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// 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.
package x509

import (
        "bytes"
        "crypto"
        "crypto/dsa"
        "crypto/rsa"
        "crypto/sha1"
        "crypto/x509/pkix"
        "encoding/asn1"
        "encoding/pem"
        "errors"
        "io"
        "math/big"
        "time"
)

// 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".
func ParsePKIXPublicKey(derBytes []byte) (pub interface{}, err error) {
        var pki publicKeyInfo
        if _, err = asn1.Unmarshal(derBytes, &pki); err != nil {
                return
        }
        algo := getPublicKeyAlgorithmFromOID(pki.Algorithm.Algorithm)
        if algo == UnknownPublicKeyAlgorithm {
                return nil, errors.New("ParsePKIXPublicKey: unknown public key algorithm")
        }
        return parsePublicKey(algo, &pki)
}

// MarshalPKIXPublicKey serialises a public key to DER-encoded PKIX format.
func MarshalPKIXPublicKey(pub interface{}) ([]byte, error) {
        var pubBytes []byte

        switch pub := pub.(type) {
        case *rsa.PublicKey:
                pubBytes, _ = asn1.Marshal(rsaPublicKey{
                        N: pub.N,
                        E: pub.E,
                })
        default:
                return nil, errors.New("MarshalPKIXPublicKey: unknown public key type")
        }

        pkix := pkixPublicKey{
                Algo: pkix.AlgorithmIdentifier{
                        Algorithm: []int{1, 2, 840, 113549, 1, 1, 1},
                        // This is a NULL parameters value which is technically
                        // superfluous, but most other code includes it and, by
                        // doing this, we match their public key hashes.
                        Parameters: asn1.RawValue{
                                Tag: 5,
                        },
                },
                BitString: asn1.BitString{
                        Bytes:     pubBytes,
                        BitLength: 8 * len(pubBytes),
                },
        }

        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:1,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 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
)

type PublicKeyAlgorithm int

const (
        UnknownPublicKeyAlgorithm PublicKeyAlgorithm = iota
        RSA
        DSA
)

// 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 IDENTIFER ::= { 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 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
//
// dsaWithSha356 OBJECT IDENTIFER ::= {
//    joint-iso-ccitt(2) country(16) us(840) organization(1) gov(101)
//    algorithms(4) id-dsa-with-sha2(3) 2}
//
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}
        oidSignatureDSAWithSHA1   = asn1.ObjectIdentifier{1, 2, 840, 10040, 4, 3}
        oidSignatureDSAWithSHA256 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 4, 3, 2}
)

func getSignatureAlgorithmFromOID(oid asn1.ObjectIdentifier) SignatureAlgorithm {
        switch {
        case oid.Equal(oidSignatureMD2WithRSA):
                return MD2WithRSA
        case oid.Equal(oidSignatureMD5WithRSA):
                return MD5WithRSA
        case oid.Equal(oidSignatureSHA1WithRSA):
                return SHA1WithRSA
        case oid.Equal(oidSignatureSHA256WithRSA):
                return SHA256WithRSA
        case oid.Equal(oidSignatureSHA384WithRSA):
                return SHA384WithRSA
        case oid.Equal(oidSignatureSHA512WithRSA):
                return SHA512WithRSA
        case oid.Equal(oidSignatureDSAWithSHA1):
                return DSAWithSHA1
        case oid.Equal(oidSignatureDSAWithSHA256):
                return DSAWithSHA256
        }
        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 }
var (
        oidPublicKeyRsa = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 1}
        oidPublicKeyDsa = asn1.ObjectIdentifier{1, 2, 840, 10040, 4, 1}
)

func getPublicKeyAlgorithmFromOID(oid asn1.ObjectIdentifier) PublicKeyAlgorithm {
        switch {
        case oid.Equal(oidPublicKeyRsa):
                return RSA
        case oid.Equal(oidPublicKeyDsa):
                return DSA
        }
        return UnknownPublicKeyAlgorithm
}

// 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}
        oidExtKeyUsageTimeStamping    = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 8}
        oidExtKeyUsageOCSPSigning     = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 9}
)

// 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
        ExtKeyUsageTimeStamping
        ExtKeyUsageOCSPSigning
)

// 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

        ExtKeyUsage        []ExtKeyUsage           // Sequence of extended key usages.
        UnknownExtKeyUsage []asn1.ObjectIdentifier // Encountered extended key usages unknown to this package.

