scrypt VS libsodium.js

Check out the scrypt encryption tool.

Given that the encryption algorithm is open source (https://github.com/Tarsnap/scrypt) can you try to explain what you mean here?

I wouldn't use OpenSSL personally. If you just need simple but secure symmetric encryption, checkout the scrypt(1) encryption utility from Tarsnap. If you need support for public keys, check out age(1).

scrypt if you strictly only need symmetric encryption.

^ "scrypt page on the Tarsnap website". Retrieved 21 January 2014.

First of all I know that "implementing your own cryptography is bad". However, at some point, one does stumble upon a use-case that is not (well) covered by existing tools.

Now, assuming one has already done his due-diligence and has read (and hopefully understood at least the main ideas of) cryptography related articles / posts / etc. (especially in the area pertaining to what one wants to build), and thus we can assume one is not a complete newbie in this mater, however, nor is he an expert. Basically we can assume he is an "amateur".

Where would one go with his design to ask for feedback about it, in the hope to at least eliminate some weaknesses that one (as a non expert) might have overlooked. (I'm not speaking here about "proofs" or "audits".)

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More specifically ---- but please let's not get into this right now, this being just an example ---- I'm trying to implement something similar to `scrypt` (the encryption utility, that uses the `scrypt` PBKDF, ) or `age` (), as a replacement to my current solution that relies on GnuPG.

It entirely does and that's exactly my point. Most "hashes" are designed to be fast, for data validation/checking whatever. For securing data (passwords, anonymisation, etc) you want a "hash" to be as slow as possible. Scrypt for example is designed to be extremely slow and use much memory (making GPU-based parallelisation useless and driving up the cost of CPU-based work). The default settings for five-second hashes changes their 18 hour estimate to a bit over two years... and that's assuming you don't turn it up further.

If you want to encrypt the data on the client, so that no one, not even the server is able to decrypt it, then you could use a javascript client side encryption library like https://github.com/jedisct1/libsodium.js/. In this case the server would see opaque random looking bytea data from the client. Note this means the database can't search within that data, it can only serve the encrypted data back to the client for local decryption.

And libsodium for cryptography.