The Asn1 application provides:

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.

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

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

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:

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:

, )]]>

or

, )]]>

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

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>      

It is common that asn1 modules import defined types, values and other entities from another asn1 module.

Earlier versions of the asn1 compiler required that modules that were imported from had to be compiled before the module that imported. This caused problems when asn1 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.

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:

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).      

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 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.

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:

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

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 []. 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.

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 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'.

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.

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.

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.

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.

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]
      

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 [], there is an easy-to-understand description of the usage of OBJECT IDENTIFIER.

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

      "This is the value of an Object descriptor"

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.

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>>}     

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.

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.

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.

{asn1_ExtAlt, BytesForOpenType}
        

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.

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

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

The EXTERNAL type had a slightly different associated type before 1994. [] 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.

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>>} 

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:

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
}      

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'}}}.