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PyASN1 codecs
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<h3>
2. PyASN1 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>

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

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<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>
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<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>

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<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>
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<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>

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<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>
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<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>

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<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>
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<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>

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

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<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>
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<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>

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

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<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>
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<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>

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

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<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>
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<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>

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<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>
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<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>

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<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>
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<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>

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<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>
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<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>

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