Right, but AES is a primitive used in a lot of protocols, and they might need to do something different with the IV. The source of randomness is also traditionally something that people want control over, because some platforms can have terrible randomness.

Even high-level libraries like crypto_secretbox still take the nonce separately. They do have a combined mode that prepends the authentication code to the cipher text, and most people who just want to encrypt something should probably look at a higher-level interface like this one instead of directly using raw AES libraries.

That being said, providing an interface where the IV is optional and the default value is a constant instead of random is still insane. That wouldn't be out of place in some Underhanded Crypto Contest where the goal is to create subtle bugs.

As someone else has already said, how you want to handle the IV depends on the application. There is no good default method useful for every case.

It is very rarely necessary to prepend an IV to the cipher text, because normally every application provides something that is useful as an IV, e.g. some unique serial number or some other kind of unique identifier.

For instance, I have an archive of data stored on tape cartridges. I have on my computer a database that allows me to search for information stored on the tapes, which tells me e.g. that the file that I want is in "Tape 174 file 103".

Each tape cartridge (6 TB per cartridge LTO-7) stores about 120 files of 50 GB each, inside which the actual archived data reside.

The archive files are encrypted. Both the 256-bit decryption key and the 128-bit CTR IV are generated simultaneously with a one-way hash function (SHA-384) by hashing some secret data (which is not used for any other purpose) concatenated with the unique name of the file, e.g. "Tape 174 file 103". Thus for any of the encrypted files, both the AES key and the CTR IV are unique and never shared with any other kind of encrypted data.

An AES-CTR encryption/decryption function, e.g. for AES-GCM, should always have separate input parameters for key & IV, without default values, to allow you to use whatever is more suitable in your environment for deriving them.

Hashing a combination of secret data and unique data, like in the example above, is usually a good method for deriving a pair of key and IV for file encryption. For things like encrypting packets of a communication connection, the key is derived only once, to avoid the overhead of switching keys and only the IV changes from packet to packet. Standards like GCM specify how this should be handled. If the value of the IV is known to an adversary, that is normally not harmful, but it is even better when the adversary does not know the value of the IV. (Cryptographic algorithms are designed to resist attacks where the attacker has maximum information, but in practice you also try to minimize the information available to the attacker.)

Trail of Bits is charging hefty sums for audits. I suppose they could provide some patches.

This argument doesn't hold because paid cryptography libraries aren't any better and equally provide their code as-is.

So the next and final commit should be a message at the top of the README saying the library is unmaintained and contains serious vulnerabilities and users are advised to move to alternative XYZ.