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ASN.1
User's Guide
Version 3.0

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Chapters

- Asn1
  - Top of chapter
  - Introduction
  - Getting Started with Asn1
  - The Asn1 Application User Interface
  - Multi-file Compilation
  - A quick note about tags
  - The ASN.1 Types
  - Naming of Records in .hrl Files
  - ASN.1 Values
  - Macros
  - ASN.1 Information Objects (X.681)
  - Parameterization (X.683)
- Specialized Decodes
  - Top of chapter
  - Exclusive Decode
  - Selective Decode
  - Performance

# 1 Asn1

### 1.1 Introduction

#### Features

The Asn1 application provides:

- An ASN.1 compiler for Erlang, which generates encode and decode functions to be used by Erlang programs sending and receiving ASN.1 specified data.
- Run-time functions used by the generated code.
- Support for the following encoding rules:
  - Basic Encoding Rules (BER)
  - Distinguished Encoding Rules (DER), a specialized form of BER that is used in security-conscious applications.
  - Packed Encoding Rules (PER); both the aligned and unaligned variant.

#### Overview

ASN.1 (Abstract Syntax Notation One) is a formal language for describing data structures to be exchanged between distributed computer systems. The purpose of ASN.1 is to have a platform and programming language independent notation to express types using a standardized set of rules for the transformation of values of a defined type into a stream of bytes. This stream of bytes can then be sent on any type of communication channel. This way, two applications written in different programming languages running on different computers with different internal representation of data can exchange instances of structured data types.

#### Prerequisites

It is assumed that the reader is familiar with the ASN.1 notation as documented in the standard definition [X.680] which is the primary text. It may also be helpful, but not necessary, to read the standard definitions [X.681] [X.682] [X.683] [X.690] [X.691].

A good book explaining those reference texts is [DUBUISSON], which is free to download at http://www.oss.com/asn1/dubuisson.html.

#### Capabilities

This application covers all features of ASN.1 up to the 1997 edition of the specification. In the 2002 edition of ASN.1 a number of new features were introduced. The following features of the 2002 edition are fully or partly supported as shown below:

- Decimal notation (e.g., "1.5e3") for REAL values. The NR1, NR2 and NR3 formats as explained in ISO6093 are supported.

- The RELATIVE-OID type for relative object identifiers is fully supported.

- The subtype constraint (CONTAINING/ENCODED BY) to constrain the content of an octet string or a bit string is parsed when compiling, but no further action is taken. This constraint is not a PER-visible constraint.

- The subtype constraint by regular expressions (PATTERN) for character string types is parsed when compiling, but no further action is taken. This constraint is not a PER-visible constraint.

- Multiple-line comments as in C, /* ... */, are supported.

### 1.2 Getting Started with Asn1

#### A First Example

The following example demonstrates the basic functionality used to run the Erlang ASN.1 compiler.

Create a file called People.asn containing the following:

    People DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
      Person ::= SEQUENCE {
        name PrintableString,
        location INTEGER {home(0),field(1),roving(2)},
        age INTEGER OPTIONAL
      }
    END

This file (People.asn) must be compiled before it can be used. The ASN.1 compiler checks that the syntax is correct and that the text represents proper ASN.1 code before generating an abstract syntax tree. The code-generator then uses the abstract syntax tree in order to generate code.

The generated Erlang files will be placed in the current directory or in the directory specified with the {outdir,Dir} option. The following shows how the compiler can be called from the Erlang shell:

    1> asn1ct:compile("People", [ber]).
    ok
    2>

The verbose option can be given to have information about the generated files printed:

    2> asn1ct:compile("People", [ber,verbose]).
    Erlang ASN.1 compiling "People.asn"
    --{generated,"People.asn1db"}--
    --{generated,"People.hrl"}--
    --{generated,"People.erl"}--
    ok
    3>

The ASN.1 module People is now accepted and the abstract syntax tree is saved in the People.asn1db file; the generated Erlang code is compiled using the Erlang compiler and loaded into the Erlang run-time system. Now there is an API for encode/2 and decode/2 in the module People, which is invoked by:
'People':encode(<Type name>, <Value>)
or
'People':decode(<Type name>, <Value>)

Assume there is a network application which receives instances of the ASN.1 defined type Person, modifies and sends them back again:

    receive
       {Port,{data,Bytes}} ->
           case 'People':decode('Person',Bytes) of
               {ok,P} ->
                   {ok,Answer} = 'People':encode('Person',mk_answer(P)),
                   Port ! {self(),{command,Answer}};
               {error,Reason} ->
                   exit({error,Reason})
           end
        end,

In the example above, a series of bytes is received from an external source and the bytes are then decoded into a valid Erlang term. This was achieved with the call 'People':decode('Person',Bytes) which returned an Erlang value of the ASN.1 type Person. Then an answer was constructed and encoded using 'People':encode('Person',Answer) which takes an instance of a defined ASN.1 type and transforms it to a binary according to the BER or PER encoding rules.
The encoder and the decoder can also be run from the shell.

