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|
| +<html>
|
| +<title>
|
| +PyASN1 programmer's manual
|
| +</title>
|
| +<head>
|
| +</head>
|
| +<body>
|
| +<center>
|
| +<table width=60%>
|
| +<tr>
|
| +<td>
|
| +
|
| +<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>
|
|
|
Chromium Code Reviews
Issue 1264873003:
Add gsutil/third_party to telemetry/third_party/gsutilz/third_party. (Closed)
Base URL: https://chromium.googlesource.com/chromium/src.git@master