path: root/lib/asn1/doc/src/asn1_ug.xml

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Asn1</title>
    <prepared>Kenneth Lundin</prepared>
    <docno></docno>
    <date>1999-03-25</date>
    <rev>D</rev>
    <file>asn1_ug.xml</file>
  </header>

  <section>
    <title>Introduction</title>

    <section>
      <title>Features</title>
      <p>The Asn1 application provides:</p>
      <list type="bulleted">
        <item>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.</item>
        <item>Run-time functions used by the generated code.</item>
        <item>Support for the following encoding rules:
	<list>
	  <item>
	    Basic Encoding Rules (<em>BER</em>)
	  </item>
	  <item>
	    Distinguished Encoding Rules (<em>DER</em>), a specialized
	    form of BER that is used in security-conscious
	    applications.
	  </item>
	  <item>
	    Packed Encoding Rules (<em>PER</em>); both the aligned and
	    unaligned variant.
	  </item>
	  </list>
	</item>
      </list>
    </section>

    <section>
      <title>Overview</title>
      <p>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.</p>
    </section>

    <section>
      <title>Prerequisites</title>
      <p>It is assumed that the reader is familiar with the ASN.1
      notation as documented in the standard definition [<cite
      id="X.680"></cite>] which is the primary text. It may also be
      helpful, but not necessary, to read the standard definitions
      [<cite id="X.681"></cite>] [<cite id="X.682"></cite>] [<cite
      id="X.683"></cite>] [<cite id="X.690"></cite>] [<cite
      id="X.691"></cite>]. </p>
      <p>A good book explaining those reference texts is
        [<cite id="DUBUISSON"></cite>], which is free to download at
        <url href="http://www.oss.com/asn1/dubuisson.html">http://www.oss.com/asn1/dubuisson.html</url>.
        </p>
    </section>

    <section>
      <title>Capabilities</title>
      <p>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:</p>
      <list type="bulleted">
	<item>
	  <p>Decimal notation (e.g., "1.5e3") for REAL values. The
	  NR1, NR2 and NR3 formats as explained in ISO6093 are
	  supported.</p>
	</item>
	<item>
	  <p>The RELATIVE-OID type for relative object identifiers is
	  fully supported.</p>
	</item>
	<item>
	  <p>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.</p>
	</item>
	<item>
	  <p>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.</p>
	</item>
	<item>
	  <p>Multiple-line comments as in C, <c>/* ... */</c>, are
	  supported.</p>
	</item>
      </list>
    </section>

  </section>

  <section>
    <title>Getting Started with Asn1</title>

    <section>
      <title>A First Example</title>
      <p>The following example demonstrates the basic functionality used to run 
        the Erlang ASN.1 compiler.</p>
      <p>First, create a file called <c>People.asn</c> containing the following:</p>
      <pre>
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      </pre>
      <p>This file (<c>people.asn</c>) 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.
        </p>
      <p>The generated Erlang  files will be placed in the current directory or
        in the directory specified with the <c>{outdir,Dir}</c> option.
        The following shows how the compiler
        can be called from the  Erlang shell:</p>
      <pre>
1><input>asn1ct:compile("People", [ber]).</input>
ok
2>      </pre>

      <p>The <c>verbose</c> option can be given to have information
      about the generated files printed:</p>
      <pre>
2><input>asn1ct:compile("People", [ber,verbose]).</input>
Erlang ASN.1 compiling "People.asn" 
--{generated,"People.asn1db"}--
--{generated,"People.hrl"}--
--{generated,"People.erl"}--
ok
3>      </pre>

      <p>The ASN.1 module People is now accepted and the abstract syntax tree
        is saved in the <c>People.asn1db</c> file, the
        generated Erlang code is compiled using the Erlang compiler and
        loaded into  the Erlang runtime system. Now there is a user interface
        for <c>encode/2</c> and <c>decode/2</c> in the module People,
	which is invoked by:
                <br></br>
<c><![CDATA['People':encode(<Type name>,<Value>),]]></c>        <br></br>

        or        <br></br>
<c><![CDATA['People':decode(<Type name>,<Value>),]]></c></p>
      <p>Assume there is a network
        application which receives instances of the  ASN.1 defined
        type Person,  modifies and sends them back again:</p>
      <code type="none">
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,      </code>
      <p>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
        <c>'People':decode('Person',Bytes)</c> which   returned
        an Erlang value of the ASN.1 type <c>Person</c>.  Then an answer was
        constructed  and encoded using
        <c>'People':encode('Person',Answer)</c> which takes an
        instance of a defined ASN.1 type and transforms it to a
        binary according to the BER or PER
        encoding-rules. 
                <br></br>
The encoder and the decoder can also be run from
        the shell.</p>
      <pre>
2> <input>Rockstar = {'Person',"Some Name",roving,50}.</input>
{'Person',"Some Name",roving,50}
3> <input>{ok,Bin} = 'People':encode('Person',Rockstar).</input>
{ok,&lt;&lt;243,17,19,9,83,111,109,101,32,78,97,109,101,2,1,2,
      2,1,50&gt;&gt;}
4> <input>{ok,Person} = 'People':decode('Person',Bin).</input>
{ok,{'Person',"Some Name",roving,50}}
5>      </pre>
    </section>

    <section>
      <title>Module dependencies</title>
      <p>It is common that asn1 modules import defined types, values and
        other entities from another asn1 module.</p>
      <p>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.</p>
      <p>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.</p>
    </section>
  </section>

  <section>
    <title>The Asn1 Application User Interface</title>
    <p>The Asn1 application provides two separate user interfaces:</p>
    <list type="bulleted">
      <item>
        <p>The module <c>asn1ct</c> which provides the compile-time functions 
          (including the compiler).</p>
      </item>
      <item>
        <p>The module <c>asn1rt_nif</c> which provides the run-time functions
	for the ASN.1 decoder for the BER back-end.</p>
      </item>
    </list>
    <p>The reason for the division  of the interface into  compile-time 
      and  run-time 
      is that only run-time modules (<c>asn1rt*</c>) need to be loaded in
      an embedded system. 
      </p>

