RSA, crypto.SHA1},
  	{SHA256WithRSA, "SHA256-RSA", oidSignatureSHA256WithRSA, RSA, crypto.SHA256},
  	{SHA384WithRSA, "SHA384-RSA", oidSignatureSHA384WithRSA, RSA, crypto.SHA384},
  	{SHA512WithRSA, "SHA512-RSA", oidSignatureSHA512WithRSA, RSA, crypto.SHA512},
  	{SHA256WithRSAPSS, "SHA256-RSAPSS", oidSignatureRSAPSS, RSA, crypto.SHA256},
  	{SHA384WithRSAPSS, "SHA384-RSAPSS", oidSignatureRSAPSS, RSA, crypto.SHA384},
  	{SHA512WithRSAPSS, "SHA512-RSAPSS", oidSignatureRSAPSS, RSA, crypto.SHA512},
  	{DSAWithSHA1, "DSA-SHA1", oidSignatureDSAWithSHA1, DSA, crypto.SHA1},
  	{DSAWithSHA256, "DSA-SHA256", oidSignatureDSAWithSHA256, DSA, crypto.SHA256},
  	{ECDSAWithSHA1, "ECDSA-SHA1", oidSignatureECDSAWithSHA1, ECDSA, crypto.SHA1},
  	{ECDSAWithSHA256, "ECDSA-SHA256", oidSignatureECDSAWithSHA256, ECDSA, crypto.SHA256},
  	{ECDSAWithSHA384, "ECDSA-SHA384", oidSignatureECDSAWithSHA384, ECDSA, crypto.SHA384},
  	{ECDSAWithSHA512, "ECDSA-SHA512", oidSignatureECDSAWithSHA512, ECDSA, crypto.SHA512},
  }
  
  // pssParameters reflects the parameters in an AlgorithmIdentifier that
  // specifies RSA PSS. See https://tools.ietf.org/html/rfc3447#appendix-A.2.3
  type pssParameters struct {
  	// The following three fields are not marked as
  	// optional because the default values specify SHA-1,
  	// which is no longer suitable for use in signatures.
  	Hash         pkix.AlgorithmIdentifier `asn1:"explicit,tag:0"`
  	MGF          pkix.AlgorithmIdentifier `asn1:"explicit,tag:1"`
  	SaltLength   int                      `asn1:"explicit,tag:2"`
  	TrailerField int                      `asn1:"optional,explicit,tag:3,default:1"`
  }
  
  // rsaPSSParameters returns an asn1.RawValue suitable for use as the Parameters
  // in an AlgorithmIdentifier that specifies RSA PSS.
  func rsaPSSParameters(hashFunc crypto.Hash) asn1.RawValue {
  	var hashOID asn1.ObjectIdentifier
  
  	switch hashFunc {
  	case crypto.SHA256:
  		hashOID = oidSHA256
  	case crypto.SHA384:
  		hashOID = oidSHA384
  	case crypto.SHA512:
  		hashOID = oidSHA512
  	}
  
  	params := pssParameters{
  		Hash: pkix.AlgorithmIdentifier{
  			Algorithm:  hashOID,
  			Parameters: asn1.NullRawValue,
  		},
  		MGF: pkix.AlgorithmIdentifier{
  			Algorithm: oidMGF1,
  		},
  		SaltLength:   hashFunc.Size(),
  		TrailerField: 1,
  	}
  
  	mgf1Params := pkix.AlgorithmIdentifier{
  		Algorithm:  hashOID,
  		Parameters: asn1.NullRawValue,
  	}
  
  	var err error
  	params.MGF.Parameters.FullBytes, err = asn1.Marshal(mgf1Params)
  	if err != nil {
  		panic(err)
  	}
  
  	serialized, err := asn1.Marshal(params)
  	if err != nil {
  		panic(err)
  	}
  
  	return asn1.RawValue{FullBytes: serialized}
  }
  
  func getSignatureAlgorithmFromAI(ai pkix.AlgorithmIdentifier) SignatureAlgorithm {
  	if !ai.Algorithm.Equal(oidSignatureRSAPSS) {
  		for _, details := range signatureAlgorithmDetails {
  			if ai.Algorithm.Equal(details.oid) {
  				return details.algo
  			}
  		}
  		return UnknownSignatureAlgorithm
  	}
  
  	// RSA PSS is special because it encodes important parameters
  	// in the Parameters.
  
  	var params pssParameters
  	if _, err := asn1.Unmarshal(ai.Parameters.FullBytes, &params); err != nil {
  		return UnknownSignatureAlgorithm
  	}
  
  	var mgf1HashFunc pkix.AlgorithmIdentifier
  	if _, err := asn1.Unmarshal(params.MGF.Parameters.FullBytes, &mgf1HashFunc); err != nil {
  		return UnknownSignatureAlgorithm
  	}
  
  	// PSS is greatly overburdened with options. This code forces
  	// them into three buckets by requiring that the MGF1 hash
  	// function always match the message hash function (as
  	// recommended in
  	// https://tools.ietf.org/html/rfc3447#section-8.1), that the
  	// salt length matches the hash length, and that the trailer
  	// field has the default value.
  	if !bytes.Equal(params.Hash.Parameters.FullBytes, asn1.NullBytes) ||
  		!params.MGF.Algorithm.Equal(oidMGF1) ||
  		!mgf1HashFunc.Algorithm.Equal(params.Hash.Algorithm) ||
  		!bytes.Equal(mgf1HashFunc.Parameters.FullBytes, asn1.NullBytes) ||
  		params.TrailerField != 1 {
  		return UnknownSignatureAlgorithm
  	}
  
  	switch {
  	case params.Hash.Algorithm.Equal(oidSHA256) && params.SaltLength == 32:
  		return SHA256WithRSAPSS
  	case params.Hash.Algorithm.Equal(oidSHA384) && params.SaltLength == 48:
  		return SHA384WithRSAPSS
  	case params.Hash.Algorithm.Equal(oidSHA512) && params.SaltLength == 64:
  		return SHA512WithRSAPSS
  	}
  
  	return UnknownSignatureAlgorithm
  }
  
  // RFC 3279, 2.3 Public Key Algorithms
  //
  // pkcs-1 OBJECT IDENTIFIER ::== { iso(1) member-body(2) us(840)
  //    rsadsi(113549) pkcs(1) 1 }
  //
  // rsaEncryption OBJECT IDENTIFIER ::== { pkcs1-1 1 }
  //
  // id-dsa OBJECT IDENTIFIER ::== { iso(1) member-body(2) us(840)
  //    x9-57(10040) x9cm(4) 1 }
  //
  // RFC 5480, 2.1.1 Unrestricted Algorithm Identifier and Parameters
  //
  // id-ecPublicKey OBJECT IDENTIFIER ::= {
  //       iso(1) member-body(2) us(840) ansi-X9-62(10045) keyType(2) 1 }
  var (
  	oidPublicKeyRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 1}
  	oidPublicKeyDSA   = asn1.ObjectIdentifier{1, 2, 840, 10040, 4, 1}
  	oidPublicKeyECDSA = asn1.ObjectIdentifier{1, 2, 840, 10045, 2, 1}
  )
  
  func getPublicKeyAlgorithmFromOID(oid asn1.ObjectIdentifier) PublicKeyAlgorithm {
  	switch {
  	case oid.Equal(oidPublicKeyRSA):
  		return RSA
  	case oid.Equal(oidPublicKeyDSA):
  		return DSA
  	case oid.Equal(oidPublicKeyECDSA):
  		return ECDSA
  	}
  	return UnknownPublicKeyAlgorithm
  }
  
  // RFC 5480, 2.1.1.1. Named Curve
  //
  // secp224r1 OBJECT IDENTIFIER ::= {
  //   iso(1) identified-organization(3) certicom(132) curve(0) 33 }
  //
  // secp256r1 OBJECT IDENTIFIER ::= {
  //   iso(1) member-body(2) us(840) ansi-X9-62(10045) curves(3)
  //   prime(1) 7 }
  //
  // secp384r1 OBJECT IDENTIFIER ::= {
  //   iso(1) identified-organization(3) certicom(132) curve(0) 34 }
  //
  // secp521r1 OBJECT IDENTIFIER ::= {
  //   iso(1) identified-organization(3) certicom(132) curve(0) 35 }
  //
  // NB: secp256r1 is equivalent to prime256v1
  var (
  	oidNamedCurveP224 = asn1.ObjectIdentifier{1, 3, 132, 0, 33}
  	oidNamedCurveP256 = asn1.ObjectIdentifier{1, 2, 840, 10045, 3, 1, 7}
  	oidNamedCurveP384 = asn1.ObjectIdentifier{1, 3, 132, 0, 34}
  	oidNamedCurveP521 = asn1.ObjectIdentifier{1, 3, 132, 0, 35}
  )
  
  func namedCurveFromOID(oid asn1.ObjectIdentifier) elliptic.Curve {
  	switch {
  	case oid.Equal(oidNamedCurveP224):
  		return elliptic.P224()
  	case oid.Equal(oidNamedCurveP256):
  		return elliptic.P256()
  	case oid.Equal(oidNamedCurveP384):
  		return elliptic.P384()
  	case oid.Equal(oidNamedCurveP521):
  		return elliptic.P521()
  	}
  	return nil
  }
  
  func oidFromNamedCurve(curve elliptic.Curve) (asn1.ObjectIdentifier, bool) {
  	switch curve {
  	case elliptic.P224():
  		return oidNamedCurveP224, true
  	case elliptic.P256():
  		return oidNamedCurveP256, true
  	case elliptic.P384():
  		return oidNamedCurveP384, true
  	case elliptic.P521():
  		return oidNamedCurveP521, true
  	}
  
  	return nil, false
  }
  
  // KeyUsage represents the set of actions that are valid for a given key. It's
  // a bitmap of the KeyUsage* constants.
  type KeyUsage int
  
  const (
  	KeyUsageDigitalSignature KeyUsage = 1 << iota
  	KeyUsageContentCommitment
  	KeyUsageKeyEncipherment
  	KeyUsageDataEncipherment
  	KeyUsageKeyAgreement
  	KeyUsageCertSign
  	KeyUsageCRLSign
  	KeyUsageEncipherOnly
  	KeyUsageDecipherOnly
  )
  
  // RFC 5280, 4.2.1.12  Extended Key Usage
  //
  // anyExtendedKeyUsage OBJECT IDENTIFIER ::= { id-ce-extKeyUsage 0 }
  //
  // id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }
  //
  // id-kp-serverAuth             OBJECT IDENTIFIER ::= { id-kp 1 }
  // id-kp-clientAuth             OBJECT IDENTIFIER ::= { id-kp 2 }
  // id-kp-codeSigning            OBJECT IDENTIFIER ::= { id-kp 3 }
  // id-kp-emailProtection        OBJECT IDENTIFIER ::= { id-kp 4 }
  // id-kp-timeStamping           OBJECT IDENTIFIER ::= { id-kp 8 }
  // id-kp-OCSPSigning            OBJECT IDENTIFIER ::= { id-kp 9 }
  var (
  	oidExtKeyUsageAny                            = asn1.ObjectIdentifier{2, 5, 29, 37, 0}
  	oidExtKeyUsageServerAuth                     = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 1}
  	oidExtKeyUsageClientAuth                     = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 2}
  	oidExtKeyUsageCodeSigning                    = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 3}
  	oidExtKeyUsageEmailProtection                = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 4}
  	oidExtKeyUsageIPSECEndSystem                 = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 5}
  	oidExtKeyUsageIPSECTunnel                    = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 6}
  	oidExtKeyUsageIPSECUser                      = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 7}
  	oidExtKeyUsageTimeStamping                   = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 8}
  	oidExtKeyUsageOCSPSigning                    = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 9}
  	oidExtKeyUsageMicrosoftServerGatedCrypto     = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 311, 10, 3, 3}
  	oidExtKeyUsageNetscapeServerGatedCrypto      = asn1.ObjectIdentifier{2, 16, 840, 1, 113730, 4, 1}
  	oidExtKeyUsageMicrosoftCommercialCodeSigning = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 311, 2, 1, 22}
  	oidExtKeyUsageMicrosoftKernelCodeSigning     = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 311, 61, 1, 1}
  )
  