    2> Rockstar = {'Person',"Some Name",roving,50}.
    {'Person',"Some Name",roving,50}
    3> {ok,Bin} = 'People':encode('Person',Rockstar).
    {ok,<<243,17,19,9,83,111,109,101,32,78,97,109,101,2,1,2,
          2,1,50>>}
    4> {ok,Person} = 'People':decode('Person',Bin).
    {ok,{'Person',"Some Name",roving,50}}
    5>

#### Module dependencies

It is common that ASN.1 modules import defined types, values and other entities from another ASN.1 module.

Earlier versions of the ASN.1 compiler required that modules that were imported from had to be compiled before the module that imported. This caused problems when ASN.1 modules had circular dependencies.

Referenced modules are now parsed when the compiler finds an entity that is imported. There will not be any code generated for the referenced module. However, the compiled module rely on that the referenced modules also will be compiled.

### 1.3 The Asn1 Application User Interface

The Asn1 application provides two separate user interfaces:

- The module asn1ct which provides the compile-time functions (including the compiler).

- The module asn1rt_nif which provides the run-time functions for the ASN.1 decoder for the BER back-end.

The reason for the division of the interface into compile-time and run-time is that only run-time modules (asn1rt*) need to be loaded in an embedded system.

#### Compile-time Functions

The ASN.1 compiler can be invoked directly from the command-line by means of the erlc program. This is convenient when compiling many ASN.1 files from the command-line or when using Makefiles. Here are some examples of how the erlc command can be used to invoke the ASN.1 compiler:

    erlc Person.asn
    erlc -bper Person.asn
    erlc -bber ../Example.asn
    erlc -o ../asnfiles -I ../asnfiles -I /usr/local/standards/asn1 Person.asn

The useful options for the ASN.1 compiler are:

-b[ber | per | uper]

  Choice of encoding rules, if omitted ber is the default.

-o OutDirectory

  Where to put the generated files, default is the current directory.

-I IncludeDir

  Where to search for .asn1db files and ASN.1 source specs in order to resolve references to other modules. This option can be repeated many times if there are several places to search in. The compiler will always search the current directory first.

+der

  DER encoding rule. Only when using -ber option.

+asn1config

  This functionality works together with the ber option. It enables the specialized decodes, see the Specialized Decode chapter.

+undec_rest

  A buffer that holds a message being decoded may also have trailing bytes. If those trailing bytes are important they can be returned along with the decoded value by compiling the ASN.1 specification with the +undec_rest option. The return value from the decoder will be {ok,Value,Rest} where Rest is a binary containing the trailing bytes.

+'Any Erlc Option'

  You may add any option to the Erlang compiler when compiling the generated Erlang files. Any option unrecognized by the ASN.1 compiler will be passed to the Erlang compiler.

For a complete description of erlc see Erts Reference Manual.

The compiler and other compile-time functions can also be invoked from the Erlang shell. Below follows a brief description of the primary functions, for a complete description of each function see the Asn1 Reference Manual, the asn1ct module.

The compiler is invoked by using asn1ct:compile/1 with default options, or asn1ct:compile/2 if explicit options are given. Example:

    asn1ct:compile("H323-MESSAGES.asn1").

which equals:

    asn1ct:compile("H323-MESSAGES.asn1",[ber]).

If one wants PER encoding:

    asn1ct:compile("H323-MESSAGES.asn1",[per]).

The generic encode and decode functions can be invoked like this:

    'H323-MESSAGES':encode('SomeChoiceType',{call,"octetstring"}).
    'H323-MESSAGES':decode('SomeChoiceType',Bytes).

#### Run-time Functions

When an ASN.1 specification is compiled with the ber option, the module asn1rt_nif module and the NIF library in asn1/priv_dir will be needed at run-time.

By invoking the function info/0 in a generated module, one gets information about which compiler options were used.

#### Errors

Errors detected at compile time appear on the screen together with a line number indicating where in the source file the error was detected. If no errors are found, an Erlang ASN.1 module will be created.

The run-time encoders and decoders execute within a catch and returns {ok, Data} or {error, {asn1, Description}} where Description is an Erlang term describing the error.

### 1.4 Multi-file Compilation

There are various reasons for using multi-file compilation:

- You want to choose the name for the generated module, perhaps because you need to compile the same specs for different encoding rules.
- You want only one resulting module.

You need to specify which ASN.1 specs you will compile in a module that must have the extension .set.asn. You chose name of the module and provide the names of the ASN.1 specs. For instance, if you have the specs File1.asn, File2.asn and File3.asn your module MyModule.set.asn will look like:

    File1.asn
    File2.asn
    File3.asn

If you compile with:

    ~> erlc MyModule.set.asn

the result will be one merged module MyModule.erl with the generated code from the three ASN.1 specs.

