asn1

This module provides ASN.1 encoder and decoder.

class asn1.Classes(value)

An enumeration.

class asn1.Decoder

ASN.1 decoder. Understands BER (and DER which is a subset).

enter() → None

This method enters the constructed type that is at the current decoding offset.

Note

It is an error to call Decoder.enter() if the to be decoded ASN.1 tag is not of a constructed type.

Returns

None

eof() → bool

Return True if we are at the end of input.

Returns

bool – True if all input has been decoded, and False otherwise.

leave() → None

This method leaves the last constructed type that was Decoder.enter()-ed.

Note

It is an error to call Decoder.leave() if the current ASN.1 tag is not of a constructed type.

Returns

None

peek() → asn1.Tag

This method returns the current ASN.1 tag (i.e. the tag that a subsequent Decoder.read() call would return) without updating the decoding offset. In case no more data is available from the input, this method returns None to signal end-of-file.

This method is useful if you don’t know whether the next tag will be a primitive or a constructed tag. Depending on the return value of peek, you would decide to either issue a Decoder.read() in case of a primitive type, or an Decoder.enter() in case of a constructed type.

Note

Because this method does not advance the current offset in the input, calling it multiple times in a row will return the same value for all calls.

Returns

Tag – The current ASN.1 tag.

Raises

Error –

read(tagnr: Number = None)

This method decodes one ASN.1 tag from the input and returns it as a (tag, value) tuple. tag is a 3-tuple (nr, typ, cls), while value is a Python object representing the ASN.1 value. The offset in the input is increased so that the next Decoder.read() call will return the next tag. In case no more data is available from the input, this method returns None to signal end-of-file.

Returns

Tag, value – The current ASN.1 tag and its value.

Raises

Error –

start(data: bytes) → None

This method instructs the decoder to start decoding the ASN.1 input data, which must be a passed in as plain Python bytes. This method may be called at any time to start a new decoding job. If this method is called while currently decoding another input, that decoding context is discarded.

Note

It is not necessary to specify the encoding because the decoder assumes the input is in BER or DER format.

Parameters

data (bytes) – ASN.1 input, in BER or DER format, to be decoded.

Returns

None

Raises

Error –

class asn1.Encoder

ASN.1 encoder. Uses DER encoding.

enter(nr: int, cls: Optional[int] = None) → None

This method starts the construction of a constructed type.

Parameters

• nr (int) – The desired ASN.1 type. Use Numbers enumeration.

• cls (int) – This optional parameter specifies the class of the constructed type. The default class to use is the universal class. Use Classes enumeration.

Returns

None

Raises

Error –

leave() → None

This method completes the construction of a constructed type and writes the encoded representation to the output buffer.

output() → bytes

This method returns the encoded ASN.1 data as plain Python bytes. This method can be called multiple times, also during encoding. In the latter case the data that has been encoded so far is returned.

Note

It is an error to call this method if the encoder is still constructing a constructed type, i.e. if Encoder.enter() has been called more times that Encoder.leave().

Returns

bytes – The DER encoded ASN.1 data.

Raises

Error –

start() → None

This method instructs the encoder to start encoding a new ASN.1 output. This method may be called at any time to reset the encoder, and resets the current output (if any).

write(value: object, nr: Optional[int] = None, typ: Optional[int] = None, cls: Optional[int] = None) → None

This method encodes one ASN.1 tag and writes it to the output buffer.

Note

Normally, value will be the only parameter to this method. In this case Python-ASN1 will autodetect the correct ASN.1 type from the type of value, and will output the encoded value based on this type.

Parameters

• value (any) – The value of the ASN.1 tag to write. Python-ASN1 will try to autodetect the correct ASN.1 type from the type of value.

• nr (int) – If the desired ASN.1 type cannot be autodetected or is autodetected wrongly, the nr parameter can be provided to specify the ASN.1 type to be used. Use Numbers enumeration.

• typ (int) – This optional parameter can be used to write constructed types to the output by setting it to indicate the constructed encoding type. In this case, value must already be valid ASN.1 encoded data as plain Python bytes. This is not normally how constructed types should be encoded though, see Encoder.enter() and Encoder.leave() for the recommended way of doing this. Use Types enumeration.

• cls (int) – This parameter can be used to override the class of the value. The default class is the universal class. Use Classes enumeration.

Returns

None

Raises

Error –

exception asn1.Error

ASN.11 encoding or decoding error.

class asn1.Numbers(value)

An enumeration.

class asn1.Tag(nr, typ, cls)

A named tuple to represent ASN.1 tags as returned by Decoder.peek() and Decoder.read().

cls

Alias for field number 2

nr

Alias for field number 0

typ

Alias for field number 1

class asn1.Types(value)

An enumeration.