  // ExtKeyUsage represents an extended set of actions that are valid for a given key.
  // Each of the ExtKeyUsage* constants define a unique action.
  type ExtKeyUsage int
  
  const (
  	ExtKeyUsageAny ExtKeyUsage = iota
  	ExtKeyUsageServerAuth
  	ExtKeyUsageClientAuth
  	ExtKeyUsageCodeSigning
  	ExtKeyUsageEmailProtection
  	ExtKeyUsageIPSECEndSystem
  	ExtKeyUsageIPSECTunnel
  	ExtKeyUsageIPSECUser
  	ExtKeyUsageTimeStamping
  	ExtKeyUsageOCSPSigning
  	ExtKeyUsageMicrosoftServerGatedCrypto
  	ExtKeyUsageNetscapeServerGatedCrypto
  	ExtKeyUsageMicrosoftCommercialCodeSigning
  	ExtKeyUsageMicrosoftKernelCodeSigning
  )
  
  // extKeyUsageOIDs contains the mapping between an ExtKeyUsage and its OID.
  var extKeyUsageOIDs = []struct {
  	extKeyUsage ExtKeyUsage
  	oid         asn1.ObjectIdentifier
  }{
  	{ExtKeyUsageAny, oidExtKeyUsageAny},
  	{ExtKeyUsageServerAuth, oidExtKeyUsageServerAuth},
  	{ExtKeyUsageClientAuth, oidExtKeyUsageClientAuth},
  	{ExtKeyUsageCodeSigning, oidExtKeyUsageCodeSigning},
  	{ExtKeyUsageEmailProtection, oidExtKeyUsageEmailProtection},
  	{ExtKeyUsageIPSECEndSystem, oidExtKeyUsageIPSECEndSystem},
  	{ExtKeyUsageIPSECTunnel, oidExtKeyUsageIPSECTunnel},
  	{ExtKeyUsageIPSECUser, oidExtKeyUsageIPSECUser},
  	{ExtKeyUsageTimeStamping, oidExtKeyUsageTimeStamping},
  	{ExtKeyUsageOCSPSigning, oidExtKeyUsageOCSPSigning},
  	{ExtKeyUsageMicrosoftServerGatedCrypto, oidExtKeyUsageMicrosoftServerGatedCrypto},
  	{ExtKeyUsageNetscapeServerGatedCrypto, oidExtKeyUsageNetscapeServerGatedCrypto},
  	{ExtKeyUsageMicrosoftCommercialCodeSigning, oidExtKeyUsageMicrosoftCommercialCodeSigning},
  	{ExtKeyUsageMicrosoftKernelCodeSigning, oidExtKeyUsageMicrosoftKernelCodeSigning},
  }
  
  func extKeyUsageFromOID(oid asn1.ObjectIdentifier) (eku ExtKeyUsage, ok bool) {
  	for _, pair := range extKeyUsageOIDs {
  		if oid.Equal(pair.oid) {
  			return pair.extKeyUsage, true
  		}
  	}
  	return
  }
  
  func oidFromExtKeyUsage(eku ExtKeyUsage) (oid asn1.ObjectIdentifier, ok bool) {
  	for _, pair := range extKeyUsageOIDs {
  		if eku == pair.extKeyUsage {
  			return pair.oid, true
  		}
  	}
  	return
  }
  
  // A Certificate represents an X.509 certificate.
  type Certificate struct {
  	Raw                     []byte // Complete ASN.1 DER content (certificate, signature algorithm and signature).
  	RawTBSCertificate       []byte // Certificate part of raw ASN.1 DER content.
  	RawSubjectPublicKeyInfo []byte // DER encoded SubjectPublicKeyInfo.
  	RawSubject              []byte // DER encoded Subject
  	RawIssuer               []byte // DER encoded Issuer
  
  	Signature          []byte
  	SignatureAlgorithm SignatureAlgorithm
  
  	PublicKeyAlgorithm PublicKeyAlgorithm
  	PublicKey          interface{}
  
  	Version             int
  	SerialNumber        *big.Int
  	Issuer              pkix.Name
  	Subject             pkix.Name
  	NotBefore, NotAfter time.Time // Validity bounds.
  	KeyUsage            KeyUsage
  
  	// Extensions contains raw X.509 extensions. When parsing certificates,
  	// this can be used to extract non-critical extensions that are not
  	// parsed by this package. When marshaling certificates, the Extensions
  	// field is ignored, see ExtraExtensions.
  	Extensions []pkix.Extension
  
  	// ExtraExtensions contains extensions to be copied, raw, into any
  	// marshaled certificates. Values override any extensions that would
  	// otherwise be produced based on the other fields. The ExtraExtensions
  	// field is not populated when parsing certificates, see Extensions.
  	ExtraExtensions []pkix.Extension
  
  	// UnhandledCriticalExtensions contains a list of extension IDs that
  	// were not (fully) processed when parsing. Verify will fail if this
  	// slice is non-empty, unless verification is delegated to an OS
  	// library which understands all the critical extensions.
  	//
  	// Users can access these extensions using Extensions and can remove
  	// elements from this slice if they believe that they have been
  	// handled.
  	UnhandledCriticalExtensions []asn1.ObjectIdentifier
  
  	ExtKeyUsage        []ExtKeyUsage           // Sequence of extended key usages.
  	UnknownExtKeyUsage []asn1.ObjectIdentifier // Encountered extended key usages unknown to this package.
  
  	// BasicConstraintsValid indicates whether IsCA, MaxPathLen,
  	// and MaxPathLenZero are valid.
  	BasicConstraintsValid bool
  	IsCA                  bool
  
  	// MaxPathLen and MaxPathLenZero indicate the presence and
  	// value of the BasicConstraints' "pathLenConstraint".
  	//
  	// When parsing a certificate, a positive non-zero MaxPathLen
  	// means that the field was specified, -1 means it was unset,
  	// and MaxPathLenZero being true mean that the field was
  	// explicitly set to zero. The case of MaxPathLen==0 with MaxPathLenZero==false
  	// should be treated equivalent to -1 (unset).
  	//
  	// When generating a certificate, an unset pathLenConstraint
  	// can be requested with either MaxPathLen == -1 or using the
  	// zero value for both MaxPathLen and MaxPathLenZero.
  	MaxPathLen int
  	// MaxPathLenZero indicates that BasicConstraintsValid==true
  	// and MaxPathLen==0 should be interpreted as an actual
  	// maximum path length of zero. Otherwise, that combination is
  	// interpreted as MaxPathLen not being set.
  	MaxPathLenZero bool
  
  	SubjectKeyId   []byte
  	AuthorityKeyId []byte
  
  	// RFC 5280, 4.2.2.1 (Authority Information Access)
  	OCSPServer            []string
  	IssuingCertificateURL []string
  
  	// Subject Alternate Name values. (Note that these values may not be valid
  	// if invalid values were contained within a parsed certificate. For
  	// example, an element of DNSNames may not be a valid DNS domain name.)
  	DNSNames       []string
  	EmailAddresses []string
  	IPAddresses    []net.IP
  	URIs           []*url.URL
  
  	// Name constraints
  	PermittedDNSDomainsCritical bool // if true then the name constraints are marked critical.
  	PermittedDNSDomains         []string
  	ExcludedDNSDomains          []string
  	PermittedIPRanges           []*net.IPNet
  	ExcludedIPRanges            []*net.IPNet
  	PermittedEmailAddresses     []string
  	ExcludedEmailAddresses      []string
  	PermittedURIDomains         []string
  	ExcludedURIDomains          []string
  
  	// CRL Distribution Points
  	CRLDistributionPoints []string
  
  	PolicyIdentifiers []asn1.ObjectIdentifier
  }
  
  // ErrUnsupportedAlgorithm results from attempting to perform an operation that
  // involves algorithms that are not currently implemented.
  var ErrUnsupportedAlgorithm = errors.New("x509: cannot verify signature: algorithm unimplemented")
  
  // An InsecureAlgorithmError
  type InsecureAlgorithmError SignatureAlgorithm
  
  func (e InsecureAlgorithmError) Error() string {
  	return fmt.Sprintf("x509: cannot verify signature: insecure algorithm %v", SignatureAlgorithm(e))
  }
  
  // ConstraintViolationError results when a requested usage is not permitted by
  // a certificate. For example: checking a signature when the public key isn't a
  // certificate signing key.
  type ConstraintViolationError struct{}
  
  func (ConstraintViolationError) Error() string {
  	return "x509: invalid signature: parent certificate cannot sign this kind of certificate"
  }
  
  func (c *Certificate) Equal(other *Certificate) bool {
  	return bytes.Equal(c.Raw, other.Raw)
  }
  
  func (c *Certificate) hasSANExtension() bool {
  	return oidInExtensions(oidExtensionSubjectAltName, c.Extensions)
  }
  
