summaryrefslogtreecommitdiff
path: root/src/pkg/encoding/asn1/asn1.go
blob: ec7f91c1bba5ebd8909aecabd24e7e6078b977a2 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
// 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"
	"strconv"
	"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 "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

// 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 parseInt32(bytes []byte) (int32, error) {
	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 {
	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 reports whether 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
}

func (oi ObjectIdentifier) String() string {
	var s string

	for i, v := range oi {
		if i > 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)
	ret, err = time.Parse("0601021504Z0700", s)
	if err != nil {
		ret, err = time.Parse("060102150405Z0700", s)
	}
	if err == nil && 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) {
	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)
		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
			}
		}
	}

	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)
			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
	}

	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 := 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, 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
}

// 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:
//
//	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
}