/usr/share/doc/python-pyasn1/pyasn1-tutorial.html

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<title>
PyASN1 programmer's manual
</title>
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</head>
<body>
<center>
<table width=60%>
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<h3>
PyASN1 programmer's manual
</h3>

<p align=right>
<i>written by <a href=mailto:ilya@glas.net>Ilya Etingof</a>, 2011-2012</i>
</p>

<p>
Free and open-source pyasn1 library makes it easier for programmers and
network engineers to develop, debug and experiment with ASN.1-based protocols
using Python programming language as a tool.
</p>

<p>
Abstract Syntax Notation One 
(<a href=http://en.wikipedia.org/wiki/Abstract_Syntax_Notation_1x>ASN.1</a>)
is a set of 
<a href=http://www.itu.int/ITU-T/studygroups/com17/languages/X.680-X.693-0207w.zip>
ITU standards</a> concered with provisioning instrumentation for developing
data exchange protocols in a robust, clear and interoperabable way for
various IT systems and applications. Most of the efforts are targeting the
following areas:
<ul>
<li>Data structures: the standard introduces a collection of basic data types
(similar to integers, bits, strings, arrays and records in a programming
language) that can be used for defining complex, possibly nested data
structures representing domain-specific data units.
<li>Serialization protocols: domain-specific data units expressed in ASN.1
types could be converted into a series of octets for storage or transmission
over the wire and then recovered back into their structured form on the
receiving end. This process is immune to various hardware and software
related dependencies.
<li>Data description language: could be used to describe particular set of
domain-specific data structures and their relationships. Such a description
could be passed to an ASN.1 compiler for automated generation of program
code that represents ASN.1 data structures in language-native environment
and handles data serialization issues.
</ul>
</p>

<p>
This tutorial and algorithms, implemented by pyasn1 library, are
largely based on the information read in the book
<a href="http://www.oss.com/asn1/dubuisson.html">
ASN.1 - Communication between heterogeneous systems</a>
by Olivier Dubuisson. Another relevant resource is
<a href=ftp://ftp.rsasecurity.com/pub/pkcs/ascii/layman.asc>
A Layman's Guide to a Subset of ASN.1, BER, and DER</a> by Burton S. Kaliski.
It's advised to refer to these books for more in-depth knowledge on the
subject of ASN.1.
</p>

<p>
As of this writing, pyasn1 library implements most of standard ASN.1 data
structures in a rather detailed and feature-rich manner. Another highly
important capability of the library is its data serialization facilities.
The last component of the standard - ASN.1 compiler is planned for
implementation in the future.
</p>

</p>
The pyasn1 library was designed to follow the pre-1995 ASN.1 specification
(also known as X.208). Later, post 1995, revision (X.680) introduced
significant changes most of which have not yet been supported by pyasn1.
</p>

<h3>
Table of contents
</h3>

<p>
<ul>
<li><a href="#1">1. Data model for ASN.1 types</a>
<li><a href="#1.1">1.1 Scalar types</a>
<li><a href="#1.1.1">1.1.1 Boolean type</a>
<li><a href="#1.1.2">1.1.2 Null type</a>
<li><a href="#1.1.3">1.1.3 Integer type</a>
<li><a href="#1.1.4">1.1.4 Enumerated type</a>
<li><a href="#1.1.5">1.1.5 Real type</a>
<li><a href="#1.1.6">1.1.6 Bit string type</a>
<li><a href="#1.1.7">1.1.7 OctetString type</a>
<li><a href="#1.1.8">1.1.8 ObjectIdentifier type</a>
<li><a href="#1.1.9">1.1.9 Character string types</a>
<li><a href="#1.1.10">1.1.10 Useful types</a>
<li><a href="#1.2">1.2 Tagging</a>
<li><a href="#1.3">1.3 Constructed types</a>
<li><a href="#1.3.1">1.3.1 Sequence and Set types</a>
<li><a href="#1.3.2">1.3.2 SequenceOf and SetOf types</a>
<li><a href="#1.3.3">1.3.3 Choice type</a>
<li><a href="#1.3.4">1.3.4 Any type</a>
<li><a href="#1.4">1.4 Subtype constraints</a>
<li><a href="#1.4.1">1.4.1 Single value constraint</a>
<li><a href="#1.4.2">1.4.2 Value range constraint</a>
<li><a href="#1.4.3">1.4.3 Size constraint</a>
<li><a href="#1.4.4">1.4.4 Alphabet constraint</a>
<li><a href="#1.4.5">1.4.5 Constraint combinations</a>
<li><a href="#1.5">1.5 Types relationships</a>
<li><a href="#2">2. Codecs</a>
<li><a href="#2.1">2.1 Encoders</a>
<li><a href="#2.2">2.2 Decoders</a>
<li><a href="#2.2.1">2.2.1 Decoding untagged types</a>
<li><a href="#2.2.2">2.2.2 Ignoring unknown types</a>
<li><a href="#3">3. Feedback and getting help</a>
</ul>


<a name="1"></a>
<h3>
1. Data model for ASN.1 types
</h3>

<p>
All ASN.1 types could be categorized into two groups: scalar (also called
simple or primitive) and constructed. The first group is populated by
well-known types like Integer or String. Members of constructed group
hold other types (simple or constructed) as their inner components, thus
they are semantically close to a programming language records or lists.
</p>

<p>
In pyasn1, all ASN.1 types and values are implemented as Python objects.
The same pyasn1 object can represent either ASN.1 type and/or value
depending of the presense of value initializer on object instantiation.
We will further refer to these as <i>pyasn1 type object</i> versus <i>pyasn1
value object</i>.
</p>

<p>
Primitive ASN.1 types are implemented as immutable scalar objects. There values
could be used just like corresponding native Python values (integers,
strings/bytes etc) and freely mixed with them in expressions.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> asn1IntegerValue = univ.Integer(12)
>>> asn1IntegerValue - 2
10
>>> univ.OctetString('abc') == 'abc'
True   # Python 2
>>> univ.OctetString(b'abc') == b'abc'
True   # Python 3
</pre>
</td></tr></table>

<p>
It would be an error to perform an operation on a pyasn1 type object
as it holds no value to deal with:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> asn1IntegerType = univ.Integer()
>>> asn1IntegerType - 2
...
pyasn1.error.PyAsn1Error: No value for __coerce__()
</pre>
</td></tr></table>

<a name="1.1"></a>
<h4>
1.1 Scalar types
</h4>

<p>
In the sub-sections that follow we will explain pyasn1 mapping to those
primitive ASN.1 types. Both, ASN.1 notation and corresponding pyasn1
syntax will be given in each case.
</p>

<a name="1.1.1"></a>
<h4>
1.1.1 Boolean type
</h4>

<p>
This is the simplest type those values could be either True or False.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
;; type specification
FunFactorPresent ::= BOOLEAN

;; values declaration and assignment
pythonFunFactor FunFactorPresent ::= TRUE
cobolFunFactor FunFactorPresent :: FALSE
</pre>
</td></tr></table>

<p>
And here's pyasn1 version of it:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> class FunFactorPresent(univ.Boolean): pass
... 
>>> pythonFunFactor = FunFactorPresent(True)
>>> cobolFunFactor = FunFactorPresent(False)
>>> pythonFunFactor
FunFactorPresent('True(1)')
>>> cobolFunFactor
FunFactorPresent('False(0)')
>>> pythonFunFactor == cobolFunFactor
False
>>>
</pre>
</td></tr></table>

<a name="1.1.2"></a>
<h4>
1.1.2 Null type
</h4>

<p>
The NULL type is sometimes used to express the absense of any information.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
;; type specification
Vote ::= CHOICE {
  agreed BOOLEAN,
  skip NULL
}
</td></tr></table>

;; value declaration and assignment
myVote Vote ::= skip:NULL
</pre>

<p>
We will explain the CHOICE type later in this paper, meanwhile the NULL
type:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> skip = univ.Null()
>>> skip
Null('')
>>>
</pre>
</td></tr></table>

