Change the createCipher to createDecipheriv as it is deprecated

[Y0ngg4n]

What kind of change does this PR introduce?

createCipher is deprecated and @aumetra helped me to change it to the new function definition.
It also adds a flake.nix for nixos development.

Why was this change needed?

createCipher is deprecated

Other information:

eg: Did you discuss this change with anybody before working on it (not required, but can be a good idea for bigger changes). Any plans for the future, etc?

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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[aumetra]

(side note: In order to use AES-CBC correctly here, I'd generate a random IV and append it to the end of the cipher text. Then in the decryption you can retrieve it by splitting off the last 16 bytes of the ciphertext. I forgot to mention it that evening when I helped you out, @Y0ngg4n)

(side side note (this goes more to the maintainers): Is there a specific reason why CBC is used as the mode, as opposed to AES-GCM-SIV? Or was it just used because it came to mind first?)

[Eshwar1212-maker]

Nice PR! It seems the maintainer for this repo doesnt review PRs? Is he busy? I see alot of good ones and i made a few as well.

[Y0ngg4n]

Nice PR! It seems the maintainer for this repo doesnt review PRs? Is he busy? I see alot of good ones and i made a few as well.

I dont know :(