low-level DER formatting in python

asn1.py

def der_tag(tag_type: int, composed = False, tag_class = 0):
	assert 0 <= tag_class < 4 and 0 <= tag_type < 31  # FIXME: long tag types not implemented
	return bytes([ tag_class << 6 | int(composed) << 5 | tag_type ])
def der_length(x: int):
	if x < 128: return bytes([x])
	n = (x.bit_length() + 7) // 8
	assert n < 127
	return bytes([ 128 | n ]) + x.to_bytes(n)
def der_wrap(payload: bytes, raw: bool, tag_type: int, composed = False, tag_class = 0):
	if raw: return payload
	return (der_tag(tag_type, composed, tag_class) + der_length(len(payload))) + payload

der_null = der_wrap(b'', False, 5)
def der_integer(x: int, raw=False):
	n = (~x if x < 0 else x).bit_length() // 8 + 1
	return der_wrap(x.to_bytes(n, signed=True), raw, 2)
def der_bitstring(x: bytes, unused_bits=0, raw=False):
	return der_wrap(bytes([unused_bits]) + x, raw, 3)
def der_octetstring(x: bytes, raw=False):
	return der_wrap(x, raw, 4)
def der_oid(*oid: int, raw=False):
	assert all(i >= 0 for i in oid)
	assert oid[0] < 2 and (oid[0] == 1 or oid[1] < 40)
	oid = (oid[0]*40 + oid[1], *oid[2:])
	return der_wrap(b''.join(map(oid_varint, oid)), raw, 6)
def der_utf8string(x: str, raw=False):
	return der_wrap(x.encode('utf-8'), raw, 12)
def der_ia5string(x: str, raw=False):
	return der_wrap(x.encode('ascii'), raw, 22)
def der_sequence(*tags: bytes, raw=False):
	return der_wrap(b''.join(tags), raw, 16, True)

def oid_varint(x: int):
	n = max((x.bit_length() + 6) // 7, 1)
	return bytes((x >> (i*7)) & 127 | (min(i, 1) << 7) for i in reversed(range(n)))