    <section>
      <title>Compile-time Functions</title>
      <p>The ASN.1 compiler can be invoked directly from the command-line
        by means of the <c>erlc</c> 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 <c>erlc</c> command can be used to invoke the
        ASN.1 compiler:</p>
      <pre>
erlc Person.asn
erlc -bper Person.asn
erlc -bber ../Example.asn
erlc -o ../asnfiles -I ../asnfiles -I /usr/local/standards/asn1 Person.asn      </pre>
      <p>The useful options for the ASN.1 compiler are:</p>
      <taglist>
        <tag><c>-b[ber | per | uper]</c></tag>
        <item>
          <p>Choice of encoding rules, if omitted <c>ber</c> is the
          default.</p>
        </item>
        <tag><c>-o OutDirectory</c></tag>
        <item>
          <p>Where to put the generated files, default is the current
            directory.</p>
        </item>
        <tag><c>-I IncludeDir</c></tag>
        <item>
          <p>Where to search for <c>.asn1db</c> files and asn1
            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.</p>
        </item>
        <tag><c>+der</c></tag>
        <item>
          <p>DER encoding rule. Only when using <c>-ber</c> option.</p>
        </item>
        <tag><c>+asn1config</c></tag>
        <item>
          <p>This functionality works together with the flags
            <c>ber</c>. It enables the
            specialized decodes, see the <seealso marker="asn1_spec">Specialized Decode</seealso> chapter.
            </p>
        </item>
        <tag><c>+undec_rest</c></tag>
        <item>
          <p>A buffer that holds a message, being decoded may
            also have some following bytes. Now it is possible to get
            those following bytes returned together with the decoded
            value. If an asn1 spec is compiled with this option a tuple
            <c>{ok,Value,Rest}</c> is returned. <c>Rest</c> may be a
            list or a binary. Earlier versions of the compiler ignored
            those following bytes.</p>
        </item>
        <tag><c>+'Any Erlc Option'</c></tag>
        <item>
          <p>You may add any option to the Erlang compiler when
            compiling the generated Erlang files. Any option
            unrecognised by the asn1 compiler will be passed to the
            Erlang compiler.</p>
        </item>
      </taglist>
      <p>For a complete description of <c>erlc</c> see Erts Reference Manual.</p>
      <p>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 
        <seealso marker="asn1ct">the Asn1 Reference Manual</seealso>, the
        <c>asn1ct</c> module.</p>
      <p>The compiler is invoked by using <c>asn1ct:compile/1</c> with
        default options, or <c>asn1ct:compile/2</c> if explicit options
        are given.
        Example:</p>
      <pre>
asn1ct:compile("H323-MESSAGES.asn1").      </pre>
      <p>which equals:</p>
      <pre>
asn1ct:compile("H323-MESSAGES.asn1",[ber]).      </pre>
      <p>If one wants PER encoding:</p>
      <pre>
asn1ct:compile("H323-MESSAGES.asn1",[per]).      </pre>
      <p>The generic encode and decode functions can be invoked like this:</p>
      <pre>
asn1ct:encode('H323-MESSAGES','SomeChoiceType',{call,"octetstring"}).
asn1ct:decode('H323-MESSAGES','SomeChoiceType',Bytes).      </pre>
      <p>Or, preferable like:</p>
      <pre>
'H323-MESSAGES':encode('SomeChoiceType',{call,"octetstring"}).
'H323-MESSAGES':decode('SomeChoiceType',Bytes).      </pre>
    </section>

    <section>
      <title>Run-time Functions</title>
      <p>When an ASN.1 specification is compiled with the <c>ber</c>
      option, the module <c>asn1rt_nif</c> module and the NIF library in
      <c>asn1/priv_dir</c> will be needed at run-time.</p>
      <p>By invoking the function <c>info/0</c> in a generated module, one
        gets information about which compiler options were used.</p>
    </section>

    <section>
      <title>Errors</title>
      <p>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.</p>
      <p>The run-time encoders and decoders execute within a catch and
      returns <c>{ok, Data}</c> or
        <c>{error, {asn1, Description}}</c> where
        <c>Description</c> is
        an Erlang term describing the error. </p>
    </section>
  </section>

  <section>
    <marker id="inlineExamples"></marker>
    <title>Multi File Compilation</title>
    <p>There are various reasons for using a multi file compilation:</p>
    <list type="bulleted">
      <item>You want to choose name for the generated module by
       any reason. Maybe you need to compile the same specs for
       different encoding/decoding standards.</item>
      <item>You want only one resulting module.</item>
    </list>
    <p>You need to specify which asn1 specs you will
      compile in a module that must have the extension
      <c>.set.asn</c>. You chose name of the module and provide the
      names of the asn1 specs. For instance, if you have the specs
      <c>File1.asn</c>, <c>File2.asn</c> and <c>File3.asn</c> your
      module <c>MyModule.set.asn</c> will look like:</p>
    <pre>
File1.asn
File2.asn
File3.asn    </pre>
    <p>If you compile with:</p>
    <code type="none">
~> erlc MyModule.set.asn    </code>
    <p>the result will be one merged module <c>MyModule.erl</c> with
      the generated code from the three asn1 specs.
      </p>
  </section>