        BasicConstraintsValid bool // if true then the next two fields are valid.
        IsCA                  bool
        MaxPathLen            int

        SubjectKeyId   []byte
        AuthorityKeyId []byte

        // Subject Alternate Name values
        DNSNames       []string
        EmailAddresses []string

        // Name constraints
        PermittedDNSDomainsCritical bool // if true then the name constraints are marked critical.
        PermittedDNSDomains         []string

        PolicyIdentifiers []asn1.ObjectIdentifier
}

// UnsupportedAlgorithmError results from attempting to perform an operation
// that involves algorithms that are not currently implemented.
type UnsupportedAlgorithmError struct{}

func (UnsupportedAlgorithmError) Error() string {
        return "cannot verify signature: algorithm unimplemented"
}

// 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 "invalid signature: parent certificate cannot sign this kind of certificate"
}

func (c *Certificate) Equal(other *Certificate) bool {
        return bytes.Equal(c.Raw, other.Raw)
}

// CheckSignatureFrom verifies that the signature on c is a valid signature
// from parent.
func (c *Certificate) CheckSignatureFrom(parent *Certificate) (err 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."
        if parent.Version == 3 && !parent.BasicConstraintsValid ||
                parent.BasicConstraintsValid && !parent.IsCA {
                return ConstraintViolationError{}
        }

        if parent.KeyUsage != 0 && parent.KeyUsage&KeyUsageCertSign == 0 {
                return ConstraintViolationError{}
        }

        if parent.PublicKeyAlgorithm == UnknownPublicKeyAlgorithm {
                return UnsupportedAlgorithmError{}
        }

        // 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) (err error) {
        var hashType crypto.Hash

        switch algo {
        case SHA1WithRSA, DSAWithSHA1:
                hashType = crypto.SHA1
        case SHA256WithRSA, DSAWithSHA256:
                hashType = crypto.SHA256
        case SHA384WithRSA:
                hashType = crypto.SHA384
        case SHA512WithRSA:
                hashType = crypto.SHA512
        default:
                return UnsupportedAlgorithmError{}
        }

        h := hashType.New()
        if h == nil {
                return UnsupportedAlgorithmError{}
        }

        h.Write(signed)
        digest := h.Sum(nil)

        switch pub := c.PublicKey.(type) {
        case *rsa.PublicKey:
                return rsa.VerifyPKCS1v15(pub, hashType, digest, signature)
        case *dsa.PublicKey:
                dsaSig := new(dsaSignature)
                if _, err := asn1.Unmarshal(signature, dsaSig); err != nil {
                        return err
                }
                if dsaSig.R.Sign() <= 0 || dsaSig.S.Sign() <= 0 {
                        return errors.New("DSA signature contained zero or negative values")
                }
                if !dsa.Verify(pub, digest, dsaSig.R, dsaSig.S) {
                        return errors.New("DSA verification failure")
                }
                return
        }
        return UnsupportedAlgorithmError{}
}

// CheckCRLSignature checks that the signature in crl is from c.
func (c *Certificate) CheckCRLSignature(crl *pkix.CertificateList) (err error) {
        algo := getSignatureAlgorithmFromOID(crl.SignatureAlgorithm.Algorithm)
        return c.CheckSignature(algo, crl.TBSCertList.Raw, crl.SignatureValue.RightAlign())
}

type UnhandledCriticalExtension struct{}

func (h UnhandledCriticalExtension) Error() string {
        return "unhandled critical extension"
}

type basicConstraints struct {
        IsCA       bool `asn1:"optional"`
        MaxPathLen int  `asn1:"optional"`
}

// RFC 5280 4.2.1.4
type policyInformation struct {
        Policy asn1.ObjectIdentifier
        // policyQualifiers omitted
}

// RFC 5280, 4.2.1.10
type nameConstraints struct {
        Permitted []generalSubtree `asn1:"optional,tag:0"`
        Excluded  []generalSubtree `asn1:"optional,tag:1"`
}

type generalSubtree struct {
        Name string `asn1:"tag:2,optional,ia5"`
        Min  int    `asn1:"optional,tag:0"`
        Max  int    `asn1:"optional,tag:1"`
}

func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, error) {
        asn1Data := keyData.PublicKey.RightAlign()
        switch algo {
        case RSA:
                p := new(rsaPublicKey)
                _, err := asn1.Unmarshal(asn1Data, p)
                if err != nil {
                        return nil, err
                }

                pub := &rsa.PublicKey{
                        E: p.E,
                        N: p.N,
                }
                return pub, nil
        case DSA:
                var p *big.Int
                _, err := asn1.Unmarshal(asn1Data, &p)
                if err != nil {
                        return nil, err
                }
                paramsData := keyData.Algorithm.Parameters.FullBytes
                params := new(dsaAlgorithmParameters)
                _, err = asn1.Unmarshal(paramsData, params)
                if err != nil {
                        return nil, err
                }
                if p.Sign() <= 0 || params.P.Sign() <= 0 || params.Q.Sign() <= 0 || params.G.Sign() <= 0 {
                        return nil, errors.New("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
        default:
                return nil, nil
        }
        panic("unreachable")
}

