From 5f8de423f190bbb79a62f804151bc24824fa32d8 Mon Sep 17 00:00:00 2001 From: "Matt A. Tobin" Date: Fri, 2 Feb 2018 04:16:08 -0500 Subject: Add m-esr52 at 52.6.0 --- python/pyasn1/doc/pyasn1-tutorial.html | 2405 ++++++++++++++++++++++++++++++++ 1 file changed, 2405 insertions(+) create mode 100644 python/pyasn1/doc/pyasn1-tutorial.html (limited to 'python/pyasn1/doc/pyasn1-tutorial.html') diff --git a/python/pyasn1/doc/pyasn1-tutorial.html b/python/pyasn1/doc/pyasn1-tutorial.html new file mode 100644 index 000000000..2eb82f1e9 --- /dev/null +++ b/python/pyasn1/doc/pyasn1-tutorial.html @@ -0,0 +1,2405 @@ + + +PyASN1 programmer's manual + + + + +
+ + + + +
+ +

+PyASN1 programmer's manual +

+ +

+written by Ilya Etingof, 2011-2012 +

+ +

+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. +

+ +

+Abstract Syntax Notation One +(ASN.1) +is a set of + +ITU standards 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: +

    +
  • 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. +
  • 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. +
  • 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. +
+

+ +

+This tutorial and algorithms, implemented by pyasn1 library, are +largely based on the information read in the book + +ASN.1 - Communication between heterogeneous systems +by Olivier Dubuisson. Another relevant resource is + +A Layman's Guide to a Subset of ASN.1, BER, and DER by Burton S. Kaliski. +It's advised to refer to these books for more in-depth knowledge on the +subject of ASN.1. +

+ +

+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. +

+ +

+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. +

+ +

+Table of contents +

+ +

+

+ + + +

+1. Data model for ASN.1 types +

+ +

+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. +

+ +

+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 pyasn1 type object versus pyasn1 +value object. +

+ +

+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. +

+ +
+
+>>> 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
+
+
+ +

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

+ +
+
+>>> from pyasn1.type import univ
+>>> asn1IntegerType = univ.Integer()
+>>> asn1IntegerType - 2
+...
+pyasn1.error.PyAsn1Error: No value for __coerce__()
+
+
+ + +

+1.1 Scalar types +

+ +

+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. +

+ + +

+1.1.1 Boolean type +

+ +

+This is the simplest type those values could be either True or False. +

+ +
+
+;; type specification
+FunFactorPresent ::= BOOLEAN
+
+;; values declaration and assignment
+pythonFunFactor FunFactorPresent ::= TRUE
+cobolFunFactor FunFactorPresent :: FALSE
+
+
+ +

+And here's pyasn1 version of it: +

+ +
+
+>>> 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
+>>>
+
+
+ + +

+1.1.2 Null type +

+ +

+The NULL type is sometimes used to express the absense of any information. +

+ +
+
+;; type specification
+Vote ::= CHOICE {
+  agreed BOOLEAN,
+  skip NULL
+}
+
+ +;; value declaration and assignment +myVote Vote ::= skip:NULL + + +

+We will explain the CHOICE type later in this paper, meanwhile the NULL +type: +

+ +
+
+>>> from pyasn1.type import univ
+>>> skip = univ.Null()
+>>> skip
+Null('')
+>>>
+
+
+ + +

+1.1.3 Integer type +

+ +

+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. +

+ +
+
+;; values specification
+age-of-universe INTEGER ::= 13750000000
+mean-martian-surface-temperature INTEGER ::= -63
+
+
+ +

+A rather strigntforward mapping into pyasn1: +

+ +
+
+>>> from pyasn1.type import univ
+>>> ageOfUniverse = univ.Integer(13750000000)
+>>> ageOfUniverse
+Integer(13750000000)
+>>>
+>>> meanMartianSurfaceTemperature = univ.Integer(-63)
+>>> meanMartianSurfaceTemperature
+Integer(-63)
+>>>
+
+
+ +

+ASN.1 allows to assign human-friendly names to particular values of +an INTEGER type. +

+ +
+
+Temperature ::= INTEGER {
+  freezing(0),
+  boiling(100) 
+}
+
+
+ +

+The Temperature type expressed in pyasn1: +

+ +
+
+>>> 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
+>>>
+
+
+ +

+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). +

+ + +

+1.1.4 Enumerated type +

+ +

+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. +

+ +
+
+error-status ::= ENUMERATED {
+  no-error(0),
+  authentication-error(10),
+  authorization-error(20),
+  general-failure(51)
+}
+
+
+ +

+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). +

+ +
+
+>>> 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
+>>>
+
+
+ +

+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. +

+ + +

+1.1.5 Real type +

+ +

+Values of the Real type are a three-component tuple of mantissa, base and +exponent. All three are integers. +

