def find_block_length(encryption_oracle):
    """Returns the length of a block for the block cipher used by the encryption_oracle.
    To find the length of a block, we encrypt increasingly longer plaintexts until the size of the
    output ciphertext increases too. When this happens, we can then easily compute the length of a
    block as the difference between this new length of the ciphertext and its initial one.
    """
    my_text = b''
    ciphertext = encryption_oracle.encrypt(my_text)
    initial_len = len(ciphertext)
    new_len = initial_len

    while new_len == initial_len:
        my_text += b'A'
        ciphertext = encryption_oracle.encrypt(my_text)
        new_len = len(ciphertext)

    return new_len - initial_len