<a name="1.1.3"></a>
<h4>
1.1.3 Integer type
</h4>

<p>
ASN.1 defines the values of Integer type as negative or positive of whatever
length. This definition plays nicely with Python as the latter places no
limit on Integers. However, some ASN.1 implementations may impose certain
limits of integer value ranges. Keep that in mind when designing new
data structures.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
;; values specification
age-of-universe INTEGER ::= 13750000000
mean-martian-surface-temperature INTEGER ::= -63
</pre>
</td></tr></table>

<p>
A rather strigntforward mapping into pyasn1:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> ageOfUniverse = univ.Integer(13750000000)
>>> ageOfUniverse
Integer(13750000000)
>>>
>>> meanMartianSurfaceTemperature = univ.Integer(-63)
>>> meanMartianSurfaceTemperature
Integer(-63)
>>>
</pre>
</td></tr></table>

<p>
ASN.1 allows to assign human-friendly names to particular values of
an INTEGER type.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
Temperature ::= INTEGER {
  freezing(0),
  boiling(100) 
}
</pre>
</td></tr></table>

<p>
The Temperature type expressed in pyasn1:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedval
>>> class Temperature(univ.Integer):
...   namedValues = namedval.NamedValues(('freezing', 0), ('boiling', 100))
...
>>> t = Temperature(0)
>>> t
Temperature('freezing(0)')
>>> t + 1
Temperature(1)
>>> t + 100
Temperature('boiling(100)')
>>> t = Temperature('boiling')
>>> t
Temperature('boiling(100)')
>>> Temperature('boiling') / 2
Temperature(50)
>>> -1 < Temperature('freezing')
True
>>> 47 > Temperature('boiling')
False
>>>
</pre>
</td></tr></table>

<p>
These values labels have no effect on Integer type operations, any value
still could be assigned to a type (information on value constraints will
follow further in this paper).
</p>

<a name="1.1.4"></a>
<h4>
1.1.4 Enumerated type
</h4>

<p>
ASN.1 Enumerated type differs from an Integer type in a number of ways.
Most important is that its instance can only hold a value that belongs
to a set of values specified on type declaration.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
error-status ::= ENUMERATED {
  no-error(0),
  authentication-error(10),
  authorization-error(20),
  general-failure(51)
}
</pre>
</td></tr></table>

<p>
When constructing Enumerated type we will use two pyasn1 features: values
labels (as mentioned above) and value constraint (will be described in
more details later on).
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedval, constraint
>>> class ErrorStatus(univ.Enumerated):
...   namedValues = namedval.NamedValues(
...        ('no-error', 0),
...        ('authentication-error', 10),
...        ('authorization-error', 20),
...        ('general-failure', 51)
...   )
...   subtypeSpec = univ.Enumerated.subtypeSpec + \
...                    constraint.SingleValueConstraint(0, 10, 20, 51)
...
>>> errorStatus = univ.ErrorStatus('no-error')
>>> errorStatus
ErrorStatus('no-error(0)')
>>> errorStatus == univ.ErrorStatus('general-failure')
False
>>> univ.ErrorStatus('non-existing-state')
Traceback (most recent call last):
...
pyasn1.error.PyAsn1Error: Can't coerce non-existing-state into integer
>>>
</pre>
</td></tr></table>

<p>
Particular integer values associated with Enumerated value states
have no meaning. They should not be used as such or in any kind of
math operation. Those integer values are only used by codecs to
transfer state from one entity to another.
</p>

<a name="1.1.5"></a>
<h4>
1.1.5 Real type
</h4>

<p>
Values of the Real type are a three-component tuple of mantissa, base and 
exponent. All three are integers.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
pi ::= REAL { mantissa 314159, base 10, exponent -5 }
</pre>
</td></tr></table>

<p>
Corresponding pyasn1 objects can be initialized with either a three-component
tuple or a Python float. Infinite values could be expressed in a way, 
compatible with Python float type.

</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> pi = univ.Real((314159, 10, -5))
>>> pi
Real((314159, 10,-5))
>>> float(pi)
3.14159
>>> pi == univ.Real(3.14159)
True
>>> univ.Real('inf')
Real('inf')
>>> univ.Real('-inf') == float('-inf')
True
>>>
</pre>
</td></tr></table>

<p>
If a Real object is initialized from a Python float or yielded by a math
operation, the base is set to decimal 10 (what affects encoding).
</p>

<a name="1.1.6"></a>
<h4>
1.1.6 Bit string type
</h4>

<p>
ASN.1 BIT STRING type holds opaque binary data of an arbitrarily length.
A BIT STRING value could be initialized by either a binary (base 2) or 
hex (base 16) value.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
public-key BIT STRING ::= '1010111011110001010110101101101
                           1011000101010000010110101100010
                           0110101010000111101010111111110'B

signature  BIT STRING ::= 'AF01330CD932093392100B39FF00DE0'H
</pre>
</td></tr></table>

<p>
The pyasn1 BitString objects can initialize from native ASN.1 notation
(base 2 or base 16 strings) or from a Python tuple of binary components.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> publicKey = univ.BitString(
...          "'1010111011110001010110101101101"
...          "1011000101010000010110101100010"
...          "0110101010000111101010111111110'B"
)
>>> publicKey
BitString("'10101110111100010101101011011011011000101010000010110101100010\
0110101010000111101010111111110'B")
>>> signature = univ.BitString(
...          "'AF01330CD932093392100B39FF00DE0'H"
... )
>>> signature
BitString("'101011110000000100110011000011001101100100110010000010010011001\
1100100100001000000001011001110011111111100000000110111100000'B")
>>> fingerprint = univ.BitString(
...          (1, 0, 1, 1 ,0, 1, 1, 1, 0, 1, 0, 1)
... )
>>> fingerprint
BitString("'101101110101'B")
>>>
</pre>
</td></tr></table>

<p>
Another BIT STRING initialization method supported by ASN.1 notation
is to specify only 1-th bits along with their human-friendly label
and bit offset relative to the beginning of the bit string. With this 
method, all not explicitly mentioned bits are doomed to be zeros.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
bit-mask  BIT STRING ::= {
  read-flag(0),
  write-flag(2),
  run-flag(4)
}
</pre>
</td></tr></table>

<p>
To express this in pyasn1, we will employ the named values feature (as with
Enumeration type).
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedval
>>> class BitMask(univ.BitString):
...   namedValues = namedval.NamedValues(
...        ('read-flag', 0),
...        ('write-flag', 2),
...        ('run-flag', 4)
... )
>>> bitMask = BitMask('read-flag,run-flag')
>>> bitMask
BitMask("'10001'B")
>>> tuple(bitMask)
(1, 0, 0, 0, 1)
>>> bitMask[4]
1
>>>
</pre>
</td></tr></table>

<p>
The BitString objects mimic the properties of Python tuple type in part
of immutable sequence object protocol support.
</p>

<a name="1.1.7"></a>
<h4>
1.1.7 OctetString type
</h4>

<p>
The OCTET STRING type is a confusing subject. According to ASN.1
specification, this type is similar to BIT STRING, the major difference
is that the former operates in 8-bit chunks of data. What is important
to note, is that OCTET STRING was NOT designed to handle text strings - the
standard provides many other types specialized for text content. For that
reason, ASN.1 forbids to initialize OCTET STRING values with "quoted text
strings", only binary or hex initializers, similar to BIT STRING ones,
are allowed.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
thumbnail OCTET STRING ::= '1000010111101110101111000000111011'B
thumbnail OCTET STRING ::= 'FA9823C43E43510DE3422'H
</pre>
</td></tr></table>

<p>
However, ASN.1 users (e.g. protocols designers) seem to ignore the original
purpose of the OCTET STRING type - they used it for handling all kinds of
data, including text strings.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
welcome-message OCTET STRING ::= "Welcome to ASN.1 wilderness!"
</pre>
</td></tr></table>

