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Source file src/encoding/asn1/common.go

Documentation: encoding/asn1

  // 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
  
  import (
  	"reflect"
  	"strconv"
  	"strings"
  )
  
  // ASN.1 objects have metadata preceding them:
  //   the tag: the type of the object
  //   a flag denoting if this object is compound or not
  //   the class type: the namespace of the tag
  //   the length of the object, in bytes
  
  // Here are some standard tags and classes
  
  // ASN.1 tags represent the type of the following object.
  const (
  	TagBoolean         = 1
  	TagInteger         = 2
  	TagBitString       = 3
  	TagOctetString     = 4
  	TagNull            = 5
  	TagOID             = 6
  	TagEnum            = 10
  	TagUTF8String      = 12
  	TagSequence        = 16
  	TagSet             = 17
  	TagPrintableString = 19
  	TagT61String       = 20
  	TagIA5String       = 22
  	TagUTCTime         = 23
  	TagGeneralizedTime = 24
  	TagGeneralString   = 27
  )
  
  // ASN.1 class types represent the namespace of the tag.
  const (
  	ClassUniversal       = 0
  	ClassApplication     = 1
  	ClassContextSpecific = 2
  	ClassPrivate         = 3
  )
  
  type tagAndLength struct {
  	class, tag, length int
  	isCompound         bool
  }
  
  // ASN.1 has IMPLICIT and EXPLICIT tags, which can be translated as "instead
  // of" and "in addition to". When not specified, every primitive type has a
  // default tag in the UNIVERSAL class.
  //
  // For example: a BIT STRING is tagged [UNIVERSAL 3] by default (although ASN.1
  // doesn't actually have a UNIVERSAL keyword). However, by saying [IMPLICIT
  // CONTEXT-SPECIFIC 42], that means that the tag is replaced by another.
  //
  // On the other hand, if it said [EXPLICIT CONTEXT-SPECIFIC 10], then an
  // /additional/ tag would wrap the default tag. This explicit tag will have the
  // compound flag set.
  //
  // (This is used in order to remove ambiguity with optional elements.)
  //
  // You can layer EXPLICIT and IMPLICIT tags to an arbitrary depth, however we
  // don't support that here. We support a single layer of EXPLICIT or IMPLICIT
  // tagging with tag strings on the fields of a structure.
  
  // fieldParameters is the parsed representation of tag string from a structure field.
  type fieldParameters struct {
  	optional     bool   // true iff the field is OPTIONAL
  	explicit     bool   // true iff an EXPLICIT tag is in use.
  	application  bool   // true iff an APPLICATION tag is in use.
  	defaultValue *int64 // a default value for INTEGER typed fields (maybe nil).
  	tag          *int   // the EXPLICIT or IMPLICIT tag (maybe nil).
  	stringType   int    // the string tag to use when marshaling.
  	timeType     int    // the time tag to use when marshaling.
  	set          bool   // true iff this should be encoded as a SET
  	omitEmpty    bool   // true iff this should be omitted if empty when marshaling.
  
  	// Invariants:
  	//   if explicit is set, tag is non-nil.
  }
  
  // Given a tag string with the format specified in the package comment,
  // parseFieldParameters will parse it into a fieldParameters structure,
  // ignoring unknown parts of the string.
  func parseFieldParameters(str string) (ret fieldParameters) {
  	for _, part := range strings.Split(str, ",") {
  		switch {
  		case part == "optional":
  			ret.optional = true
  		case part == "explicit":
  			ret.explicit = true
  			if ret.tag == nil {
  				ret.tag = new(int)
  			}
  		case part == "generalized":
  			ret.timeType = TagGeneralizedTime
  		case part == "utc":
  			ret.timeType = TagUTCTime
  		case part == "ia5":
  			ret.stringType = TagIA5String
  		case part == "printable":
  			ret.stringType = TagPrintableString
  		case part == "utf8":
  			ret.stringType = TagUTF8String
  		case strings.HasPrefix(part, "default:"):
  			i, err := strconv.ParseInt(part[8:], 10, 64)
  			if err == nil {
  				ret.defaultValue = new(int64)
  				*ret.defaultValue = i
  			}
  		case strings.HasPrefix(part, "tag:"):
  			i, err := strconv.Atoi(part[4:])
  			if err == nil {
  				ret.tag = new(int)
  				*ret.tag = i
  			}
  		case part == "set":
  			ret.set = true
  		case part == "application":
  			ret.application = true
  			if ret.tag == nil {
  				ret.tag = new(int)
  			}
  		case part == "omitempty":
  			ret.omitEmpty = true
  		}
  	}
  	return
  }
  
  // Given a reflected Go type, getUniversalType returns the default tag number
  // and expected compound flag.
  func getUniversalType(t reflect.Type) (tagNumber int, isCompound, ok bool) {
  	switch t {
  	case objectIdentifierType:
  		return TagOID, false, true
  	case bitStringType:
  		return TagBitString, false, true
  	case timeType:
  		return TagUTCTime, false, true
  	case enumeratedType:
  		return TagEnum, false, true
  	case bigIntType:
  		return TagInteger, false, true
  	}
  	switch t.Kind() {
  	case reflect.Bool:
  		return TagBoolean, false, true
  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  		return TagInteger, false, true
  	case reflect.Struct:
  		return TagSequence, true, true
  	case reflect.Slice:
  		if t.Elem().Kind() == reflect.Uint8 {
  			return TagOctetString, false, true
  		}
  		if strings.HasSuffix(t.Name(), "SET") {
  			return TagSet, true, true
  		}
  		return TagSequence, true, true
  	case reflect.String:
  		return TagPrintableString, false, true
  	}
  	return 0, false, false
  }
  

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