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python/pyasn1/doc/scalar.html
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python/pyasn1/doc/scalar.html
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<html>
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<title>
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PyASN1 data model and scalar types
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</title>
|
||||
<head>
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<table width=60%>
|
||||
<tr>
|
||||
<td>
|
||||
|
||||
<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
|
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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>
|
||||
|
||||
<hr>
|
||||
|
||||
</td>
|
||||
</tr>
|
||||
</table>
|
||||
</center>
|
||||
</body>
|
||||
</html>
|
||||
Loading…
Add table
Add a link
Reference in a new issue