The Asn1 application provides:

ASN.1 (Abstract Syntax Notation 1) 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 a communication channel set up by the lower layers in the stack of communication protocols e.g. TCP/IP or encapsulated within UDP packets. This way, two different applications written in two completely different programming languages running on different computers with different internal representation of data can exchange instances of structured data types (instead of exchanging bytes or bits). This makes programming faster and easier since no code has to be written to process the transport format of the data.

To write a network application which processes ASN.1 encoded messages, it is prudent and sometimes essential to have a set of off-line development tools such as an ASN.1 compiler which can generate the encode and decode logic for the specific ASN.1 data types. It is also necessary to combine this with some general language-specific runtime support for ASN.1 encoding and decoding.

The ASN.1 compiler must be directed towards a target language or a set of closely related languages. This manual describes a compiler which is directed towards the functional language Erlang. In order to use this compiler, familiarity with the language Erlang is essential. Therefore, the runtime support for ASN.1 is also closely related to the language Erlang and consist of a number of functions, which the compiler uses. The types in ASN.1 and how to represent values of those types in Erlang are described in this manual.

The following document is structured so that the first part describes how to use ASN.1 compiler, and then there are descriptions of all the primitive and constructed ASN.1 types and their representation in Erlang,

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 very good book explaining those reference texts 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 where introduced of which some are supported while others are not. For example the ECN (Encoding Control Notation) and XML notation are still unsupported. Though, the other features of the 2002 edition are fully or partly supported as shown below:

It should also be added here that the encoding formats supported are BER, DER, PER aligned basic variant and PER unaligned basic variant.

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

First, create a file called People.asn containing the following:

People DEFINITIONS IMPLICIT TAGS ::=

BEGIN
EXPORTS Person;

Person ::= [PRIVATE 19] 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_bin]).
Erlang ASN.1 compiling "People.asn" 
--{generated,"People.asn1db"}--
--{generated,"People.hrl"}--
--{generated,"People.erl"}--
ok
2>      

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 runtime system. Now there is a user interface of encode/2 and decode/2 in the module People, which is invoked by:

,),]]>

or

,),]]>

Alternatively one can use the ,,)]]> and ,,)]]> calls. However, they are not as efficient as the previous methods since they result in an additional apply/3 call.

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 (possibly) nested list of bytes according to the BER or PER encoding-rules.

The encoder and the decoder can also be run from the shell. The following dialogue with the shell illustrates how the functions asn1rt:encode/3 and asn1rt:decode/3 are used.

2> Rockstar = {'Person',"Some Name",roving,50}.
{'Person',"Some Name",roving,50}
3> {ok,Bytes} = asn1rt:encode('People','Person',Rockstar).
{ok,[<<243>>,
     [17],
     [19,9,"Some Name"],
     [2,1,[2]],
     [2,1,"2"]]}
4> Bin = list_to_binary(Bytes).
<<243,17,19,9,83,111,109,101,32,78,97,109,101,2,1,2,2,1,50>>
5> {ok,Person} = asn1rt:decode('People','Person',Bin).
{ok,{'Person',"Some Name",roving,50}}
6>      

Notice that the result from encode is a nested list which must be turned into a binary before the call to decode. A binary is necessary as input to decode since the module was compiled with the ber_bin option The reason for returning a nested list is that it is faster to produce and the list_to_binary operation is performed automatically when the list is sent via the Erlang port mechanism.

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.

Now are referenced modules 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_bin Person.asn
erlc -bber_bin +optimize ../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.

For preferred option use see Preferred Option Use section.

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 with optimizations:

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

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

asn1ct:encode('H323-MESSAGES','SomeChoiceType',{call,"octetstring"}).
asn1ct:decode('H323-MESSAGES','SomeChoiceType',Bytes).      

Or, preferable like:

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

It may not be obvious which compile options best fit a situation. This section describes the format of the result of encode and decode. It also gives some performance statistics when using certain options. Finally there is a recommendation which option combinations should be used.

