Package asn1

import "encoding/asn1"

Overview ▾

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.


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
    TagNumericString   = 18
    TagPrintableString = 19
    TagT61String       = 20
    TagIA5String       = 22
    TagUTCTime         = 23
    TagGeneralizedTime = 24
    TagGeneralString   = 27
    TagBMPString       = 30

ASN.1 class types represent the namespace of the tag.

const (
    ClassUniversal       = 0
    ClassApplication     = 1
    ClassContextSpecific = 2
    ClassPrivate         = 3


NullBytes contains bytes representing the DER-encoded ASN.1 NULL type.

var NullBytes = []byte{TagNull, 0}

NullRawValue is a RawValue with its Tag set to the ASN.1 NULL type tag (5).

var NullRawValue = RawValue{Tag: TagNull}

func Marshal

func Marshal(val any) ([]byte, error)

Marshal returns the ASN.1 encoding of val.

In addition to the struct tags recognized by Unmarshal, the following can be used:

ia5:         causes strings to be marshaled as ASN.1, IA5String values
omitempty:   causes empty slices to be skipped
printable:   causes strings to be marshaled as ASN.1, PrintableString values
utf8:        causes strings to be marshaled as ASN.1, UTF8String values
utc:         causes time.Time to be marshaled as ASN.1, UTCTime values
generalized: causes time.Time to be marshaled as ASN.1, GeneralizedTime values

func MarshalWithParams 1.10

func MarshalWithParams(val any, params string) ([]byte, error)

MarshalWithParams allows field parameters to be specified for the top-level element. The form of the params is the same as the field tags.

func Unmarshal

func Unmarshal(b []byte, val any) (rest []byte, err error)

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. If val is nil or not a pointer, Unmarshal returns an error.

After parsing b, any bytes that were leftover and not used to fill val will be returned in rest. When parsing a SEQUENCE into a struct, any trailing elements of the SEQUENCE that do not have matching fields in val will not be included in rest, as these are considered valid elements of the SEQUENCE and not trailing data.

The following tags on struct fields have special meaning to Unmarshal:

application specifies that an APPLICATION tag is used
private     specifies that a PRIVATE tag is used
default:x   sets the default value for optional integer fields (only used if optional is also present)
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

When decoding an ASN.1 value with an IMPLICIT tag into a string field, Unmarshal will default to a PrintableString, which doesn't support characters such as '@' and '&'. To force other encodings, use the following tags:

ia5     causes strings to be unmarshaled as ASN.1 IA5String values
numeric causes strings to be unmarshaled as ASN.1 NumericString values
utf8    causes strings to be unmarshaled as ASN.1 UTF8String values

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 name of a slice type ends with "SET" then it's treated as if the "set" tag was set on it. This results in interpreting the type as a SET OF x rather than a SEQUENCE OF x. 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 UnmarshalWithParams

func UnmarshalWithParams(b []byte, val any, params string) (rest []byte, err error)

UnmarshalWithParams allows field parameters to be specified for the top-level element. The form of the params is the same as the field tags.

type BitString

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.

func (BitString) At

func (b BitString) At(i int) int

At returns the bit at the given index. If the index is out of range it returns 0.

func (BitString) RightAlign

func (b BitString) RightAlign() []byte

RightAlign returns a slice where the padding bits are at the beginning. The slice may share memory with the BitString.

type Enumerated

An Enumerated is represented as a plain int.

type Enumerated int

type Flag

A Flag accepts any data and is set to true if present.

type Flag bool

type ObjectIdentifier

An ObjectIdentifier represents an ASN.1 OBJECT IDENTIFIER.

type ObjectIdentifier []int

func (ObjectIdentifier) Equal

func (oi ObjectIdentifier) Equal(other ObjectIdentifier) bool

Equal reports whether oi and other represent the same identifier.

func (ObjectIdentifier) String 1.3

func (oi ObjectIdentifier) String() string

type RawContent

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

type RawValue

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

type StructuralError

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 (StructuralError) Error

func (e StructuralError) Error() string

type SyntaxError

A SyntaxError suggests that the ASN.1 data is invalid.

type SyntaxError struct {
    Msg string

func (SyntaxError) Error

func (e SyntaxError) Error() string