  // Entrust have a broken root certificate (CN=Entrust.net Certification
  // Authority (2048)) which isn't marked as a CA certificate and is thus invalid
  // according to PKIX.
  // We recognise this certificate by its SubjectPublicKeyInfo and exempt it
  // from the Basic Constraints requirement.
  // See http://www.entrust.net/knowledge-base/technote.cfm?tn=7869
  //
  // TODO(agl): remove this hack once their reissued root is sufficiently
  // widespread.
  var entrustBrokenSPKI = []byte{
  	0x30, 0x82, 0x01, 0x22, 0x30, 0x0d, 0x06, 0x09,
  	0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01,
  	0x01, 0x05, 0x00, 0x03, 0x82, 0x01, 0x0f, 0x00,
  	0x30, 0x82, 0x01, 0x0a, 0x02, 0x82, 0x01, 0x01,
  	0x00, 0x97, 0xa3, 0x2d, 0x3c, 0x9e, 0xde, 0x05,
  	0xda, 0x13, 0xc2, 0x11, 0x8d, 0x9d, 0x8e, 0xe3,
  	0x7f, 0xc7, 0x4b, 0x7e, 0x5a, 0x9f, 0xb3, 0xff,
  	0x62, 0xab, 0x73, 0xc8, 0x28, 0x6b, 0xba, 0x10,
  	0x64, 0x82, 0x87, 0x13, 0xcd, 0x57, 0x18, 0xff,
  	0x28, 0xce, 0xc0, 0xe6, 0x0e, 0x06, 0x91, 0x50,
  	0x29, 0x83, 0xd1, 0xf2, 0xc3, 0x2a, 0xdb, 0xd8,
  	0xdb, 0x4e, 0x04, 0xcc, 0x00, 0xeb, 0x8b, 0xb6,
  	0x96, 0xdc, 0xbc, 0xaa, 0xfa, 0x52, 0x77, 0x04,
  	0xc1, 0xdb, 0x19, 0xe4, 0xae, 0x9c, 0xfd, 0x3c,
  	0x8b, 0x03, 0xef, 0x4d, 0xbc, 0x1a, 0x03, 0x65,
  	0xf9, 0xc1, 0xb1, 0x3f, 0x72, 0x86, 0xf2, 0x38,
  	0xaa, 0x19, 0xae, 0x10, 0x88, 0x78, 0x28, 0xda,
  	0x75, 0xc3, 0x3d, 0x02, 0x82, 0x02, 0x9c, 0xb9,
  	0xc1, 0x65, 0x77, 0x76, 0x24, 0x4c, 0x98, 0xf7,
  	0x6d, 0x31, 0x38, 0xfb, 0xdb, 0xfe, 0xdb, 0x37,
  	0x02, 0x76, 0xa1, 0x18, 0x97, 0xa6, 0xcc, 0xde,
  	0x20, 0x09, 0x49, 0x36, 0x24, 0x69, 0x42, 0xf6,
  	0xe4, 0x37, 0x62, 0xf1, 0x59, 0x6d, 0xa9, 0x3c,
  	0xed, 0x34, 0x9c, 0xa3, 0x8e, 0xdb, 0xdc, 0x3a,
  	0xd7, 0xf7, 0x0a, 0x6f, 0xef, 0x2e, 0xd8, 0xd5,
  	0x93, 0x5a, 0x7a, 0xed, 0x08, 0x49, 0x68, 0xe2,
  	0x41, 0xe3, 0x5a, 0x90, 0xc1, 0x86, 0x55, 0xfc,
  	0x51, 0x43, 0x9d, 0xe0, 0xb2, 0xc4, 0x67, 0xb4,
  	0xcb, 0x32, 0x31, 0x25, 0xf0, 0x54, 0x9f, 0x4b,
  	0xd1, 0x6f, 0xdb, 0xd4, 0xdd, 0xfc, 0xaf, 0x5e,
  	0x6c, 0x78, 0x90, 0x95, 0xde, 0xca, 0x3a, 0x48,
  	0xb9, 0x79, 0x3c, 0x9b, 0x19, 0xd6, 0x75, 0x05,
  	0xa0, 0xf9, 0x88, 0xd7, 0xc1, 0xe8, 0xa5, 0x09,
  	0xe4, 0x1a, 0x15, 0xdc, 0x87, 0x23, 0xaa, 0xb2,
  	0x75, 0x8c, 0x63, 0x25, 0x87, 0xd8, 0xf8, 0x3d,
  	0xa6, 0xc2, 0xcc, 0x66, 0xff, 0xa5, 0x66, 0x68,
  	0x55, 0x02, 0x03, 0x01, 0x00, 0x01,
  }
  
  // CheckSignatureFrom verifies that the signature on c is a valid signature
  // from parent.
  func (c *Certificate) CheckSignatureFrom(parent *Certificate) error {
  	// RFC 5280, 4.2.1.9:
  	// "If the basic constraints extension is not present in a version 3
  	// certificate, or the extension is present but the cA boolean is not
  	// asserted, then the certified public key MUST NOT be used to verify
  	// certificate signatures."
  	// (except for Entrust, see comment above entrustBrokenSPKI)
  	if (parent.Version == 3 && !parent.BasicConstraintsValid ||
  		parent.BasicConstraintsValid && !parent.IsCA) &&
  		!bytes.Equal(c.RawSubjectPublicKeyInfo, entrustBrokenSPKI) {
  		return ConstraintViolationError{}
  	}
  
  	if parent.KeyUsage != 0 && parent.KeyUsage&KeyUsageCertSign == 0 {
  		return ConstraintViolationError{}
  	}
  
  	if parent.PublicKeyAlgorithm == UnknownPublicKeyAlgorithm {
  		return ErrUnsupportedAlgorithm
  	}
  
  	// TODO(agl): don't ignore the path length constraint.
  
  	return parent.CheckSignature(c.SignatureAlgorithm, c.RawTBSCertificate, c.Signature)
  }
  
  // CheckSignature verifies that signature is a valid signature over signed from
  // c's public key.
  func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature []byte) error {
  	return checkSignature(algo, signed, signature, c.PublicKey)
  }
  
  func (c *Certificate) hasNameConstraints() bool {
  	for _, e := range c.Extensions {
  		if len(e.Id) == 4 && e.Id[0] == 2 && e.Id[1] == 5 && e.Id[2] == 29 && e.Id[3] == 30 {
  			return true
  		}
  	}
  
  	return false
  }
  
  func (c *Certificate) getSANExtension() ([]byte, bool) {
  	for _, e := range c.Extensions {
  		if len(e.Id) == 4 && e.Id[0] == 2 && e.Id[1] == 5 && e.Id[2] == 29 && e.Id[3] == 17 {
  			return e.Value, true
  		}
  	}
  
  	return nil, false
  }
  
  func signaturePublicKeyAlgoMismatchError(expectedPubKeyAlgo PublicKeyAlgorithm, pubKey interface{}) error {
  	return fmt.Errorf("x509: signature algorithm specifies an %s public key, but have public key of type %T", expectedPubKeyAlgo.String(), pubKey)
  }
  
  // CheckSignature verifies that signature is a valid signature over signed from
  // a crypto.PublicKey.
  func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey crypto.PublicKey) (err error) {
  	var hashType crypto.Hash
  	var pubKeyAlgo PublicKeyAlgorithm
  
  	for _, details := range signatureAlgorithmDetails {
  		if details.algo == algo {
  			hashType = details.hash
  			pubKeyAlgo = details.pubKeyAlgo
  		}
  	}
  
  	switch hashType {
  	case crypto.Hash(0):
  		return ErrUnsupportedAlgorithm
  	case crypto.MD5:
  		return InsecureAlgorithmError(algo)
  	}
  
  	if !hashType.Available() {
  		return ErrUnsupportedAlgorithm
  	}
  	h := hashType.New()
  
  	h.Write(signed)
  	digest := h.Sum(nil)
  
  	switch pub := publicKey.(type) {
  	case *rsa.PublicKey:
  		if pubKeyAlgo != RSA {
  			return signaturePublicKeyAlgoMismatchError(pubKeyAlgo, pub)
  		}
  		if algo.isRSAPSS() {
  			return rsa.VerifyPSS(pub, hashType, digest, signature, &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash})
  		} else {
  			return rsa.VerifyPKCS1v15(pub, hashType, digest, signature)
  		}
  	case *dsa.PublicKey:
  		if pubKeyAlgo != DSA {
  			return signaturePublicKeyAlgoMismatchError(pubKeyAlgo, pub)
  		}
  		dsaSig := new(dsaSignature)
  		if rest, err := asn1.Unmarshal(signature, dsaSig); err != nil {
  			return err
  		} else if len(rest) != 0 {
  			return errors.New("x509: trailing data after DSA signature")
  		}
  		if dsaSig.R.Sign() <= 0 || dsaSig.S.Sign() <= 0 {
  			return errors.New("x509: DSA signature contained zero or negative values")
  		}
  		if !dsa.Verify(pub, digest, dsaSig.R, dsaSig.S) {
  			return errors.New("x509: DSA verification failure")
  		}
  		return
  	case *ecdsa.PublicKey:
  		if pubKeyAlgo != ECDSA {
  			return signaturePublicKeyAlgoMismatchError(pubKeyAlgo, pub)
  		}
  		ecdsaSig := new(ecdsaSignature)
  		if rest, err := asn1.Unmarshal(signature, ecdsaSig); err != nil {
  			return err
  		} else if len(rest) != 0 {
  			return errors.New("x509: trailing data after ECDSA signature")
  		}
  		if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
  			return errors.New("x509: ECDSA signature contained zero or negative values")
  		}
  		if !ecdsa.Verify(pub, digest, ecdsaSig.R, ecdsaSig.S) {
  			return errors.New("x509: ECDSA verification failure")
  		}
  		return
  	}
  	return ErrUnsupportedAlgorithm
  }
  
  // CheckCRLSignature checks that the signature in crl is from c.
  func (c *Certificate) CheckCRLSignature(crl *pkix.CertificateList) error {
  	algo := getSignatureAlgorithmFromAI(crl.SignatureAlgorithm)
  	return c.CheckSignature(algo, crl.TBSCertList.Raw, crl.SignatureValue.RightAlign())
  }
  
  type UnhandledCriticalExtension struct{}
  
  func (h UnhandledCriticalExtension) Error() string {
  	return "x509: unhandled critical extension"
  }
  
  type basicConstraints struct {
  	IsCA       bool `asn1:"optional"`
  	MaxPathLen int  `asn1:"optional,default:-1"`
  }
  
  // RFC 5280 4.2.1.4
  type policyInformation struct {
  	Policy asn1.ObjectIdentifier
  	// policyQualifiers omitted
  }
  
  const (
  	nameTypeEmail = 1
  	nameTypeDNS   = 2
  	nameTypeURI   = 6
  	nameTypeIP    = 7
  )
  
  // RFC 5280, 4.2.2.1
  type authorityInfoAccess struct {
  	Method   asn1.ObjectIdentifier
  	Location asn1.RawValue
  }
  
  // RFC 5280, 4.2.1.14
  type distributionPoint struct {
  	DistributionPoint distributionPointName `asn1:"optional,tag:0"`
  	Reason            asn1.BitString        `asn1:"optional,tag:1"`
  	CRLIssuer         asn1.RawValue         `asn1:"optional,tag:2"`
  }
  
  type distributionPointName struct {
  	FullName     []asn1.RawValue  `asn1:"optional,tag:0"`
  	RelativeName pkix.RDNSequence `asn1:"optional,tag:1"`
  }
  
  func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, error) {
  	asn1Data := keyData.PublicKey.RightAlign()
  	switch algo {
  	case RSA:
  		// RSA public keys must have a NULL in the parameters
  		// (https://tools.ietf.org/html/rfc3279#section-2.3.1).
  		if !bytes.Equal(keyData.Algorithm.Parameters.FullBytes, asn1.NullBytes) {
  			return nil, errors.New("x509: RSA key missing NULL parameters")
  		}
  
  		p := new(pkcs1PublicKey)
  		rest, err := asn1.Unmarshal(asn1Data, p)
  		if err != nil {
  			return nil, err
  		}
  		if len(rest) != 0 {
  			return nil, errors.New("x509: trailing data after RSA public key")
  		}
  
  		if p.N.Sign() <= 0 {
  			return nil, errors.New("x509: RSA modulus is not a positive number")
  		}
  		if p.E <= 0 {
  			return nil, errors.New("x509: RSA public exponent is not a positive number")
  		}
  
  		pub := &rsa.PublicKey{
  			E: p.E,
  			N: p.N,
  		}
  		return pub, nil
  	case DSA:
  		var p *big.Int
  		rest, err := asn1.Unmarshal(asn1Data, &p)
  		if err != nil {
  			return nil, err
  		}
  		if len(rest) != 0 {
  			return nil, errors.New("x509: trailing data after DSA public key")
  		}
  		paramsData := keyData.Algorithm.Parameters.FullBytes
  		params := new(dsaAlgorithmParameters)
  		rest, err = asn1.Unmarshal(paramsData, params)
  		if err != nil {
  			return nil, err
  		}
  		if len(rest) != 0 {
  			return nil, errors.New("x509: trailing data after DSA parameters")
  		}
  		if p.Sign() <= 0 || params.P.Sign() <= 0 || params.Q.Sign() <= 0 || params.G.Sign() <= 0 {
  			return nil, errors.New("x509: zero or negative DSA parameter")
  		}
  		pub := &dsa.PublicKey{
  			Parameters: dsa.Parameters{
  				P: params.P,
  				Q: params.Q,
  				G: params.G,
  			},
  			Y: p,
  		}
  		return pub, nil
  	case ECDSA:
  		paramsData := keyData.Algorithm.Parameters.FullBytes
  		namedCurveOID := new(asn1.ObjectIdentifier)
  		rest, err := asn1.Unmarshal(paramsData, namedCurveOID)
  		if err != nil {
  			return nil, err
  		}
  		if len(rest) != 0 {
  			return nil, errors.New("x509: trailing data after ECDSA parameters")
  		}
  		namedCurve := namedCurveFromOID(*namedCurveOID)
  		if namedCurve == nil {
  			return nil, errors.New("x509: unsupported elliptic curve")
  		}
  		x, y := elliptic.Unmarshal(namedCurve, asn1Data)
  		if x == nil {
  			return nil, errors.New("x509: failed to unmarshal elliptic curve point")
  		}
  		pub := &ecdsa.PublicKey{
  			Curve: namedCurve,
  			X:     x,
  			Y:     y,
  		}
  		return pub, nil
  	default:
  		return nil, nil
  	}
  }
  
  func forEachSAN(extension []byte, callback func(tag int, data []byte) error) error {
  	// RFC 5280, 4.2.1.6
  