### 1.5 A quick note about tags

Tags used to be important for all users of ASN.1, because it was necessary to manually add tags to certain constructs in order for the ASN.1 specification to be valid. Here is an example of an old-style specification:

    Tags DEFINITIONS ::=
    BEGIN
      Afters ::= CHOICE { cheese [0] IA5String,
                          dessert [1] IA5String }
    END

Without the tags (the numbers in square brackets) the ASN.1 compiler would refuse to compile the file.

In 1994 the global tagging mode AUTOMATIC TAGS was introduced. By putting AUTOMATIC TAGS in the module header, the ASN.1 compiler will automatically add tags when needed. Here is the same specification in AUTOMATIC TAGS mode:

    Tags DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
      Afters ::= CHOICE { cheese IA5String,
                          dessert IA5String }
    END

Tags will not be mentioned any more in this manual.

### 1.6 The ASN.1 Types

This section describes the ASN.1 types including their functionality, purpose and how values are assigned in Erlang.

ASN.1 has both primitive and constructed types:

Primitive types | Constructed types
BOOLEAN | SEQUENCE
INTEGER | SET
REAL | CHOICE
NULL | SET OF and SEQUENCE OF
ENUMERATED | ANY
BIT STRING | ANY DEFINED BY
OCTET STRING | EXTERNAL
Character Strings | EMBEDDED PDV
OBJECT IDENTIFIER | CHARACTER STRING
Object Descriptor |
The TIME types |

Table 1.1: The supported ASN.1 types
Note

Values of each ASN.1 type has its own representation in Erlang described in the following subsections. Users shall provide these values for encoding according to the representation, as in the example below.

    Operational ::= BOOLEAN --ASN.1 definition

In Erlang code it may look like:

    Val = true,
    {ok,Bytes} = MyModule:encode('Operational', Val),

Below follows a description of how values of each type can be represented in Erlang.

#### BOOLEAN

Booleans in ASN.1 express values that can be either TRUE or FALSE. The meanings assigned to TRUE or FALSE is beyond the scope of this text.
In ASN.1 it is possible to have:

    Operational ::= BOOLEAN

Assigning a value to the type Operational in Erlang is possible by using the following Erlang code:

    Myvar1 = true,

Thus, in Erlang the atoms true and false are used to encode a boolean value.

#### INTEGER

ASN.1 itself specifies indefinitely large integers, and the Erlang systems with versions 4.3 and higher, support very large integers, in practice indefinitely large integers.

The concept of sub-typing can be applied to integers as well as to other ASN.1 types. The details of sub-typing are not explained here, for further info see [X.680]. A variety of syntaxes are allowed when defining a type as an integer:

    T1 ::= INTEGER
    T2 ::= INTEGER (-2..7)
    T3 ::= INTEGER (0..MAX)
    T4 ::= INTEGER (0<..MAX)
    T5 ::= INTEGER (MIN<..-99)
    T6 ::= INTEGER {red(0),blue(1),white(2)}

The Erlang representation of an ASN.1 INTEGER is an integer or an atom if a so called Named Number List (see T6 above) is specified.

Below is an example of Erlang code which assigns values for the above types:

    T1value = 0,
    T2value = 6,
    T6value1 = blue,
    T6value2 = 0,
    T6value3 = white

The Erlang variables above are now bound to valid instances of ASN.1 defined types. This style of value can be passed directly to the encoder for transformation into a series of bytes.

The decoder will return an atom if the value corresponds to a symbol in the Named Number List.

#### REAL

The following ASN.1 type is used for real numbers:

    R1 ::= REAL

It can be assigned a value in Erlang as:

    R1value1 = "2.14",
    R1value2 = {256,10,-2},

In the last line note that the tuple {256,10,-2} is the real number 2.56 in a special notation, which will encode faster than simply stating the number as "2.56". The arity three tuple is {Mantissa,Base,Exponent} i.e. Mantissa * Base^Exponent.

#### NULL

Null is suitable in cases where supply and recognition of a value is important but the actual value is not.

    Notype ::= NULL

The NULL type can be assigned in Erlang:

    N1 = 'NULL',

The actual value is the quoted atom 'NULL'.

#### ENUMERATED

The enumerated type can be used, when the value we wish to describe, may only take one of a set of predefined values.

    DaysOfTheWeek ::= ENUMERATED {
        sunday(1),monday(2),tuesday(3),
        wednesday(4),thursday(5),friday(6),saturday(7) }

For example to assign a weekday value in Erlang use the same atom as in the Enumerations of the type definition:

    Day1 = saturday,

The enumerated type is very similar to an integer type, when defined with a set of predefined values. An enumerated type differs from an integer in that it may only have specified values, whereas an integer can also have any other value.

#### BIT STRING

The BIT STRING type can be used to model information which is made up of arbitrary length series of bits. It is intended to be used for a selection of flags, not for binary files.
In ASN.1 BIT STRING definitions may look like:

    Bits1 ::= BIT STRING
    Bits2 ::= BIT STRING {foo(0),bar(1),gnu(2),gnome(3),punk(14)}

There are two notations available for representation of BIT STRING values in Erlang and as input to the encode functions.