  <section>
    <marker id="ASN1Types"></marker>
    <title>The ASN.1 Types</title>
    <p>This section describes the ASN.1 types including their
      functionality, purpose and how values are assigned in Erlang.
      </p>
    <p>ASN.1 has both primitive and constructed types:</p>
    <p></p>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Primitive types</em></cell>
        <cell align="left" valign="middle"><em>Constructed types</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#BOOLEAN">BOOLEAN</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#SEQUENCE">SEQUENCE</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#INTEGER">INTEGER</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#SET">SET</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#REAL">REAL</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#CHOICE">CHOICE</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#NULL">NULL</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#SOF">SET OF and SEQUENCE OF</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#ENUMERATED">ENUMERATED</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#ANY">ANY</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#BIT STRING">BIT STRING</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#ANY">ANY DEFINED BY</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#OCTET STRING">OCTET STRING</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#NegotiationTypes">EXTERNAL</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#Character Strings">Character Strings</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#NegotiationTypes">EMBEDDED PDV</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#OBJECT IDENTIFIER">OBJECT IDENTIFIER</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#NegotiationTypes">CHARACTER STRING</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#Object Descriptor">Object Descriptor</seealso></cell>
        <cell align="left" valign="middle"></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#The TIME types">The TIME types</seealso></cell>
        <cell align="left" valign="middle"></cell>
      </row>
      <tcaption>The supported ASN.1 types</tcaption>
    </table>
    <marker id="TypeNameValue"></marker>
    <note>
      <p>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.</p>
    </note>
    <pre>
Operational ::= BOOLEAN --ASN.1 definition    </pre>
    <p>In Erlang code it may look like:</p>
    <pre>
Val = true,
{ok,Bytes}=asn1rt:encode(MyModule,'Operational',Val),    </pre>
    <p>Below follows a description of how
      values of each type can be represented in Erlang. 
      </p>

    <section>
      <marker id="BOOLEAN"></marker>
      <title>BOOLEAN</title>
      <p>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.          <br></br>

        In ASN.1 it is possible to have:</p>
      <pre>
Operational ::= BOOLEAN
      </pre>
      <p>Assigning a value to the type Operational in Erlang is possible by 
        using the following Erlang code:</p>
      <code type="erl">
Myvar1 = true,
      </code>
      <p>Thus, in Erlang the atoms <c>true</c> and <c>false</c> are used
        to encode a boolean value.</p>
    </section>

    <section>
      <marker id="INTEGER"></marker>
      <title>INTEGER</title>
      <p>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.</p>
      <p>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  [<cite id="X.680"></cite>]. A variety 
        of syntaxes are allowed when defining a  type as an integer:</p>
      <pre>
T1 ::= INTEGER
T2 ::= INTEGER (-2..7)
T3 ::= INTEGER (0..MAX)
T4 ::= INTEGER (0&lt;..MAX)
T5 ::= INTEGER (MIN&lt;..-99)
T6 ::= INTEGER {red(0),blue(1),white(2)}
      </pre>
      <p>The Erlang representation of an ASN.1 INTEGER is an integer or
        an atom if a so called <c>Named Number List</c> (see T6 above)
        is specified.</p>
      <p>Below is an example of Erlang code which assigns values for the
        above types: </p>
      <pre>
T1value = 0,
T2value = 6,
T6value1 = blue,
T6value2 = 0,
T6value3 = white
      </pre>
      <p>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.</p>
      <p>The decoder will return an atom if the value corresponds to a
        symbol in the Named Number List.</p>
    </section>

    <section>
      <marker id="REAL"></marker>
      <title>REAL</title>
      <p>In this version reals are not implemented. When they are, 
        the following
        ASN.1 type is used:</p>
      <pre>
R1 ::= REAL
      </pre>
      <p>Can be assigned a value in Erlang as:</p>
      <pre>
R1value1 = 2.14,
R1value2 = {256,10,-2},
      </pre>
      <p>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 
        <c>{Mantissa,Base,Exponent}</c> i.e. Mantissa * Base^Exponent.</p>
    </section>

    <section>
      <marker id="NULL"></marker>
      <title>NULL</title>
      <p>Null is suitable in cases where supply and recognition of a value 
        is important but the actual value is not.</p>
      <pre>
Notype ::= NULL
      </pre>
      <p>The NULL type can be assigned in Erlang:</p>
      <pre>
N1 = 'NULL',
      </pre>
      <p>The actual value is the quoted atom 'NULL'.</p>
    </section>

    <section>
      <marker id="ENUMERATED"></marker>
      <title>ENUMERATED</title>
      <p>The enumerated type can be used, when the value we wish to 
        describe, may only take one of a set of predefined values.</p>
      <pre>
DaysOfTheWeek ::= ENUMERATED { 
    sunday(1),monday(2),tuesday(3),
    wednesday(4),thursday(5),friday(6),saturday(7) }
      </pre>
      <p>For example to assign a weekday value in Erlang use the same atom
        as in the <c>Enumerations</c> of the type definition:</p>
      <pre>
Day1 = saturday,
      </pre>
      <p>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.</p>
    </section>

    <section>
      <marker id="BIT STRING"></marker>
      <title>BIT STRING</title>
      <p>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.        <br></br>

        In ASN.1 BIT STRING definitions may look like:
        </p>
      <pre>
Bits1 ::= BIT STRING
Bits2 ::= BIT STRING {foo(0),bar(1),gnu(2),gnome(3),punk(14)}
      </pre>
      <p>There are two notations available for representation of
        BIT STRING values in Erlang and as input to the encode functions.</p>
      <list type="ordered">
	<item>A bitstring. By default, a BIT STRING with no
	  symbolic names will be decoded to an Erlang bitstring.</item>
        <item>A list of atoms corresponding to atoms in the <c>NamedBitList</c> 
         in the BIT STRING definition. A BIT STRING with symbolic
	 names will always be decoded to this format.</item>
      </list>
      <p>Example:</p>
      <pre>
Bits1Val1 = &lt;&lt;0:1,1:1,0:1,1:1,1:1&gt;&gt;,
Bits2Val1 = [gnu,punk],
Bits2Val2 = &lt;&lt;2#1110:4&gt;&gt;,
Bits2Val3 = [bar,gnu,gnome],
      </pre>
      <p><c>Bits2Val2</c> and <c>Bits2Val3</c> above denote the same value.</p>
      <p><c>Bits2Val1</c> is assigned symbolic values. The assignment means 
        that the bits corresponding to <c>gnu</c> and <c>punk</c> 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.</p>
      <p>BIT STRINGS may also be sub-typed with, for example, a SIZE
        specification:</p>
      <pre>
Bits3 ::= BIT STRING (SIZE(0..31))      </pre>
      <p>This means that no bit higher than 31 can ever be set.</p>