  	// SubjectAltName ::= GeneralNames
  	//
  	// GeneralNames ::= SEQUENCE SIZE (1..MAX) OF GeneralName
  	//
  	// GeneralName ::= CHOICE {
  	//      otherName                       [0]     OtherName,
  	//      rfc822Name                      [1]     IA5String,
  	//      dNSName                         [2]     IA5String,
  	//      x400Address                     [3]     ORAddress,
  	//      directoryName                   [4]     Name,
  	//      ediPartyName                    [5]     EDIPartyName,
  	//      uniformResourceIdentifier       [6]     IA5String,
  	//      iPAddress                       [7]     OCTET STRING,
  	//      registeredID                    [8]     OBJECT IDENTIFIER }
  	var seq asn1.RawValue
  	rest, err := asn1.Unmarshal(extension, &seq)
  	if err != nil {
  		return err
  	} else if len(rest) != 0 {
  		return errors.New("x509: trailing data after X.509 extension")
  	}
  	if !seq.IsCompound || seq.Tag != 16 || seq.Class != 0 {
  		return asn1.StructuralError{Msg: "bad SAN sequence"}
  	}
  
  	rest = seq.Bytes
  	for len(rest) > 0 {
  		var v asn1.RawValue
  		rest, err = asn1.Unmarshal(rest, &v)
  		if err != nil {
  			return err
  		}
  
  		if err := callback(v.Tag, v.Bytes); err != nil {
  			return err
  		}
  	}
  
  	return nil
  }
  
  func parseSANExtension(value []byte) (dnsNames, emailAddresses []string, ipAddresses []net.IP, uris []*url.URL, err error) {
  	err = forEachSAN(value, func(tag int, data []byte) error {
  		switch tag {
  		case nameTypeEmail:
  			emailAddresses = append(emailAddresses, string(data))
  		case nameTypeDNS:
  			dnsNames = append(dnsNames, string(data))
  		case nameTypeURI:
  			uri, err := url.Parse(string(data))
  			if err != nil {
  				return fmt.Errorf("x509: cannot parse URI %q: %s", string(data), err)
  			}
  			if len(uri.Host) > 0 {
  				if _, ok := domainToReverseLabels(uri.Host); !ok {
  					return fmt.Errorf("x509: cannot parse URI %q: invalid domain", string(data))
  				}
  			}
  			uris = append(uris, uri)
  		case nameTypeIP:
  			switch len(data) {
  			case net.IPv4len, net.IPv6len:
  				ipAddresses = append(ipAddresses, data)
  			default:
  				return errors.New("x509: cannot parse IP address of length " + strconv.Itoa(len(data)))
  			}
  		}
  
  		return nil
  	})
  
  	return
  }
  
  // isValidIPMask returns true iff mask consists of zero or more 1 bits, followed by zero bits.
  func isValidIPMask(mask []byte) bool {
  	seenZero := false
  
  	for _, b := range mask {
  		if seenZero {
  			if b != 0 {
  				return false
  			}
  
  			continue
  		}
  
  		switch b {
  		case 0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe:
  			seenZero = true
  		case 0xff:
  		default:
  			return false
  		}
  	}
  
  	return true
  }
  
  func parseNameConstraintsExtension(out *Certificate, e pkix.Extension) (unhandled bool, err error) {
  	// RFC 5280, 4.2.1.10
  
  	// NameConstraints ::= SEQUENCE {
  	//      permittedSubtrees       [0]     GeneralSubtrees OPTIONAL,
  	//      excludedSubtrees        [1]     GeneralSubtrees OPTIONAL }
  	//
  	// GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree
  	//
  	// GeneralSubtree ::= SEQUENCE {
  	//      base                    GeneralName,
  	//      minimum         [0]     BaseDistance DEFAULT 0,
  	//      maximum         [1]     BaseDistance OPTIONAL }
  	//
  	// BaseDistance ::= INTEGER (0..MAX)
  
  	outer := cryptobyte.String(e.Value)
  	var toplevel, permitted, excluded cryptobyte.String
  	var havePermitted, haveExcluded bool
  	if !outer.ReadASN1(&toplevel, cryptobyte_asn1.SEQUENCE) ||
  		!outer.Empty() ||
  		!toplevel.ReadOptionalASN1(&permitted, &havePermitted, cryptobyte_asn1.Tag(0).ContextSpecific().Constructed()) ||
  		!toplevel.ReadOptionalASN1(&excluded, &haveExcluded, cryptobyte_asn1.Tag(1).ContextSpecific().Constructed()) ||
  		!toplevel.Empty() {
  		return false, errors.New("x509: invalid NameConstraints extension")
  	}
  
  	if !havePermitted && !haveExcluded || len(permitted) == 0 && len(excluded) == 0 {
  		// https://tools.ietf.org/html/rfc5280#section-4.2.1.10:
  		//   “either the permittedSubtrees field
  		//   or the excludedSubtrees MUST be
  		//   present”
  		return false, errors.New("x509: empty name constraints extension")
  	}
  
  	getValues := func(subtrees cryptobyte.String) (dnsNames []string, ips []*net.IPNet, emails, uriDomains []string, err error) {
  		for !subtrees.Empty() {
  			var seq, value cryptobyte.String
  			var tag cryptobyte_asn1.Tag
  			if !subtrees.ReadASN1(&seq, cryptobyte_asn1.SEQUENCE) ||
  				!seq.ReadAnyASN1(&value, &tag) {
  				return nil, nil, nil, nil, fmt.Errorf("x509: invalid NameConstraints extension")
  			}
  
  			var (
  				dnsTag   = cryptobyte_asn1.Tag(2).ContextSpecific()
  				emailTag = cryptobyte_asn1.Tag(1).ContextSpecific()
  				ipTag    = cryptobyte_asn1.Tag(7).ContextSpecific()
  				uriTag   = cryptobyte_asn1.Tag(6).ContextSpecific()
  			)
  
  			switch tag {
  			case dnsTag:
  				domain := string(value)
  				if err := isIA5String(domain); err != nil {
  					return nil, nil, nil, nil, errors.New("x509: invalid constraint value: " + err.Error())
  				}
  
  				trimmedDomain := domain
  				if len(trimmedDomain) > 0 && trimmedDomain[0] == '.' {
  					// constraints can have a leading
  					// period to exclude the domain
  					// itself, but that's not valid in a
  					// normal domain name.
  					trimmedDomain = trimmedDomain[1:]
  				}
  				if _, ok := domainToReverseLabels(trimmedDomain); !ok {
  					return nil, nil, nil, nil, fmt.Errorf("x509: failed to parse dnsName constraint %q", domain)
  				}
  				dnsNames = append(dnsNames, domain)
  
  			case ipTag:
  				l := len(value)
  				var ip, mask []byte
  
  				switch l {
  				case 8:
  					ip = value[:4]
  					mask = value[4:]
  
  				case 32:
  					ip = value[:16]
  					mask = value[16:]
  
  				default:
  					return nil, nil, nil, nil, fmt.Errorf("x509: IP constraint contained value of length %d", l)
  				}
  
  				if !isValidIPMask(mask) {
  					return nil, nil, nil, nil, fmt.Errorf("x509: IP constraint contained invalid mask %x", mask)
  				}
  
  				ips = append(ips, &net.IPNet{IP: net.IP(ip), Mask: net.IPMask(mask)})
  
  			case emailTag:
  				constraint := string(value)
  				if err := isIA5String(constraint); err != nil {
  					return nil, nil, nil, nil, errors.New("x509: invalid constraint value: " + err.Error())
  				}
  
  				// If the constraint contains an @ then
  				// it specifies an exact mailbox name.
  				if strings.Contains(constraint, "@") {
  					if _, ok := parseRFC2821Mailbox(constraint); !ok {
  						return nil, nil, nil, nil, fmt.Errorf("x509: failed to parse rfc822Name constraint %q", constraint)
  					}
  				} else {
  					// Otherwise it's a domain name.
  					domain := constraint
  					if len(domain) > 0 && domain[0] == '.' {
  						domain = domain[1:]
  					}
  					if _, ok := domainToReverseLabels(domain); !ok {
  						return nil, nil, nil, nil, fmt.Errorf("x509: failed to parse rfc822Name constraint %q", constraint)
  					}
  				}
  				emails = append(emails, constraint)
  
  			case uriTag:
  				domain := string(value)
  				if err := isIA5String(domain); err != nil {
  					return nil, nil, nil, nil, errors.New("x509: invalid constraint value: " + err.Error())
  				}
  
  				if net.ParseIP(domain) != nil {
  					return nil, nil, nil, nil, fmt.Errorf("x509: failed to parse URI constraint %q: cannot be IP address", domain)
  				}
  
  				trimmedDomain := domain
  				if len(trimmedDomain) > 0 && trimmedDomain[0] == '.' {
  					// constraints can have a leading
  					// period to exclude the domain itself,
  					// but that's not valid in a normal
  					// domain name.
  					trimmedDomain = trimmedDomain[1:]
  				}
  				if _, ok := domainToReverseLabels(trimmedDomain); !ok {
  					return nil, nil, nil, nil, fmt.Errorf("x509: failed to parse URI constraint %q", domain)
  				}
  				uriDomains = append(uriDomains, domain)
  
  			default:
  				unhandled = true
  			}
  		}
  
  		return dnsNames, ips, emails, uriDomains, nil
  	}
  
  	if out.PermittedDNSDomains, out.PermittedIPRanges, out.PermittedEmailAddresses, out.PermittedURIDomains, err = getValues(permitted); err != nil {
  		return false, err
  	}
  	if out.ExcludedDNSDomains, out.ExcludedIPRanges, out.ExcludedEmailAddresses, out.ExcludedURIDomains, err = getValues(excluded); err != nil {
  		return false, err
  	}
  	out.PermittedDNSDomainsCritical = e.Critical
  
