asn1andr

Documentation

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.

Index

• Constants
• Variables
• func Marshal(val interface{}) ([]byte, error)
• func MarshalWithParams(val interface{}, params string) ([]byte, error)
• func Unmarshal(b []byte, val interface{}) (rest []byte, err error)
• func UnmarshalWithParams(b []byte, val interface{}, params string) (rest []byte, err error)

Constants

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.

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

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

Variables

var NullBytes = []byte{TagNull, 0}

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

var NullRawValue = asn1.NullRawValue

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

Functions

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, 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

func MarshalWithParams(val interface{}, 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 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.

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 (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 asn1.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

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.