      <section>
	<title>Deprecated representations for BIT STRING</title>
	<p>In addition to the representations described above, the
	following deprecated representations are available if the
	specification has been compiled with the
	<c>legacy_erlang_types</c> option:</p>
	<list type="ordered">
	  <item>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
	  <em>legacy_bit_string</em> option has been given.
	  </item>
	  <item>As <c>{Unused,Binary}</c> where <c>Unused</c> denotes
	  how many trailing zero-bits 0 to 7 that are unused in the
	  least significant byte in <c>Binary</c>. This format is
	  accepted as input to the encode functions, and a <c>BIT
	  STRING</c> will be decoded to this format if
	  <em>compact_bit_string</em> has been given.
	  </item>
	  <item>A hexadecimal number (or an integer). This format
	  should be avoided, since it is easy to misinterpret a BIT
	  STRING value in this format.
	  </item>
	</list>
      </section>
    </section>

    <section>
      <marker id="OCTET STRING"></marker>
      <title>OCTET STRING</title>
      <p>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.</p>
      <p>It is possible to have the following ASN.1 type definitions:</p>
      <pre>
O1 ::= OCTET STRING
O2 ::= OCTET STRING (SIZE(28))      </pre>
      <p>With the following example assignments in Erlang:</p>
      <pre>
O1Val = &lt;&lt;17,13,19,20,0,0,255,254&gt;&gt;,
O2Val = &lt;&lt;"must be exactly 28 chars...."&gt;&gt;,</pre>
       <p>By default, an OCTET STRING is always represented as
       an Erlang binary. If the specification has been compiled with
       the <c>legacy_erlang_types</c> option, the encode functions
       will accept both lists and binaries, and the decode functions
       will decode an OCTET STRING to a list.</p>
    </section>

    <section>
      <marker id="Character Strings"></marker>
      <title>Character Strings</title>
      <p>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.</p>
      <p>However, when using for instance the IA5String (which closely
        resembles ASCII)  the byte 65 (in decimal
        notation)  <em>means</em> the character 'A'.
        </p>
      <p>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. 
        </p>
      <p>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.
        </p>
      <p>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.
        </p>
      <p>Please note that <em>all</em> the Character strings are 
        supported and it is possible to use the following ASN.1 type 
        definitions:</p>
      <pre>
Digs ::= NumericString (SIZE(1..3))
TextFile ::= IA5String (SIZE(0..64000))      </pre>
      <p>and the following Erlang assignments:</p>
      <pre>
DigsVal1 = "456",
DigsVal2 = "123",
TextFileVal1 = "abc...xyz...",
TextFileVal2 = [88,76,55,44,99,121 .......... a lot of characters here ....]        </pre>
      <p>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:</p>
      <p>In a file <c>PrimStrings.asn1</c> the type <c>BMP</c> is defined as
                <br></br>
<c>BMP ::= BMPString</c> then using BER encoding (<c>ber</c>
        option)the input/output format will be:</p>
      <pre>
1> <input>{ok,Bytes1} = asn1rt:encode('PrimStrings','BMP',[{0,0,53,53},{0,0,45,56}]).</input>
{ok,[30,4,"55-8"]}
2> <input>asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes1)).</input>
{ok,[{0,0,53,53},{0,0,45,56}]}
3> <input>{ok,Bytes2} = asn1rt:encode('PrimStrings','BMP',[{0,0,53,53},{0,0,0,65}]).</input>
{ok,[30,4,[53,53,0,65]]}
4> <input>asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes2)).</input>
{ok,[{0,0,53,53},65]}
5> <input>{ok,Bytes3} = asn1rt:encode('PrimStrings','BMP',"BMP string").</input>
{ok,[30,20,[0,66,0,77,0,80,0,32,0,115,0,116,0,114,0,105,0,110,0,103]]}
6> <input>asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes3)).</input>
{ok,"BMP string"}      </pre>
      <p>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, <c>utf8_binary_to_list</c> and
        <c>utf8_list_to_binary</c>, are in the <c>asn1rt</c> module. In
        the example below we assume an asn1 definition <c>UTF ::= UTF8String</c> in a module <c>UTF.asn</c>:</p>
      <pre>
1> <input>asn1ct:compile('UTF',[ber]).</input>
Erlang ASN.1 version "1.4.3.3" compiling "UTF.asn" 
Compiler Options: [ber]
--{generated,"UTF.asn1db"}--
--{generated,"UTF.erl"}--
ok
2> <input>UTF8Val1 = "hello".</input>
"hello"
3> <input>{ok,UTF8bin1} = asn1rt:utf8_list_to_binary(UTF8Val1).</input>
{ok,&lt;&lt;104,101,108,108,111&gt;&gt;}
4> <input>{ok,B}='UTF':encode('UTF',UTF8bin1).</input>
{ok,[12,
     5,
     &lt;&lt;104,101,108,108,111&gt;&gt;]}
5> <input>Bin = list_to_binary(B).</input>
&lt;&lt;12,5,104,101,108,108,111&gt;&gt;
6> <input>{ok,UTF8bin1}='UTF':decode('UTF',Bin).</input>
{ok,&lt;&lt;104,101,108,108,111&gt;&gt;}
7> <input>asn1rt:utf8_binary_to_list(UTF8bin1).</input>
{ok,"hello"}           
8> <input>UTF8Val2 = [16#00,16#100,16#ffff,16#ffffff].</input>
[0,256,65535,16777215]
9> <input>{ok,UTF8bin2} = asn1rt:utf8_list_to_binary(UTF8Val2).</input>
{ok,&lt;&lt;0,196,128,239,191,191,248,191,191,191,191&gt;&gt;}
10> <input>{ok,B2} = 'UTF':encode('UTF',UTF8bin2).</input>
{ok,[12,
     11,
     &lt;&lt;0,196,128,239,191,191,248,191,191,191,191&gt;&gt;]}
11> <input>Bin2 = list_to_binary(B2).</input>
&lt;&lt;12,11,0,196,128,239,191,191,248,191,191,191,191&gt;&gt;
12> <input>{ok,UTF8bin2} = 'UTF':decode('UTF',Bin2).</input>
{ok,&lt;&lt;0,196,128,239,191,191,248,191,191,191,191&gt;&gt;}
13> <input>asn1rt:utf8_binary_to_list(UTF8bin2).</input>
{ok,[0,256,65535,16777215]}
14>       </pre>
    </section>