  	return unhandled, nil
  }
  
  func parseCertificate(in *certificate) (*Certificate, error) {
  	out := new(Certificate)
  	out.Raw = in.Raw
  	out.RawTBSCertificate = in.TBSCertificate.Raw
  	out.RawSubjectPublicKeyInfo = in.TBSCertificate.PublicKey.Raw
  	out.RawSubject = in.TBSCertificate.Subject.FullBytes
  	out.RawIssuer = in.TBSCertificate.Issuer.FullBytes
  
  	out.Signature = in.SignatureValue.RightAlign()
  	out.SignatureAlgorithm =
  		getSignatureAlgorithmFromAI(in.TBSCertificate.SignatureAlgorithm)
  
  	out.PublicKeyAlgorithm =
  		getPublicKeyAlgorithmFromOID(in.TBSCertificate.PublicKey.Algorithm.Algorithm)
  	var err error
  	out.PublicKey, err = parsePublicKey(out.PublicKeyAlgorithm, &in.TBSCertificate.PublicKey)
  	if err != nil {
  		return nil, err
  	}
  
  	out.Version = in.TBSCertificate.Version + 1
  	out.SerialNumber = in.TBSCertificate.SerialNumber
  
  	var issuer, subject pkix.RDNSequence
  	if rest, err := asn1.Unmarshal(in.TBSCertificate.Subject.FullBytes, &subject); err != nil {
  		return nil, err
  	} else if len(rest) != 0 {
  		return nil, errors.New("x509: trailing data after X.509 subject")
  	}
  	if rest, err := asn1.Unmarshal(in.TBSCertificate.Issuer.FullBytes, &issuer); err != nil {
  		return nil, err
  	} else if len(rest) != 0 {
  		return nil, errors.New("x509: trailing data after X.509 subject")
  	}
  
  	out.Issuer.FillFromRDNSequence(&issuer)
  	out.Subject.FillFromRDNSequence(&subject)
  
  	out.NotBefore = in.TBSCertificate.Validity.NotBefore
  	out.NotAfter = in.TBSCertificate.Validity.NotAfter
  
  	for _, e := range in.TBSCertificate.Extensions {
  		out.Extensions = append(out.Extensions, e)
  		unhandled := false
  
  		if len(e.Id) == 4 && e.Id[0] == 2 && e.Id[1] == 5 && e.Id[2] == 29 {
  			switch e.Id[3] {
  			case 15:
  				// RFC 5280, 4.2.1.3
  				var usageBits asn1.BitString
  				if rest, err := asn1.Unmarshal(e.Value, &usageBits); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 KeyUsage")
  				}
  
  				var usage int
  				for i := 0; i < 9; i++ {
  					if usageBits.At(i) != 0 {
  						usage |= 1 << uint(i)
  					}
  				}
  				out.KeyUsage = KeyUsage(usage)
  
  			case 19:
  				// RFC 5280, 4.2.1.9
  				var constraints basicConstraints
  				if rest, err := asn1.Unmarshal(e.Value, &constraints); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 BasicConstraints")
  				}
  
  				out.BasicConstraintsValid = true
  				out.IsCA = constraints.IsCA
  				out.MaxPathLen = constraints.MaxPathLen
  				out.MaxPathLenZero = out.MaxPathLen == 0
  				// TODO: map out.MaxPathLen to 0 if it has the -1 default value? (Issue 19285)
  			case 17:
  				out.DNSNames, out.EmailAddresses, out.IPAddresses, out.URIs, err = parseSANExtension(e.Value)
  				if err != nil {
  					return nil, err
  				}
  
  				if len(out.DNSNames) == 0 && len(out.EmailAddresses) == 0 && len(out.IPAddresses) == 0 && len(out.URIs) == 0 {
  					// If we didn't parse anything then we do the critical check, below.
  					unhandled = true
  				}
  
  			case 30:
  				unhandled, err = parseNameConstraintsExtension(out, e)
  				if err != nil {
  					return nil, err
  				}
  
  			case 31:
  				// RFC 5280, 4.2.1.13
  
  				// CRLDistributionPoints ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint
  				//
  				// DistributionPoint ::= SEQUENCE {
  				//     distributionPoint       [0]     DistributionPointName OPTIONAL,
  				//     reasons                 [1]     ReasonFlags OPTIONAL,
  				//     cRLIssuer               [2]     GeneralNames OPTIONAL }
  				//
  				// DistributionPointName ::= CHOICE {
  				//     fullName                [0]     GeneralNames,
  				//     nameRelativeToCRLIssuer [1]     RelativeDistinguishedName }
  
  				var cdp []distributionPoint
  				if rest, err := asn1.Unmarshal(e.Value, &cdp); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 CRL distribution point")
  				}
  
  				for _, dp := range cdp {
  					// Per RFC 5280, 4.2.1.13, one of distributionPoint or cRLIssuer may be empty.
  					if len(dp.DistributionPoint.FullName) == 0 {
  						continue
  					}
  
  					for _, fullName := range dp.DistributionPoint.FullName {
  						if fullName.Tag == 6 {
  							out.CRLDistributionPoints = append(out.CRLDistributionPoints, string(fullName.Bytes))
  						}
  					}
  				}
  
  			case 35:
  				// RFC 5280, 4.2.1.1
  				var a authKeyId
  				if rest, err := asn1.Unmarshal(e.Value, &a); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 authority key-id")
  				}
  				out.AuthorityKeyId = a.Id
  
  			case 37:
  				// RFC 5280, 4.2.1.12.  Extended Key Usage
  
  				// id-ce-extKeyUsage OBJECT IDENTIFIER ::= { id-ce 37 }
  				//
  				// ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId
  				//
  				// KeyPurposeId ::= OBJECT IDENTIFIER
  
  				var keyUsage []asn1.ObjectIdentifier
  				if rest, err := asn1.Unmarshal(e.Value, &keyUsage); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 ExtendedKeyUsage")
  				}
  
  				for _, u := range keyUsage {
  					if extKeyUsage, ok := extKeyUsageFromOID(u); ok {
  						out.ExtKeyUsage = append(out.ExtKeyUsage, extKeyUsage)
  					} else {
  						out.UnknownExtKeyUsage = append(out.UnknownExtKeyUsage, u)
  					}
  				}
  
  			case 14:
  				// RFC 5280, 4.2.1.2
  				var keyid []byte
  				if rest, err := asn1.Unmarshal(e.Value, &keyid); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 key-id")
  				}
  				out.SubjectKeyId = keyid
  
  			case 32:
  				// RFC 5280 4.2.1.4: Certificate Policies
  				var policies []policyInformation
  				if rest, err := asn1.Unmarshal(e.Value, &policies); err != nil {
  					return nil, err
  				} else if len(rest) != 0 {
  					return nil, errors.New("x509: trailing data after X.509 certificate policies")
  				}
  				out.PolicyIdentifiers = make([]asn1.ObjectIdentifier, len(policies))
  				for i, policy := range policies {
  					out.PolicyIdentifiers[i] = policy.Policy
  				}
  
  			default:
  				// Unknown extensions are recorded if critical.
  				unhandled = true
  			}
  		} else if e.Id.Equal(oidExtensionAuthorityInfoAccess) {
  			// RFC 5280 4.2.2.1: Authority Information Access
  			var aia []authorityInfoAccess
  			if rest, err := asn1.Unmarshal(e.Value, &aia); err != nil {
  				return nil, err
  			} else if len(rest) != 0 {
  				return nil, errors.New("x509: trailing data after X.509 authority information")
  			}
  
  			for _, v := range aia {
  				// GeneralName: uniformResourceIdentifier [6] IA5String
  				if v.Location.Tag != 6 {
  					continue
  				}
  				if v.Method.Equal(oidAuthorityInfoAccessOcsp) {
  					out.OCSPServer = append(out.OCSPServer, string(v.Location.Bytes))
  				} else if v.Method.Equal(oidAuthorityInfoAccessIssuers) {
  					out.IssuingCertificateURL = append(out.IssuingCertificateURL, string(v.Location.Bytes))
  				}
  			}
  		} else {
  			// Unknown extensions are recorded if critical.
  			unhandled = true
  		}
  
  		if e.Critical && unhandled {
  			out.UnhandledCriticalExtensions = append(out.UnhandledCriticalExtensions, e.Id)
  		}
  	}
  
  	return out, nil
  }
  
  // ParseCertificate parses a single certificate from the given ASN.1 DER data.
  func ParseCertificate(asn1Data []byte) (*Certificate, error) {
  	var cert certificate
  	rest, err := asn1.Unmarshal(asn1Data, &cert)
  	if err != nil {
  		return nil, err
  	}
  	if len(rest) > 0 {
  		return nil, asn1.SyntaxError{Msg: "trailing data"}
  	}
  
  	return parseCertificate(&cert)
  }
  
  // ParseCertificates parses one or more certificates from the given ASN.1 DER
  // data. The certificates must be concatenated with no intermediate padding.
  func ParseCertificates(asn1Data []byte) ([]*Certificate, error) {
  	var v []*certificate
  
  	for len(asn1Data) > 0 {
  		cert := new(certificate)
  		var err error
  		asn1Data, err = asn1.Unmarshal(asn1Data, cert)
  		if err != nil {
  			return nil, err
  		}
  		v = append(v, cert)
  	}
  
  	ret := make([]*Certificate, len(v))
  	for i, ci := range v {
  		cert, err := parseCertificate(ci)
  		if err != nil {
  			return nil, err
  		}
  		ret[i] = cert
  	}
  
  	return ret, nil
  }
  
  func reverseBitsInAByte(in byte) byte {
  	b1 := in>>4 | in<<4
  	b2 := b1>>2&0x33 | b1<<2&0xcc
  	b3 := b2>>1&0x55 | b2<<1&0xaa
  	return b3
  }
  
  // asn1BitLength returns the bit-length of bitString by considering the
  // most-significant bit in a byte to be the "first" bit. This convention
  // matches ASN.1, but differs from almost everything else.
  func asn1BitLength(bitString []byte) int {
  	bitLen := len(bitString) * 8
  
  	for i := range bitString {
  		b := bitString[len(bitString)-i-1]
  
  		for bit := uint(0); bit < 8; bit++ {
  			if (b>>bit)&1 == 1 {
  				return bitLen
  			}
  			bitLen--
  		}
  	}
  
  	return 0
  }
  
  var (
  	oidExtensionSubjectKeyId          = []int{2, 5, 29, 14}
  	oidExtensionKeyUsage              = []int{2, 5, 29, 15}
  	oidExtensionExtendedKeyUsage      = []int{2, 5, 29, 37}
  	oidExtensionAuthorityKeyId        = []int{2, 5, 29, 35}
  	oidExtensionBasicConstraints      = []int{2, 5, 29, 19}
  	oidExtensionSubjectAltName        = []int{2, 5, 29, 17}
  	oidExtensionCertificatePolicies   = []int{2, 5, 29, 32}
  	oidExtensionNameConstraints       = []int{2, 5, 29, 30}
  	oidExtensionCRLDistributionPoints = []int{2, 5, 29, 31}
  	oidExtensionAuthorityInfoAccess   = []int{1, 3, 6, 1, 5, 5, 7, 1, 1}
  )
  
  var (
  	oidAuthorityInfoAccessOcsp    = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 48, 1}
  	oidAuthorityInfoAccessIssuers = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 48, 2}
  )
  