+ +
+
+pi ::= REAL { mantissa 314159, base 10, exponent -5 }
+
+
+ +

+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. + +

+ +
+
+>>> 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
+>>>
+
+
+ +

+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). +

+ + +

+1.1.6 Bit string type +

+ +

+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. +

+ +
+
+public-key BIT STRING ::= '1010111011110001010110101101101
+                           1011000101010000010110101100010
+                           0110101010000111101010111111110'B
+
+signature  BIT STRING ::= 'AF01330CD932093392100B39FF00DE0'H
+
+
+ +

+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. +

+ +
+
+>>> 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")
+>>>
+
+
+ +

+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. +

+ +
+
+bit-mask  BIT STRING ::= {
+  read-flag(0),
+  write-flag(2),
+  run-flag(4)
+}
+
+
+ +

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

+ +
+
+>>> 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
+>>>
+
+
+ +

+The BitString objects mimic the properties of Python tuple type in part +of immutable sequence object protocol support. +

+ + +

+1.1.7 OctetString type +

+ +

+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. +

+ +
+
+thumbnail OCTET STRING ::= '1000010111101110101111000000111011'B
+thumbnail OCTET STRING ::= 'FA9823C43E43510DE3422'H
+
+
+ +

+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. +

+ +
+
+welcome-message OCTET STRING ::= "Welcome to ASN.1 wilderness!"
+
+
+ +

+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. +

+ +
+
+>>> from pyasn1.type import univ
+>>> thumbnail = univ.OctetString(
+...    binValue='1000010111101110101111000000111011'
+... )
+>>> thumbnail
+OctetString(hexValue='85eebcec0')
+>>> thumbnail = univ.OctetString(
+...    hexValue='FA9823C43E43510DE3422'
+... )
+>>> thumbnail
+OctetString(hexValue='fa9823c43e4351de34220')
+>>>
+
+
+ +

+Most frequent usage of the OctetString class is to instantiate it with +a text string. +

+ +
+
+>>> 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')
+>>> 
+
+
+ +

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

+ +

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

+ +

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

+ +
+
+>>> univ.OctetString((77, 101, 101, 103, 111))
+OctetString(b'Meego')
+
+
+ +

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

+ +
+
+>>> octetString = univ.OctetString('ABCDEF')
+>>> octetString.asNumbers()
+(65, 66, 67, 68, 69, 70)
+>>> octetString.asOctets()
+b'ABCDEF'
+
+
+ + +

+1.1.8 ObjectIdentifier type +

+ +

+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. +

+ +
+
+internet-id OBJECT IDENTIFIER ::= {
+  iso(1) identified-organization(3) dod(6) internet(1)
+}
+
+
+ +

+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. +

+ +
+
+>>> 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')
+
+
+ +

+A more human-friendly "dotted" notation is also supported. +

+ +
+
+>>> from pyasn1.type import univ
+>>> univ.ObjectIdentifier('1.3.6.1')
+ObjectIdentifier('1.3.6.1')
+
+
+ +

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

+ +

+The ObjectIdentifier objects mimic the properties of Python tuple type in +part of immutable sequence object protocol support. +

+ + +

+1.1.9 Character string types +

+ +

+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. +

+ +

+The two types are specific to ASN.1 are NumericString and PrintableString. +

+ +
+
+welcome-message ::= PrintableString {
+  "Welcome to ASN.1 text types"
+}
+
+dial-pad-numbers ::= NumericString {
+  "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
+}
+
+
+ +

+Their pyasn1 implementations are: +

+ +
+
+>>> 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')
+>>>
+
+
+ +

+The following types came to ASN.1 from ISO standards on character sets. +

+ +
+
+>>> 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')
+>>>
+
+
+ +

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

+ +
+
+>>> 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
+>>>
+
+
+ +

+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. +

+ + +

+1.1.10 Useful types +

+ +

+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. +

+ +

+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. +

+ +
+
+>>> from pyasn1.type import useful
+>>> descrBER = useful.ObjectDescriptor(
+      "Basic encoding of a single ASN.1 type"
+)
+>>> 
+
+
+ +

+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. +

+ +
+
+;; 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"
+
+
+ +
+
+>>> from pyasn1.type import useful
+>>> moscowTime = useful.GeneralizedTime("20110308120000.0")
+>>> utcTime = useful.UTCTime("9803081200Z")
+>>> 
+
+
+ +

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

+ + +

+1.2 Tagging +

+ +

+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. +

+ +

+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. +

+ +

+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 tag 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. +

+ +

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

+ +

+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). +

+ +
+
+MyIntegerType ::= [12] INTEGER
+MyOctetString ::= [APPLICATION 0] OCTET STRING
+
+
+ +