<p>
In pyasn1, we have taken a liberal approach and allowed both BIT STRING
style and quoted text initializers for the OctetString objects. To avoid
possible collisions, quoted text is the default initialization syntax.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> thumbnail = univ.OctetString(
...    binValue='1000010111101110101111000000111011'
... )
>>> thumbnail
OctetString(hexValue='85eebcec0')
>>> thumbnail = univ.OctetString(
...    hexValue='FA9823C43E43510DE3422'
... )
>>> thumbnail
OctetString(hexValue='fa9823c43e4351de34220')
>>>
</pre>
</td></tr></table>

<p>
Most frequent usage of the OctetString class is to instantiate it with
a text string.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> welcomeMessage = univ.OctetString('Welcome to ASN.1 wilderness!')
>>> welcomeMessage
OctetString(b'Welcome to ASN.1 wilderness!')
>>> print('%s' % welcomeMessage)
Welcome to ASN.1 wilderness!
>>> welcomeMessage[11:16]
OctetString(b'ASN.1')
>>> 
</pre>
</td></tr></table>

<p>
OctetString objects support the immutable sequence object protocol.
In other words, they behave like Python 3 bytes (or Python 2 strings).
</p>

<p>
When running pyasn1 on Python 3, it's better to use the bytes objects for
OctetString instantiation, as it's more reliable and efficient.
</p>

<p>
Additionally, OctetString's can also be instantiated with a sequence of
8-bit integers (ASCII codes).
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> univ.OctetString((77, 101, 101, 103, 111))
OctetString(b'Meego')
</pre>
</td></tr></table>

<p>
It is sometimes convenient to express OctetString instances as 8-bit
characters (Python 3 bytes or Python 2 strings) or 8-bit integers.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> octetString = univ.OctetString('ABCDEF')
>>> octetString.asNumbers()
(65, 66, 67, 68, 69, 70)
>>> octetString.asOctets()
b'ABCDEF'
</pre>
</td></tr></table>

<a name="1.1.8"></a>
<h4>
1.1.8 ObjectIdentifier type
</h4>

<p>
Values of the OBJECT IDENTIFIER type are sequences of integers that could
be used to identify virtually anything in the world. Various ASN.1-based
protocols employ OBJECT IDENTIFIERs for their own identification needs.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
internet-id OBJECT IDENTIFIER ::= {
  iso(1) identified-organization(3) dod(6) internet(1)
}
</pre>
</td></tr></table>

<p>
One of the natural ways to map OBJECT IDENTIFIER type into a Python
one is to use Python tuples of integers. So this approach is taken by
pyasn1.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> internetId = univ.ObjectIdentifier((1, 3, 6, 1))
>>> internetId
ObjectIdentifier('1.3.6.1')
>>> internetId[2]
6
>>> internetId[1:3]
ObjectIdentifier('3.6')
</pre>
</td></tr></table>

<p>
A more human-friendly "dotted" notation is also supported.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> univ.ObjectIdentifier('1.3.6.1')
ObjectIdentifier('1.3.6.1')
</pre>
</td></tr></table>

<p>
Symbolic names of the arcs of object identifier, sometimes present in
ASN.1 specifications, are not preserved and used in pyasn1 objects.
</p>

<p>
The ObjectIdentifier objects mimic the properties of Python tuple type in
part of immutable sequence object protocol support.
</p>

<a name="1.1.9"></a>
<h4>
1.1.9 Character string types
</h4>

<p>
ASN.1 standard introduces a diverse set of text-specific types. All of them
were designed to handle various types of characters. Some of these types seem
be obsolete nowdays, as their target technologies are gone. Another issue
to be aware of is that raw OCTET STRING type is sometimes used in practice
by ASN.1 users instead of specialized character string types, despite
explicit prohibition imposed by ASN.1 specification.
</p>

<p>
The two types are specific to ASN.1 are NumericString and PrintableString.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
welcome-message ::= PrintableString {
  "Welcome to ASN.1 text types"
}

dial-pad-numbers ::= NumericString {
  "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
}
</pre>
</td></tr></table>

<p>
Their pyasn1 implementations are:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import char
>>> '%s' % char.PrintableString("Welcome to ASN.1 text types")
'Welcome to ASN.1 text types'
>>> dialPadNumbers = char.NumericString(
      "0" "1" "2" "3" "4" "5" "6" "7" "8" "9"
)
>>> dialPadNumbers
NumericString(b'0123456789')
>>>
</pre>
</td></tr></table>

<p>
The following types came to ASN.1 from ISO standards on character sets.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import char
>>> char.VisibleString("abc")
VisibleString(b'abc')
>>> char.IA5String('abc')
IA5String(b'abc')
>>> char.TeletexString('abc')
TeletexString(b'abc')
>>> char.VideotexString('abc')
VideotexString(b'abc')
>>> char.GraphicString('abc')
GraphicString(b'abc')
>>> char.GeneralString('abc')
GeneralString(b'abc')
>>>
</pre>
</td></tr></table>

<p>
The last three types are relatively recent addition to the family of
character string types: UniversalString, BMPString, UTF8String.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import char
>>> char.UniversalString("abc")
UniversalString(b'abc')
>>> char.BMPString('abc')
BMPString(b'abc')
>>> char.UTF8String('abc')
UTF8String(b'abc')
>>> utf8String = char.UTF8String('У попа была собака')
>>> utf8String
UTF8String(b'\xd0\xa3 \xd0\xbf\xd0\xbe\xd0\xbf\xd0\xb0 \xd0\xb1\xd1\x8b\xd0\xbb\xd0\xb0 \
\xd1\x81\xd0\xbe\xd0\xb1\xd0\xb0\xd0\xba\xd0\xb0')
>>> print(utf8String)
У попа была собака
>>>
</pre>
</td></tr></table>

<p>
In pyasn1, all character type objects behave like Python strings. None of
them is currently constrained in terms of valid alphabet so it's up to
the data source to keep an eye on data validation for these types.
</p>

<a name="1.1.10"></a>
<h4>
1.1.10 Useful types
</h4>

<p>
There are three so-called useful types defined in the standard:
ObjectDescriptor, GeneralizedTime, UTCTime. They all are subtypes
of GraphicString or VisibleString types therefore useful types are
character string types.
</p>

<p>
It's advised by the ASN.1 standard to have an instance of ObjectDescriptor
type holding a human-readable description of corresponding instance of
OBJECT IDENTIFIER type. There are no formal linkage between these instances
and provision for ObjectDescriptor uniqueness in the standard.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import useful
>>> descrBER = useful.ObjectDescriptor(
      "Basic encoding of a single ASN.1 type"
)
>>> 
</pre>
</td></tr></table>

<p>
GeneralizedTime and UTCTime types are designed to hold a human-readable
timestamp in a universal and unambiguous form. The former provides
more flexibility in notation while the latter is more strict but has
Y2K issues.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
;; Mar 8 2010 12:00:00 MSK
moscow-time GeneralizedTime ::= "20110308120000.0"
;; Mar 8 2010 12:00:00 UTC
utc-time GeneralizedTime ::= "201103081200Z"
;; Mar 8 1999 12:00:00 UTC
utc-time UTCTime ::= "9803081200Z"
</pre>
</td></tr></table>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import useful
>>> moscowTime = useful.GeneralizedTime("20110308120000.0")
>>> utcTime = useful.UTCTime("9803081200Z")
>>> 
</pre>
</td></tr></table>

<p>
Despite their intended use, these types possess no special, time-related,
handling in pyasn1. They are just printable strings.
</p>

<a name="1.2"></a>
<h4>
1.2 Tagging
</h4>

<p>
In order to continue with the Constructed ASN.1 types, we will first have
to introduce the concept of tagging (and its pyasn1 implementation), as
some of the Constructed types rely upon the tagging feature.
</p>

<p>
When a value is coming into an ASN.1-based system (received from a network
or read from some storage), the receiving entity has to determine the
type of the value to interpret and verify it accordingly.
</p>

<p>
Historically, the first data serialization protocol introduced in
ASN.1 was BER (Basic Encoding Rules). According to BER, any serialized
value is packed into a triplet of (Type, Length, Value) where Type is a 
code that identifies the value (which is called <i>tag</i> in ASN.1),
length is the number of bytes occupied by the value in its serialized form
and value is ASN.1 value in a form suitable for serial transmission or storage.
</p>

