// 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.
// The asn1 package 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";
"os";
"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) String() 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) String() 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 os.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 os.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, os.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;
}
// 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 array and returns it.
func parseBitString(bytes []byte) (ret BitString, err os.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
// parseObjectIdentifier parses an OBJECT IDENTIFER from the given bytes and
// returns it. An object identifer is a sequence of variable length integers
// that are assigned in a hierarachy.
func parseObjectIdentifier(bytes []byte) (s []int, err os.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;
}
// parseBase128Int parses a base-128 encoded int from the given offset in the
// given byte array. It returns the value and the new offset.
func parseBase128Int(bytes []byte, initOffset int) (ret, offset int, err os.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 isDigit(b byte) bool { return '0' <= b && b <= '9' }
// twoDigits returns the value of two, base 10 digits.
func twoDigits(bytes []byte, max int) (int, bool) {
for i := 0; i < 2; i++ {
if !isDigit(bytes[i]) {
return 0, false
}
}
value := (int(bytes[0])-'0')*10 + int(bytes[1]-'0');
if value > max {
return 0, false
}
return value, true;
}
// parseUTCTime parses the UTCTime from the given byte array and returns the
// resulting time.
func parseUTCTime(bytes []byte) (ret *time.Time, err os.Error) {
// A UTCTime can take the following formats:
//
// 1111111
// 01234567890123456
//
// YYMMDDhhmmZ
// YYMMDDhhmm+hhmm
// YYMMDDhhmm-hhmm
// YYMMDDhhmmssZ
// YYMMDDhhmmss+hhmm
// YYMMDDhhmmss-hhmm
if len(bytes) < 11 {
err = SyntaxError{"UTCTime too short"};
return;
}
ret = new(time.Time);
var ok1, ok2, ok3, ok4, ok5 bool;
year, ok1 := twoDigits(bytes[0:2], 99);
// RFC 5280, section 5.1.2.4 says that years 2050 or later use another date
// scheme.
if year >= 50 {
ret.Year = 1900 + int64(year)
} else {
ret.Year = 2000 + int64(year)
}
ret.Month, ok2 = twoDigits(bytes[2:4], 12);
ret.Day, ok3 = twoDigits(bytes[4:6], 31);
ret.Hour, ok4 = twoDigits(bytes[6:8], 23);
ret.Minute, ok5 = twoDigits(bytes[8:10], 59);
if !ok1 || !ok2 || !ok3 || !ok4 || !ok5 {
goto Error
}
bytes = bytes[10:];
switch bytes[0] {
case '0', '1', '2', '3', '4', '5', '6':
if len(bytes) < 3 {
goto Error
}
ret.Second, ok1 = twoDigits(bytes[0:2], 60); // 60, not 59, because of leap seconds.
if !ok1 {
goto Error
}
bytes = bytes[2:];
}
if len(bytes) == 0 {
goto Error
}
switch bytes[0] {
case 'Z':
if len(bytes) != 1 {
goto Error
}
return;
case '-', '+':
if len(bytes) != 5 {
goto Error
}
hours, ok1 := twoDigits(bytes[1:3], 12);
minutes, ok2 := twoDigits(bytes[3:5], 59);
if !ok1 || !ok2 {
goto Error
}
sign := 1;
if bytes[0] == '-' {
sign = -1
}
ret.ZoneOffset = sign * (60 * (hours*60 + minutes));
default:
goto Error
}
return;
Error:
err = SyntaxError{"invalid UTCTime"};
return;
}
// PrintableString
// parsePrintableString parses a ASN.1 PrintableString from the given byte
// array and returns it.
func parsePrintableString(bytes []byte) (ret string, err os.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 == '?'
}
// IA5String
// parseIA5String parses a ASN.1 IA5String (ASCII string) from the given
// byte array and returns it.
func parseIA5String(bytes []byte) (ret string, err os.Error) {
for _, b := range bytes {
if b >= 0x80 {
err = SyntaxError{"IA5String contains invalid character"};
return;
}
}
ret = string(bytes);
return;
}
// A RawValue represents an undecoded ASN.1 object.
type RawValue struct {
Class, Tag int;
IsCompound bool;
Bytes []byte;
}
// 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 array. 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 os.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);
}
}
// We magically map SET and SET OF to SEQUENCE and SEQUENCE OF
// because we treat everything as ordered.
if ret.tag == tagSet {
ret.tag = tagSequence
}
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 array and returns them as a
// slice of Go values of the given type.