The default option is ber. It is the same backend as ber_bin except that the result of encode is transformed to a flat list. Below is a table that gives the different formats of input and output of encode and decode using the allowed combinations of coding and optimization options: (EAVF stands for how ASN1 values are represented in Erlang which is described in the ASN1 Types chapter)

Encode / decode speed comparison in one user case for the above alternatives (except DER) is showed in the table below. The DER alternatives are slower than their corresponding BER alternative.

The compile options ber, per and driver are kept for backwards compatibility and should not be used in new code. The nif implementation which replaces the linked-in driver has been shown to be about 5-15% faster.

You are strongly recommended to use the appropriate alternative of the bold typed options. The optimize and nif options does not affect the encode or decode result, just the time spent in run-time. When ber_bin and nif or per_bin and optimize is combined the C-code nif is used in chosen parts of encode / decode procedure.

A brief description of the major functions is given here. For a complete description of each function see the Asn1 Reference Manual, the asn1rt module.

The generic run-time encode and decode functions can be invoked as below:

asn1rt:encode('H323-MESSAGES','SomeChoiceType',{call,"octetstring"}).
asn1rt:decode('H323-MESSAGES','SomeChoiceType',Bytes).      

Or, preferable like:

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

The asn1 nif is enabled in two occasions: encoding of asn1 values when the asn1 spec is compiled with per_bin and optimize or decode of encoded asn1 values when the asn1 spec is compiled with ber_bin, optimize and nif. In those cases the nif will be loaded automatically at the first call to encode/decode. If one doesn't want the performance overhead of the nif being loaded at the first call it is possible to load the nif separately by loading the asn1rt_nif module.

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 as default.

The run-time encoders and decoders (in the asn1rt module) do 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.

In this version reals are not implemented. When they are, the following ASN.1 type is used:

R1 ::= REAL
      

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 four different notations available for representation of BIT STRING values in Erlang and as input to the encode functions.

Bits1Val1 = [0,1,0,1,1],
Bits1Val2 = 16#1A,
Bits1Val3 = {3,<<0:1,1:1,0:1,1:1,1:1,0:3>>}
      

Note that Bits1Val1, Bits1Val2 and Bits1Val3 denote the same value.

Bits2Val1 = [gnu,punk],
Bits2Val2 = 2#1110,
Bits2Val3 = [bar,gnu,gnome],
Bits2Val4 = [0,1,1,1]
      

The above Bits2Val2, Bits2Val3 and Bits2Val4 also all 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.

The compact notation equivalent to the empty BIT STRING is >}]]>, which in the other notations is [] or 0.

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....",      

Observe that O1Val is assigned a series of numbers between 0 and 255 i.e. octets. O2Val is assigned using the string notation.

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.

Please note that all the Character strings are supported and it is possible to use the following ASN.1 type definitions:

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

and the following 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:

In a file PrimStrings.asn1 the type BMP is defined as

BMP ::= BMPString then using BER encoding (ber_bin option)the input/output format will be:

1> {ok,Bytes1} = asn1rt:encode('PrimStrings','BMP',[{0,0,53,53},{0,0,45,56}]).
{ok,[30,4,"55-8"]}
2> asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes1)).
{ok,[{0,0,53,53},{0,0,45,56}]}
3> {ok,Bytes2} = asn1rt:encode('PrimStrings','BMP',[{0,0,53,53},{0,0,0,65}]).
{ok,[30,4,[53,53,0,65]]}
4> asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes2)).
{ok,[{0,0,53,53},65]}
5> {ok,Bytes3} = asn1rt:encode('PrimStrings','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]]}
6> asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes3)).
{ok,"BMP string"}      

The UTF8String is represented in Erlang as a list of integers, where each integer represents the unicode value of one character. When a value shall be encoded one first has to transform it to a UTF8 encoded binary, then it can be encoded by asn1. When decoding the result is a UTF8 encoded binary, which may be transformed to an integer list. The transformation functions, utf8_binary_to_list and utf8_list_to_binary, are in the asn1rt module. In the example below we assume an asn1 definition UTF ::= UTF8String in a module UTF.asn:

1> asn1ct:compile('UTF',[ber_bin]).
Erlang ASN.1 version "1.4.3.3" compiling "UTF.asn" 
Compiler Options: [ber_bin]
--{generated,"UTF.asn1db"}--
--{generated,"UTF.erl"}--
ok
2> UTF8Val1 = "hello".
"hello"
3> {ok,UTF8bin1} = asn1rt:utf8_list_to_binary(UTF8Val1).
{ok,<<104,101,108,108,111>>}
4> {ok,B}='UTF':encode('UTF',UTF8bin1).
{ok,[12,
     5,
     <<104,101,108,108,111>>]}
5> Bin = list_to_binary(B).
<<12,5,104,101,108,108,111>>
6> {ok,UTF8bin1}='UTF':decode('UTF',Bin).
{ok,<<104,101,108,108,111>>}
7> asn1rt:utf8_binary_to_list(UTF8bin1).
{ok,"hello"}           
8> UTF8Val2 = [16#00,16#100,16#ffff,16#ffffff].
[0,256,65535,16777215]
9> {ok,UTF8bin2} = asn1rt:utf8_list_to_binary(UTF8Val2).
{ok,<<0,196,128,239,191,191,248,191,191,191,191>>}
10> {ok,B2} = 'UTF':encode('UTF',UTF8bin2).
{ok,[12,
     11,
     <<0,196,128,239,191,191,248,191,191,191,191>>]}
11> Bin2 = list_to_binary(B2).
<<12,11,0,196,128,239,191,191,248,191,191,191,191>>
12> {ok,UTF8bin2} = 'UTF':decode('UTF',Bin2).
{ok,<<0,196,128,239,191,191,248,191,191,191,191>>}
13> asn1rt:utf8_binary_to_list(UTF8bin2).
{ok,[0,256,65535,16777215]}
14>       

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 i.e.

"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. In case of BER encoding it is optional to encode the value if it equals the default value. If the application uses the atom asn1_DEFAULT as value or if the value is a primitive value that equals the default value the encoding omits the bytes for this value, which is more efficient and it results in fever bytes to send to the receiving application.

For instance, if the following types exists in a file "File.asn":

Seq1 ::= SEQUENCE {
    a INTEGER DEFAULT 1,
    b Seq2 DEFAULT {aa TRUE, bb 15}
}

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

Some values and the corresponding encoding in an Erlang terminal is shown below:

1> asn1ct:compile('File').
Erlang ASN.1 version "1.3.2" compiling "File.asn1" 
Compiler Options: []
--{generated,"File.asn1db"}--
--{generated,"File.hrl"}--
--{generated,"File.erl"}--
ok
2> 'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).
{ok,["0",[0],[[],[]]]}
3> lists:flatten(["0",[0],[[],[]]]).
[48,0]
4> 'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).
{ok,["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]}
5> lists:flatten(["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]).
[48,8,161,6,128,1,255,129,1,15]
6>      

The result after command line 3, in the example above,shows that the encoder omits the encoding of default values when they are specific by asn1_DEFAULT. Line 5 shows that even primitive values that equals the default value are detected and not encoded. But the constructed value of component b in Seq1 is not recognized as the default value. Checking of default values in BER is not done in case of complex values, because it would be to expensive.

But, the DER encoding format has stronger requirements regarding default values both for SET and SEQUENCE. A more elaborate and time expensive check of default values will take place. The following is an example with the same types and values as above but with der encoding format.

1> asn1ct:compile('File',[der]).
Erlang ASN.1 version "1.3.2" compiling "File.asn1" 
Compiler Options: [der]
--{generated,"File.asn1db"}--
--{generated,"File.hrl"}--
--{generated,"File.erl"}--
ok
2> 'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).
{ok,["0",[0],[[],[]]]}
3> lists:flatten(["0",[0],[[],[]]]).
[48,0]
4> 'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).
{ok,["0",[0],[[],[]]]}
5> lists:flatten(["0",[0],[[],[]]]).
[48,0]
6> 
      

Line 5 shows that even values of constructed types is checked and if it equals the default value it will not be encoded.

