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// 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
const (
tagBoolean = 1
tagInteger = 2
tagBitString = 3
tagOctetString = 4
tagOID = 6
tagEnum = 10
tagUTF8String = 12
tagSequence = 16
tagSet = 17
tagPrintableString = 19
tagT61String = 20
tagIA5String = 22
tagUTCTime = 23
tagGeneralizedTime = 24
tagGeneralString = 27
)
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.
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 == "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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