<p>
For that reason almost every ASN.1 type has a tag (which is actually a
BER type) associated with it by default.
</p>

<p>
An ASN.1 tag could be viewed as a tuple of three numbers:
(Class, Format, Number). While Number identifies a tag, Class component 
is used to create scopes for Numbers. Four scopes are currently defined:
UNIVERSAL, context-specific, APPLICATION and PRIVATE. The Format component
is actually a one-bit flag - zero for tags associated with scalar types,
and one for constructed types (will be discussed later on).
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
MyIntegerType ::= [12] INTEGER
MyOctetString ::= [APPLICATION 0] OCTET STRING
</pre>
</td></tr></table>

<p>
In pyasn1, tags are implemented as immutable, tuple-like objects:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import tag
>>> myTag = tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 10)
>>> myTag
Tag(tagClass=128, tagFormat=0, tagId=10)
>>> tuple(myTag)
(128, 0, 10)
>>> myTag[2]
10
>>> myTag == tag.Tag(tag.tagClassApplication, tag.tagFormatSimple, 10)
False
>>>
</pre>
</td></tr></table>

<p>
Default tag, associated with any ASN.1 type, could be extended or replaced
to make new type distinguishable from its ancestor. The standard provides
two modes of tag mangling - IMPLICIT and EXPLICIT.
</p>

<p>
EXPLICIT mode works by appending new tag to the existing ones thus creating
an ordered set of tags. This set will be considered as a whole for type
identification and encoding purposes. Important property of EXPLICIT tagging
mode is that it preserves base type information in encoding what makes it
possible to completely recover type information from encoding.
</p>

<p>
When tagging in IMPLICIT mode, the outermost existing tag is dropped and
replaced with a new one.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
MyIntegerType ::= [12] IMPLICIT INTEGER
MyOctetString ::= [APPLICATION 0] EXPLICIT OCTET STRING
</pre>
</td></tr></table>

<p>
To model both modes of tagging, a specialized container TagSet object (holding
zero, one or more Tag objects) is used in pyasn1.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import tag
>>> tagSet = tag.TagSet(
...   tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 10), # base tag
...   tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 10)  # effective tag
... )
>>> tagSet
TagSet(Tag(tagClass=128, tagFormat=0, tagId=10))
>>> tagSet.getBaseTag()
Tag(tagClass=128, tagFormat=0, tagId=10)
>>> tagSet = tagSet.tagExplicitly(
...    tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 20)
... )
>>> tagSet
TagSet(Tag(tagClass=128, tagFormat=0, tagId=10), 
       Tag(tagClass=128, tagFormat=32, tagId=20))
>>> tagSet = tagSet.tagExplicitly(
...    tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 30)
... )
>>> tagSet
TagSet(Tag(tagClass=128, tagFormat=0, tagId=10), 
       Tag(tagClass=128, tagFormat=32, tagId=20), 
       Tag(tagClass=128, tagFormat=32, tagId=30))
>>> tagSet = tagSet.tagImplicitly(
...    tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 40)
... )
>>> tagSet
TagSet(Tag(tagClass=128, tagFormat=0, tagId=10),
       Tag(tagClass=128, tagFormat=32, tagId=20),
       Tag(tagClass=128, tagFormat=32, tagId=40))
>>> 
</pre>
</td></tr></table>

<p>
As a side note: the "base tag" concept (accessible through the getBaseTag()
method) is specific to pyasn1 -- the base tag is used to identify the original
ASN.1 type of an object in question. Base tag is never occurs in encoding
and is mostly used internally by pyasn1 for choosing type-specific data 
processing algorithms. The "effective tag" is the one that always appears in
encoding and is used on tagSets comparation.
</p>

<p>
Any two TagSet objects could be compared to see if one is a derivative
of the other. Figuring this out is also useful in cases when a type-specific
data processing algorithms are to be chosen.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import tag
>>> tagSet1 = tag.TagSet(
...   tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 10) # base tag
...   tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 10) # effective tag
... )
>>> tagSet2 = tagSet1.tagExplicitly(
...    tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 20)
... )
>>> tagSet1.isSuperTagSetOf(tagSet2)
True
>>> tagSet2.isSuperTagSetOf(tagSet1)
False
>>> 
</pre>
</td></tr></table>

<p>
We will complete this discussion on tagging with a real-world example. The
following ASN.1 tagged type:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
MyIntegerType ::= [12] EXPLICIT INTEGER
</pre>
</td></tr></table>

<p>
could be expressed in pyasn1 like this:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, tag
>>> class MyIntegerType(univ.Integer):
...   tagSet = univ.Integer.tagSet.tagExplicitly(
...        tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 12)
...        )
>>> myInteger = MyIntegerType(12345)
>>> myInteger.getTagSet()
TagSet(Tag(tagClass=0, tagFormat=0, tagId=2), 
       Tag(tagClass=128, tagFormat=32, tagId=12))
>>>
</pre>
</td></tr></table>

<p>
Referring to the above code, the tagSet class attribute is a property of any
pyasn1 type object that assigns default tagSet to a pyasn1 value object. This
default tagSet specification can be ignored and effectively replaced by some
other tagSet value passed on object instantiation.
</p>

<p>
It's important to understand that the tag set property of pyasn1 type/value
object can never be modifed in place. In other words, a pyasn1 type/value
object can never change its tags. The only way is to create a new pyasn1
type/value object and associate different tag set with it.
</p>


<a name="1.3"></a>
<h4>
1.3 Constructed types
</h4>

<p>
Besides scalar types, ASN.1 specifies so-called constructed ones - these
are capable of holding one or more values of other types, both scalar
and constructed.
</p>

<p>
In pyasn1 implementation, constructed ASN.1 types behave like 
Python sequences, and also support additional component addressing methods,
specific to particular constructed type.
</p>

<a name="1.3.1"></a>
<h4>
1.3.1 Sequence and Set types
</h4>

<p>
The Sequence and Set types have many similar properties:
</p>
<ul>
<li>they can hold any number of inner components of different types
<li>every component has a human-friendly identifier
<li>any component can have a default value
<li>some components can be absent.
</ul>

<p>
However, Sequence type guarantees the ordering of Sequence value components
to match their declaration order. By contrast, components of the
Set type can be ordered to best suite application's needs.
<p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
Record ::= SEQUENCE {
  id        INTEGER,
  room  [0] INTEGER OPTIONAL,
  house [1] INTEGER DEFAULT 0
}
</pre>
</td></tr></table>

<p>
Up to this moment, the only method we used for creating new pyasn1 types
is Python sub-classing. With this method, a new, named Python class is created
what mimics type derivation in ASN.1 grammar. However, ASN.1 also allows for
defining anonymous subtypes (room and house components in the example above).
To support anonymous subtyping in pyasn1, a cloning operation on an existing
pyasn1 type object can be invoked what creates a new instance of original
object with possibly modified properties.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedtype, tag
>>> class Record(univ.Sequence):
...   componentType = namedtype.NamedTypes(
...     namedtype.NamedType('id', univ.Integer()),
...     namedtype.OptionalNamedType(
...       'room',
...       univ.Integer().subtype(implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 0))
...     ),
...     namedtype.DefaultedNamedType(
...       'house', 
...       univ.Integer(0).subtype(implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 1))
...     )
...   )
>>>
</pre>
</td></tr></table>

<p>
All pyasn1 constructed type classes have a class attribute <b>componentType</b>
that represent default type specification. Its value is a NamedTypes object.
</p>

<p>
The NamedTypes class instance holds a sequence of NameType, OptionalNamedType
or DefaultedNamedType objects which, in turn, refer to pyasn1 type objects that
represent inner SEQUENCE components specification.
</p>

<p>
Finally, invocation of a subtype() method of pyasn1 type objects in the code
above returns an implicitly tagged copy of original object.
</p>