The SET type is an unusual construct and normally the SEQUENCE type is more appropriate to use. Set is also inefficient compared with SEQUENCE, as the components can be in any order. Hence, it must be possible to distinguish every component in 'SET', both when encoding and decoding a value of a type defined to be a SET. The tags of all components must be different from each other in order to be easily recognizable.

A SET may be defined as:

Pdu2 ::= SET {
    a INTEGER,
    b BOOLEAN,
    c ENUMERATED {on(0),off(1)} }      

A SET is represented as an Erlang record. For each SEQUENCE and SET in an ASN.1 module an Erlang record declaration is generated. For Pdu2 above a record is defined like this:

-record('Pdu2',{a, b, c}).      

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

Values can be assigned in Erlang as demonstrated below:

V = #'Pdu2'{a=44,b=false,c=off}.      

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

The difference between SET and SEQUENCE is that the order of the components (in the BER encoded format) is undefined for SET and defined as the lexical order from the ASN.1 definition for SEQUENCE. The ASN.1 compiler for Erlang will always encode a SET in the lexical order. The decode routines can handle SET components encoded in any order but will always return the result as a record. Since all components of the SET must be distinguishable both in the encoding phase as well as the decoding phase the following type is not allowed in a module with EXPLICIT or IMPLICIT as tag-default :

Bad ::= SET {i INTEGER,
             j INTEGER }      

The ASN.1 to Erlang compiler rejects the above type. We shall not explain the concept of tag further here, we refer to [].

Encoding of a SET with components with DEFAULT values behaves similar as a SEQUENCE, see above. The DER encoding format restrictions on DEFAULT values is the same for SET as for SEQUENCE, and is supported by the compiler, see above.

Moreover, in DER the elements of a SET will be sorted. If a component is an un-tagged choice the sorting have to take place in run-time. This fact emphasizes the following recommendation if DER encoding format is used.

The concept of SET is an unusual construct and one cannot think of one single application where the set type is essential. (Imagine if someone "invented'' the shuffled array in 'C') People tend to think that 'SET' sounds nicer and more mathematical than 'SEQUENCE' and hence use it when 'SEQUENCE' would have been more appropriate. It is also most inefficient, since every correct implementation of SET must always be prepared to accept the components in any order. So, if possible use SEQUENCE instead of 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.

The CHOICE type is a space saver and is similar to the concept of a 'union' in the C-language. As with the previous SET-type, the tags of all components of a CHOICE need to be distinct. If AUTOMATIC TAGS are defined for the module (which is preferable) the tags can be omitted completely in the ASN.1 specification of a CHOICE.

Assume:

T ::= CHOICE {
        x [0] REAL,
        y [1] INTEGER,
        z [2] OBJECT IDENTIFIER }
      

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.

It is also allowed to have a CHOICE type tagged as follow:

C ::= [PRIVATE 111] CHOICE {
        C1,
        C2 }

C1 ::= CHOICE { 
         a [0] INTEGER,
         b [1] BOOLEAN }

C2 ::= CHOICE {
         c [2] INTEGER,
         d [3] OCTET STRING }      

In this case, the top type C appears to have no tags at all in its components, however, both C1 and C2 are also defined as CHOICE types and they have distinct tags among themselves. Hence, the above type C is both legal and allowed.

{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:

B ::= SEQUENCE {
        a Arr1,
        b [0] T }

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

T ::= CHOICE {
        x [0] REAL,
        y [1] INTEGER,
        z [2] OBJECT IDENTIFIER }      

The above example can be assigned like this in Erlang:

V2 = #'B'{a=[4,5,6,7,8], b={x,7.77}}.
      

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 [0] INTEGER,
       b [1] 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 [0] 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'}}}.