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 =
                getSignatureAlgorithmFromOID(in.TBSCertificate.SignatureAlgorithm.Algorithm)

        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
        }

        if in.TBSCertificate.SerialNumber.Sign() < 0 {
                return nil, errors.New("negative serial number")
        }

        out.Version = in.TBSCertificate.Version + 1
        out.SerialNumber = in.TBSCertificate.SerialNumber

        var issuer, subject pkix.RDNSequence
        if _, err := asn1.Unmarshal(in.TBSCertificate.Subject.FullBytes, &subject); err != nil {
                return nil, err
        }
        if _, err := asn1.Unmarshal(in.TBSCertificate.Issuer.FullBytes, &issuer); err != nil {
                return nil, err
        }

        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 {
                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
                                _, err := asn1.Unmarshal(e.Value, &usageBits)

                                if err == nil {
                                        var usage int
                                        for i := 0; i < 9; i++ {
                                                if usageBits.At(i) != 0 {
                                                        usage |= 1 << uint(i)
                                                }
                                        }
                                        out.KeyUsage = KeyUsage(usage)
                                        continue
                                }
                        case 19:
                                // RFC 5280, 4.2.1.9
                                var constraints basicConstraints
                                _, err := asn1.Unmarshal(e.Value, &constraints)

                                if err == nil {
                                        out.BasicConstraintsValid = true
                                        out.IsCA = constraints.IsCA
                                        out.MaxPathLen = constraints.MaxPathLen
                                        continue
                                }
                        case 17:
                                // 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
                                _, err := asn1.Unmarshal(e.Value, &seq)
                                if err != nil {
                                        return nil, err
                                }
                                if !seq.IsCompound || seq.Tag != 16 || seq.Class != 0 {
                                        return nil, asn1.StructuralError{Msg: "bad SAN sequence"}
                                }

                                parsedName := false

                                rest := seq.Bytes
                                for len(rest) > 0 {
                                        var v asn1.RawValue
                                        rest, err = asn1.Unmarshal(rest, &v)
                                        if err != nil {
                                                return nil, err
                                        }
                                        switch v.Tag {
                                        case 1:
                                                out.EmailAddresses = append(out.EmailAddresses, string(v.Bytes))
                                                parsedName = true
                                        case 2:
                                                out.DNSNames = append(out.DNSNames, string(v.Bytes))
                                                parsedName = true
                                        }
                                }

                                if parsedName {
                                        continue
                                }
                                // If we didn't parse any of the names then we
                                // fall through to the critical check below.

                        case 30:
                                // 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)

                                var constraints nameConstraints
                                _, err := asn1.Unmarshal(e.Value, &constraints)
                                if err != nil {
                                        return nil, err
                                }

                                if len(constraints.Excluded) > 0 && e.Critical {
                                        return out, UnhandledCriticalExtension{}
                                }

                                for _, subtree := range constraints.Permitted {
                                        if subtree.Min > 0 || subtree.Max > 0 || len(subtree.Name) == 0 {
                                                if e.Critical {
                                                        return out, UnhandledCriticalExtension{}
                                                }
                                                continue
                                        }
                                        out.PermittedDNSDomains = append(out.PermittedDNSDomains, subtree.Name)
                                }
                                continue

                        case 35:
                                // RFC 5280, 4.2.1.1
                                var a authKeyId
                                _, err = asn1.Unmarshal(e.Value, &a)
                                if err != nil {
                                        return nil, err
                                }
                                out.AuthorityKeyId = a.Id
                                continue

                        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
                                _, err = asn1.Unmarshal(e.Value, &keyUsage)
                                if err != nil {
                                        return nil, err
                                }