    <section>
      <marker id="OBJECT IDENTIFIER"></marker>
      <title>OBJECT IDENTIFIER</title>
      <p>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:</p>
      <pre>
Oid ::= OBJECT IDENTIFIER
      </pre>
      <p>Therefore, the example below is a valid Erlang instance of the 
        type 'Oid'.</p>
      <pre>
OidVal1 = {1,2,55},
      </pre>
      <p>The OBJECT IDENTIFIER value is simply a tuple with the
        consecutive values which must be integers. 
        </p>
      <p>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.
        </p>
      <p>The OBJECT IDENTIFIER is a very important type and it is
        widely used within different standards to uniquely identify
        various objects. In [<cite id="DUBUISSON"></cite>], there is an 
        easy-to-understand description of the usage of 
        OBJECT IDENTIFIER.</p>
      <p></p>
    </section>

    <section>
      <marker id="Object Descriptor"></marker>
      <title>Object Descriptor</title>
      <p>Values of this type can be assigned a value as an ordinary string i.e.        <br></br>

        "This is the value of an Object descriptor"</p>
    </section>

    <section>
      <marker id="The TIME types"></marker>
      <title>The TIME Types</title>
      <p>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".</p>
      <p>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.</p>
    </section>

    <section>
      <marker id="SEQUENCE"></marker>
      <title>SEQUENCE</title>
      <p>The structured types of ASN.1 are constructed from other types 
        in a manner similar to the concepts of array and struct in  C.
                <br></br>
 A SEQUENCE in ASN.1 is
        comparable with a struct in C and a record in Erlang. 
        A SEQUENCE may be defined as:</p>
      <pre>
Pdu ::= SEQUENCE {
   a INTEGER,
   b REAL,
   c OBJECT IDENTIFIER,
   d NULL }      </pre>
      <p>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 <c>SET</c> in an ASN.1 module an Erlang 
        record declaration is generated. For <c>Pdu</c> above, a record 
        like this is defined:</p>
      <pre>
-record('Pdu',{a, b, c, d}).      </pre>
      <p>The record declarations for a module <c>M</c> are placed in a 
        separate <c>M.hrl</c> file.</p>
      <p>Values can be assigned in Erlang as shown below:</p>
      <pre>
MyPdu = #'Pdu'{a=22,b=77.99,c={0,1,2,3,4},d='NULL'}.      </pre>
      <p>The decode functions will return a record as result when decoding 
        a <c>SEQUENCE</c> or a <c>SET</c>.
                <marker id="DEFAULT"></marker>
</p>
      <p>A <c>SEQUENCE</c> and a <c>SET</c> may contain a component with a
        <c>DEFAULT</c> 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.</p>
      <p>For instance, if the following types exists in a file "File.asn":</p>
      <pre>
Seq1 ::= SEQUENCE {
    a INTEGER DEFAULT 1,
    b Seq2 DEFAULT {aa TRUE, bb 15}
}

Seq2 ::= SEQUENCE {
    aa BOOLEAN,
    bb INTEGER
}
      </pre>
      <p>Some values and the corresponding encoding in an Erlang terminal
        is shown below:</p>
      <pre>
1> <input>asn1ct:compile('File').</input>
Erlang ASN.1 version "1.3.2" compiling "File.asn1" 
Compiler Options: []
--{generated,"File.asn1db"}--
--{generated,"File.hrl"}--
--{generated,"File.erl"}--
ok
2> <input>'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input>
{ok,["0",[0],[[],[]]]}
3> <input>lists:flatten(["0",[0],[[],[]]]).</input>
[48,0]
4> <input>'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).</input>
{ok,["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]}
5> <input>lists:flatten(["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]).</input>
[48,8,161,6,128,1,255,129,1,15]
6>      </pre>
      <p>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 <c>b</c> in <c>Seq1</c> is not recognized as the
        default value. Checking of default values in <c>BER</c> is not done
        in case of complex values, because it would be to expensive.
                <marker id="DEFAULT DER"></marker>
</p>
      <p>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.</p>
      <pre>
1> <input>asn1ct:compile('File',[der]).</input>
Erlang ASN.1 version "1.3.2" compiling "File.asn1" 
Compiler Options: [der]
--{generated,"File.asn1db"}--
--{generated,"File.hrl"}--
--{generated,"File.erl"}--
ok
2> <input>'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input>
{ok,["0",[0],[[],[]]]}
3> <input>lists:flatten(["0",[0],[[],[]]]).</input>
[48,0]
4> <input>'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).</input>
{ok,["0",[0],[[],[]]]}
5> <input>lists:flatten(["0",[0],[[],[]]]).</input>
[48,0]
6> 
      </pre>
      <p>Line 5 shows that even values of constructed types is checked and if 
        it equals the default value it will not be encoded.</p>
    </section>

    <section>
      <marker id="SET"></marker>
      <title>SET</title>
      <p>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.</p>
      <p>A SET may be defined as:</p>
      <pre>
Pdu2 ::= SET {
    a INTEGER,
    b BOOLEAN,
    c ENUMERATED {on(0),off(1)} }      </pre>
      <p>A SET is represented as an Erlang record.
        For each SEQUENCE and <c>SET</c> in
        an ASN.1 module an Erlang record declaration is generated. For
        <c>Pdu2</c> above a record is defined like this:</p>
      <pre>
-record('Pdu2',{a, b, c}).      </pre>
      <p>The record declarations for a module <c>M</c> are placed in a 
        separate <c>M.hrl</c> file.</p>
      <p>Values can be assigned in Erlang as demonstrated below:</p>
      <pre>
V = #'Pdu2'{a=44,b=false,c=off}.      </pre>
      <p>The decode functions will return a record as result when decoding 
        a SET.
        </p>
      <p>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 :</p>
      <p></p>
      <pre>
Bad ::= SET {i INTEGER,
             j INTEGER }      </pre>
      <p>The  ASN.1 to Erlang compiler  rejects the above type. We
        shall not explain the concept of tag further here, we refer to
        [<cite id="X.680"></cite>].
        </p>
      <p>Encoding of a SET with components with DEFAULT values behaves
        similar as a SEQUENCE, <seealso marker="#DEFAULT">see above</seealso>. The DER encoding format restrictions on DEFAULT 
        values is the same for SET as for SEQUENCE, and is supported by 
        the compiler, <seealso marker="#DEFAULT DER">see above</seealso>.</p>
      <p>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.</p>
      <p>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.</p>
    </section>

