I need to create an app that includes facts that is encrypted (using AES, but which is irrelevant). The knowledge is decrypted in a world-wide-web browser that involves a mix of two keys: 1. A important regarded only to the user, these kinds of as a passphrase (let us phone it `Ku`) 2. A important saved in a server and is only offered to the browser soon after sure credentials (`Ks`) are confirmed. The trouble I am striving to resolve is that no party (consumer or server) really should be equipped to decrypt the info making use of only a person of the two keys. Only the browser is allowed to decrypt the important after the user has typed a passphrase (which is not despatched to the server) and acquired 50 percent of the essential from the server (after authenticating himself). `Ku` is a consumer-outlined passphrase this is with any luck , sturdy enough (with more than enough entropy). `Ks` can be generated randomly, e.g. be a string lengthy enough (these types of as 32 random bytes, perhaps encoded as foundation64). There are two tips I’m looking at and would like your opinion on them: 1. Use a crucial that is `KDF(Ku || Ks)`, i.e. concatenate the two keys (as basic strings) and use then a KDF to derive a symmetric important. (`Ks` could be base64 encoded in this case – it would nonetheless be 256 little bit entropy) 2. Use a (static) ECDH to complete a critical match. In this circumstance `Ks` would be the private vital of the server. `Ku` is the user’s personal critical wrapped with a passphrase (that is, converting the user’s passphrase to a critical with a KDF, then we use AES-KW per RFC-3394 to help you save the personal EC- to pack the essential). Ideas on the above? Or is there a far better possibility? (As for KDF, that will in all probability be Argon2id, but any solid KDF should really get the job done)
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