Subversion Repositories openrisc

// 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 asn1 implements parsing of DER-encoded ASN.1 data structures,
// as defined in ITU-T Rec X.690.
//
// See also ``A Layman's Guide to a Subset of ASN.1, BER, and DER,''
// http://luca.ntop.org/Teaching/Appunti/asn1.html.
package asn1

// ASN.1 is a syntax for specifying abstract objects and BER, DER, PER, XER etc
// are different encoding formats for those objects. Here, we'll be dealing
// with DER, the Distinguished Encoding Rules. DER is used in X.509 because
// it's fast to parse and, unlike BER, has a unique encoding for every object.
// When calculating hashes over objects, it's important that the resulting
// bytes be the same at both ends and DER removes this margin of error.
//
// ASN.1 is very complex and this package doesn't attempt to implement
// everything by any means.

import (
        "fmt"
        "math/big"
        "reflect"
        "time"
)

// A StructuralError suggests that the ASN.1 data is valid, but the Go type
// which is receiving it doesn't match.
type StructuralError struct {
        Msg string
}

func (e StructuralError) Error() string { return "ASN.1 structure error: " + e.Msg }

// A SyntaxError suggests that the ASN.1 data is invalid.
type SyntaxError struct {
        Msg string
}

func (e SyntaxError) Error() string { return "ASN.1 syntax error: " + e.Msg }

// We start by dealing with each of the primitive types in turn.

// BOOLEAN

func parseBool(bytes []byte) (ret bool, err error) {
        if len(bytes) != 1 {
                err = SyntaxError{"invalid boolean"}
                return
        }

        return bytes[0] != 0, nil
}

// INTEGER

// parseInt64 treats the given bytes as a big-endian, signed integer and
// returns the result.
func parseInt64(bytes []byte) (ret int64, err error) {
        if len(bytes) > 8 {
                // We'll overflow an int64 in this case.
                err = StructuralError{"integer too large"}
                return
        }
        for bytesRead := 0; bytesRead < len(bytes); bytesRead++ {
                ret <<= 8
                ret |= int64(bytes[bytesRead])
        }

        // Shift up and down in order to sign extend the result.
        ret <<= 64 - uint8(len(bytes))*8
        ret >>= 64 - uint8(len(bytes))*8
        return
}

// parseInt treats the given bytes as a big-endian, signed integer and returns
// the result.
func parseInt(bytes []byte) (int, error) {
        ret64, err := parseInt64(bytes)
        if err != nil {
                return 0, err
        }
        if ret64 != int64(int(ret64)) {
                return 0, StructuralError{"integer too large"}
        }
        return int(ret64), nil
}

var bigOne = big.NewInt(1)

// parseBigInt treats the given bytes as a big-endian, signed integer and returns
// the result.
func parseBigInt(bytes []byte) *big.Int {
        ret := new(big.Int)
        if len(bytes) > 0 && bytes[0]&0x80 == 0x80 {
                // This is a negative number.
                notBytes := make([]byte, len(bytes))
                for i := range notBytes {
                        notBytes[i] = ^bytes[i]
                }
                ret.SetBytes(notBytes)
                ret.Add(ret, bigOne)
                ret.Neg(ret)
                return ret
        }
        ret.SetBytes(bytes)
        return ret
}

// BIT STRING

// BitString is the structure to use when you want an ASN.1 BIT STRING type. A
// bit string is padded up to the nearest byte in memory and the number of
// valid bits is recorded. Padding bits will be zero.
type BitString struct {
        Bytes     []byte // bits packed into bytes.
        BitLength int    // length in bits.
}

// At returns the bit at the given index. If the index is out of range it
// returns false.
func (b BitString) At(i int) int {
        if i < 0 || i >= b.BitLength {
                return 0
        }
        x := i / 8
        y := 7 - uint(i%8)
        return int(b.Bytes[x]>>y) & 1
}