    <section>
      <title>Notes about Extend-ability for SEQUENCE and SET</title>
      <p>When a SEQUENCE or SET contains an extension marker and 
        extension components like this:</p>
      <pre>
SExt ::= SEQUENCE {
           a INTEGER,
           ...,
           b BOOLEAN }
      </pre>
      <p>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 <c>b</c>. Thus, incoming messages that will be decoded
        may have more or fever components than this one.  
        </p>
      <p>The component <c>b</c> 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.
        </p>
      <p>During decoding the <c>b</c> field of the record will get the decoded 
        value of the <c>b</c> 
        component if present and otherwise the value <c>asn1_NOVALUE</c>.</p>
    </section>

    <section>
      <marker id="CHOICE"></marker>
      <title>CHOICE</title>
      <p>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.
        </p>
      <p>Assume:</p>
      <pre>
T ::= CHOICE {
        x [0] REAL,
        y [1] INTEGER,
        z [2] OBJECT IDENTIFIER }
      </pre>
      <p>It is then possible to assign values:</p>
      <pre>
TVal1 = {y,17},
TVal2 = {z,{0,1,2}},
      </pre>
      <p>A CHOICE value is  always represented as the tuple
        <c>{ChoiceAlternative, Val}</c> where <c>ChoiceAlternative</c>
        is an atom denoting the selected choice
        alternative.
        </p>
      <p>It is also allowed to have a CHOICE type tagged as follow:</p>
      <p></p>
      <pre>
C ::= [PRIVATE 111] CHOICE {
        C1,
        C2 }

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

C2 ::= CHOICE {
         c [2] INTEGER,
         d [3] OCTET STRING }      </pre>
      <p>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.
        </p>

      <section>
        <title>Extendable CHOICE</title>
        <p>When a CHOICE contains an extension marker and the decoder detects
          an unknown alternative of the CHOICE the value is represented as:</p>
        <pre>
{asn1_ExtAlt, BytesForOpenType}
        </pre>
        <p>Where <c>BytesForOpenType</c> is a list of bytes constituting the 
          encoding of the "unknown" CHOICE alternative. </p>
      </section>
    </section>

    <section>
      <marker id="SOF"></marker>
      <title>SET OF and SEQUENCE OF</title>
      <p>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:</p>
      <pre>
Arr1 ::= SET SIZE (5) OF INTEGER (4..9) 
Arr2 ::= SEQUENCE OF OCTET STRING      </pre>
      <p>We may have the following in Erlang:</p>
      <pre>
Arr1Val = [4,5,6,7,8],
Arr2Val = ["abc",[14,34,54],"Octets"],      </pre>
      <p>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.</p>
      <p>However, in case of a value of the type <c>SET OF</c>, 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 <c>SEQUENCE OF</c> instead of <c>SET OF</c> if it is possible.</p>
    </section>

    <section>
      <marker id="ANY"></marker>
      <title>ANY and ANY DEFINED BY</title>
      <p>The types <c>ANY</c> and <c>ANY DEFINED BY</c> 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.</p>
      <p>A value of this type is encoded as an <c>open type</c>.</p>
      <p>Instead of <c>ANY</c>/<c>ANY DEFINED BY</c> one should use
        <c>information object class</c>, <c>table constraints</c> and
        <c>parameterization</c>. In particular the construct
        <c>TYPE-IDENTIFIER.@Type</c> accomplish the same as the
        deprecated <c>ANY</c>.</p>
      <p>See also <seealso marker="#Information Object">Information object</seealso></p>
    </section>

    <section>
      <marker id="NegotiationTypes"></marker>
      <title>EXTERNAL, EMBEDDED PDV and CHARACTER STRING</title>
      <p>These types are used in presentation layer negotiation. They are
        encoded according to their associated type, see [<cite id="X.680"></cite>].</p>
      <p>The <c>EXTERNAL</c> type had a slightly different associated type
        before 1994. [<cite id="X.691"></cite>] 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 
        <c>EXTERNAL</c> type values of either format for encoding. Decoded
        values are always returned on the newer format.</p>
    </section>

    <section>
      <title>Embedded Named Types</title>
      <p>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
        <c>#'T'{'C'=Value}</c>. 
        Where <c>Value</c> may be a value of yet another type T2.</p>
      <p>For example:</p>
      <pre>
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 }      </pre>
      <p>The above example can be assigned like this in Erlang:</p>
      <pre>
V2 = #'B'{a=[4,5,6,7,8], b={x,7.77}}.
      </pre>
    </section>
  </section>

  <section>
    <title>Naming of Records in .hrl Files</title>
    <p>When an asn1 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.</p>
    <p>Though there are some special cases of this functionality that
      are presented below.</p>

