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If you have the technical knowledge, you can just read the protocols, find out if they make sense, and then implement them yourself. Most of them are quite straight forward so it's not possible to hide a backdoor like Dual_EC_DRBG in the protocol.

If you are not so technical then you have to decide who to trust. For example, you may trust that open source software has been vetted enough and build one from source. Or trust that the built artefacts downloaded from github is good enough. Or trust that the software downloaded from a website not marked as fraud by Google Chrome is good enough. Etc.

In any case, the more technical knowledge you have, the more confidence you can have by doing due diligence yourself.