+In pyasn1, tags are implemented as immutable, tuple-like objects: +

+ +
+
+>>> 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
+>>>
+
+
+ +

+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. +

+ +

+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. +

+ +

+When tagging in IMPLICIT mode, the outermost existing tag is dropped and +replaced with a new one. +

+ +
+
+MyIntegerType ::= [12] IMPLICIT INTEGER
+MyOctetString ::= [APPLICATION 0] EXPLICIT OCTET STRING
+
+
+ +

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

+ +
+
+>>> 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))
+>>> 
+
+
+ +

+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. +

+ +

+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. +

+ +
+
+>>> 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
+>>> 
+
+
+ +

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

+ +
+
+MyIntegerType ::= [12] EXPLICIT INTEGER
+
+
+ +

+could be expressed in pyasn1 like this: +

+ +
+
+>>> 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))
+>>>
+
+
+ +

+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. +

+ +

+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. +

+ + + +

+1.3 Constructed types +

+ +

+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. +

+ +

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

+ + +

+1.3.1 Sequence and Set types +

+ +

+The Sequence and Set types have many similar properties: +

+
    +
  • they can hold any number of inner components of different types +
  • every component has a human-friendly identifier +
  • any component can have a default value +
  • some components can be absent. +
+ +

+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. +

+ +
+
+Record ::= SEQUENCE {
+  id        INTEGER,
+  room  [0] INTEGER OPTIONAL,
+  house [1] INTEGER DEFAULT 0
+}
+
+
+ +

+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. +

+ +
+
+>>> 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))
+...     )
+...   )
+>>>
+
+
+ +

+All pyasn1 constructed type classes have a class attribute componentType +that represent default type specification. Its value is a NamedTypes object. +

+ +

+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. +

+ +

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

+ +

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

+ +
+
+>>> 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
+
+
+ +

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

+ +
+
+>>> 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
+>>>
+
+
+ +

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

+ +
+
+>>> 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
+>>>
+
+
+ + +

+1.3.2 SequenceOf and SetOf types +

+ +

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

+ +
+
+Progression ::= SEQUENCE OF INTEGER
+
+arithmeticProgression Progression ::= { 1, 3, 5, 7 }
+
+
+ +

+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. +

+ +

+To specify inner component type, the componentType class attribute +should refer to another pyasn1 type object. +

+ +
+
+>>> 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)
+>>>
+
+
+ +

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

+ + +

+1.3.3 Choice type +

+ +

+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. +

+ +
+
+CodeOrMessage ::= CHOICE {
+  code    INTEGER,
+  message OCTET STRING
+}
+
+
+ +

+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. +

+ +
+
+>>> 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'
+>>>
+
+
+ +

+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): +

+ +
+
+>>> codeOrMessage.getName()
+'message'
+>>> codeOrMessage.getComponent()
+OctetString(b'my string value')
+>>>
+
+
+ + +

+1.3.4 Any type +

+ +

+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. +

+ +
+
+Error ::= SEQUENCE {
+  code      INTEGER,
+  parameter ANY DEFINED BY code
+}
+
+
+ +

+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). +

+ +

+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. +

+ +
+
+>>> 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'
+>>>
+
+
+ +

+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. +

+ +

+There will be some more talk and code snippets covering Any type in the codecs +chapters that follow. +

+ + +

+1.4 Subtype constraints +

+ +

+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. +

+ +

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

+ +

+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. +

+ +

+A handful of different flavors of constraints 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. +

+ + +

+1.4.1 Single value constraint +

+ +

+This kind of constraint allows for limiting type to a finite, specified set +of values. +

+ +
+
+DialButton ::= OCTET STRING (
+  "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
+)
+
+
+ +

+Its pyasn1 implementation would look like: +

+ +
+
+>>> 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
+>>> 
+
+
+ +

+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. +

+ +
+
+>>> 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
+>>> 
+
+
+ +

+Constrained pyasn1 value object can never hold a violating value. +

+ + +

+1.4.2 Value range constraint +

+ +

+A pair of values, compliant to a type to be constrained, denote low and upper +bounds of allowed range of values of a type. +

+ +
+
+Teenagers ::= INTEGER (13..19)
+
+
+ +

+And in pyasn1 terms: +

+ +
+
+>>> 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
+>>> 
+
+
+ +

+Value range constraint usually applies numeric types. +

+ + +

+1.4.3 Size constraint +

+ +

+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. +

+ +
+
+TwoBits ::= BIT STRING (SIZE (2))
+
+
+ +

+Express the same grammar in pyasn1: +

+ +
+
+>>> 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)
+>>> 
+
+
+ +

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

+ + +

+1.4.4 Alphabet constraint +

+ +

+The permitted alphabet constraint is similar to Single value constraint +but constraint applies to individual characters of a value. +