// RightAlign returns a slice where the padding bits are at the beginning. The
// slice may share memory with the BitString.
func (b BitString) RightAlign() []byte {
        shift := uint(8 - (b.BitLength % 8))
        if shift == 8 || len(b.Bytes) == 0 {
                return b.Bytes
        }

        a := make([]byte, len(b.Bytes))
        a[0] = b.Bytes[0] >> shift
        for i := 1; i < len(b.Bytes); i++ {
                a[i] = b.Bytes[i-1] << (8 - shift)
                a[i] |= b.Bytes[i] >> shift
        }

        return a
}

// parseBitString parses an ASN.1 bit string from the given byte slice and returns it.
func parseBitString(bytes []byte) (ret BitString, err error) {
        if len(bytes) == 0 {
                err = SyntaxError{"zero length BIT STRING"}
                return
        }
        paddingBits := int(bytes[0])
        if paddingBits > 7 ||
                len(bytes) == 1 && paddingBits > 0 ||
                bytes[len(bytes)-1]&((1<<bytes[0])-1) != 0 {
                err = SyntaxError{"invalid padding bits in BIT STRING"}
                return
        }
        ret.BitLength = (len(bytes)-1)*8 - paddingBits
        ret.Bytes = bytes[1:]
        return
}

// OBJECT IDENTIFIER

// An ObjectIdentifier represents an ASN.1 OBJECT IDENTIFIER.
type ObjectIdentifier []int

// Equal returns true iff oi and other represent the same identifier.
func (oi ObjectIdentifier) Equal(other ObjectIdentifier) bool {
        if len(oi) != len(other) {
                return false
        }
        for i := 0; i < len(oi); i++ {
                if oi[i] != other[i] {
                        return false
                }
        }

        return true
}

// parseObjectIdentifier parses an OBJECT IDENTIFIER from the given bytes and
// returns it. An object identifier is a sequence of variable length integers
// that are assigned in a hierarchy.
func parseObjectIdentifier(bytes []byte) (s []int, err error) {
        if len(bytes) == 0 {
                err = SyntaxError{"zero length OBJECT IDENTIFIER"}
                return
        }

        // In the worst case, we get two elements from the first byte (which is
        // encoded differently) and then every varint is a single byte long.
        s = make([]int, len(bytes)+1)

        // The first byte is 40*value1 + value2:
        s[0] = int(bytes[0]) / 40
        s[1] = int(bytes[0]) % 40
        i := 2
        for offset := 1; offset < len(bytes); i++ {
                var v int
                v, offset, err = parseBase128Int(bytes, offset)
                if err != nil {
                        return
                }
                s[i] = v
        }
        s = s[0:i]
        return
}

// ENUMERATED

// An Enumerated is represented as a plain int.
type Enumerated int

// FLAG

// A Flag accepts any data and is set to true if present.
type Flag bool

// parseBase128Int parses a base-128 encoded int from the given offset in the
// given byte slice. It returns the value and the new offset.
func parseBase128Int(bytes []byte, initOffset int) (ret, offset int, err error) {
        offset = initOffset
        for shifted := 0; offset < len(bytes); shifted++ {
                if shifted > 4 {
                        err = StructuralError{"base 128 integer too large"}
                        return
                }
                ret <<= 7
                b := bytes[offset]
                ret |= int(b & 0x7f)
                offset++
                if b&0x80 == 0 {
                        return
                }
        }
        err = SyntaxError{"truncated base 128 integer"}
        return
}

// UTCTime

func parseUTCTime(bytes []byte) (ret time.Time, err error) {
        s := string(bytes)
        ret, err = time.Parse("0601021504Z0700", s)
        if err == nil {
                return
        }
        ret, err = time.Parse("060102150405Z0700", s)
        return
}

// parseGeneralizedTime parses the GeneralizedTime from the given byte slice
// and returns the resulting time.
func parseGeneralizedTime(bytes []byte) (ret time.Time, err error) {
        return time.Parse("20060102150405Z0700", string(bytes))
}

// PrintableString

// parsePrintableString parses a ASN.1 PrintableString from the given byte
// array and returns it.
func parsePrintableString(bytes []byte) (ret string, err error) {
        for _, b := range bytes {
                if !isPrintable(b) {
                        err = SyntaxError{"PrintableString contains invalid character"}
                        return
                }
        }
        ret = string(bytes)
        return
}

// isPrintable returns true iff the given b is in the ASN.1 PrintableString set.
func isPrintable(b byte) bool {
        return 'a' <= b && b <= 'z' ||
                'A' <= b && b <= 'Z' ||
                '0' <= b && b <= '9' ||
                '\'' <= b && b <= ')' ||
                '+' <= b && b <= '/' ||
                b == ' ' ||
                b == ':' ||
                b == '=' ||
                b == '?' ||
                // This is technically not allowed in a PrintableString.
                // However, x509 certificates with wildcard strings don't
                // always use the correct string type so we permit it.
                b == '*'
}