  // oidNotInExtensions returns whether an extension with the given oid exists in
  // extensions.
  func oidInExtensions(oid asn1.ObjectIdentifier, extensions []pkix.Extension) bool {
  	for _, e := range extensions {
  		if e.Id.Equal(oid) {
  			return true
  		}
  	}
  	return false
  }
  
  // marshalSANs marshals a list of addresses into a the contents of an X.509
  // SubjectAlternativeName extension.
  func marshalSANs(dnsNames, emailAddresses []string, ipAddresses []net.IP, uris []*url.URL) (derBytes []byte, err error) {
  	var rawValues []asn1.RawValue
  	for _, name := range dnsNames {
  		rawValues = append(rawValues, asn1.RawValue{Tag: nameTypeDNS, Class: 2, Bytes: []byte(name)})
  	}
  	for _, email := range emailAddresses {
  		rawValues = append(rawValues, asn1.RawValue{Tag: nameTypeEmail, Class: 2, Bytes: []byte(email)})
  	}
  	for _, rawIP := range ipAddresses {
  		// If possible, we always want to encode IPv4 addresses in 4 bytes.
  		ip := rawIP.To4()
  		if ip == nil {
  			ip = rawIP
  		}
  		rawValues = append(rawValues, asn1.RawValue{Tag: nameTypeIP, Class: 2, Bytes: ip})
  	}
  	for _, uri := range uris {
  		rawValues = append(rawValues, asn1.RawValue{Tag: nameTypeURI, Class: 2, Bytes: []byte(uri.String())})
  	}
  	return asn1.Marshal(rawValues)
  }
  
  func isIA5String(s string) error {
  	for _, r := range s {
  		if r >= utf8.RuneSelf {
  			return fmt.Errorf("x509: %q cannot be encoded as an IA5String", s)
  		}
  	}
  
  	return nil
  }
  
  func buildExtensions(template *Certificate, subjectIsEmpty bool, authorityKeyId []byte) (ret []pkix.Extension, err error) {
  	ret = make([]pkix.Extension, 10 /* maximum number of elements. */)
  	n := 0
  
  	if template.KeyUsage != 0 &&
  		!oidInExtensions(oidExtensionKeyUsage, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionKeyUsage
  		ret[n].Critical = true
  
  		var a [2]byte
  		a[0] = reverseBitsInAByte(byte(template.KeyUsage))
  		a[1] = reverseBitsInAByte(byte(template.KeyUsage >> 8))
  
  		l := 1
  		if a[1] != 0 {
  			l = 2
  		}
  
  		bitString := a[:l]
  		ret[n].Value, err = asn1.Marshal(asn1.BitString{Bytes: bitString, BitLength: asn1BitLength(bitString)})
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if (len(template.ExtKeyUsage) > 0 || len(template.UnknownExtKeyUsage) > 0) &&
  		!oidInExtensions(oidExtensionExtendedKeyUsage, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionExtendedKeyUsage
  
  		var oids []asn1.ObjectIdentifier
  		for _, u := range template.ExtKeyUsage {
  			if oid, ok := oidFromExtKeyUsage(u); ok {
  				oids = append(oids, oid)
  			} else {
  				panic("internal error")
  			}
  		}
  
  		oids = append(oids, template.UnknownExtKeyUsage...)
  
  		ret[n].Value, err = asn1.Marshal(oids)
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if template.BasicConstraintsValid && !oidInExtensions(oidExtensionBasicConstraints, template.ExtraExtensions) {
  		// Leaving MaxPathLen as zero indicates that no maximum path
  		// length is desired, unless MaxPathLenZero is set. A value of
  		// -1 causes encoding/asn1 to omit the value as desired.
  		maxPathLen := template.MaxPathLen
  		if maxPathLen == 0 && !template.MaxPathLenZero {
  			maxPathLen = -1
  		}
  		ret[n].Id = oidExtensionBasicConstraints
  		ret[n].Value, err = asn1.Marshal(basicConstraints{template.IsCA, maxPathLen})
  		ret[n].Critical = true
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if len(template.SubjectKeyId) > 0 && !oidInExtensions(oidExtensionSubjectKeyId, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionSubjectKeyId
  		ret[n].Value, err = asn1.Marshal(template.SubjectKeyId)
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if len(authorityKeyId) > 0 && !oidInExtensions(oidExtensionAuthorityKeyId, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionAuthorityKeyId
  		ret[n].Value, err = asn1.Marshal(authKeyId{authorityKeyId})
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if (len(template.OCSPServer) > 0 || len(template.IssuingCertificateURL) > 0) &&
  		!oidInExtensions(oidExtensionAuthorityInfoAccess, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionAuthorityInfoAccess
  		var aiaValues []authorityInfoAccess
  		for _, name := range template.OCSPServer {
  			aiaValues = append(aiaValues, authorityInfoAccess{
  				Method:   oidAuthorityInfoAccessOcsp,
  				Location: asn1.RawValue{Tag: 6, Class: 2, Bytes: []byte(name)},
  			})
  		}
  		for _, name := range template.IssuingCertificateURL {
  			aiaValues = append(aiaValues, authorityInfoAccess{
  				Method:   oidAuthorityInfoAccessIssuers,
  				Location: asn1.RawValue{Tag: 6, Class: 2, Bytes: []byte(name)},
  			})
  		}
  		ret[n].Value, err = asn1.Marshal(aiaValues)
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if (len(template.DNSNames) > 0 || len(template.EmailAddresses) > 0 || len(template.IPAddresses) > 0 || len(template.URIs) > 0) &&
  		!oidInExtensions(oidExtensionSubjectAltName, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionSubjectAltName
  		// https://tools.ietf.org/html/rfc5280#section-4.2.1.6
  		// “If the subject field contains an empty sequence ... then
  		// subjectAltName extension ... is marked as critical”
  		ret[n].Critical = subjectIsEmpty
  		ret[n].Value, err = marshalSANs(template.DNSNames, template.EmailAddresses, template.IPAddresses, template.URIs)
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if len(template.PolicyIdentifiers) > 0 &&
  		!oidInExtensions(oidExtensionCertificatePolicies, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionCertificatePolicies
  		policies := make([]policyInformation, len(template.PolicyIdentifiers))
  		for i, policy := range template.PolicyIdentifiers {
  			policies[i].Policy = policy
  		}
  		ret[n].Value, err = asn1.Marshal(policies)
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	if (len(template.PermittedDNSDomains) > 0 || len(template.ExcludedDNSDomains) > 0 ||
  		len(template.PermittedIPRanges) > 0 || len(template.ExcludedIPRanges) > 0 ||
  		len(template.PermittedEmailAddresses) > 0 || len(template.ExcludedEmailAddresses) > 0 ||
  		len(template.PermittedURIDomains) > 0 || len(template.ExcludedURIDomains) > 0) &&
  		!oidInExtensions(oidExtensionNameConstraints, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionNameConstraints
  		ret[n].Critical = template.PermittedDNSDomainsCritical
  
  		ipAndMask := func(ipNet *net.IPNet) []byte {
  			maskedIP := ipNet.IP.Mask(ipNet.Mask)
  			ipAndMask := make([]byte, 0, len(maskedIP)+len(ipNet.Mask))
  			ipAndMask = append(ipAndMask, maskedIP...)
  			ipAndMask = append(ipAndMask, ipNet.Mask...)
  			return ipAndMask
  		}
  
  		serialiseConstraints := func(dns []string, ips []*net.IPNet, emails []string, uriDomains []string) (der []byte, err error) {
  			var b cryptobyte.Builder
  
  			for _, name := range dns {
  				if err = isIA5String(name); err != nil {
  					return nil, err
  				}
  
  				b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) {
  					b.AddASN1(cryptobyte_asn1.Tag(2).ContextSpecific(), func(b *cryptobyte.Builder) {
  						b.AddBytes([]byte(name))
  					})
  				})
  			}
  
  			for _, ipNet := range ips {
  				b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) {
  					b.AddASN1(cryptobyte_asn1.Tag(7).ContextSpecific(), func(b *cryptobyte.Builder) {
  						b.AddBytes(ipAndMask(ipNet))
  					})
  				})
  			}
  
  			for _, email := range emails {
  				if err = isIA5String(email); err != nil {
  					return nil, err
  				}
  
  				b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) {
  					b.AddASN1(cryptobyte_asn1.Tag(1).ContextSpecific(), func(b *cryptobyte.Builder) {
  						b.AddBytes([]byte(email))
  					})
  				})
  			}
  
  			for _, uriDomain := range uriDomains {
  				if err = isIA5String(uriDomain); err != nil {
  					return nil, err
  				}
  
  				b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) {
  					b.AddASN1(cryptobyte_asn1.Tag(6).ContextSpecific(), func(b *cryptobyte.Builder) {
  						b.AddBytes([]byte(uriDomain))
  					})
  				})
  			}
  
  			return b.Bytes()
  		}
  
  		permitted, err := serialiseConstraints(template.PermittedDNSDomains, template.PermittedIPRanges, template.PermittedEmailAddresses, template.PermittedURIDomains)
  		if err != nil {
  			return nil, err
  		}
  
  		excluded, err := serialiseConstraints(template.ExcludedDNSDomains, template.ExcludedIPRanges, template.ExcludedEmailAddresses, template.ExcludedURIDomains)
  		if err != nil {
  			return nil, err
  		}
  
  		var b cryptobyte.Builder
  		b.AddASN1(cryptobyte_asn1.SEQUENCE, func(b *cryptobyte.Builder) {
  			if len(permitted) > 0 {
  				b.AddASN1(cryptobyte_asn1.Tag(0).ContextSpecific().Constructed(), func(b *cryptobyte.Builder) {
  					b.AddBytes(permitted)
  				})
  			}
  
  			if len(excluded) > 0 {
  				b.AddASN1(cryptobyte_asn1.Tag(1).ContextSpecific().Constructed(), func(b *cryptobyte.Builder) {
  					b.AddBytes(excluded)
  				})
  			}
  		})
  
  		ret[n].Value, err = b.Bytes()
  		if err != nil {
  			return nil, err
  		}
  		n++
  	}
  
  	if len(template.CRLDistributionPoints) > 0 &&
  		!oidInExtensions(oidExtensionCRLDistributionPoints, template.ExtraExtensions) {
  		ret[n].Id = oidExtensionCRLDistributionPoints
  
  		var crlDp []distributionPoint
  		for _, name := range template.CRLDistributionPoints {
  			dp := distributionPoint{
  				DistributionPoint: distributionPointName{
  					FullName: []asn1.RawValue{
  						asn1.RawValue{Tag: 6, Class: 2, Bytes: []byte(name)},
  					},
  				},
  			}
  			crlDp = append(crlDp, dp)
  		}
  
  		ret[n].Value, err = asn1.Marshal(crlDp)
  		if err != nil {
  			return
  		}
  		n++
  	}
  
  	// Adding another extension here? Remember to update the maximum number
  	// of elements in the make() at the top of the function.
  
