BIT STRING¶

The ASN.1 BIT STRING type represents an arbitrary-length sequence of bits. It is used to model flags, bitmasks, and other bit-level data. In the generated Python bindings, all BIT STRING types implement type behavior described in _Asn1BitStrType.

Named BIT STRING types additionally expose named boolean attributes for convenient flag access.

Example ASN.1 definitions:

ExampleBitString ::= BIT STRING

ExampleNamedBitString ::= BIT STRING {
    zero(0),
    one(1),
    two(2)
}

These generate Python classes similar to:

class ExampleBitString(_Asn1BitStrType):
    pass

class ExampleNamedBitString(_Asn1BitStrType):
    V_zero: bool  # bit 0
    V_one: bool   # bit 1
    V_two: bool   # bit 2

Conceptual Representation¶

class _Asn1BitStrType¶

Base class for all ASN.1 BIT STRING types.

__init__(self, size: int = ...) → None¶: Initializes a BIT STRING with the given number of bytes.

property value: bitarray.bitarray | bytes¶

Gets or sets the raw value of the BIT STRING.

Assignment accepts either a bytes object or a bitarray.bitarray.
The returned object can be modified directly, but must be reassigned to take effect.

clear() → None¶: Clears all bits (sets them to 0).

set(bit: int, flag: bool) → None¶: Sets the given bit position to True or False.

get(bit: int) → bool¶: Returns the boolean state of the given bit position.

size() → int¶: Returns the current number of bytes in the BIT STRING.

resize(size: int) → None¶: Adjusts the number of bits in the BIT STRING.

Usage Notes¶

Unlike integer masks, bits can be manipulated directly using _Asn1BitStrType.set() and _Asn1BitStrType.get().
```
bs = ExampleBitString(size=8)
bs.set(3, True)   # set bit 3
print(bs.get(3))  # True
```

Named BIT STRING types provide boolean attributes for each flag:

flags = ExampleNamedBitString()
flags.V_one = True
if flags.V_one:
    print("Flag 'one' is set")

For very small BIT STRING types (≤ 1 byte): If they are encoded using two bytes of space, the queried bit will always be positioned on the last octet internally.