I don't know that you can rely on random with CTR mode. It's maybe OK for AES since it has a large enough block size, but CTR can be used for smaller block ciphers as well where that isn't the case.

Perhaps something like:

**MUST** be a unique value, any reuse of the nonce with the same key compromises the security of *every* message encrypted with that key. Must be the same number of bytes as the ``block_size`` of the cipher with a given ``key``. The nonce does not need to be kept secret and may be included alongside the ciphertext.