common.go

Documentation: encoding/asn1

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package asn1
     6  
     7  import (
     8  	"reflect"
     9  	"strconv"
    10  	"strings"
    11  )
    12  
    13  // ASN.1 objects have metadata preceding them:
    14  //   the tag: the type of the object
    15  //   a flag denoting if this object is compound or not
    16  //   the class type: the namespace of the tag
    17  //   the length of the object, in bytes
    18  
    19  // Here are some standard tags and classes
    20  
    21  // ASN.1 tags represent the type of the following object.
    22  const (
    23  	TagBoolean         = 1
    24  	TagInteger         = 2
    25  	TagBitString       = 3
    26  	TagOctetString     = 4
    27  	TagNull            = 5
    28  	TagOID             = 6
    29  	TagEnum            = 10
    30  	TagUTF8String      = 12
    31  	TagSequence        = 16
    32  	TagSet             = 17
    33  	TagNumericString   = 18
    34  	TagPrintableString = 19
    35  	TagT61String       = 20
    36  	TagIA5String       = 22
    37  	TagUTCTime         = 23
    38  	TagGeneralizedTime = 24
    39  	TagGeneralString   = 27
    40  )
    41  
    42  // ASN.1 class types represent the namespace of the tag.
    43  const (
    44  	ClassUniversal       = 0
    45  	ClassApplication     = 1
    46  	ClassContextSpecific = 2
    47  	ClassPrivate         = 3
    48  )
    49  
    50  type tagAndLength struct {
    51  	class, tag, length int
    52  	isCompound         bool
    53  }
    54  
    55  // ASN.1 has IMPLICIT and EXPLICIT tags, which can be translated as "instead
    56  // of" and "in addition to". When not specified, every primitive type has a
    57  // default tag in the UNIVERSAL class.
    58  //
    59  // For example: a BIT STRING is tagged [UNIVERSAL 3] by default (although ASN.1
    60  // doesn't actually have a UNIVERSAL keyword). However, by saying [IMPLICIT
    61  // CONTEXT-SPECIFIC 42], that means that the tag is replaced by another.
    62  //
    63  // On the other hand, if it said [EXPLICIT CONTEXT-SPECIFIC 10], then an
    64  // /additional/ tag would wrap the default tag. This explicit tag will have the
    65  // compound flag set.
    66  //
    67  // (This is used in order to remove ambiguity with optional elements.)
    68  //
    69  // You can layer EXPLICIT and IMPLICIT tags to an arbitrary depth, however we
    70  // don't support that here. We support a single layer of EXPLICIT or IMPLICIT
    71  // tagging with tag strings on the fields of a structure.
    72  
    73  // fieldParameters is the parsed representation of tag string from a structure field.
    74  type fieldParameters struct {
    75  	optional     bool   // true iff the field is OPTIONAL
    76  	explicit     bool   // true iff an EXPLICIT tag is in use.
    77  	application  bool   // true iff an APPLICATION tag is in use.
    78  	defaultValue *int64 // a default value for INTEGER typed fields (maybe nil).
    79  	tag          *int   // the EXPLICIT or IMPLICIT tag (maybe nil).
    80  	stringType   int    // the string tag to use when marshaling.
    81  	timeType     int    // the time tag to use when marshaling.
    82  	set          bool   // true iff this should be encoded as a SET
    83  	omitEmpty    bool   // true iff this should be omitted if empty when marshaling.
    84  
    85  	// Invariants:
    86  	//   if explicit is set, tag is non-nil.
    87  }
    88  
    89  // Given a tag string with the format specified in the package comment,
    90  // parseFieldParameters will parse it into a fieldParameters structure,
    91  // ignoring unknown parts of the string.
    92  func parseFieldParameters(str string) (ret fieldParameters) {
    93  	for _, part := range strings.Split(str, ",") {
    94  		switch {
    95  		case part == "optional":
    96  			ret.optional = true
    97  		case part == "explicit":
    98  			ret.explicit = true
    99  			if ret.tag == nil {
   100  				ret.tag = new(int)
   101  			}
   102  		case part == "generalized":
   103  			ret.timeType = TagGeneralizedTime
   104  		case part == "utc":
   105  			ret.timeType = TagUTCTime
   106  		case part == "ia5":
   107  			ret.stringType = TagIA5String
   108  		case part == "printable":
   109  			ret.stringType = TagPrintableString
   110  		case part == "numeric":
   111  			ret.stringType = TagNumericString
   112  		case part == "utf8":
   113  			ret.stringType = TagUTF8String
   114  		case strings.HasPrefix(part, "default:"):
   115  			i, err := strconv.ParseInt(part[8:], 10, 64)
   116  			if err == nil {
   117  				ret.defaultValue = new(int64)
   118  				*ret.defaultValue = i
   119  			}
   120  		case strings.HasPrefix(part, "tag:"):
   121  			i, err := strconv.Atoi(part[4:])
   122  			if err == nil {
   123  				ret.tag = new(int)
   124  				*ret.tag = i
   125  			}
   126  		case part == "set":
   127  			ret.set = true
   128  		case part == "application":
   129  			ret.application = true
   130  			if ret.tag == nil {
   131  				ret.tag = new(int)
   132  			}
   133  		case part == "omitempty":
   134  			ret.omitEmpty = true
   135  		}
   136  	}
   137  	return
   138  }
   139  
   140  // Given a reflected Go type, getUniversalType returns the default tag number
   141  // and expected compound flag.
   142  func getUniversalType(t reflect.Type) (matchAny bool, tagNumber int, isCompound, ok bool) {
   143  	switch t {
   144  	case rawValueType:
   145  		return true, -1, false, true
   146  	case objectIdentifierType:
   147  		return false, TagOID, false, true
   148  	case bitStringType:
   149  		return false, TagBitString, false, true
   150  	case timeType:
   151  		return false, TagUTCTime, false, true
   152  	case enumeratedType:
   153  		return false, TagEnum, false, true
   154  	case bigIntType:
   155  		return false, TagInteger, false, true
   156  	}
   157  	switch t.Kind() {
   158  	case reflect.Bool:
   159  		return false, TagBoolean, false, true
   160  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   161  		return false, TagInteger, false, true
   162  	case reflect.Struct:
   163  		return false, TagSequence, true, true
   164  	case reflect.Slice:
   165  		if t.Elem().Kind() == reflect.Uint8 {
   166  			return false, TagOctetString, false, true
   167  		}
   168  		if strings.HasSuffix(t.Name(), "SET") {
   169  			return false, TagSet, true, true
   170  		}
   171  		return false, TagSequence, true, true
   172  	case reflect.String:
   173  		return false, TagPrintableString, false, true
   174  	}
   175  	return false, 0, false, false
   176  }
   177
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