- A bitstring. By default, a BIT STRING with no symbolic names will be decoded to an Erlang bitstring.
- A list of atoms corresponding to atoms in the NamedBitList in the BIT STRING definition. A BIT STRING with symbolic names will always be decoded to this format.

Example:

    Bits1Val1 = <<0:1,1:1,0:1,1:1,1:1>>,
    Bits2Val1 = [gnu,punk],
    Bits2Val2 = <<2#1110:4>>,
    Bits2Val3 = [bar,gnu,gnome],

Bits2Val2 and Bits2Val3 above denote the same value.

Bits2Val1 is assigned symbolic values. The assignment means that the bits corresponding to gnu and punk i.e. bits 2 and 14 are set to 1 and the rest set to 0. The symbolic values appear as a list of values. If a named value appears, which is not specified in the type definition, a run-time error will occur.

BIT STRINGS may also be sub-typed with, for example, a SIZE specification:

    Bits3 ::= BIT STRING (SIZE(0..31))

This means that no bit higher than 31 can ever be set.

#### Deprecated representations for BIT STRING

In addition to the representations described above, the following deprecated representations are available if the specification has been compiled with the legacy_erlang_types option:

- A list of binary digits (0 or 1). This format is accepted as input to the encode functions, and a BIT STRING will be decoded to this format if the legacy_bit_string option has been given.
- As {Unused,Binary} where Unused denotes how many trailing zero-bits 0 to 7 that are unused in the least significant byte in Binary. This format is accepted as input to the encode functions, and a BIT STRING will be decoded to this format if compact_bit_string has been given.
- A hexadecimal number (or an integer). This format should be avoided, since it is easy to misinterpret a BIT STRING value in this format.

#### OCTET STRING

The OCTET STRING is the simplest of all ASN.1 types. The OCTET STRING only moves or transfers e.g. binary files or other unstructured information complying to two rules. Firstly, the bytes consist of octets and secondly, encoding is not required.

It is possible to have the following ASN.1 type definitions:

    O1 ::= OCTET STRING
    O2 ::= OCTET STRING (SIZE(28))

With the following example assignments in Erlang:

    O1Val = <<17,13,19,20,0,0,255,254>>,
    O2Val = <<"must be exactly 28 chars....">>,

By default, an OCTET STRING is always represented as an Erlang binary. If the specification has been compiled with the legacy_erlang_types option, the encode functions will accept both lists and binaries, and the decode functions will decode an OCTET STRING to a list.

#### Character Strings

ASN.1 supports a wide variety of character sets. The main difference between OCTET STRINGS and the Character strings is that OCTET STRINGS have no imposed semantics on the bytes delivered.

However, when using for instance the IA5String (which closely resembles ASCII) the byte 65 (in decimal notation) means the character 'A'.

For example, if a defined type is to be a VideotexString and an octet is received with the unsigned integer value X, then the octet should be interpreted as specified in the standard ITU-T T.100,T.101.

The ASN.1 to Erlang compiler will not determine the correct interpretation of each BER (Basic Encoding Rules) string octet value with different Character strings. Interpretation of octets is the responsibility of the application. Therefore, from the BER string point of view, octets appear to be very similar to character strings and are compiled in the same way.

It should be noted that when PER (Packed Encoding Rules) is used, there is a significant difference in the encoding scheme between OCTET STRINGS and other strings. The constraints specified for a type are especially important for PER, where they affect the encoding.

Here are some examples:

    Digs ::= NumericString (SIZE(1..3))
    TextFile ::= IA5String (SIZE(0..64000))

with corresponding Erlang assignments:

    DigsVal1 = "456",
    DigsVal2 = "123",
    TextFileVal1 = "abc...xyz...",
    TextFileVal2 = [88,76,55,44,99,121 .......... a lot of characters here ....]

The Erlang representation for "BMPString" and "UniversalString" is either a list of ASCII values or a list of quadruples. The quadruple representation associates to the Unicode standard representation of characters. The ASCII characters are all represented by quadruples beginning with three zeros like {0,0,0,65} for the 'A' character. When decoding a value for these strings the result is a list of quadruples, or integers when the value is an ASCII character.

The following example shows how it works. We have the following specification in the file PrimStrings.asn1.

    PrimStrings DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
       BMP ::= BMPString
    END

Encoding and decoding some strings:

    1> asn1ct:compile('PrimStrings', [ber]).
    ok
    2> {ok,Bytes1} = 'PrimStrings':encode('BMP', [{0,0,53,53},{0,0,45,56}]).
    {ok,<<30,4,53,54,45,56>>}
    3> 'PrimStrings':decode('BMP', Bytes1).
    {ok,[{0,0,53,53},{0,0,45,56}]}
    4> {ok,Bytes2} = 'PrimStrings':encode('BMP', [{0,0,53,53},{0,0,0,65}]).
    {ok,<<30,4,53,53,0,65>>}
    5> 'PrimStrings':decode('BMP', Bytes2).
    {ok,[{0,0,53,53},65]}
    6> {ok,Bytes3} = 'PrimStrings':encode('BMP', "BMP string").
    {ok,<<30,20,0,66,0,77,0,80,0,32,0,115,0,116,0,114,0,105,0,110,0,103>>}
    7> 'PrimStrings':decode('BMP', Bytes3).
    {ok,"BMP string"}

The UTF8String type is represented as a UTF-8 encoded binary in Erlang. Such binaries can be created directly using the binary syntax or by converting from a list of Unicode code points using the unicode:characters_to_binary/1 function.