// IA5String

// parseIA5String parses a ASN.1 IA5String (ASCII string) from the given
// byte slice and returns it.
func parseIA5String(bytes []byte) (ret string, err error) {
        for _, b := range bytes {
                if b >= 0x80 {
                        err = SyntaxError{"IA5String contains invalid character"}
                        return
                }
        }
        ret = string(bytes)
        return
}

// T61String

// parseT61String parses a ASN.1 T61String (8-bit clean string) from the given
// byte slice and returns it.
func parseT61String(bytes []byte) (ret string, err error) {
        return string(bytes), nil
}

// UTF8String

// parseUTF8String parses a ASN.1 UTF8String (raw UTF-8) from the given byte
// array and returns it.
func parseUTF8String(bytes []byte) (ret string, err error) {
        return string(bytes), nil
}

// A RawValue represents an undecoded ASN.1 object.
type RawValue struct {
        Class, Tag int
        IsCompound bool
        Bytes      []byte
        FullBytes  []byte // includes the tag and length
}

// RawContent is used to signal that the undecoded, DER data needs to be
// preserved for a struct. To use it, the first field of the struct must have
// this type. It's an error for any of the other fields to have this type.
type RawContent []byte

// Tagging

// parseTagAndLength parses an ASN.1 tag and length pair from the given offset
// into a byte slice. It returns the parsed data and the new offset. SET and
// SET OF (tag 17) are mapped to SEQUENCE and SEQUENCE OF (tag 16) since we
// don't distinguish between ordered and unordered objects in this code.
func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset int, err error) {
        offset = initOffset
        b := bytes[offset]
        offset++
        ret.class = int(b >> 6)
        ret.isCompound = b&0x20 == 0x20
        ret.tag = int(b & 0x1f)

        // If the bottom five bits are set, then the tag number is actually base 128
        // encoded afterwards
        if ret.tag == 0x1f {
                ret.tag, offset, err = parseBase128Int(bytes, offset)
                if err != nil {
                        return
                }
        }
        if offset >= len(bytes) {
                err = SyntaxError{"truncated tag or length"}
                return
        }
        b = bytes[offset]
        offset++
        if b&0x80 == 0 {
                // The length is encoded in the bottom 7 bits.
                ret.length = int(b & 0x7f)
        } else {
                // Bottom 7 bits give the number of length bytes to follow.
                numBytes := int(b & 0x7f)
                // We risk overflowing a signed 32-bit number if we accept more than 3 bytes.
                if numBytes > 3 {
                        err = StructuralError{"length too large"}
                        return
                }
                if numBytes == 0 {
                        err = SyntaxError{"indefinite length found (not DER)"}
                        return
                }
                ret.length = 0
                for i := 0; i < numBytes; i++ {
                        if offset >= len(bytes) {
                                err = SyntaxError{"truncated tag or length"}
                                return
                        }
                        b = bytes[offset]
                        offset++
                        ret.length <<= 8
                        ret.length |= int(b)
                }
        }

        return
}

// parseSequenceOf is used for SEQUENCE OF and SET OF values. It tries to parse
// a number of ASN.1 values from the given byte slice and returns them as a
// slice of Go values of the given type.
func parseSequenceOf(bytes []byte, sliceType reflect.Type, elemType reflect.Type) (ret reflect.Value, err error) {
        expectedTag, compoundType, ok := getUniversalType(elemType)
        if !ok {
                err = StructuralError{"unknown Go type for slice"}
                return
        }

        // First we iterate over the input and count the number of elements,
        // checking that the types are correct in each case.
        numElements := 0
        for offset := 0; offset < len(bytes); {
                var t tagAndLength
                t, offset, err = parseTagAndLength(bytes, offset)
                if err != nil {
                        return
                }
                // We pretend that GENERAL STRINGs are PRINTABLE STRINGs so
                // that a sequence of them can be parsed into a []string.
                if t.tag == tagGeneralString {
                        t.tag = tagPrintableString
                }
                if t.class != classUniversal || t.isCompound != compoundType || t.tag != expectedTag {
                        err = StructuralError{"sequence tag mismatch"}
                        return
                }
                if invalidLength(offset, t.length, len(bytes)) {
                        err = SyntaxError{"truncated sequence"}
                        return
                }
                offset += t.length
                numElements++
        }
        ret = reflect.MakeSlice(sliceType, numElements, numElements)
        params := fieldParameters{}
        offset := 0
        for i := 0; i < numElements; i++ {
                offset, err = parseField(ret.Index(i), bytes, offset, params)
                if err != nil {
                        return
                }
        }
        return
}