<p>
Once a SEQUENCE or SET type is decleared with pyasn1, it can be instantiated
and initialized (continuing the above code):
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> record = Record()
>>> record.setComponentByName('id', 123)
>>> print(record.prettyPrint())
Record:
 id=123
>>> 
>>> record.setComponentByPosition(1, 321)
>>> print(record.prettyPrint())
Record:
 id=123
 room=321
>>>
>>> record.setDefaultComponents()
>>> print(record.prettyPrint())
Record:
 id=123
 room=321
 house=0
</pre>
</td></tr></table>

<p>
Inner components of pyasn1 Sequence/Set objects could be accessed using the
following methods:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> record.getComponentByName('id')
Integer(123)
>>> record.getComponentByPosition(1)
Integer(321)
>>> record[2]
Integer(0)
>>> for idx in range(len(record)):
...   print(record.getNameByPosition(idx), record.getComponentByPosition(idx))
id 123
room 321
house 0
>>>
</pre>
</td></tr></table>

<p>
The Set type share all the properties of Sequence type, and additionally
support by-tag component addressing (as all Set components have distinct
types).
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedtype, tag
>>> class Gamer(univ.Set):
...   componentType = namedtype.NamedTypes(
...     namedtype.NamedType('score', univ.Integer()),
...     namedtype.NamedType('player', univ.OctetString()),
...     namedtype.NamedType('id', univ.ObjectIdentifier())
...   )
>>> gamer = Gamer()
>>> gamer.setComponentByType(univ.Integer().getTagSet(), 121343)
>>> gamer.setComponentByType(univ.OctetString().getTagSet(), 'Pascal')
>>> gamer.setComponentByType(univ.ObjectIdentifier().getTagSet(), (1,3,7,2))
>>> print(gamer.prettyPrint())
Gamer:
 score=121343
 player=b'Pascal'
 id=1.3.7.2
>>>
</pre>
</td></tr></table>

<a name="1.3.2"></a>
<h4>
1.3.2 SequenceOf and SetOf types
</h4>

<p>
Both, SequenceOf and SetOf types resemble an unlimited size list of components.
All the components must be of the same type.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
Progression ::= SEQUENCE OF INTEGER

arithmeticProgression Progression ::= { 1, 3, 5, 7 }
</pre>
</td></tr></table>

<p>
SequenceOf and SetOf types are expressed by the very similar pyasn1 type 
objects. Their components can only be addressed by position and they
both have a property of automatic resize.
</p>

<p>
To specify inner component type, the <b>componentType</b> class attribute
should refer to another pyasn1 type object.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> class Progression(univ.SequenceOf):
...   componentType = univ.Integer()
>>> arithmeticProgression = Progression()
>>> arithmeticProgression.setComponentByPosition(1, 111)
>>> print(arithmeticProgression.prettyPrint())
Progression:
-empty- 111
>>> arithmeticProgression.setComponentByPosition(0, 100)
>>> print(arithmeticProgression.prettyPrint())
Progression:
100 111
>>>
>>> for idx in range(len(arithmeticProgression)):
...    arithmeticProgression.getComponentByPosition(idx)
Integer(100)
Integer(111)
>>>
</pre>
</td></tr></table>

<p>
Any scalar or constructed pyasn1 type object can serve as an inner component.
Missing components are prohibited in SequenceOf/SetOf value objects.
</p>

<a name="1.3.3"></a>
<h4>
1.3.3 Choice type
</h4>

<p>
Values of ASN.1 CHOICE type can contain only a single value of a type from a 
list of possible alternatives. Alternatives must be ASN.1 types with
distinct tags for the whole structure to remain unambiguous. Unlike most
other types, CHOICE is an untagged one, e.g. it has no base tag of its own.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
CodeOrMessage ::= CHOICE {
  code    INTEGER,
  message OCTET STRING
}
</pre>
</td></tr></table>

<p>
In pyasn1 implementation, Choice object behaves like Set but accepts only
a single inner component at a time. It also offers a few additional methods
specific to its behaviour.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedtype
>>> class CodeOrMessage(univ.Choice):
...   componentType = namedtype.NamedTypes(
...     namedtype.NamedType('code', univ.Integer()),
...     namedtype.NamedType('message', univ.OctetString())
...   )
>>>
>>> codeOrMessage = CodeOrMessage()
>>> print(codeOrMessage.prettyPrint())
CodeOrMessage:
>>> codeOrMessage.setComponentByName('code', 123)
>>> print(codeOrMessage.prettyPrint())
CodeOrMessage:
 code=123
>>> codeOrMessage.setComponentByName('message', 'my string value')
>>> print(codeOrMessage.prettyPrint())
CodeOrMessage:
 message=b'my string value'
>>>
</pre>
</td></tr></table>

<p>
Since there could be only a single inner component value in the pyasn1 Choice
value object, either of the following methods could be used for fetching it
(continuing previous code):
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> codeOrMessage.getName()
'message'
>>> codeOrMessage.getComponent()
OctetString(b'my string value')
>>>
</pre>
</td></tr></table>

<a name="1.3.4"></a>
<h4>
1.3.4 Any type
</h4>

<p>
The ASN.1 ANY type is a kind of wildcard or placeholder that matches
any other type without knowing it in advance. Like CHOICE type, ANY
has no base tag.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
Error ::= SEQUENCE {
  code      INTEGER,
  parameter ANY DEFINED BY code
}
</pre>
</td></tr></table>

<p>
The ANY type is frequently used in specifications, where exact type is not
yet agreed upon between communicating parties or the number of possible
alternatives of a type is infinite.
Sometimes an auxiliary selector is kept around to help parties indicate
the kind of ANY payload in effect ("code" in the example above).
</p>

<p>
Values of the ANY type contain serialized ASN.1 value(s) in form of
an octet string. Therefore pyasn1 Any value object share the properties of
pyasn1 OctetString object.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> someValue = univ.Any(b'\x02\x01\x01')
>>> someValue
Any(b'\x02\x01\x01')
>>> str(someValue)
'\x02\x01\x01'
>>> bytes(someValue)
b'\x02\x01\x01'
>>>
</pre>
</td></tr></table>

<p>
Receiving application is supposed to explicitly deserialize the content of Any
value object, possibly using auxiliary selector for figuring out its ASN.1
type to pick appropriate decoder.
</p>

<p>
There will be some more talk and code snippets covering Any type in the codecs
chapters that follow.
</p>

<a name="1.4"></a>
<h4>
1.4 Subtype constraints
</h4>

<p>
Most ASN.1 types can correspond to an infinite set of values. To adapt to
particular application's data model and needs, ASN.1 provides a mechanism
for limiting the infinite set to values, that make sense in particular case.
</p>

<p>
Imposing value constraints on an ASN.1 type can also be seen as creating
a subtype from its base type.
</p>

<p>
In pyasn1, constraints take shape of immutable objects capable
of evaluating given value against constraint-specific requirements.
Constraint object is a property of pyasn1 type. Like TagSet property,
associated with every pyasn1 type, constraints can never be modified
in place. The only way to modify pyasn1 type constraint is to associate
new constraint object to a new pyasn1 type object.
</p>

<p>
A handful of different flavors of <i>constraints</i> are defined in ASN.1.
We will discuss them one by one in the following chapters and also explain
how to combine and apply them to types.
</p>

<a name="1.4.1"></a>
<h4>
1.4.1 Single value constraint
</h4>

<p>
This kind of constraint allows for limiting type to a finite, specified set
of values.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
DialButton ::= OCTET STRING (
  "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
)
</pre>
</td></tr></table>

<p>
Its pyasn1 implementation would look like:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import constraint
>>> c = constraint.SingleValueConstraint(
  '0','1','2','3','4','5','6','7','8','9'
)
>>> c
SingleValueConstraint(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
>>> c('0')
>>> c('A')
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError: 
  SingleValueConstraint(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) failed at: A
>>> 
</pre>
</td></tr></table>