Here are some examples showing how UTF-8 encoded binaries can be created and manipulated:

    1> Gs = "Мой маленький Гном".
    [1052,1086,1081,32,1084,1072,1083,1077,1085,1100,1082,1080,
     1081,32,1043,1085,1086,1084]
    2> Gbin = unicode:characters_to_binary(Gs).
    <<208,156,208,190,208,185,32,208,188,208,176,208,187,208,
      181,208,189,209,140,208,186,208,184,208,185,32,208,147,
      208,...>>
    3> Gbin = <<"Мой маленький Гном"/utf8>>.
    <<208,156,208,190,208,185,32,208,188,208,176,208,187,208,
      181,208,189,209,140,208,186,208,184,208,185,32,208,147,
      208,...>>
    4> Gs = unicode:characters_to_list(Gbin).
    [1052,1086,1081,32,1084,1072,1083,1077,1085,1100,1082,1080,
     1081,32,1043,1085,1086,1084]

See the unicode module for more details.

In the following example we will use this ASN.1 specification:

    UTF DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
       UTF ::= UTF8String
    END

Encoding and decoding a string with Unicode characters:

    5> asn1ct:compile('UTF', [ber]).
    ok
    6> {ok,Bytes1} = 'UTF':encode('UTF', <<"Гном"/utf8>>).
    {ok,<<12,8,208,147,208,189,208,190,208,188>>}
    7> {ok,Bin1} = 'UTF':decode('UTF', Bytes1).
    {ok,<<208,147,208,189,208,190,208,188>>}
    8> io:format("~ts\n", [Bin1]).
    Гном
    ok
    9> unicode:characters_to_list(Bin1).
    [1043,1085,1086,1084]

#### OBJECT IDENTIFIER

The OBJECT IDENTIFIER is used whenever a unique identity is required. An ASN.1 module, a transfer syntax, etc. is identified with an OBJECT IDENTIFIER. Assume the example below:

    Oid ::= OBJECT IDENTIFIER

Therefore, the example below is a valid Erlang instance of the type 'Oid'.

    OidVal1 = {1,2,55},

The OBJECT IDENTIFIER value is simply a tuple with the consecutive values which must be integers.

The first value is limited to the values 0, 1 or 2 and the second value must be in the range 0..39 when the first value is 0 or 1.

The OBJECT IDENTIFIER is a very important type and it is widely used within different standards to uniquely identify various objects. In [DUBUISSON], there is an easy-to-understand description of the usage of OBJECT IDENTIFIER.

#### Object Descriptor

Values of this type can be assigned a value as an ordinary string like this:

          "This is the value of an Object descriptor"

#### The TIME Types

Two different time types are defined within ASN.1, Generalized Time and UTC (Universal Time Coordinated), both are assigned a value as an ordinary string within double quotes i.e. "19820102070533.8".

In case of DER encoding the compiler does not check the validity of the time values. The DER requirements upon those strings is regarded as a matter for the application to fulfill.

#### SEQUENCE

The structured types of ASN.1 are constructed from other types in a manner similar to the concepts of array and struct in C.
A SEQUENCE in ASN.1 is comparable with a struct in C and a record in Erlang. A SEQUENCE may be defined as:

    Pdu ::= SEQUENCE {
       a INTEGER,
       b REAL,
       c OBJECT IDENTIFIER,
       d NULL }

This is a 4-component structure called 'Pdu'. The major format for representation of SEQUENCE in Erlang is the record format. For each SEQUENCE and SET in an ASN.1 module an Erlang record declaration is generated. For Pdu above, a record like this is defined:

    -record('Pdu',{a, b, c, d}).

The record declarations for a module M are placed in a separate M.hrl file.

Values can be assigned in Erlang as shown below:

    MyPdu = #'Pdu'{a=22,b=77.99,c={0,1,2,3,4},d='NULL'}.

The decode functions will return a record as result when decoding a SEQUENCE or a SET.

A SEQUENCE and a SET may contain a component with a DEFAULT key word followed by the actual value that is the default value. The DEFAULT keyword means that the application doing the encoding can omit encoding of the value, thus resulting in fewer bytes to send to the receiving application.

An application can use the atom asn1_DEFAULT to indicate that the encoding should be omitted for that position in the SEQUENCE.

Depending on the encoding rules, the encoder may also compare the given value to the default value and automatically omit the encoding if they are equal. How much effort the encoder makes to to compare the values depends on the encoding rules. The DER encoding rules forbids encoding a value equal to the default value, so it has a more thorough and time-consuming comparison than the encoders for the other encoding rules.