+ +
+
+MorseCode ::= PrintableString (FROM ("."|"-"|" "))
+
+
+ +

+And in pyasn1: +

+ +
+
+>>> 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: "?"
+>>> 
+
+
+ +

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

+ + +

+1.4.5 Constraint combinations +

+ +

+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. +

+ +

+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. +

+ +

+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: +

+ +
+
+PhoneNumber ::= NumericString (FROM ("0".."9")) (SIZE 11)
+
+
+ +

+Constraint intersection object serves the logic above: +

+ +
+
+>>> 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"
+>>>
+
+
+ +

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

+ +
+
+CapitalOrSmall ::= IA5String (FROM ('A','B','C') | FROM ('a','b','c'))
+
+
+ +

+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: +

+ +
+
+>>> 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"
+>>>
+
+
+ +

+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. +

+ +
+
+NoZero ::= INTEGER (ALL EXCEPT 0)
+
+
+ +

+In pyasn1 the above definition would read: +

+ +
+
+>>> 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
+>>>
+
+
+ +

+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. +

+ + +

+1.5 Types relationships +

+ +

+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: +tag set and subtype constraints. One pyasn1 type is considered +to be a derivative of another if their TagSet and Constraint objects are +a derivation of one another. +

+ +

+The following example illustrates the concept (we use the same tagset but +different constraints for simplicity): +

+ +
+
+>>> 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
+>>>
+
+
+ +

+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: +isSameTypeWith() and isSuperTypeOf(). 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. +

+ + +

+2. Codecs +

+ +

+In ASN.1 context, +codec +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 substrate or essence. +

+ +

+In pyasn1 implementation, substrate takes shape of Python 3 bytes or +Python 2 string objects. +

+ +

+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 +Apache ASN.1 project +. +

+ +

+As of this writing, codecs implemented in pyasn1 are all stateless, mostly +to keep the code simple. +

+ +

+The pyasn1 package currently supports +BER codec and +its variations -- +CER and +DER. +More ASN.1 codecs are planned for implementation in the future. +

+ + +

+2.1 Encoders +

+ +

+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. +

+ +

+The following code will create a pyasn1 Integer object and serialize it with +BER encoder: +

+ +
+
+>>> from pyasn1.type import univ
+>>> from pyasn1.codec.ber import encoder
+>>> encoder.encode(univ.Integer(123456))
+b'\x02\x03\x01\xe2@'
+>>>
+
+
+ +

+BER standard also defines a so-called indefinite length 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. +

+ +

+Constructed encoding 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. +

+ +

+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. +

+ +
+
+>>> 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'
+
+
+ +

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

+ +

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

+ +
+
+>>> 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'
+>>>
+
+
+ + +

+2.2 Decoders +

+ +

+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. +

+ +
+
+>>> 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'')
+>>>
+
+
+ +

+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. +

+ +

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

+ +
+
+>>> 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'')
+>>>
+
+
+ +

+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. +

+ +

+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 guide 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). +

+ +
+
+>>> from pyasn1.codec.ber import decoder
+>>> decoder.decode(b'\x02\x01\x0c', asn1Spec=univ.Integer())
+Integer(12), b''
+>>>
+
+
+ +

+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: +

+ +
+
+>>> 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'')
+>>>
+
+
+ +

+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. +

+ +

+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. +

+ +
+
+>>> 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
+>>> 
+
+
+ +

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

+ +
+
+>>> 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'')
+>>> 
+
+
+ + +

+2.2.1 Decoding untagged types +

+ +

+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. +

+ +

+To explain the issue, we will first prepare a Choice object to deal with: +

+ +
+
+>>> 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'
+>>>
+
+
+ +

+Let's now encode this Choice object and then decode its substrate +with and without pyasn1 specification object: +

+ +
+
+>>> 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'
+>>>
+
+
+ +

+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. +

+ +

+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. +

+ +

+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. +

+ +

+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. +

+ +

+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. +

+ +
+
+>>> 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'
+>>>
+
+
+ +

+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: +

+ +
+
+>>> 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'')
+>>>
+
+
+ +

+Both CHOICE and ANY types are widely used in practice. Reader is welcome to +take a look at + +ASN.1 specifications of X.509 applications for more information. +

+ + +

+2.2.2 Ignoring unknown types +

+ +

+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. +

+ +
+
+>>> 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'), '')
+>>>
+
+
+ +

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

+ + +

+3. Feedback and getting help +

+ +

+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 +pyasn1 mailing list +or better yet fix the issue and send +me the patch. +

+ +

+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 + +pyasn1-modules package. 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. +

+ +

+And finally, the latest pyasn1 package revision is available for free +download from +project home and +also from the +Python package repository. +

+ +
+ +
+
+ + -- cgit v1.2.3