Run Format

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
    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

func Marshal

func Marshal(val interface{}) ([]byte, error)

Marshal returns the ASN.1 encoding of val.

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

ia5:		causes strings to be marshaled as ASN.1, IA5 strings
omitempty:	causes empty slices to be skipped
printable:	causes strings to be marshaled as ASN.1, PrintableString strings.
utf8:		causes strings to be marshaled as ASN.1, UTF8 strings

func Unmarshal

func Unmarshal(b []byte, val interface{}) (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.

An ASN.1 INTEGER can be written to an int, int32, int64, or *big.Int (from the math/big package). If the encoded value does not fit in the Go type, Unmarshal returns a parse error.

An ASN.1 BIT STRING can be written to a BitString.

An ASN.1 OCTET STRING can be written to a []byte.

An ASN.1 OBJECT IDENTIFIER can be written to an ObjectIdentifier.

An ASN.1 ENUMERATED can be written to an Enumerated.

An ASN.1 UTCTIME or GENERALIZEDTIME can be written to a time.Time.

An ASN.1 PrintableString or IA5String can be written to a string.

Any of the above ASN.1 values can be written to an interface{}. The value stored in the interface has the corresponding Go type. For integers, that type is int64.

An ASN.1 SEQUENCE OF x or SET OF x can be written to a slice if an x can be written to the slice's element type.

An ASN.1 SEQUENCE or SET can be written to a struct if each of the elements in the sequence can be written to the corresponding element in the struct.

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

application	specifies that a APPLICATION 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

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 type name of a slice element ends with "SET" then it's treated as if the "set" tag was set on it. 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 interface{}, 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 false.

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

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