In the following example we will use this ASN.1 specification:

    File DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
    Seq1 ::= SEQUENCE {
        a INTEGER DEFAULT 1,
        b Seq2 DEFAULT {aa TRUE, bb 15}
    }

    Seq2 ::= SEQUENCE {
        aa BOOLEAN,
        bb INTEGER
    }

    Seq3 ::= SEQUENCE {
        bs BIT STRING {a(0), b(1), c(2)} DEFAULT {a, c}
    }
    END

Here is an example where the BER encoder is able to omit encoding of the default values:

    1> asn1ct:compile('File', [ber]).
    ok
    2> 'File':encode('Seq1', {'Seq1',asn1_DEFAULT,asn1_DEFAULT}).
    {ok,<<48,0>>}
    3> 'File':encode('Seq1', {'Seq1',1,{'Seq2',true,15}}).
    {ok,<<48,0>>}

And here is an example with a named BIT STRING where the BER encoder will not omit the encoding:

    4> 'File':encode('Seq3', {'Seq3',asn1_DEFAULT).
    {ok,<<48,0>>}
    5> 'File':encode('Seq3', {'Seq3',<<16#101:3>>).
    {ok,<<48,4,128,2,5,160>>}

The DER encoder will omit the encoding for the same BIT STRING:

    6> asn1ct:compile('File', [ber,der]).
    ok
    7> 'File':encode('Seq3', {'Seq3',asn1_DEFAULT).
    {ok,<<48,0>>}
    8> 'File':encode('Seq3', {'Seq3',<<16#101:3>>).
    {ok,<<48,0>>}

#### SET

In Erlang, the SET type is used exactly as SEQUENCE. Note that if the BER or DER encoding rules are used, decoding a SET is slower than decoding a SEQUENCE because the components must be sorted.

#### Notes about extensibility for SEQUENCE and SET

When a SEQUENCE or SET contains an extension marker and extension components like this:

    SExt ::= SEQUENCE {
               a INTEGER,
               ...,
               b BOOLEAN }

It means that the type may get more components in newer versions of the ASN.1 spec. In this case it has got a new component b. Thus, incoming messages that will be decoded may have more or fever components than this one.

The component b will be treated as an original component when encoding a message. In this case, as it is not an optional element, it must be encoded.

During decoding the b field of the record will get the decoded value of the b component if present and otherwise the value asn1_NOVALUE.

#### CHOICE

The CHOICE type is a space saver and is similar to the concept of a 'union' in the C language.

Assume:

    SomeModuleName DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
    T ::= CHOICE {
            x REAL,
            y INTEGER,
            z OBJECT IDENTIFIER }
    END

It is then possible to assign values:

    TVal1 = {y,17},
    TVal2 = {z,{0,1,2}},

A CHOICE value is always represented as the tuple {ChoiceAlternative, Val} where ChoiceAlternative is an atom denoting the selected choice alternative.

#### Extensible CHOICE

When a CHOICE contains an extension marker and the decoder detects an unknown alternative of the CHOICE the value is represented as:

    {asn1_ExtAlt, BytesForOpenType}

Where BytesForOpenType is a list of bytes constituting the encoding of the "unknown" CHOICE alternative.

#### SET OF and SEQUENCE OF

The SET OF and SEQUENCE OF types correspond to the concept of an array found in several programming languages. The Erlang syntax for both of these types is straight forward. For example:

    Arr1 ::= SET SIZE (5) OF INTEGER (4..9)
    Arr2 ::= SEQUENCE OF OCTET STRING

We may have the following in Erlang:

    Arr1Val = [4,5,6,7,8],
    Arr2Val = ["abc",[14,34,54],"Octets"],

Please note that the definition of the SET OF type implies that the order of the components is undefined, but in practice there is no difference between SET OF and SEQUENCE OF. The ASN.1 compiler for Erlang does not randomize the order of the SET OF components before encoding.

However, in case of a value of the type SET OF, the DER encoding format requires the elements to be sent in ascending order of their encoding, which implies an expensive sorting procedure in run-time. Therefore it is strongly recommended to use SEQUENCE OF instead of SET OF if it is possible.

#### ANY and ANY DEFINED BY

The types ANY and ANY DEFINED BY have been removed from the standard since 1994. It is recommended not to use these types any more. They may, however, exist in some old ASN.1 modules. The idea with this type was to leave a "hole" in a definition where one could put unspecified data of any kind, even non ASN.1 data.

A value of this type is encoded as an open type.

Instead of ANY/ANY DEFINED BY one should use information object class, table constraints and parameterization. In particular the construct TYPE-IDENTIFIER.@Type accomplish the same as the deprecated ANY.

See also Information object

#### EXTERNAL, EMBEDDED PDV and CHARACTER STRING

These types are used in presentation layer negotiation. They are encoded according to their associated type, see [X.680].