  	return append(ret[:n], template.ExtraExtensions...), nil
  }
  
  func subjectBytes(cert *Certificate) ([]byte, error) {
  	if len(cert.RawSubject) > 0 {
  		return cert.RawSubject, nil
  	}
  
  	return asn1.Marshal(cert.Subject.ToRDNSequence())
  }
  
  // signingParamsForPublicKey returns the parameters to use for signing with
  // priv. If requestedSigAlgo is not zero then it overrides the default
  // signature algorithm.
  func signingParamsForPublicKey(pub interface{}, requestedSigAlgo SignatureAlgorithm) (hashFunc crypto.Hash, sigAlgo pkix.AlgorithmIdentifier, err error) {
  	var pubType PublicKeyAlgorithm
  
  	switch pub := pub.(type) {
  	case *rsa.PublicKey:
  		pubType = RSA
  		hashFunc = crypto.SHA256
  		sigAlgo.Algorithm = oidSignatureSHA256WithRSA
  		sigAlgo.Parameters = asn1.NullRawValue
  
  	case *ecdsa.PublicKey:
  		pubType = ECDSA
  
  		switch pub.Curve {
  		case elliptic.P224(), elliptic.P256():
  			hashFunc = crypto.SHA256
  			sigAlgo.Algorithm = oidSignatureECDSAWithSHA256
  		case elliptic.P384():
  			hashFunc = crypto.SHA384
  			sigAlgo.Algorithm = oidSignatureECDSAWithSHA384
  		case elliptic.P521():
  			hashFunc = crypto.SHA512
  			sigAlgo.Algorithm = oidSignatureECDSAWithSHA512
  		default:
  			err = errors.New("x509: unknown elliptic curve")
  		}
  
  	default:
  		err = errors.New("x509: only RSA and ECDSA keys supported")
  	}
  
  	if err != nil {
  		return
  	}
  
  	if requestedSigAlgo == 0 {
  		return
  	}
  
  	found := false
  	for _, details := range signatureAlgorithmDetails {
  		if details.algo == requestedSigAlgo {
  			if details.pubKeyAlgo != pubType {
  				err = errors.New("x509: requested SignatureAlgorithm does not match private key type")
  				return
  			}
  			sigAlgo.Algorithm, hashFunc = details.oid, details.hash
  			if hashFunc == 0 {
  				err = errors.New("x509: cannot sign with hash function requested")
  				return
  			}
  			if requestedSigAlgo.isRSAPSS() {
  				sigAlgo.Parameters = rsaPSSParameters(hashFunc)
  			}
  			found = true
  			break
  		}
  	}
  
  	if !found {
  		err = errors.New("x509: unknown SignatureAlgorithm")
  	}
  
  	return
  }
  
  // emptyASN1Subject is the ASN.1 DER encoding of an empty Subject, which is
  // just an empty SEQUENCE.
  var emptyASN1Subject = []byte{0x30, 0}
  
  // CreateCertificate creates a new X.509v3 certificate based on a template.
  // The following members of template are used: AuthorityKeyId,
  // BasicConstraintsValid, DNSNames, ExcludedDNSDomains, ExtKeyUsage,
  // IsCA, KeyUsage, MaxPathLen, MaxPathLenZero, NotAfter, NotBefore,
  // PermittedDNSDomains, PermittedDNSDomainsCritical, SerialNumber,
  // SignatureAlgorithm, Subject, SubjectKeyId, and UnknownExtKeyUsage.
  //
  // The certificate is signed by parent. If parent is equal to template then the
  // certificate is self-signed. The parameter pub is the public key of the
  // signee and priv is the private key of the signer.
  //
  // The returned slice is the certificate in DER encoding.
  //
  // All keys types that are implemented via crypto.Signer are supported (This
  // includes *rsa.PublicKey and *ecdsa.PublicKey.)
  //
  // The AuthorityKeyId will be taken from the SubjectKeyId of parent, if any,
  // unless the resulting certificate is self-signed. Otherwise the value from
  // template will be used.
  func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv interface{}) (cert []byte, err error) {
  	key, ok := priv.(crypto.Signer)
  	if !ok {
  		return nil, errors.New("x509: certificate private key does not implement crypto.Signer")
  	}
  
  	if template.SerialNumber == nil {
  		return nil, errors.New("x509: no SerialNumber given")
  	}
  
  	hashFunc, signatureAlgorithm, err := signingParamsForPublicKey(key.Public(), template.SignatureAlgorithm)
  	if err != nil {
  		return nil, err
  	}
  
  	publicKeyBytes, publicKeyAlgorithm, err := marshalPublicKey(pub)
  	if err != nil {
  		return nil, err
  	}
  
  	asn1Issuer, err := subjectBytes(parent)
  	if err != nil {
  		return
  	}
  
  	asn1Subject, err := subjectBytes(template)
  	if err != nil {
  		return
  	}
  
  	authorityKeyId := template.AuthorityKeyId
  	if !bytes.Equal(asn1Issuer, asn1Subject) && len(parent.SubjectKeyId) > 0 {
  		authorityKeyId = parent.SubjectKeyId
  	}
  
  	extensions, err := buildExtensions(template, bytes.Equal(asn1Subject, emptyASN1Subject), authorityKeyId)
  	if err != nil {
  		return
  	}
  
  	encodedPublicKey := asn1.BitString{BitLength: len(publicKeyBytes) * 8, Bytes: publicKeyBytes}
  	c := tbsCertificate{
  		Version:            2,
  		SerialNumber:       template.SerialNumber,
  		SignatureAlgorithm: signatureAlgorithm,
  		Issuer:             asn1.RawValue{FullBytes: asn1Issuer},
  		Validity:           validity{template.NotBefore.UTC(), template.NotAfter.UTC()},
  		Subject:            asn1.RawValue{FullBytes: asn1Subject},
  		PublicKey:          publicKeyInfo{nil, publicKeyAlgorithm, encodedPublicKey},
  		Extensions:         extensions,
  	}
  
  	tbsCertContents, err := asn1.Marshal(c)
  	if err != nil {
  		return
  	}
  
  	c.Raw = tbsCertContents
  
  	h := hashFunc.New()
  	h.Write(tbsCertContents)
  	digest := h.Sum(nil)
  
  	var signerOpts crypto.SignerOpts
  	signerOpts = hashFunc
  	if template.SignatureAlgorithm != 0 && template.SignatureAlgorithm.isRSAPSS() {
  		signerOpts = &rsa.PSSOptions{
  			SaltLength: rsa.PSSSaltLengthEqualsHash,
  			Hash:       hashFunc,
  		}
  	}
  
  	var signature []byte
  	signature, err = key.Sign(rand, digest, signerOpts)
  	if err != nil {
  		return
  	}
  
  	return asn1.Marshal(certificate{
  		nil,
  		c,
  		signatureAlgorithm,
  		asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
  	})
  }
  
  // pemCRLPrefix is the magic string that indicates that we have a PEM encoded
  // CRL.
  var pemCRLPrefix = []byte("-----BEGIN X509 CRL")
  
  // pemType is the type of a PEM encoded CRL.
  var pemType = "X509 CRL"
  
  // ParseCRL parses a CRL from the given bytes. It's often the case that PEM
  // encoded CRLs will appear where they should be DER encoded, so this function
  // will transparently handle PEM encoding as long as there isn't any leading
  // garbage.
  func ParseCRL(crlBytes []byte) (*pkix.CertificateList, error) {
  	if bytes.HasPrefix(crlBytes, pemCRLPrefix) {
  		block, _ := pem.Decode(crlBytes)
  		if block != nil && block.Type == pemType {
  			crlBytes = block.Bytes
  		}
  	}
  	return ParseDERCRL(crlBytes)
  }
  
  // ParseDERCRL parses a DER encoded CRL from the given bytes.
  func ParseDERCRL(derBytes []byte) (*pkix.CertificateList, error) {
  	certList := new(pkix.CertificateList)
  	if rest, err := asn1.Unmarshal(derBytes, certList); err != nil {
  		return nil, err
  	} else if len(rest) != 0 {
  		return nil, errors.New("x509: trailing data after CRL")
  	}
  	return certList, nil
  }
  
  // CreateCRL returns a DER encoded CRL, signed by this Certificate, that
  // contains the given list of revoked certificates.
  func (c *Certificate) CreateCRL(rand io.Reader, priv interface{}, revokedCerts []pkix.RevokedCertificate, now, expiry time.Time) (crlBytes []byte, err error) {
  	key, ok := priv.(crypto.Signer)
  	if !ok {
  		return nil, errors.New("x509: certificate private key does not implement crypto.Signer")
  	}
  
  	hashFunc, signatureAlgorithm, err := signingParamsForPublicKey(key.Public(), 0)
  	if err != nil {
  		return nil, err
  	}
  
  	// Force revocation times to UTC per RFC 5280.
  	revokedCertsUTC := make([]pkix.RevokedCertificate, len(revokedCerts))
  	for i, rc := range revokedCerts {
  		rc.RevocationTime = rc.RevocationTime.UTC()
  		revokedCertsUTC[i] = rc
  	}
  
  	tbsCertList := pkix.TBSCertificateList{
  		Version:             1,
  		Signature:           signatureAlgorithm,
  		Issuer:              c.Subject.ToRDNSequence(),
  		ThisUpdate:          now.UTC(),
  		NextUpdate:          expiry.UTC(),
  		RevokedCertificates: revokedCertsUTC,
  	}
  
  	// Authority Key Id
  	if len(c.SubjectKeyId) > 0 {
  		var aki pkix.Extension
  		aki.Id = oidExtensionAuthorityKeyId
  		aki.Value, err = asn1.Marshal(authKeyId{Id: c.SubjectKeyId})
  		if err != nil {
  			return
  		}
  		tbsCertList.Extensions = append(tbsCertList.Extensions, aki)
  	}
  
  	tbsCertListContents, err := asn1.Marshal(tbsCertList)
  	if err != nil {
  		return
  	}
  
  	h := hashFunc.New()
  	h.Write(tbsCertListContents)
  	digest := h.Sum(nil)
  
  	var signature []byte
  	signature, err = key.Sign(rand, digest, hashFunc)
  	if err != nil {
  		return
  	}
  
  	return asn1.Marshal(pkix.CertificateList{
  		TBSCertList:        tbsCertList,
  		SignatureAlgorithm: signatureAlgorithm,
  		SignatureValue:     asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
  	})
  }
  
  // CertificateRequest represents a PKCS #10, certificate signature request.
  type CertificateRequest struct {
  	Raw                      []byte // Complete ASN.1 DER content (CSR, signature algorithm and signature).
  	RawTBSCertificateRequest []byte // Certificate request info part of raw ASN.1 DER content.
  	RawSubjectPublicKeyInfo  []byte // DER encoded SubjectPublicKeyInfo.
  	RawSubject               []byte // DER encoded Subject.
  