var (
        bitStringType        = reflect.TypeOf(BitString{})
        objectIdentifierType = reflect.TypeOf(ObjectIdentifier{})
        enumeratedType       = reflect.TypeOf(Enumerated(0))
        flagType             = reflect.TypeOf(Flag(false))
        timeType             = reflect.TypeOf(time.Time{})
        rawValueType         = reflect.TypeOf(RawValue{})
        rawContentsType      = reflect.TypeOf(RawContent(nil))
        bigIntType           = reflect.TypeOf(new(big.Int))
)

// invalidLength returns true iff offset + length > sliceLength, or if the
// addition would overflow.
func invalidLength(offset, length, sliceLength int) bool {
        return offset+length < offset || offset+length > sliceLength
}

// parseField is the main parsing function. Given a byte slice and an offset
// into the array, it will try to parse a suitable ASN.1 value out and store it
// in the given Value.
func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParameters) (offset int, err error) {
        offset = initOffset
        fieldType := v.Type()

        // If we have run out of data, it may be that there are optional elements at the end.
        if offset == len(bytes) {
                if !setDefaultValue(v, params) {
                        err = SyntaxError{"sequence truncated"}
                }
                return
        }

        // Deal with raw values.
        if fieldType == rawValueType {
                var t tagAndLength
                t, offset, err = parseTagAndLength(bytes, offset)
                if err != nil {
                        return
                }
                if invalidLength(offset, t.length, len(bytes)) {
                        err = SyntaxError{"data truncated"}
                        return
                }
                result := RawValue{t.class, t.tag, t.isCompound, bytes[offset : offset+t.length], bytes[initOffset : offset+t.length]}
                offset += t.length
                v.Set(reflect.ValueOf(result))
                return
        }

        // Deal with the ANY type.
        if ifaceType := fieldType; ifaceType.Kind() == reflect.Interface && ifaceType.NumMethod() == 0 {
                var t tagAndLength
                t, offset, err = parseTagAndLength(bytes, offset)
                if err != nil {
                        return
                }
                if invalidLength(offset, t.length, len(bytes)) {
                        err = SyntaxError{"data truncated"}
                        return
                }
                var result interface{}
                if !t.isCompound && t.class == classUniversal {
                        innerBytes := bytes[offset : offset+t.length]
                        switch t.tag {
                        case tagPrintableString:
                                result, err = parsePrintableString(innerBytes)
                        case tagIA5String:
                                result, err = parseIA5String(innerBytes)
                        case tagT61String:
                                result, err = parseT61String(innerBytes)
                        case tagUTF8String:
                                result, err = parseUTF8String(innerBytes)
                        case tagInteger:
                                result, err = parseInt64(innerBytes)
                        case tagBitString:
                                result, err = parseBitString(innerBytes)
                        case tagOID:
                                result, err = parseObjectIdentifier(innerBytes)
                        case tagUTCTime:
                                result, err = parseUTCTime(innerBytes)
                        case tagOctetString:
                                result = innerBytes
                        default:
                                // If we don't know how to handle the type, we just leave Value as nil.
                        }
                }
                offset += t.length
                if err != nil {
                        return
                }
                if result != nil {
                        v.Set(reflect.ValueOf(result))
                }
                return
        }
        universalTag, compoundType, ok1 := getUniversalType(fieldType)
        if !ok1 {
                err = StructuralError{fmt.Sprintf("unknown Go type: %v", fieldType)}
                return
        }

        t, offset, err := parseTagAndLength(bytes, offset)
        if err != nil {
                return
        }
        if params.explicit {
                expectedClass := classContextSpecific
                if params.application {
                        expectedClass = classApplication
                }
                if t.class == expectedClass && t.tag == *params.tag && (t.length == 0 || t.isCompound) {
                        if t.length > 0 {
                                t, offset, err = parseTagAndLength(bytes, offset)
                                if err != nil {
                                        return
                                }
                        } else {
                                if fieldType != flagType {
                                        err = StructuralError{"Zero length explicit tag was not an asn1.Flag"}
                                        return
                                }
                                v.SetBool(true)
                                return
                        }
                } else {
                        // The tags didn't match, it might be an optional element.
                        ok := setDefaultValue(v, params)
                        if ok {
                                offset = initOffset
                        } else {
                                err = StructuralError{"explicitly tagged member didn't match"}
                        }
                        return
                }
        }