<p>
As can be seen in the snippet above, if a value violates the constraint, an
exception will be thrown. A constrainted pyasn1 type object holds a
reference to a constraint object (or their combination, as will be explained
later) and calls it for value verification.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, constraint
>>> class DialButton(univ.OctetString):
...   subtypeSpec = constraint.SingleValueConstraint(
...       '0','1','2','3','4','5','6','7','8','9'
...   )
>>> DialButton('0')
DialButton(b'0')
>>> DialButton('A')
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  SingleValueConstraint(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) failed at: A
>>> 
</pre>
</td></tr></table>

<p>
Constrained pyasn1 value object can never hold a violating value.
</p>

<a name="1.4.2"></a>
<h4>
1.4.2 Value range constraint
</h4>

<p>
A pair of values, compliant to a type to be constrained, denote low and upper
bounds of allowed range of values of a type.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
Teenagers ::= INTEGER (13..19)
</pre>
</td></tr></table>

<p>
And in pyasn1 terms:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, constraint
>>> class Teenagers(univ.Integer):
...   subtypeSpec = constraint.ValueRangeConstraint(13, 19)
>>> Teenagers(14)
Teenagers(14)
>>> Teenagers(20)
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ValueRangeConstraint(13, 19) failed at: 20
>>> 
</pre>
</td></tr></table>

<p>
Value range constraint usually applies numeric types.
</p>

<a name="1.4.3"></a>
<h4>
1.4.3 Size constraint
</h4>

<p>
It is sometimes convenient to set or limit the allowed size of a data item
to be sent from one application to another to manage bandwidth and memory
consumption issues. Size constraint specifies the lower and upper bounds 
of the size of a valid value.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
TwoBits ::= BIT STRING (SIZE (2))
</pre>
</td></tr></table>

<p>
Express the same grammar in pyasn1:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, constraint
>>> class TwoBits(univ.BitString):
...   subtypeSpec = constraint.ValueSizeConstraint(2, 2)
>>> TwoBits((1,1))
TwoBits("'11'B")
>>> TwoBits((1,1,0))
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ValueSizeConstraint(2, 2) failed at: (1, 1, 0)
>>> 
</pre>
</td></tr></table>

<p>
Size constraint can be applied to potentially massive values - bit or octet
strings, SEQUENCE OF/SET OF values.
</p>

<a name="1.4.4"></a>
<h4>
1.4.4 Alphabet constraint
</h4>

<p>
The permitted alphabet constraint is similar to Single value constraint
but constraint applies to individual characters of a value. 
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
MorseCode ::= PrintableString (FROM ("."|"-"|" "))
</pre>
</td></tr></table>

<p>
And in pyasn1:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import char, constraint
>>> class MorseCode(char.PrintableString):
...   subtypeSpec = constraint.PermittedAlphabetConstraint(".", "-", " ")
>>> MorseCode("...---...")
MorseCode('...---...')
>>> MorseCode("?")
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  PermittedAlphabetConstraint(".", "-", " ") failed at: "?"
>>> 
</pre>
</td></tr></table>

<p>
Current implementation does not handle ranges of characters in constraint
(FROM "A".."Z" syntax), one has to list the whole set in a range.
</p>

<a name="1.4.5"></a>
<h4>
1.4.5 Constraint combinations
</h4>

<p>
Up to this moment, we used a single constraint per ASN.1 type. The standard,
however, allows for combining multiple individual constraints into
intersections, unions and exclusions.
</p>

<p>
In pyasn1 data model, all of these methods of constraint combinations are
implemented as constraint-like objects holding individual constraint (or
combination) objects. Like terminal constraint objects, combination objects
are capable to perform value verification at its set of enclosed constraints
according to the logic of particular combination.
</p>

<p>
Constraints intersection verification succeeds only if a value is
compliant to each constraint in a set. To begin with, the following
specification will constitute a valid telephone number:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
PhoneNumber ::= NumericString (FROM ("0".."9")) (SIZE 11)
</pre>
</td></tr></table>

<p>
Constraint intersection object serves the logic above:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import char, constraint
>>> class PhoneNumber(char.NumericString):
...   subtypeSpec = constraint.ConstraintsIntersection(
...     constraint.PermittedAlphabetConstraint('0','1','2','3','4','5','6','7','8','9'),
...     constraint.ValueSizeConstraint(11, 11)
...   )
>>> PhoneNumber('79039343212')
PhoneNumber('79039343212')
>>> PhoneNumber('?9039343212')
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ConstraintsIntersection(
    PermittedAlphabetConstraint('0','1','2','3','4','5','6','7','8','9'),
      ValueSizeConstraint(11, 11)) failed at: 
   PermittedAlphabetConstraint('0','1','2','3','4','5','6','7','8','9') failed at: "?039343212"
>>> PhoneNumber('9343212')
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ConstraintsIntersection(
    PermittedAlphabetConstraint('0','1','2','3','4','5','6','7','8','9'),
      ValueSizeConstraint(11, 11)) failed at:
  ValueSizeConstraint(10, 10) failed at: "9343212"
>>>
</pre>
</td></tr></table>

<p>
Union of constraints works by making sure that a value is compliant
to any of the constraint in a set. For instance:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
CapitalOrSmall ::= IA5String (FROM ('A','B','C') | FROM ('a','b','c'))
</pre>
</td></tr></table>

<p>
It's important to note, that a value must fully comply to any single
constraint in a set. In the specification above, a value of all small or
all capital letters is compliant, but a mix of small&capitals is not. 
Here's its pyasn1 analogue:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import char, constraint
>>> class CapitalOrSmall(char.IA5String):
...   subtypeSpec = constraint.ConstraintsUnion(
...     constraint.PermittedAlphabetConstraint('A','B','C'),
...     constraint.PermittedAlphabetConstraint('a','b','c')
...   )
>>> CapitalOrSmall('ABBA')
CapitalOrSmall('ABBA')
>>> CapitalOrSmall('abba')
CapitalOrSmall('abba')
>>> CapitalOrSmall('Abba')
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ConstraintsUnion(PermittedAlphabetConstraint('A', 'B', 'C'),
    PermittedAlphabetConstraint('a', 'b', 'c')) failed at: failed for "Abba"
>>>
</pre>
</td></tr></table>

<p>
Finally, the exclusion constraint simply negates the logic of value 
verification at a constraint. In the following example, any integer value
is allowed in a type but not zero.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
NoZero ::= INTEGER (ALL EXCEPT 0)
</pre>
</td></tr></table>

<p>
In pyasn1 the above definition would read:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, constraint
>>> class NoZero(univ.Integer):
...   subtypeSpec = constraint.ConstraintsExclusion(
...     constraint.SingleValueConstraint(0)
...   )
>>> NoZero(1)
NoZero(1)
>>> NoZero(0)
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ConstraintsExclusion(SingleValueConstraint(0)) failed at: 0
>>>
</pre>
</td></tr></table>

<p>
The depth of such a constraints tree, built with constraint combination objects
at its nodes, has not explicit limit. Value verification is performed in a
recursive manner till a definite solution is found.
</p>

<a name="1.5"></a>
<h4>
1.5 Types relationships
</h4>

<p>
In the course of data processing in an application, it is sometimes
convenient to figure out the type relationships between pyasn1 type or
value objects. Formally, two things influence pyasn1 types relationship:
<i>tag set</i> and <i>subtype constraints</i>. One pyasn1 type is considered
to be a derivative of another if their TagSet and Constraint objects are
a derivation of one another.
</p>

<p>
The following example illustrates the concept (we use the same tagset but
different constraints for simplicity):
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, constraint
>>> i1 = univ.Integer(subtypeSpec=constraint.ValueRangeConstraint(3,8))
>>> i2 = univ.Integer(subtypeSpec=constraint.ConstraintsIntersection(
...    constraint.ValueRangeConstraint(3,8),
...    constraint.ValueRangeConstraint(4,7)
... ) )
>>> i1.isSameTypeWith(i2)
False
>>> i1.isSuperTypeOf(i2)
True
>>> i1.isSuperTypeOf(i1)
True
>>> i2.isSuperTypeOf(i1)
False
>>>
</pre>
</td></tr></table>