  	Version            int
  	Signature          []byte
  	SignatureAlgorithm SignatureAlgorithm
  
  	PublicKeyAlgorithm PublicKeyAlgorithm
  	PublicKey          interface{}
  
  	Subject pkix.Name
  
  	// Attributes is the dried husk of a bug and shouldn't be used.
  	Attributes []pkix.AttributeTypeAndValueSET
  
  	// Extensions contains raw X.509 extensions. When parsing CSRs, this
  	// can be used to extract extensions that are not parsed by this
  	// package.
  	Extensions []pkix.Extension
  
  	// ExtraExtensions contains extensions to be copied, raw, into any
  	// marshaled CSR. Values override any extensions that would otherwise
  	// be produced based on the other fields but are overridden by any
  	// extensions specified in Attributes.
  	//
  	// The ExtraExtensions field is not populated when parsing CSRs, see
  	// Extensions.
  	ExtraExtensions []pkix.Extension
  
  	// Subject Alternate Name values.
  	DNSNames       []string
  	EmailAddresses []string
  	IPAddresses    []net.IP
  	URIs           []*url.URL
  }
  
  // These structures reflect the ASN.1 structure of X.509 certificate
  // signature requests (see RFC 2986):
  
  type tbsCertificateRequest struct {
  	Raw           asn1.RawContent
  	Version       int
  	Subject       asn1.RawValue
  	PublicKey     publicKeyInfo
  	RawAttributes []asn1.RawValue `asn1:"tag:0"`
  }
  
  type certificateRequest struct {
  	Raw                asn1.RawContent
  	TBSCSR             tbsCertificateRequest
  	SignatureAlgorithm pkix.AlgorithmIdentifier
  	SignatureValue     asn1.BitString
  }
  
  // oidExtensionRequest is a PKCS#9 OBJECT IDENTIFIER that indicates requested
  // extensions in a CSR.
  var oidExtensionRequest = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 14}
  
  // newRawAttributes converts AttributeTypeAndValueSETs from a template
  // CertificateRequest's Attributes into tbsCertificateRequest RawAttributes.
  func newRawAttributes(attributes []pkix.AttributeTypeAndValueSET) ([]asn1.RawValue, error) {
  	var rawAttributes []asn1.RawValue
  	b, err := asn1.Marshal(attributes)
  	if err != nil {
  		return nil, err
  	}
  	rest, err := asn1.Unmarshal(b, &rawAttributes)
  	if err != nil {
  		return nil, err
  	}
  	if len(rest) != 0 {
  		return nil, errors.New("x509: failed to unmarshal raw CSR Attributes")
  	}
  	return rawAttributes, nil
  }
  
  // parseRawAttributes Unmarshals RawAttributes intos AttributeTypeAndValueSETs.
  func parseRawAttributes(rawAttributes []asn1.RawValue) []pkix.AttributeTypeAndValueSET {
  	var attributes []pkix.AttributeTypeAndValueSET
  	for _, rawAttr := range rawAttributes {
  		var attr pkix.AttributeTypeAndValueSET
  		rest, err := asn1.Unmarshal(rawAttr.FullBytes, &attr)
  		// Ignore attributes that don't parse into pkix.AttributeTypeAndValueSET
  		// (i.e.: challengePassword or unstructuredName).
  		if err == nil && len(rest) == 0 {
  			attributes = append(attributes, attr)
  		}
  	}
  	return attributes
  }
  
  // parseCSRExtensions parses the attributes from a CSR and extracts any
  // requested extensions.
  func parseCSRExtensions(rawAttributes []asn1.RawValue) ([]pkix.Extension, error) {
  	// pkcs10Attribute reflects the Attribute structure from section 4.1 of
  	// https://tools.ietf.org/html/rfc2986.
  	type pkcs10Attribute struct {
  		Id     asn1.ObjectIdentifier
  		Values []asn1.RawValue `asn1:"set"`
  	}
  
  	var ret []pkix.Extension
  	for _, rawAttr := range rawAttributes {
  		var attr pkcs10Attribute
  		if rest, err := asn1.Unmarshal(rawAttr.FullBytes, &attr); err != nil || len(rest) != 0 || len(attr.Values) == 0 {
  			// Ignore attributes that don't parse.
  			continue
  		}
  
  		if !attr.Id.Equal(oidExtensionRequest) {
  			continue
  		}
  
  		var extensions []pkix.Extension
  		if _, err := asn1.Unmarshal(attr.Values[0].FullBytes, &extensions); err != nil {
  			return nil, err
  		}
  		ret = append(ret, extensions...)
  	}
  
  	return ret, nil
  }
  
  // CreateCertificateRequest creates a new certificate request based on a
  // template. The following members of template are used: Attributes, DNSNames,
  // EmailAddresses, ExtraExtensions, IPAddresses, URIs, SignatureAlgorithm, and
  // Subject. The private key is the private key of the signer.
  //
  // The returned slice is the certificate request in DER encoding.
  //
  // All keys types that are implemented via crypto.Signer are supported (This
  // includes *rsa.PublicKey and *ecdsa.PublicKey.)
  func CreateCertificateRequest(rand io.Reader, template *CertificateRequest, priv interface{}) (csr []byte, err error) {
  	key, ok := priv.(crypto.Signer)
  	if !ok {
  		return nil, errors.New("x509: certificate private key does not implement crypto.Signer")
  	}
  
  	var hashFunc crypto.Hash
  	var sigAlgo pkix.AlgorithmIdentifier
  	hashFunc, sigAlgo, err = signingParamsForPublicKey(key.Public(), template.SignatureAlgorithm)
  	if err != nil {
  		return nil, err
  	}
  
  	var publicKeyBytes []byte
  	var publicKeyAlgorithm pkix.AlgorithmIdentifier
  	publicKeyBytes, publicKeyAlgorithm, err = marshalPublicKey(key.Public())
  	if err != nil {
  		return nil, err
  	}
  
  	var extensions []pkix.Extension
  
  	if (len(template.DNSNames) > 0 || len(template.EmailAddresses) > 0 || len(template.IPAddresses) > 0 || len(template.URIs) > 0) &&
  		!oidInExtensions(oidExtensionSubjectAltName, template.ExtraExtensions) {
  		sanBytes, err := marshalSANs(template.DNSNames, template.EmailAddresses, template.IPAddresses, template.URIs)
  		if err != nil {
  			return nil, err
  		}
  
  		extensions = append(extensions, pkix.Extension{
  			Id:    oidExtensionSubjectAltName,
  			Value: sanBytes,
  		})
  	}
  
  	extensions = append(extensions, template.ExtraExtensions...)
  
  	var attributes []pkix.AttributeTypeAndValueSET
  	attributes = append(attributes, template.Attributes...)
  
  	if len(extensions) > 0 {
  		// specifiedExtensions contains all the extensions that we
  		// found specified via template.Attributes.
  		specifiedExtensions := make(map[string]bool)
  
  		for _, atvSet := range template.Attributes {
  			if !atvSet.Type.Equal(oidExtensionRequest) {
  				continue
  			}
  
  			for _, atvs := range atvSet.Value {
  				for _, atv := range atvs {
  					specifiedExtensions[atv.Type.String()] = true
  				}
  			}
  		}
  
  		atvs := make([]pkix.AttributeTypeAndValue, 0, len(extensions))
  		for _, e := range extensions {
  			if specifiedExtensions[e.Id.String()] {
  				// Attributes already contained a value for
  				// this extension and it takes priority.
  				continue
  			}
  
  			atvs = append(atvs, pkix.AttributeTypeAndValue{
  				// There is no place for the critical flag in a CSR.
  				Type:  e.Id,
  				Value: e.Value,
  			})
  		}
  
  		// Append the extensions to an existing attribute if possible.
  		appended := false
  		for _, atvSet := range attributes {
  			if !atvSet.Type.Equal(oidExtensionRequest) || len(atvSet.Value) == 0 {
  				continue
  			}
  
  			atvSet.Value[0] = append(atvSet.Value[0], atvs...)
  			appended = true
  			break
  		}
  
  		// Otherwise, add a new attribute for the extensions.
  		if !appended {
  			attributes = append(attributes, pkix.AttributeTypeAndValueSET{
  				Type: oidExtensionRequest,
  				Value: [][]pkix.AttributeTypeAndValue{
  					atvs,
  				},
  			})
  		}
  	}
  
  	asn1Subject := template.RawSubject
  	if len(asn1Subject) == 0 {
  		asn1Subject, err = asn1.Marshal(template.Subject.ToRDNSequence())
  		if err != nil {
  			return
  		}
  	}
  
  	rawAttributes, err := newRawAttributes(attributes)
  	if err != nil {
  		return
  	}
  
  	tbsCSR := tbsCertificateRequest{
  		Version: 0, // PKCS #10, RFC 2986
  		Subject: asn1.RawValue{FullBytes: asn1Subject},
  		PublicKey: publicKeyInfo{
  			Algorithm: publicKeyAlgorithm,
  			PublicKey: asn1.BitString{
  				Bytes:     publicKeyBytes,
  				BitLength: len(publicKeyBytes) * 8,
  			},
  		},
  		RawAttributes: rawAttributes,
  	}
  
  	tbsCSRContents, err := asn1.Marshal(tbsCSR)
  	if err != nil {
  		return
  	}
  	tbsCSR.Raw = tbsCSRContents
  
  	h := hashFunc.New()
  	h.Write(tbsCSRContents)
  	digest := h.Sum(nil)
  
  	var signature []byte
  	signature, err = key.Sign(rand, digest, hashFunc)
  	if err != nil {
  		return
  	}
  
  	return asn1.Marshal(certificateRequest{
  		TBSCSR:             tbsCSR,
  		SignatureAlgorithm: sigAlgo,
  		SignatureValue: asn1.BitString{
  			Bytes:     signature,
  			BitLength: len(signature) * 8,
  		},
  	})
  }
  
  // ParseCertificateRequest parses a single certificate request from the
  // given ASN.1 DER data.
  func ParseCertificateRequest(asn1Data []byte) (*CertificateRequest, error) {
  	var csr certificateRequest
  
  	rest, err := asn1.Unmarshal(asn1Data, &csr)
  	if err != nil {
  		return nil, err
  	} else if len(rest) != 0 {
  		return nil, asn1.SyntaxError{Msg: "trailing data"}
  	}
  
  	return parseCertificateRequest(&csr)
  }
  
  func parseCertificateRequest(in *certificateRequest) (*CertificateRequest, error) {
  	out := &CertificateRequest{
  		Raw: in.Raw,
  		RawTBSCertificateRequest: in.TBSCSR.Raw,
  		RawSubjectPublicKeyInfo:  in.TBSCSR.PublicKey.Raw,
  		RawSubject:               in.TBSCSR.Subject.FullBytes,
  
  		Signature:          in.SignatureValue.RightAlign(),
  		SignatureAlgorithm: getSignatureAlgorithmFromAI(in.SignatureAlgorithm),
  
  		PublicKeyAlgorithm: getPublicKeyAlgorithmFromOID(in.TBSCSR.PublicKey.Algorithm.Algorithm),
  
  		Version:    in.TBSCSR.Version,
  		Attributes: parseRawAttributes(in.TBSCSR.RawAttributes),
  	}
  
  	var err error
  	out.PublicKey, err = parsePublicKey(out.PublicKeyAlgorithm, &in.TBSCSR.PublicKey)
  	if err != nil {
  		return nil, err
  	}
  
  	var subject pkix.RDNSequence
  	if rest, err := asn1.Unmarshal(in.TBSCSR.Subject.FullBytes, &subject); err != nil {
  		return nil, err
  	} else if len(rest) != 0 {
  		return nil, errors.New("x509: trailing data after X.509 Subject")
  	}
  
  	out.Subject.FillFromRDNSequence(&subject)
  
  	if out.Extensions, err = parseCSRExtensions(in.TBSCSR.RawAttributes); err != nil {
  		return nil, err
  	}
  
  	for _, extension := range out.Extensions {
  		if extension.Id.Equal(oidExtensionSubjectAltName) {
  			out.DNSNames, out.EmailAddresses, out.IPAddresses, out.URIs, err = parseSANExtension(extension.Value)
  			if err != nil {
  				return nil, err
  			}
  		}
  	}
  
  	return out, nil
  }
  
  // CheckSignature reports whether the signature on c is valid.
  func (c *CertificateRequest) CheckSignature() error {
  	return checkSignature(c.SignatureAlgorithm, c.RawTBSCertificateRequest, c.Signature, c.PublicKey)
  }
  

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