package standard library
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Published: Jul 2, 2024 License: BSD-3-Clause Imports: 12 Imported by: 14,982



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,”



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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 tags represent the type of the following object.

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

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


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var NullBytes = []byte{TagNull, 0}

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

View Source
var NullRawValue = RawValue{Tag: TagNull}

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


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 added in go1.10

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.

  • An ASN.1 INTEGER can be written to an int, int32, int64, or *big.Int. 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, IA5String, or NumericString 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 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

type BitString struct {
	Bytes     []byte // bits packed into bytes.
	BitLength int    // length in bits.

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.

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

type Enumerated int

An Enumerated is represented as a plain int.

type Flag

type Flag bool

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

type ObjectIdentifier

type ObjectIdentifier []int

An ObjectIdentifier represents an ASN.1 OBJECT IDENTIFIER.

func (ObjectIdentifier) Equal

func (oi ObjectIdentifier) Equal(other ObjectIdentifier) bool

Equal reports whether oi and other represent the same identifier.

func (ObjectIdentifier) String added in go1.3

func (oi ObjectIdentifier) String() string

type RawContent

type RawContent []byte

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 RawValue

type RawValue struct {
	Class, Tag int
	IsCompound bool
	Bytes      []byte
	FullBytes  []byte // includes the tag and length

A RawValue represents an undecoded ASN.1 object.

type StructuralError

type StructuralError struct {
	Msg string

A StructuralError suggests that the ASN.1 data is valid, but the Go type which is receiving it doesn't match.

func (StructuralError) Error

func (e StructuralError) Error() string

type SyntaxError

type SyntaxError struct {
	Msg string

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

func (SyntaxError) Error

func (e SyntaxError) Error() string

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