        // Special case for strings: all the ASN.1 string types map to the Go
        // type string. getUniversalType returns the tag for PrintableString
        // when it sees a string, so if we see a different string type on the
        // wire, we change the universal type to match.
        if universalTag == tagPrintableString {
                switch t.tag {
                case tagIA5String, tagGeneralString, tagT61String, tagUTF8String:
                        universalTag = t.tag
                }
        }

        // Special case for time: UTCTime and GeneralizedTime both map to the
        // Go type time.Time.
        if universalTag == tagUTCTime && t.tag == tagGeneralizedTime {
                universalTag = tagGeneralizedTime
        }

        expectedClass := classUniversal
        expectedTag := universalTag

        if !params.explicit && params.tag != nil {
                expectedClass = classContextSpecific
                expectedTag = *params.tag
        }

        if !params.explicit && params.application && params.tag != nil {
                expectedClass = classApplication
                expectedTag = *params.tag
        }

        // We have unwrapped any explicit tagging at this point.
        if t.class != expectedClass || t.tag != expectedTag || t.isCompound != compoundType {
                // Tags don't match. Again, it could be an optional element.
                ok := setDefaultValue(v, params)
                if ok {
                        offset = initOffset
                } else {
                        err = StructuralError{fmt.Sprintf("tags don't match (%d vs %+v) %+v %s @%d", expectedTag, t, params, fieldType.Name(), offset)}
                }
                return
        }
        if invalidLength(offset, t.length, len(bytes)) {
                err = SyntaxError{"data truncated"}
                return
        }
        innerBytes := bytes[offset : offset+t.length]
        offset += t.length

        // We deal with the structures defined in this package first.
        switch fieldType {
        case objectIdentifierType:
                newSlice, err1 := parseObjectIdentifier(innerBytes)
                v.Set(reflect.MakeSlice(v.Type(), len(newSlice), len(newSlice)))
                if err1 == nil {
                        reflect.Copy(v, reflect.ValueOf(newSlice))
                }
                err = err1
                return
        case bitStringType:
                bs, err1 := parseBitString(innerBytes)
                if err1 == nil {
                        v.Set(reflect.ValueOf(bs))
                }
                err = err1
                return
        case timeType:
                var time time.Time
                var err1 error
                if universalTag == tagUTCTime {
                        time, err1 = parseUTCTime(innerBytes)
                } else {
                        time, err1 = parseGeneralizedTime(innerBytes)
                }
                if err1 == nil {
                        v.Set(reflect.ValueOf(time))
                }
                err = err1
                return
        case enumeratedType:
                parsedInt, err1 := parseInt(innerBytes)
                if err1 == nil {
                        v.SetInt(int64(parsedInt))
                }
                err = err1
                return
        case flagType:
                v.SetBool(true)
                return
        case bigIntType:
                parsedInt := parseBigInt(innerBytes)
                v.Set(reflect.ValueOf(parsedInt))
                return
        }
        switch val := v; val.Kind() {
        case reflect.Bool:
                parsedBool, err1 := parseBool(innerBytes)
                if err1 == nil {
                        val.SetBool(parsedBool)
                }
                err = err1
                return
        case reflect.Int, reflect.Int32:
                parsedInt, err1 := parseInt(innerBytes)
                if err1 == nil {
                        val.SetInt(int64(parsedInt))
                }
                err = err1
                return
        case reflect.Int64:
                parsedInt, err1 := parseInt64(innerBytes)
                if err1 == nil {
                        val.SetInt(parsedInt)
                }
                err = err1
                return
        // TODO(dfc) Add support for the remaining integer types
        case reflect.Struct:
                structType := fieldType

                if structType.NumField() > 0 &&
                        structType.Field(0).Type == rawContentsType {
                        bytes := bytes[initOffset:offset]
                        val.Field(0).Set(reflect.ValueOf(RawContent(bytes)))
                }

