Source file src/pkg/asn1/common.go

// 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 preceeding 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
    tagSequence        = 16
    tagSet             = 17
    tagPrintableString = 19
    tagIA5String       = 22
    tagUTCTime         = 23
)

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 and EXPLICIT 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

    // 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, ",", -1) {
        switch {
        case part == "optional":
            ret.optional = true
        case part == "explicit":
            ret.explicit = true
            if ret.tag == nil {
                ret.tag = new(int)
                *ret.tag = 0
            }
        case part == "ia5":
            ret.stringType = tagIA5String
        case part == "printable":
            ret.stringType = tagPrintableString
        case strings.HasPrefix(part, "default:"):
            i, err := strconv.Atoi64(part[8:])
            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
        }
    }
    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
    }
    switch t := t.(type) {
    case *reflect.BoolType:
        return tagBoolean, false, true
    case *reflect.IntType:
        return tagInteger, false, true
    case *reflect.StructType:
        return tagSequence, true, true
    case *reflect.SliceType:
        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.StringType:
        return tagPrintableString, false, true
    }
    return 0, false, false
}