The EXTERNAL type had a slightly different associated type before 1994. [X.691] states that encoding shall follow the older associate type. Therefore does generated encode/decode functions convert values of the newer format to the older format before encoding. This implies that it is allowed to use EXTERNAL type values of either format for encoding. Decoded values are always returned on the newer format.

#### Embedded Named Types

The structured types previously described may very well have other named types as their components. The general syntax to assign a value to the component C of a named ASN.1 type T in Erlang is the record syntax #'T'{'C'=Value}. Where Value may be a value of yet another type T2.

For example:

    EmbeddedExample DEFINITIONS AUTOMATIC TAGS ::=
    BEGIN
    B ::= SEQUENCE {
            a Arr1,
            b T }

    Arr1 ::= SET SIZE (5) OF INTEGER (4..9)

    T ::= CHOICE {
            x REAL,
            y INTEGER,
            z OBJECT IDENTIFIER }
            END

The SEQUENCE b can be encoded like this in Erlang:

    1> 'EmbeddedExample':encode('B', {'B',[4,5,6,7,8],{x,"7.77"}}).
    {ok,<<5,56,0,8,3,55,55,55,46,69,45,50>>}

### 1.7 Naming of Records in .hrl Files

When an ASN.1 specification is compiled all defined types of type SET or SEQUENCE will result in a corresponding record in the generated hrl file. This is because the values for SET/SEQUENCE as mentioned in sections above are represented as records.

Though there are some special cases of this functionality that are presented below.

#### Embedded Structured Types

It is also possible in ASN.1 to have components that are themselves structured types. For example, it is possible to have:

    Emb ::= SEQUENCE {
        a SEQUENCE OF OCTET STRING,
        b SET {
           a INTEGER,
           b INTEGER DEFAULT 66},
        c CHOICE {
           a INTEGER,
           b FooType } }

    FooType ::= [3] VisibleString

The following records are generated because of the type Emb:

    -record('Emb,{a, b, c}).
    -record('Emb_b',{a, b = asn1_DEFAULT}). % the embedded SET type

Values of the Emb type can be assigned like this:

    V = #'Emb'{a=["qqqq",[1,2,255]],
               b = #'Emb_b'{a=99},
               c ={b,"Can you see this"}}.

For an embedded type of type SEQUENCE/SET in a SEQUENCE/SET the record name is extended with an underscore and the component name. If the embedded structure is deeper with SEQUENCE, SET or CHOICE types in the line, each component-/alternative-name will be added to the record-name.

For example:

    Seq ::= SEQUENCE{
        a CHOICE{
            b SEQUENCE {
               c  INTEGER
            }
        }
    }

will result in the following record:

    -record('Seq_a_b',{c}).

If the structured type has a component with an embedded SEQUENCE OF/SET OF which embedded type in turn is a SEQUENCE/SET it will give a record with the SEQOF/SETOF addition as in the following example:

    Seq ::= SEQUENCE {
        a SEQUENCE OF SEQUENCE {
               b
                   }
        c SET OF SEQUENCE {
               d
                   }
    }

This results in the records:

    -record('Seq_a_SEQOF'{b}).
    -record('Seq_c_SETOF'{d}).

A parameterized type should be considered as an embedded type. Each time a such type is referenced an instance of it is defined. Thus in the following example a record with name 'Seq_b' is generated in the .hrl file and used to hold values.

    Seq ::= SEQUENCE {
        b PType{INTEGER}
    }

    PType{T} ::= SEQUENCE{
        id T
    }

#### Recursive Types

Types may refer to themselves. Suppose:

    Rec ::= CHOICE {
         nothing NULL,
         something SEQUENCE {
              a INTEGER,
              b OCTET STRING,
              c Rec }}

This type is recursive; that is, it refers to itself. This is allowed in ASN.1 and the ASN.1-to-Erlang compiler supports this recursive type. A value for this type is assigned in Erlang as shown below:

    V = {something,#'Rec_something'{a = 77,
                                    b = "some octets here",
                                    c = {nothing,'NULL'}}}.

### 1.8 ASN.1 Values

Values can be assigned to ASN.1 type within the ASN.1 code itself, as opposed to the actions taken in the previous chapter where a value was assigned to an ASN.1 type in Erlang. The full value syntax of ASN.1 is supported and [X.680] describes in detail how to assign values in ASN.1. Below is a short example:

    TT ::= SEQUENCE {
       a INTEGER,
       b SET OF OCTET STRING }

    tt TT ::= {a 77,b {"kalle","kula"}}

The value defined here could be used in several ways. Firstly, it could be used as the value in some DEFAULT component:

    SS ::= SET {
        s OBJECT IDENTIFIER,
        val TT DEFAULT tt }

It could also be used from inside an Erlang program. If the above ASN.1 code was defined in ASN.1 module Values, then the ASN.1 value tt can be reached from Erlang as a function call to 'Values':tt() as in the example below.