    <section>
      <title>Embedded Structured Types</title>
      <p>It is also possible in ASN.1 to have components that are  themselves
        structured types.
        For example, it is possible to have:</p>
      <pre>
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      </pre>
      <p>The following records are generated because of the type <c>Emb</c>:</p>
      <pre>
-record('Emb,{a, b, c}).
-record('Emb_b',{a, b = asn1_DEFAULT}). % the embedded SET type
      </pre>
      <p>Values of the <c>Emb</c> type can be assigned like this:</p>
      <code type="none">
V = #'Emb'{a=["qqqq",[1,2,255]], 
           b = #'Emb_b'{a=99}, 
           c ={b,"Can you see this"}}.
      </code>
      <p>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.</p>
      <p>For example:</p>
      <pre>
Seq ::= SEQUENCE{
    a CHOICE{
        b SEQUENCE {
           c  INTEGER
        }
    }
}      </pre>
      <p>will result in the following record:</p>
      <pre>
-record('Seq_a_b',{c}).      </pre>
      <p>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:</p>
      <pre>
Seq ::= SEQUENCE {
    a SEQUENCE OF SEQUENCE {
           b
               }
    c SET OF SEQUENCE {
           d
               }
}      </pre>
      <p>This results in the records:</p>
      <pre>
-record('Seq_a_SEQOF'{b}).
-record('Seq_c_SETOF'{d}).      </pre>
      <p>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
        <c>'Seq_b'</c> is generated in the .hrl file and used to hold
        values.</p>
      <pre>
Seq ::= SEQUENCE {
    b PType{INTEGER}
}

PType{T} ::= SEQUENCE{
    id T
}      </pre>
    </section>

    <section>
      <title>Recursive Types</title>
      <p>Types may refer to themselves. Suppose:</p>
      <pre>
Rec ::= CHOICE {
     nothing [0] NULL,
     something SEQUENCE {
          a INTEGER,
          b OCTET STRING,
          c Rec }}      </pre>
      <p>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:</p>
      <pre>
V = {something,#'Rec_something'{a = 77, 
                                b = "some octets here", 
                                c = {nothing,'NULL'}}}.      </pre>
    </section>
  </section>

  <section>
    <title>ASN.1 Values</title>
    <p>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:</p>
    <pre>
TT ::= SEQUENCE {
   a INTEGER,
   b SET OF OCTET STRING }

tt TT ::= {a 77,b {"kalle","kula"}}    </pre>
    <p>The value defined here could be used in several ways. 
      Firstly, it could be used as the value in some DEFAULT component:</p>
    <pre>
SS ::= SET {
    s [0] OBJECT IDENTIFIER,
    val TT DEFAULT tt }    </pre>
    <p>It could also be used from inside an Erlang program. If the above ASN.1
      code was defined in ASN.1 module <c>Values</c>, then the ASN.1 value
      <c>tt</c> can be reached from Erlang as
      a function call to <c>'Values':tt()</c> as in the example below.</p>
    <pre>
1> <input>Val = 'Values':tt().</input>
{'TT',77,["kalle","kula"]}
2> <input>{ok,Bytes} = 'Values':encode('TT',Val).</input>
{ok,&lt;&lt;48,18,128,1,77,161,13,4,5,107,97,108,108,101,4,4,
      107,117,108,97&gt;&gt;}
4> <input>'Values':decode('TT',Bytes).</input>
{ok,{'TT',77,["kalle","kula"]}}
5>
    </pre>
    <p>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.</p>
    <p>Furthermore, there is a macro generated for each value in the .hrl 
      file. So, the defined value <c>tt</c> can also be extracted by 
      <c>?tt</c> in application code.</p>
  </section>

  <section>
    <title>Macros</title>
    <p>MACRO is not supported as the the type is no longer part of the 
      ASN.1 standard.</p>
  </section>

  <section>
    <marker id="Information Object"></marker>
    <title>ASN.1 Information Objects (X.681)</title>
    <p>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 [<cite id="X.681"></cite>]. In the following only a brief
      explanation is given. </p>
    <p>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:</p>
    <pre>
GENERAL-PROCEDURE ::= CLASS {
      &amp;Message,
      &amp;Reply               OPTIONAL,
      &amp;Error               OPTIONAL,
      &amp;id          PrintableString UNIQUE
}
WITH SYNTAX {
      NEW MESSAGE     &amp;Message
      [REPLY           &amp;Reply]
      [ERROR           &amp;Error]
      ADDRESS          &amp;id
}    </pre>
    <p>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.</p>
    <p>The object <c>object1</c> is an instance of the CLASS 
      GENERAL-PROCEDURE and has one type field and one fixed type value 
      field. The object <c>object2</c> also has an OPTIONAL field ERROR,
      which is a type field.</p>
    <pre>
object1 GENERAL-PROCEDURE ::= {
    NEW MESSAGE      PrintableString
    ADDRESS          "home"
}

object2 GENERAL-PROCEDURE ::= {
    NEW MESSAGE INTEGER
    ERROR INTEGER
    ADDRESS "remote"
}    </pre>
    <p>The field ADDRESS is a UNIQUE field. Objects in an object set must
      have unique values in their UNIQUE field, as in GENERAL-PROCEDURES: </p>
    <pre>
GENERAL-PROCEDURES GENERAL-PROCEDURE ::= {
    object1 | object2}    </pre>
    <p>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 [<cite id="X.682"></cite>] in ASN.1 types, as in: </p>
    <pre>
StartMessage  ::= SEQUENCE {
    msgId  GENERAL-PROCEDURE.&amp;id  ({GENERAL-PROCEDURES}),
    content GENERAL-PROCEDURE.&amp;Message ({GENERAL-PROCEDURES}{@msgId}),
    }    </pre>
    <p>In the type <c>StartMessage</c> the constraint following the
      <c>content</c> field tells that in a value of type
      <c>StartMessage</c> the value in the <c>content</c> field must
      come from the same object that is chosen by the <c>msgId</c>
      field.</p>
    <p>So, the value <c>#'StartMessage'{msgId="home",content="Any Printable String"}</c> is legal to encode as a StartMessage
      value, while the value <c>#'StartMessage'{msgId="remote", content="Some String"}</c> 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.</p>
    <p><c>StartMessage</c> can in the <c>content</c> field be
      encoded with a value of any type that an object in the
      <c>GENERAL-PROCEDURES</c> object set has in its <c>NEW MESSAGE</c> field. This field refers to a type field
      <c>&amp;Message</c> in the class. The <c>msgId</c> field is always
      encoded as a PrintableString, since the field refers to a fixed type
      in the class.</p>
    <p>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:</p>
    <pre>
GENERAL-PROCEDURES GENERAL-PROCEDURE ::= {
    object1 | object2, ...}    </pre>
     <p>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:</p>