                                for _, u := range keyUsage {
                                        switch {
                                        case u.Equal(oidExtKeyUsageAny):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageAny)
                                        case u.Equal(oidExtKeyUsageServerAuth):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageServerAuth)
                                        case u.Equal(oidExtKeyUsageClientAuth):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageClientAuth)
                                        case u.Equal(oidExtKeyUsageCodeSigning):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageCodeSigning)
                                        case u.Equal(oidExtKeyUsageEmailProtection):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageEmailProtection)
                                        case u.Equal(oidExtKeyUsageTimeStamping):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageTimeStamping)
                                        case u.Equal(oidExtKeyUsageOCSPSigning):
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, ExtKeyUsageOCSPSigning)
                                        default:
                                                out.UnknownExtKeyUsage = append(out.UnknownExtKeyUsage, u)
                                        }
                                }

                                continue

                        case 14:
                                // RFC 5280, 4.2.1.2
                                var keyid []byte
                                _, err = asn1.Unmarshal(e.Value, &keyid)
                                if err != nil {
                                        return nil, err
                                }
                                out.SubjectKeyId = keyid
                                continue

                        case 32:
                                // RFC 5280 4.2.1.4: Certificate Policies
                                var policies []policyInformation
                                if _, err = asn1.Unmarshal(e.Value, &policies); err != nil {
                                        return nil, err
                                }
                                out.PolicyIdentifiers = make([]asn1.ObjectIdentifier, len(policies))
                                for i, policy := range policies {
                                        out.PolicyIdentifiers[i] = policy.Policy
                                }
                        }
                }

                if e.Critical {
                        return out, UnhandledCriticalExtension{}
                }
        }

        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
}

var (
        oidExtensionSubjectKeyId        = []int{2, 5, 29, 14}
        oidExtensionKeyUsage            = []int{2, 5, 29, 15}
        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}
)

func buildExtensions(template *Certificate) (ret []pkix.Extension, err error) {
        ret = make([]pkix.Extension, 7 /* maximum number of elements. */ )
        n := 0

        if template.KeyUsage != 0 {
                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
                }

                ret[n].Value, err = asn1.Marshal(asn1.BitString{Bytes: a[0:l], BitLength: l * 8})
                if err != nil {
                        return
                }
                n++
        }

        if template.BasicConstraintsValid {
                ret[n].Id = oidExtensionBasicConstraints
                ret[n].Value, err = asn1.Marshal(basicConstraints{template.IsCA, template.MaxPathLen})
                ret[n].Critical = true
                if err != nil {
                        return
                }
                n++
        }

        if len(template.SubjectKeyId) > 0 {
                ret[n].Id = oidExtensionSubjectKeyId
                ret[n].Value, err = asn1.Marshal(template.SubjectKeyId)
                if err != nil {
                        return
                }
                n++
        }

        if len(template.AuthorityKeyId) > 0 {
                ret[n].Id = oidExtensionAuthorityKeyId
                ret[n].Value, err = asn1.Marshal(authKeyId{template.AuthorityKeyId})
                if err != nil {
                        return
                }
                n++
        }

        if len(template.DNSNames) > 0 {
                ret[n].Id = oidExtensionSubjectAltName
                rawValues := make([]asn1.RawValue, len(template.DNSNames))
                for i, name := range template.DNSNames {
                        rawValues[i] = asn1.RawValue{Tag: 2, Class: 2, Bytes: []byte(name)}
                }
                ret[n].Value, err = asn1.Marshal(rawValues)
                if err != nil {
                        return
                }
                n++
        }

        if len(template.PolicyIdentifiers) > 0 {
                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 {
                ret[n].Id = oidExtensionNameConstraints
                ret[n].Critical = template.PermittedDNSDomainsCritical

                var out nameConstraints
                out.Permitted = make([]generalSubtree, len(template.PermittedDNSDomains))
                for i, permitted := range template.PermittedDNSDomains {
                        out.Permitted[i] = generalSubtree{Name: permitted}
                }
                ret[n].Value, err = asn1.Marshal(out)
                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 ret[0:n], nil
}

var (
        oidSHA1WithRSA = []int{1, 2, 840, 113549, 1, 1, 5}
        oidRSA         = []int{1, 2, 840, 113549, 1, 1, 1}
)

func subjectBytes(cert *Certificate) ([]byte, error) {
        if len(cert.RawSubject) > 0 {
                return cert.RawSubject, nil
        }

