pmaterer / asn1-v1

ASN.1 BER encoding and decoding.

asn1

ASN.1 BER serialization Go module.

Basic Encoding Rules (BER)

BER uses a type-length-value encoding scheme. Encoded data will be structured as follows:

The Identifier octets identify the type of thing encoded. The Length octets identify the length of the thing encoded. Finally, the Content octets contain the encoded thing.

Identifier octets

The identifier octets encode the ASN.1 tag class, whether it is primitive or constructed, and tag number.

Class

The tag's class is set in bits 8 and 7 in the first octet. Possible values:

Class	Bit 8	Bit 7
Universal	0	0
Application	0	1
Context-specific	1	0
Private	1	1

Primitive/Constructed

Bit 6 sets whether the value is primitive or constructed:

Bit 6 value
0	primitive
1	constructed

Primitive encodings represent the value directly. For instance, an INTEGER tag is primitive, and the underlying content value is an encoded integer.

Constructed encodings represent a concatenation of other encoded values.

Tag number

For tag numbers <= 30 (the universal tags), the last 5 bits are used to encode the tag number, and the identifier will be 1 octet total. For tag numbers >= 31 the last 5 bits of the first octet are encoded to 11111. The subsequent octets are then the base 128 encoding of the tag number.

Length octets

BER supports length octets in two possible forms: definite and indefinite. This module only supports the definite form.

Content Encoding

Types

ASN.1 Type	Description	Tag	Go Types
BOOLEAN	Simple boolean value	1	bool
INTEGER	A signed integer value, with no limits	2	int, int8, int16, int32, int64
BIT STRING	Arbitrary string of bits	3
OCTET STRING	A value of zero or more bytes	4	[]byte
OBJECT IDENTIFIER		6	ObjectIdentifier
REAL		9	float32, float64
Strings	See below	See below	string

Strings

ASN.1 has many string types:

Type	Tag	Description
BMPString	30
GeneralString	27
GraphicString	25
IA5String	22	First 128 chracters of ASCII alphabet
NumericString	18
PrintableString	19	A restricted subset of ASCII alphabet
T61String	20
UniversalString	28
UTF8String	12
VideotexString	21
VisibleString	26

Explicit/Implicit Tags

ASN.1 has IMPLICIT and EXPLICIT tags.

IMPLICIT tags are derived from other types by changing the tag of the underlying type. ASN.1 notation:

[[class] number] IMPLICIT Type

***class = *** UNIVERSAL | APPLICATION | PRIVATE

Note: The IMPLICIT keyword is optional in ASN.1 modules.

If there is no class name, the tag's class is Context-specific (which can only be a component of a structured or CHOICE type).

Implicit Exaple

ASN.1 notation:

[5] IMPLICIT UTF8String

1. Encode "hi"
2. The identifier octet would be encoded as:
   1. 10 - Context-specific defaults
   2. 0 - primitive
   3. 101 - 5

Todo

• Handle pointers