     <pre>
{asn1_OPENTYPE,Binary}</pre>
     <p>where <c>Binary</c> is an Erlang binary that contains the encoded
     data. (If the option <c>legacy_erlang_types</c> has been given,
     just the binary will be returned.)</p>
  </section>

  <section>
    <title>Parameterization (X.683)</title>
    <p>Parameterization, which is defined in the standard [<cite id="X.683"></cite>], 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.</p>
    <p>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. </p>
    <p>One example of use of parameterization is:</p>
    <pre>
General{Type} ::= SEQUENCE
{
     number     INTEGER,
     string     Type
}
      
T1 ::= General{PrintableString}

T2 ::= General{BIT STRING}
    </pre>
    <p>An example of a value that can be encoded as type T1 is {12,"hello"}.</p>
    <p>Observe that the compiler not generates encode/decode functions for
      parameterized types, only for the instances of the parameterized 
      types. So, if a file contains the types General{}, T1 and T2 above,
      encode/decode functions will only be generated for T1 and T2.
      </p>
  </section>

  <section>
    <title>Tags</title>
    <p>Every built-in ASN.1 type, except CHOICE and ANY have a universal tag.
      This is a unique number that clearly identifies the type.      <br></br>
 
      It is essential for all users of ASN.1  to
      understand  all the details about tags.</p>
    <p>Tags are implicitly encoded in the BER encoding as shown below, but 
      are hardly not accounted for in the PER encoding. In PER tags are
      used for instance to sort the components of a SET.</p>
    <p>There are four different types of tags.</p>
    <taglist>
      <tag><em>universal</em></tag>
      <item>
        <p>For types whose meaning is the same in all 
          applications. Such as integers, sequences and so on; that is, all the built in
          types.</p>
      </item>
      <tag><em>application</em></tag>
      <item>
        <p>For application specific types for example, the types in
          X.400 Message handling service have this sort of tag.</p>
      </item>
      <tag><em>private</em></tag>
      <item>
        <p>For your own private types.</p>
      </item>
      <tag><em>context</em></tag>
      <item>
        <p>This is used to distinguish otherwise indistinguishable
          types in a specific context. For example, if we have two
          components of a 
          CHOICE type that are both <c>INTEGER</c> values, there is no
          way for the decoder to 
          decipher which component was actually chosen, since both
          components will be
          tagged as <c>INTEGER</c>. When this or similar situations occur,  
          one or both of the components should be given a  context specific
          to resolve the ambiguity.</p>
      </item>
    </taglist>
    <p>The tag in the case of the 'Apdu' type [PRIVATE 1] is encoded to a
      sequence of bytes  making it possible for  a
      decoder to  look at the (initial) bytes that arrive and determine
      whether the rest of the bytes  must be of the type associated
      with that particular sequence of bytes.  This means that each
      tag must be uniquely associated with <em>only</em> one  ASN.1
      type.
      </p>
    <p>Immediately following the tag is a sequence of bytes
      informing the  decoder of the length of the instance. This is
      sometimes referred to  as TLV  (Tag length value) encoding.
      Hence, the structure of a BER encoded series of bytes is as shown in the table below.</p>
    <p></p>
    <table>
      <row>
        <cell align="left" valign="middle">Tag</cell>
        <cell align="left" valign="middle">Len</cell>
        <cell align="left" valign="middle">Value</cell>
      </row>
      <tcaption>Structure of a BER encoded series of bytes</tcaption>
    </table>
  </section>

  <section>
    <title>Encoding Rules</title>
    <p>When the first recommendation on ASN.1 was released 1988 it was
      accompanied with the Basic Encoding Rules, BER, as the only 
      alternative for encoding.
      BER is a somewhat verbose protocol. It adopts a so-called TLV (type, 
      length, value) approach to encoding in which every element of the 
      encoding carries some type information, some length information and 
      then the value of that element. Where the element is itself 
      structured, then the Value part of the element is itself a series of 
      embedded TLV components, to whatever depth is necessary. In summary, 
      BER is not a compact encoding but is relatively fast and easy to 
      produce.</p>
    <p>The DER (Distinguished Encoding Rule) encoding format was included in
      the standard in 1994. It is a specialized form of BER, which gives 
      the encoder the option to encode some entities differently. For
      instance, is the value for TRUE any octet with any bit set to one. But,
      DER does not leave any such choices. The value for TRUE in the DER 
      case is encoded as the octet <c>11111111</c>. So, the same value 
      encoded by two different DER encoders must result in the same bit 
      stream.</p>
    <p>A more compact encoding is achieved with the Packed Encoding
    Rules PER which was introduced together with the revised
    recommendation in 1994. PER takes a rather different approach from
    that taken by BER. The first difference is that the tag part in
    the TLV is omitted from the encodings, and any tags in the
    notation are not encoded. The potential ambiguities are resolved
    as follows:</p>
    <list type="bulleted">
      <item>
        <p>A CHOICE is encoded by first encoding a choice index which
          identifies the chosen
          alternative by its position in the notation.</p>
      </item>
      <item>
        <p>The elements of a SEQUENCE are transmitted in textual
        order. OPTIONAL or DEFAULT elements are preceded by a bit map
        to identify which elements are present. After sorting the
        elements of a SET in the "canonical tag order" as defined in
        X.680 8.6 they are treated as a SEQUENCE regarding OPTIONAL
        and DEFAULT elements. A SET is transferred in the sorted
        order.</p>
      </item>
    </list>
    <p>A second difference is that PER takes full account of the sub-typing
      information in that the encoded bytes are affected by the constraints.
      The BER encoded bytes are unaffected by the constraints.
      PER uses the sub-typing information to for example omit length fields 
      whenever possible. </p>
    <p>The run-time functions, sometimes take the constraints into account
      both for BER and PER. For instance are SIZE constrained strings checked.</p>
    <p>There are two variants of PER, <em>aligned</em> and <em>unaligned</em>.
      In summary, PER results in compact encodings which require much more
      computation to produce than  BER.
      </p>
  </section>
</chapter>