<p>
As can be seen in the above code snippet, there are two methods of any pyasn1
type/value object that test types for their relationship:
<b>isSameTypeWith</b>() and <b>isSuperTypeOf</b>(). The former is 
self-descriptive while the latter yields true if the argument appears
to be a pyasn1 object which has tagset and constraints derived from those
of the object being called.
</p>

<a name="2"></a>
<h3>
2. Codecs
</h3>

<p>
In ASN.1 context, 
<a href=http://en.wikipedia.org/wiki/Codec>codec</a>
is a program that transforms between concrete data structures and a stream
of octets, suitable for transmission over the wire. This serialized form of
data is sometimes called <i>substrate</i> or <i>essence</i>.
</p>

<p>
In pyasn1 implementation, substrate takes shape of Python 3 bytes or 
Python 2 string objects.
</p>

<p>
One of the properties of a codec is its ability to cope with incomplete
data and/or substrate what implies codec to be stateful. In other words, 
when decoder runs out of substrate and data item being recovered is still 
incomplete, stateful codec would suspend and complete data item recovery 
whenever the rest of substrate becomes available. Similarly, stateful encoder
would encode data items in multiple steps waiting for source data to
arrive. Codec restartability is especially important when application deals
with large volumes of data and/or runs on low RAM. For an interesting
discussion on codecs options and design choices, refer to
<a href=http://directory.apache.org/subprojects/asn1/>Apache ASN.1 project</a>
.
</p>

<p>
As of this writing, codecs implemented in pyasn1 are all stateless, mostly
to keep the code simple.
</p>

<p>
The pyasn1 package currently supports 
<a href=http://en.wikipedia.org/wiki/Basic_encoding_rules>BER</a> codec and
its variations -- 
<a href=http://en.wikipedia.org/wiki/Canonical_encoding_rules>CER</a> and
<a href=http://en.wikipedia.org/wiki/Distinguished_encoding_rules>DER</a>.
More ASN.1 codecs are planned for implementation in the future.
</p>

<a name="2.1"></a>
<h4>
2.1 Encoders
</h4>

<p>
Encoder is used for transforming pyasn1 value objects into substrate. Only
pyasn1 value objects could be serialized, attempts to process pyasn1 type
objects will cause encoder failure.
</p>

<p>
The following code will create a pyasn1 Integer object and serialize it with
BER encoder:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder
>>> encoder.encode(univ.Integer(123456))
b'\x02\x03\x01\xe2@'
>>>
</pre>
</td></tr></table>

<p>
BER standard also defines a so-called <i>indefinite length</i> encoding form
which makes large data items processing more memory efficient. It is mostly
useful when encoder does not have the whole value all at once and the
length of the value can not be determined at the beginning of encoding.
</p>

<p>
<i>Constructed encoding</i> is another feature of BER closely related to the
indefinite length form. In essence, a large scalar value (such as ASN.1
character BitString type) could be chopped into smaller chunks by encoder
and transmitted incrementally to limit memory consumption. Unlike indefinite
length case, the length of the whole value must be known in advance when
using constructed, definite length encoding form.
</p>

<p>
Since pyasn1 codecs are not restartable, pyasn1 encoder may only encode data
item all at once. However, even in this case, generating indefinite length 
encoding may help a low-memory receiver, running a restartable decoder,
to process a large data item.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder
>>> encoder.encode(
...   univ.OctetString('The quick brown fox jumps over the lazy dog'),
...   defMode=False,
...   maxChunkSize=8
... )
b'$\x80\x04\x08The quic\x04\x08k brown \x04\x08fox jump\x04\x08s over \
t\x04\x08he lazy \x04\x03dog\x00\x00'
>>>
>>> encoder.encode(
...   univ.OctetString('The quick brown fox jumps over the lazy dog'),
...   maxChunkSize=8
... )
b'$7\x04\x08The quic\x04\x08k brown \x04\x08fox jump\x04\x08s over \
t\x04\x08he lazy \x04\x03dog'
</pre>
</td></tr></table>

<p>
The <b>defMode</b> encoder parameter disables definite length encoding mode,
while the optional <b>maxChunkSize</b> parameter specifies desired
substrate chunk size that influences memory requirements at the decoder's end.
</p>

<p>
To use CER or DER encoders one needs to explicitly import and call them - the
APIs are all compatible.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder as ber_encoder
>>> from pyasn1.codec.cer import encoder as cer_encoder
>>> from pyasn1.codec.der import encoder as der_encoder
>>> ber_encoder.encode(univ.Boolean(True))
b'\x01\x01\x01'
>>> cer_encoder.encode(univ.Boolean(True))
b'\x01\x01\xff'
>>> der_encoder.encode(univ.Boolean(True))
b'\x01\x01\xff'
>>>
</pre>
</td></tr></table>

<a name="2.2"></a>
<h4>
2.2 Decoders
</h4>

<p>
In the process of decoding, pyasn1 value objects are created and linked to
each other, based on the information containted in the substrate. Thus,
the original pyasn1 value object(s) are recovered.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder, decoder
>>> substrate = encoder.encode(univ.Boolean(True))
>>> decoder.decode(substrate)
(Boolean('True(1)'), b'')
>>>
</pre>
</td></tr></table>

<p>
Commenting on the code snippet above, pyasn1 decoder accepts substrate
as an argument and returns a tuple of pyasn1 value object (possibly
a top-level one in case of constructed object) and unprocessed part
of input substrate.
</p>

<p>
All pyasn1 decoders can handle both definite and indefinite length
encoding modes automatically, explicit switching into one mode
to another is not required.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder, decoder
>>> substrate = encoder.encode(
...   univ.OctetString('The quick brown fox jumps over the lazy dog'),
...   defMode=False,
...   maxChunkSize=8
... )
>>> decoder.decode(substrate)
(OctetString(b'The quick brown fox jumps over the lazy dog'), b'')
>>>
</pre>
</td></tr></table>

<p>
Speaking of BER/CER/DER encoding, in many situations substrate may not contain
all necessary information needed for complete and accurate ASN.1 values
recovery. The most obvious cases include implicitly tagged ASN.1 types
and constrained types.
</p>

<p>
As discussed earlier in this handbook, when an ASN.1 type is implicitly
tagged, previous outermost tag is lost and never appears in substrate.
If it is the base tag that gets lost, decoder is unable to pick type-specific
value decoder at its table of built-in types, and therefore recover
the value part, based only on the information contained in substrate. The
approach taken by pyasn1 decoder is to use a prototype pyasn1 type object (or
a set of them) to <i>guide</i> the decoding process by matching [possibly
incomplete] tags recovered from substrate with those found in prototype pyasn1
type objects (also called pyasn1 specification object further in this paper).
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.codec.ber import decoder
>>> decoder.decode(b'\x02\x01\x0c', asn1Spec=univ.Integer())
Integer(12), b''
>>>
</pre>
</td></tr></table>

<p>
Decoder would neither modify pyasn1 specification object nor use
its current values (if it's a pyasn1 value object), but rather use it as
a hint for choosing proper decoder and as a pattern for creating new objects:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, tag
>>> from pyasn1.codec.ber import encoder, decoder
>>> i = univ.Integer(12345).subtype(
...   implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 40)
... )
>>> substrate = encoder.encode(i)
>>> substrate
b'\x9f(\x0209'
>>> decoder.decode(substrate)
Traceback (most recent call last):
...
pyasn1.error.PyAsn1Error: 
   TagSet(Tag(tagClass=128, tagFormat=0, tagId=40)) not in asn1Spec
>>> decoder.decode(substrate, asn1Spec=i)
(Integer(12345), b'')
>>>
</pre>
</td></tr></table>

<p>
Notice in the example above, that an attempt to run decoder without passing
pyasn1 specification object fails because recovered tag does not belong
to any of the built-in types.
</p>

