def decode_int(r: BytearrayReader) -> bytes:
if r.get() != 0x02:
raise ValueError
n = decode_length(r)
if n == 0:
raise ValueError
if r.peek() & 0x80:
raise ValueError # negative integer
if r.peek() == 0x00 and n > 1:
r.get()
n -= 1
if r.peek() & 0x80 == 0x00:
raise ValueError # excessive zero-padding
if r.peek() == 0x00:
raise ValueError # excessive zero-padding
return r.read(n)
def decode_length(r: BytearrayReader) -> int:
init = r.get()
if init < 0x80:
# short form encodes length in initial octet
return init
if init == 0x80 or init == 0xFF or r.peek() == 0x00:
raise ValueError # indefinite length, RFU or not shortest possible
# long form
n = 0
for _ in range(init & 0x7F):
n = n * 0x100 + r.get()
if n < 128:
raise ValueError # encoding is not the shortest possible
return n