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It occurred to me g does not need to be made public for ElGamal signing, if the value g^H(m) is stored as the password hash, generated by the client. Also (r, s) can be changed to (r, r^s) to reduce server verification load to one mod power and one precision multiply mod p, and a subtraction equality test. So on the creation of a new password (y, p, g^H(m)) is created, and each log in needs the client to generate a k value to make (r, r^s).

Password recovery would be a little complex, and involve some email backdoor based on maybe using x as a pseudo H(m), and verifying the changes via generation of y. This would of course only set the local browser to have a new password. So maybe a unique (y, p, g^H(m)) per browser local store used. Index the local storage via email address, and Bob’s yer been here before.

Also, the server can crypt any pending view using H(m) as a person’s private key, or the private key as a browser specific personal private key, or maybe even browser key with all clients using same local store x value. All using DH shared secrets. This keeps data in a database a bit more private, and sometimes encrypt to self might be useful.

Is s=H(m)(1-r)(k^-1) mod (p-1) an option? As this sets H(m)=x, eliminating another y, making (p, g^H(m)) sufficient for authentication server storage, and g is only needed if the server needs to send crypts. Along with r=g^k mod p, as some easy sign. (r, s) might have to be used, as r^s could be equated as modinverse(r) for an easy g^H(m) equality, and the requirement to calculate s from r^s is a challenge. So a secure version is not quite as server efficient.

In reality k also has to be computed to prevent (r, s) reuse. This requires the k choice is the servers. Sending k in plaintext defeats the security, so g is needed, to calculate g^z, and so g^(H(m))^z=k on both sides. A retry randomizer to hide s=0, and a protocol is possible.

This surpasses a server md5 of the password. If the md5 is client side, a server capture can log in. If the md5 is server side, the transit intercept is … but a server DB compromise also needs a web server compromise. This algorithm also needs a client side compromise, or email intercept as per.

The reuse of (r, s) can’t be prevented without knowing k, and hence H(m), therefore a shared secret as a returned value implies H(m) knowledge. So one mod power client side, and two server side.

g^k to client.
(g^k)^H(m) to server.
(g^H(m))^k = (g^k)^H(m) tests true.

Signatures are useless as challenge responses. The RSA version would have to involve a signature on H(m) and so need H(m) direct. Also, the function H can be quite interesting to study. The application of client side salt also is not needed on the server side as a decode key, and so not decoded there. DH is so cool like that. And (p-1) having a large factor is easy to arrange in the key generation. And write access is harder, most of the time, to obtain for data.

The storing of a crypt with the g^k used, locks it for H(m) keyed access. This could void data on a password reset, or a browser local storage reset, but does prevent some client’s data leak opertunities, such as DB decrypt keys. This would have multiple crypts of the symmetric key for shared data, but would this significantly reduce the shared key security? It would prevent new users accessing the said secured data without cracking the shared key. A locked share for private threads say?

Spamming your friends with g^salt and g^salt^H(m)?

The first one is a good idea, the second not so much. AI spam encoding g^salt to your and friends accounts. The critical thing is the friend doesn’t get the password. Assuming a bad friend, who registers and gets g^salt to activate, from their own chosen spoof password. An email does get sent to your email, to cancel the friend as an option, and no other problem exists excepting login to a primary mail account. As a spoof maybe would see the option to remove you from your own account.

The primary control email account would then need secondary authentication. Such as only see the spam folder, and know what to open first and in order. For password recovery, this would be ok. For initial registration, it would be first come first served anyhow.