<p>
Another important feature of guided decoder operation is the use of
values constraints possibly present in pyasn1 specification object.
To explain this, we will decode a random integer object into generic Integer
and the constrained one.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, constraint
>>> from pyasn1.codec.ber import encoder, decoder
>>> class DialDigit(univ.Integer):
...   subtypeSpec = constraint.ValueRangeConstraint(0,9)
>>> substrate = encoder.encode(univ.Integer(13))
>>> decoder.decode(substrate)
(Integer(13), b'')
>>> decoder.decode(substrate, asn1Spec=DialDigit())
Traceback (most recent call last):
...
pyasn1.type.error.ValueConstraintError:
  ValueRangeConstraint(0, 9) failed at: 13
>>> 
</pre>
</td></tr></table>

<p>
Similarily to encoders, to use CER or DER decoders application has to
explicitly import and call them - all APIs are compatible.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder as ber_encoder
>>> substrate = ber_encoder.encode(univ.OctetString('http://pyasn1.sf.net'))
>>>
>>> from pyasn1.codec.ber import decoder as ber_decoder
>>> from pyasn1.codec.cer import decoder as cer_decoder
>>> from pyasn1.codec.der import decoder as der_decoder
>>> 
>>> ber_decoder.decode(substrate)
(OctetString(b'http://pyasn1.sf.net'), b'')
>>> cer_decoder.decode(substrate)
(OctetString(b'http://pyasn1.sf.net'), b'')
>>> der_decoder.decode(substrate)
(OctetString(b'http://pyasn1.sf.net'), b'')
>>> 
</pre>
</td></tr></table>

<a name="2.2.1"></a>
<h4>
2.2.1 Decoding untagged types
</h4>

<p>
It has already been mentioned, that ASN.1 has two "special case" types:
CHOICE and ANY. They are different from other types in part of 
tagging - unless these two are additionally tagged, neither of them will
have their own tag. Therefore these types become invisible in substrate
and can not be recovered without passing pyasn1 specification object to
decoder.
</p>

<p>
To explain the issue, we will first prepare a Choice object to deal with:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, namedtype
>>> class CodeOrMessage(univ.Choice):
...   componentType = namedtype.NamedTypes(
...     namedtype.NamedType('code', univ.Integer()),
...     namedtype.NamedType('message', univ.OctetString())
...   )
>>>
>>> codeOrMessage = CodeOrMessage()
>>> codeOrMessage.setComponentByName('message', 'my string value')
>>> print(codeOrMessage.prettyPrint())
CodeOrMessage:
 message=b'my string value'
>>>
</pre>
</td></tr></table>

<p>
Let's now encode this Choice object and then decode its substrate
with and without pyasn1 specification object:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.codec.ber import encoder, decoder
>>> substrate = encoder.encode(codeOrMessage)
>>> substrate
b'\x04\x0fmy string value'
>>> encoder.encode(univ.OctetString('my string value'))
b'\x04\x0fmy string value'
>>>
>>> decoder.decode(substrate)
(OctetString(b'my string value'), b'')
>>> codeOrMessage, substrate = decoder.decode(substrate, asn1Spec=CodeOrMessage())
>>> print(codeOrMessage.prettyPrint())
CodeOrMessage:
 message=b'my string value'
>>>
</pre>
</td></tr></table>

<p>
First thing to notice in the listing above is that the substrate produced
for our Choice value object is equivalent to the substrate for an OctetString
object initialized to the same value. In other words, any information about
the Choice component is absent in encoding.
</p>

<p>
Sure enough, that kind of substrate will decode into an OctetString object,
unless original Choice type object is passed to decoder to guide the decoding
process.
</p>

<p>
Similarily untagged ANY type behaves differently on decoding phase - when
decoder bumps into an Any object in pyasn1 specification, it stops decoding
and puts all the substrate into a new Any value object in form of an octet
string. Concerned application could then re-run decoder with an additional,
more exact pyasn1 specification object to recover the contents of Any
object.
</p>

<p>
As it was mentioned elsewhere in this paper, Any type allows for incomplete
or changing ASN.1 specification to be handled gracefully by decoder and
applications.
</p>

<p>
To illustrate the working of Any type, we'll have to make the stage
by encoding a pyasn1 object and then putting its substrate into an any
object.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder, decoder
>>> innerSubstrate = encoder.encode(univ.Integer(1234))
>>> innerSubstrate
b'\x02\x02\x04\xd2'
>>> any = univ.Any(innerSubstrate)
>>> any
Any(b'\x02\x02\x04\xd2')
>>> substrate = encoder.encode(any)
>>> substrate
b'\x02\x02\x04\xd2'
>>>
</pre>
</td></tr></table>

<p>
As with Choice type encoding, there is no traces of Any type in substrate.
Obviously, the substrate we are dealing with, will decode into the inner
[Integer] component, unless pyasn1 specification is given to guide the 
decoder. Continuing previous code:
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ
>>> from pyasn1.codec.ber import encoder, decoder

>>> decoder.decode(substrate)
(Integer(1234), b'')
>>> any, substrate = decoder.decode(substrate, asn1Spec=univ.Any())
>>> any
Any(b'\x02\x02\x04\xd2')
>>> decoder.decode(str(any))
(Integer(1234), b'')
>>>
</pre>
</td></tr></table>

<p>
Both CHOICE and ANY types are widely used in practice. Reader is welcome to
take a look at 
<a href=http://www.cs.auckland.ac.nz/~pgut001/pubs/x509guide.txt>
ASN.1 specifications of X.509 applications</a> for more information.
</p>

<a name="2.2.2"></a>
<h4>
2.2.2 Ignoring unknown types
</h4>

<p>
When dealing with a loosely specified ASN.1 structure, the receiving
end may not be aware of some types present in the substrate. It may be
convenient then to turn decoder into a recovery mode. Whilst there, decoder
will not bail out when hit an unknown tag but rather treat it as an Any
type.
</p>

<table bgcolor="lightgray" border=0 width=100%><TR><TD>
<pre>
>>> from pyasn1.type import univ, tag
>>> from pyasn1.codec.ber import encoder, decoder
>>> taggedInt = univ.Integer(12345).subtype(
...   implicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 40)
... )
>>> substrate = encoder.encode(taggedInt)
>>> decoder.decode(substrate)
Traceback (most recent call last):
...
pyasn1.error.PyAsn1Error: TagSet(Tag(tagClass=128, tagFormat=0, tagId=40)) not in asn1Spec
>>>
>>> decoder.decode.defaultErrorState = decoder.stDumpRawValue
>>> decoder.decode(substrate)
(Any(b'\x9f(\x0209'), '')
>>>
</pre>
</td></tr></table>

<p>
It's also possible to configure a custom decoder, to handle unknown tags
found in substrate. This can be done by means of <b>defaultRawDecoder</b>
attribute holding a reference to type decoder object. Refer to the source
for API details.
</p>

<a name="3"></a>
<h3>
3. Feedback and getting help
</h3>

<p>
Although pyasn1 software is almost a decade old and used in many production
environments, it still may have bugs and non-implemented pieces. Anyone
who happens to run into such defect is welcome to complain to
<a href=mailto:pyasn1-users@lists.sourceforge.net>pyasn1 mailing list</a>
or better yet fix the issue and send
<a href=mailto:ilya@glas.net>me</a> the patch.
</p>

<p>
Typically, pyasn1 is used for building arbitrary protocol support into
various applications. This involves manual translation of ASN.1 data
structures into their pyasn1 implementations. To save time and effort,
data structures for some of the popular protocols are pre-programmed
and kept for further re-use in form of the
<a href=http://sourceforge.net/projects/pyasn1/files/pyasn1-modules/>
pyasn1-modules package</a>. For instance, many structures for PKI (X.509,
PKCS#*, CRMF, OCSP), LDAP and SNMP are present.
Applications authors are advised to import and use relevant modules 
from that package whenever needed protocol structures are already 
there. New protocol modules contributions are welcome.
</p>

<p>
And finally, the latest pyasn1 package revision is available for free
download from
<a href=http://sourceforge.net/projects/pyasn1/>project home</a> and
also from the 
<a href=http://pypi.python.org/pypi>Python package repository</a>.
</p>

<hr>

</td>
</tr>
</table>
</center>
</body>
</html>
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