    1> Val = 'Values':tt().
    {'TT',77,["kalle","kula"]}
    2> {ok,Bytes} = 'Values':encode('TT',Val).
    {ok,<<48,18,128,1,77,161,13,4,5,107,97,108,108,101,4,4,
          107,117,108,97>>}
    4> 'Values':decode('TT',Bytes).
    {ok,{'TT',77,["kalle","kula"]}}
    5>

The above example shows that a function is generated by the compiler that returns a valid Erlang representation of the value, even though the value is of a complex type.

Furthermore, there is a macro generated for each value in the .hrl file. So, the defined value tt can also be extracted by ?tt in application code.

### 1.9 Macros

MACRO is not supported as the the type is no longer part of the ASN.1 standard.

### 1.10 ASN.1 Information Objects (X.681)

Information Object Classes, Information Objects and Information Object Sets (in the following called classes, objects and object sets respectively) are defined in the standard definition [X.681]. In the following only a brief explanation is given.

These constructs makes it possible to define open types, i.e. values of that type can be of any ASN.1 type. It is also possible to define relationships between different types and values, since classes can hold types, values, objects, object sets and other classes in its fields. An Information Object Class may be defined in ASN.1 as:

    GENERAL-PROCEDURE ::= CLASS {
          &Message,
          &Reply               OPTIONAL,
          &Error               OPTIONAL,
          &id          PrintableString UNIQUE
    }
    WITH SYNTAX {
          NEW MESSAGE     &Message
          [REPLY           &Reply]
          [ERROR           &Error]
          ADDRESS          &id
    }

An object is an instance of a class and an object set is a set containing objects of one specified class. A definition may look like below.

The object object1 is an instance of the CLASS GENERAL-PROCEDURE and has one type field and one fixed type value field. The object object2 also has an OPTIONAL field ERROR, which is a type field.

    object1 GENERAL-PROCEDURE ::= {
        NEW MESSAGE      PrintableString
        ADDRESS          "home"
    }

    object2 GENERAL-PROCEDURE ::= {
        NEW MESSAGE INTEGER
        ERROR INTEGER
        ADDRESS "remote"
    }

The field ADDRESS is a UNIQUE field. Objects in an object set must have unique values in their UNIQUE field, as in GENERAL-PROCEDURES:

    GENERAL-PROCEDURES GENERAL-PROCEDURE ::= {
        object1 | object2}

One can not encode a class, object or object set, only referring to it when defining other ASN.1 entities. Typically one refers to a class and to object sets by table constraints and component relation constraints [X.682] in ASN.1 types, as in:

    StartMessage  ::= SEQUENCE {
        msgId  GENERAL-PROCEDURE.&id  ({GENERAL-PROCEDURES}),
        content GENERAL-PROCEDURE.&Message ({GENERAL-PROCEDURES}{@msgId}),
        }

In the type StartMessage the constraint following the content field tells that in a value of type StartMessage the value in the content field must come from the same object that is chosen by the msgId field.

So, the value #'StartMessage'{msgId="home",content="Any Printable String"} is legal to encode as a StartMessage value, while the value #'StartMessage'{msgId="remote", content="Some String"} is illegal since the constraint in StartMessage tells that when you have chosen a value from a specific object in the object set GENERAL-PROCEDURES in the msgId field you have to choose a value from that same object in the content field too. In this second case it should have been any INTEGER value.

StartMessage can in the content field be encoded with a value of any type that an object in the GENERAL-PROCEDURES object set has in its NEW MESSAGE field. This field refers to a type field &Message in the class. The msgId field is always encoded as a PrintableString, since the field refers to a fixed type in the class.

In practice, object sets are usually declared to be extensible so so that more objects can be added to the set later. Extensibility is indicated like this:

    GENERAL-PROCEDURES GENERAL-PROCEDURE ::= {
        object1 | object2, ...}

When decoding a type that uses an extensible set constraint, there is always the possibility that the value in the UNIQUE field is unknown (i.e. the type has been encoded with a later version of the ASN.1 specification). When that happens, the unencoded data will be returned wrapped in a tuple like this:

    {asn1_OPENTYPE,Binary}

where Binary is an Erlang binary that contains the encoded data. (If the option legacy_erlang_types has been given, just the binary will be returned.)

### 1.11 Parameterization (X.683)

Parameterization, which is defined in the standard [X.683], can be used when defining types, values, value sets, information object classes, information objects or information object sets. A part of a definition can be supplied as a parameter. For instance, if a Type is used in a definition with certain purpose, one want the type-name to express the intention. This can be done with parameterization.

When many types (or an other ASN.1 entity) only differs in some minor cases, but the structure of the types are similar, only one general type can be defined and the differences may be supplied through parameters.

One example of use of parameterization is:

    General{Type} ::= SEQUENCE
    {
         number     INTEGER,
         string     Type
    }

    T1 ::= General{PrintableString}

    T2 ::= General{BIT STRING}

An example of a value that can be encoded as type T1 is {12,"hello"}.

Note that the compiler does not generate encode/decode functions for parameterized types, but only for the instances of the parameterized types. Therefore, if a file contains the types General{}, T1 and T2 above, encode/decode functions will only be generated for T1 and T2.