// 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 ( "errors" "fmt" "math/big" "reflect" "strconv" "time" "unicode/utf8" ) // 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 "asn1: 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 "asn1: 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 } // DER demands that "If the encoding represents the boolean value TRUE, // its single contents octet shall have all eight bits set to one." // Thus only 0 and 255 are valid encoded values. switch bytes[0] { case 0: ret = false case 0xff: ret = true default: err = SyntaxError{"invalid boolean"} } return } // INTEGER // checkInteger returns nil if the given bytes are a valid DER-encoded // INTEGER and an error otherwise. func checkInteger(bytes []byte) error { if len(bytes) == 0 { return StructuralError{"empty integer"} } if len(bytes) == 1 { return nil } if (bytes[0] == 0 && bytes[1]&0x80 == 0) || (bytes[0] == 0xff && bytes[1]&0x80 == 0x80) { return StructuralError{"integer not minimally-encoded"} } return nil } // parseInt64 treats the given bytes as a big-endian, signed integer and // returns the result. func parseInt64(bytes []byte) (ret int64, err error) { err = checkInteger(bytes) if err != nil { return } 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 parseInt32(bytes []byte) (int32, error) { if err := checkInteger(bytes); err != nil { return 0, err } ret64, err := parseInt64(bytes) if err != nil { return 0, err } if ret64 != int64(int32(ret64)) { return 0, StructuralError{"integer too large"} } return int32(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, error) { if err := checkInteger(bytes); err != nil { return nil, err } 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, nil } ret.SetBytes(bytes) return ret, nil } // 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< 0 { s += "." } s += strconv.Itoa(v) } return s } // 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 varint is 40*value1 + value2: // According to this packing, value1 can take the values 0, 1 and 2 only. // When value1 = 0 or value1 = 1, then value2 is <= 39. When value1 = 2, // then there are no restrictions on value2. v, offset, err := parseBase128Int(bytes, 0) if err != nil { return } if v < 80 { s[0] = v / 40 s[1] = v % 40 } else { s[0] = 2 s[1] = v - 80 } i := 2 for ; offset < len(bytes); i++ { 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) formatStr := "0601021504Z0700" ret, err = time.Parse(formatStr, s) if err != nil { formatStr = "060102150405Z0700" ret, err = time.Parse(formatStr, s) } if err != nil { return } if serialized := ret.Format(formatStr); serialized != s { err = fmt.Errorf("asn1: time did not serialize back to the original value and may be invalid: given %q, but serialized as %q", s, serialized) return } if ret.Year() >= 2050 { // UTCTime only encodes times prior to 2050. See https://tools.ietf.org/html/rfc5280#section-4.1.2.5.1 ret = ret.AddDate(-100, 0, 0) } return } // parseGeneralizedTime parses the GeneralizedTime from the given byte slice // and returns the resulting time. func parseGeneralizedTime(bytes []byte) (ret time.Time, err error) { const formatStr = "20060102150405Z0700" s := string(bytes) if ret, err = time.Parse(formatStr, s); err != nil { return } if serialized := ret.Format(formatStr); serialized != s { err = fmt.Errorf("asn1: time did not serialize back to the original value and may be invalid: given %q, but serialized as %q", s, serialized) } return } // 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 reports whether 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 >= utf8.RuneSelf { 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) { if !utf8.Valid(bytes) { return "", errors.New("asn1: invalid UTF-8 string") } 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 // parseTagAndLength should not be called without at least a single // byte to read. Thus this check is for robustness: if offset >= len(bytes) { err = errors.New("asn1: internal error in parseTagAndLength") return } 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 } // Tags should be encoded in minimal form. if ret.tag < 0x1f { err = SyntaxError{"non-minimal tag"} 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) 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++ if ret.length >= 1<<23 { // We can't shift ret.length up without // overflowing. err = StructuralError{"length too large"} return } ret.length <<= 8 ret.length |= int(b) if ret.length == 0 { // DER requires that lengths be minimal. err = StructuralError{"superfluous leading zeros in length"} return } } // Short lengths must be encoded in short form. if ret.length < 0x80 { err = StructuralError{"non-minimal length"} return } } 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 } switch t.tag { case TagIA5String, TagGeneralString, TagT61String, TagUTF8String: // We pretend that various other string types are // PRINTABLE STRINGs so that a sequence of them can be // parsed into a []string. t.tag = TagPrintableString case TagGeneralizedTime, TagUTCTime: // Likewise, both time types are treated the same. t.tag = TagUTCTime } 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) // jtasn1 addition of following case case TagGeneralString: 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 TagGeneralizedTime: result, err = parseGeneralizedTime(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 offset == len(bytes) { err = StructuralError{"explicit tag has no child"} return } 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 { if t.class == ClassUniversal { switch t.tag { case TagIA5String, TagGeneralString, TagT61String, TagUTF8String: universalTag = t.tag } } else if params.stringType != 0 { universalTag = params.stringType } } // Special case for time: UTCTime and GeneralizedTime both map to the // Go type time.Time. if universalTag == TagUTCTime && t.tag == TagGeneralizedTime && t.class == ClassUniversal { universalTag = TagGeneralizedTime } if params.set { universalTag = TagSet } 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 := parseInt32(innerBytes) if err1 == nil { v.SetInt(int64(parsedInt)) } err = err1 return case flagType: v.SetBool(true) return case bigIntType: parsedInt, err1 := parseBigInt(innerBytes) if err1 == nil { v.Set(reflect.ValueOf(parsedInt)) } err = err1 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, reflect.Int64: if val.Type().Size() == 4 { parsedInt, err1 := parseInt32(innerBytes) if err1 == nil { val.SetInt(int64(parsedInt)) } err = err1 } else { 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 } // canHaveDefaultValue reports whether k is a Kind that we will set a default // value for. (A signed integer, essentially.) func canHaveDefaultValue(k reflect.Kind) bool { switch k { case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64: return true } return false } // setDefaultValue is used to install a default value, from a tag string, into // a Value. It is successful if 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 } if canHaveDefaultValue(v.Kind()) { v.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: // // application specifies that a APPLICATION tag is used // default:x sets the default value for optional integer fields // explicit specifies that an additional, explicit tag wraps the implicit one // optional marks the field as ASN.1 OPTIONAL // set causes a SET, rather than a SEQUENCE type to be expected // tag:x specifies the ASN.1 tag number; implies ASN.1 CONTEXT SPECIFIC // // 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. // // If the type name of a slice element ends with "SET" then it's treated as if // the "set" tag was set on it. This can be used with nested slices where a // struct tag cannot be given. // // 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 }