                innerOffset := 0
                for i := 0; i < structType.NumField(); i++ {
                        field := structType.Field(i)
                        if i == 0 && field.Type == rawContentsType {
                                continue
                        }
                        innerOffset, err = parseField(val.Field(i), innerBytes, innerOffset, parseFieldParameters(field.Tag.Get("asn1")))
                        if err != nil {
                                return
                        }
                }
                // We allow extra bytes at the end of the SEQUENCE because
                // adding elements to the end has been used in X.509 as the
                // version numbers have increased.
                return
        case reflect.Slice:
                sliceType := fieldType
                if sliceType.Elem().Kind() == reflect.Uint8 {
                        val.Set(reflect.MakeSlice(sliceType, len(innerBytes), len(innerBytes)))
                        reflect.Copy(val, reflect.ValueOf(innerBytes))
                        return
                }
                newSlice, err1 := parseSequenceOf(innerBytes, sliceType, sliceType.Elem())
                if err1 == nil {
                        val.Set(newSlice)
                }
                err = err1
                return
        case reflect.String:
                var v string
                switch universalTag {
                case tagPrintableString:
                        v, err = parsePrintableString(innerBytes)
                case tagIA5String:
                        v, err = parseIA5String(innerBytes)
                case tagT61String:
                        v, err = parseT61String(innerBytes)
                case tagUTF8String:
                        v, err = parseUTF8String(innerBytes)
                case tagGeneralString:
                        // GeneralString is specified in ISO-2022/ECMA-35,
                        // A brief review suggests that it includes structures
                        // that allow the encoding to change midstring and
                        // such. We give up and pass it as an 8-bit string.
                        v, err = parseT61String(innerBytes)
                default:
                        err = SyntaxError{fmt.Sprintf("internal error: unknown string type %d", universalTag)}
                }
                if err == nil {
                        val.SetString(v)
                }
                return
        }
        err = StructuralError{"unsupported: " + v.Type().String()}
        return
}

// setDefaultValue is used to install a default value, from a tag string, into
// a Value. It is successful is the field was optional, even if a default value
// wasn't provided or it failed to install it into the Value.
func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
        if !params.optional {
                return
        }
        ok = true
        if params.defaultValue == nil {
                return
        }
        switch val := v; val.Kind() {
        case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
                val.SetInt(*params.defaultValue)
        }
        return
}

// Unmarshal parses the DER-encoded ASN.1 data structure b
// and uses the reflect package to fill in an arbitrary value pointed at by val.
// Because Unmarshal uses the reflect package, the structs
// being written to must use upper case field names.
//
// An ASN.1 INTEGER can be written to an int, int32, int64,
// or *big.Int (from the math/big package).
// If the encoded value does not fit in the Go type,
// Unmarshal returns a parse error.
//
// An ASN.1 BIT STRING can be written to a BitString.
//
// An ASN.1 OCTET STRING can be written to a []byte.
//
// An ASN.1 OBJECT IDENTIFIER can be written to an
// ObjectIdentifier.
//
// An ASN.1 ENUMERATED can be written to an Enumerated.
//
// An ASN.1 UTCTIME or GENERALIZEDTIME can be written to a time.Time.
//
// An ASN.1 PrintableString or IA5String can be written to a string.
//
// Any of the above ASN.1 values can be written to an interface{}.
// The value stored in the interface has the corresponding Go type.
// For integers, that type is int64.
//
// An ASN.1 SEQUENCE OF x or SET OF x can be written
// to a slice if an x can be written to the slice's element type.
//
// An ASN.1 SEQUENCE or SET can be written to a struct
// if each of the elements in the sequence can be
// written to the corresponding element in the struct.
//
// The following tags on struct fields have special meaning to Unmarshal:
//
//      optional                marks the field as ASN.1 OPTIONAL
//      [explicit] tag:x        specifies the ASN.1 tag number; implies ASN.1 CONTEXT SPECIFIC
//      default:x               sets the default value for optional integer fields
//
// If the type of the first field of a structure is RawContent then the raw
// ASN1 contents of the struct will be stored in it.
//
// Other ASN.1 types are not supported; if it encounters them,
// Unmarshal returns a parse error.
func Unmarshal(b []byte, val interface{}) (rest []byte, err error) {
        return UnmarshalWithParams(b, val, "")
}

// UnmarshalWithParams allows field parameters to be specified for the
// top-level element. The form of the params is the same as the field tags.
func UnmarshalWithParams(b []byte, val interface{}, params string) (rest []byte, err error) {
        v := reflect.ValueOf(val).Elem()
        offset, err := parseField(v, b, 0, parseFieldParameters(params))
        if err != nil {
                return nil, err
        }
        return b[offset:], nil
}