import secrets

def xor(a: bytes, b: bytes):
    if len(a)!=len(b):
        raise Exception("Bad Length")
    return bytes([a[i]^b[i] for i in range(len(a))])

def cbcXor(blocks, key: bytes, iv: bytes = None, BS=16):
    if len(key)!=BS:
        raise Exception("Bad key len")
    if not iv:
        iv = secrets.token_bytes(16)
    lastBlock = iv
    res = []
    for block in blocks:
        if len(block)!=BS:
            raise Exception("Bad block length")
        mid = xor(block, lastBlock)
        enc = xor(mid, key)
        lastBlock = enc
        res.append(enc)
    return res

def decCbcXor(blocks, key: bytes, iv: bytes = None, BS=16):
    if len(key)!=BS:
        raise Exception("Bad key len")
    if not iv:
        iv = secrets.token_bytes(16)
    lastBlock = iv
    res = []
    for block in blocks:
        if len(block)!=BS:
            raise Exception("Bad block length")
        mid = xor(block, key)
        enc = xor(mid, lastBlock)
        lastBlock = block
        res.append(enc)
    return res

def randBlocks(num: int = 64):
    return [secrets.token_bytes(16) for i in range(num)]