        return asn1.Marshal(cert.Subject.ToRDNSequence())
}

// CreateCertificate creates a new certificate based on a template. The
// following members of template are used: SerialNumber, Subject, NotBefore,
// NotAfter, KeyUsage, BasicConstraintsValid, IsCA, MaxPathLen, SubjectKeyId,
// DNSNames, PermittedDNSDomainsCritical, PermittedDNSDomains.
//
// 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.
//
// The only supported key type is RSA (*rsa.PublicKey for pub, *rsa.PrivateKey
// for priv).
func CreateCertificate(rand io.Reader, template, parent *Certificate, pub interface{}, priv interface{}) (cert []byte, err error) {
        rsaPub, ok := pub.(*rsa.PublicKey)
        if !ok {
                return nil, errors.New("x509: non-RSA public keys not supported")
        }

        rsaPriv, ok := priv.(*rsa.PrivateKey)
        if !ok {
                return nil, errors.New("x509: non-RSA private keys not supported")
        }

        asn1PublicKey, err := asn1.Marshal(rsaPublicKey{
                N: rsaPub.N,
                E: rsaPub.E,
        })
        if err != nil {
                return
        }

        if len(parent.SubjectKeyId) > 0 {
                template.AuthorityKeyId = parent.SubjectKeyId
        }

        extensions, err := buildExtensions(template)
        if err != nil {
                return
        }

        asn1Issuer, err := subjectBytes(parent)
        if err != nil {
                return
        }

        asn1Subject, err := subjectBytes(template)
        if err != nil {
                return
        }

        encodedPublicKey := asn1.BitString{BitLength: len(asn1PublicKey) * 8, Bytes: asn1PublicKey}
        c := tbsCertificate{
                Version:            2,
                SerialNumber:       template.SerialNumber,
                SignatureAlgorithm: pkix.AlgorithmIdentifier{Algorithm: oidSHA1WithRSA},
                Issuer:             asn1.RawValue{FullBytes: asn1Issuer},
                Validity:           validity{template.NotBefore, template.NotAfter},
                Subject:            asn1.RawValue{FullBytes: asn1Subject},
                PublicKey:          publicKeyInfo{nil, pkix.AlgorithmIdentifier{Algorithm: oidRSA}, encodedPublicKey},
                Extensions:         extensions,
        }

        tbsCertContents, err := asn1.Marshal(c)
        if err != nil {
                return
        }

        c.Raw = tbsCertContents

        h := sha1.New()
        h.Write(tbsCertContents)
        digest := h.Sum(nil)

        signature, err := rsa.SignPKCS1v15(rand, rsaPriv, crypto.SHA1, digest)
        if err != nil {
                return
        }

        cert, err = asn1.Marshal(certificate{
                nil,
                c,
                pkix.AlgorithmIdentifier{Algorithm: oidSHA1WithRSA},
                asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
        })
        return
}

// 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) (certList *pkix.CertificateList, err 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) (certList *pkix.CertificateList, err error) {
        certList = new(pkix.CertificateList)
        _, err = asn1.Unmarshal(derBytes, certList)
        if err != nil {
                certList = nil
        }
        return
}

// CreateCRL returns a DER encoded CRL, signed by this Certificate, that
// contains the given list of revoked certificates.
//
// The only supported key type is RSA (*rsa.PrivateKey for priv).
func (c *Certificate) CreateCRL(rand io.Reader, priv interface{}, revokedCerts []pkix.RevokedCertificate, now, expiry time.Time) (crlBytes []byte, err error) {
        rsaPriv, ok := priv.(*rsa.PrivateKey)
        if !ok {
                return nil, errors.New("x509: non-RSA private keys not supported")
        }
        tbsCertList := pkix.TBSCertificateList{
                Version: 2,
                Signature: pkix.AlgorithmIdentifier{
                        Algorithm: oidSignatureSHA1WithRSA,
                },
                Issuer:              c.Subject.ToRDNSequence(),
                ThisUpdate:          now,
                NextUpdate:          expiry,
                RevokedCertificates: revokedCerts,
        }

        tbsCertListContents, err := asn1.Marshal(tbsCertList)
        if err != nil {
                return
        }

        h := sha1.New()
        h.Write(tbsCertListContents)
        digest := h.Sum(nil)

        signature, err := rsa.SignPKCS1v15(rand, rsaPriv, crypto.SHA1, digest)
        if err != nil {
                return
        }

        return asn1.Marshal(pkix.CertificateList{
                TBSCertList: tbsCertList,
                SignatureAlgorithm: pkix.AlgorithmIdentifier{
                        Algorithm: oidSignatureSHA1WithRSA,
                },
                SignatureValue: asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
        })
}