func parseSequenceOf(bytes []byte, sliceType *reflect.SliceType, elemType reflect.Type) (ret *reflect.SliceValue, err os.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
}
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.Elem(i), bytes, offset, params);
if err != nil {
return
}
}
return;
}
var (
bitStringType = reflect.Typeof(BitString{});
objectIdentifierType = reflect.Typeof(ObjectIdentifier{});
timeType = reflect.Typeof(&time.Time{});
rawValueType = reflect.Typeof(RawValue{});
rawContentsType = reflect.Typeof(RawContent(nil));
)
// 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 array 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 os.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]};
offset += t.length;
v.(*reflect.StructValue).Set(reflect.NewValue(result).(*reflect.StructValue));
return;
}
// Deal with the ANY type.
if ifaceType, ok := fieldType.(*reflect.InterfaceType); ok && ifaceType.NumMethod() == 0 {
ifaceValue := v.(*reflect.InterfaceValue);
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 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 {
ifaceValue.Set(reflect.NewValue(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 {
if t.class == classContextSpecific && t.tag == *params.tag && t.isCompound {
t, offset, err = parseTagAndLength(bytes, offset);
if err != nil {
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: PrintableString and IA5String both map to
// the Go type string. getUniversalType returns the tag for
// PrintableString when it sees a string so, if we see an IA5String on
// the wire, we change the universal type to match.
if universalTag == tagPrintableString && t.tag == tagIA5String {
universalTag = tagIA5String
}
expectedClass := classUniversal;
expectedTag := universalTag;
if !params.explicit && params.tag != nil {
expectedClass = classContextSpecific;
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];
// We deal with the structures defined in this package first.
switch fieldType {
case objectIdentifierType:
newSlice, err1 := parseObjectIdentifier(innerBytes);
sliceValue := v.(*reflect.SliceValue);
sliceValue.Set(reflect.MakeSlice(sliceValue.Type().(*reflect.SliceType), len(newSlice), len(newSlice)));
if err1 == nil {
reflect.ArrayCopy(sliceValue, reflect.NewValue(newSlice).(reflect.ArrayOrSliceValue))
}
offset += t.length;
err = err1;
return;
case bitStringType:
structValue := v.(*reflect.StructValue);
bs, err1 := parseBitString(innerBytes);
offset += t.length;
if err1 == nil {
structValue.Set(reflect.NewValue(bs).(*reflect.StructValue))
}
err = err1;
return;
case timeType:
ptrValue := v.(*reflect.PtrValue);
time, err1 := parseUTCTime(innerBytes);
offset += t.length;
if err1 == nil {
ptrValue.Set(reflect.NewValue(time).(*reflect.PtrValue))
}
err = err1;
return;
}
switch val := v.(type) {
case *reflect.BoolValue:
parsedBool, err1 := parseBool(innerBytes);
offset += t.length;
if err1 == nil {
val.Set(parsedBool)
}
err = err1;
return;
case *reflect.IntValue:
parsedInt, err1 := parseInt(innerBytes);
offset += t.length;
if err1 == nil {
val.Set(parsedInt)
}
err = err1;
return;
case *reflect.Int64Value:
parsedInt, err1 := parseInt64(innerBytes);
offset += t.length;
if err1 == nil {
val.Set(parsedInt)
}
err = err1;
return;
case *reflect.StructValue:
structType := fieldType.(*reflect.StructType);
if structType.NumField() > 0 &&
structType.Field(0).Type == rawContentsType {
bytes := bytes[initOffset : offset+t.length];
val.Field(0).SetValue(reflect.NewValue(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));
if err != nil {
return
}
}
offset += t.length;
// 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.SliceValue:
sliceType := fieldType.(*reflect.SliceType);
if _, ok := sliceType.Elem().(*reflect.Uint8Type); ok {
val.Set(reflect.MakeSlice(sliceType, len(innerBytes), len(innerBytes)));
reflect.ArrayCopy(val, reflect.NewValue(innerBytes).(reflect.ArrayOrSliceValue));
return;
}
newSlice, err1 := parseSequenceOf(innerBytes, sliceType, sliceType.Elem());
offset += t.length;
if err1 == nil {
val.Set(newSlice)
}
err = err1;
return;
case *reflect.StringValue:
var v string;
switch universalTag {
case tagPrintableString:
v, err = parsePrintableString(innerBytes)
case tagIA5String:
v, err = parseIA5String(innerBytes)
default:
err = SyntaxError{fmt.Sprintf("internal error: unknown string type %d", universalTag)}
}
if err == nil {
val.Set(v)
}
return;
}
err = StructuralError{"unknown Go type"};
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.(type) {
case *reflect.IntValue:
val.Set(int(*params.defaultValue))
case *reflect.Int64Value:
val.Set(int64(*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 or int64.
// 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 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(val interface{}, b []byte) (rest []byte, err os.Error) {
v := reflect.NewValue(val).(*reflect.PtrValue).Elem();
offset, err := parseField(v, b, 0, fieldParameters{});
if err != nil {
return nil, err
}
return b[offset:], nil;
}
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