srp: why M1 is not calculated according to the spec?

[masihyeganeh]

Hello.
I was using your library in my code for a couple of years. I just updated it, and saw that M1 calculation has changed.
I see this comment where it is calculated:

// M1 = H(A, B, K) this doesn't follow the spec but apparently no one does for M1
// M1 should equal = H(H(N) XOR H(g) | H(U) | s | A | B | K) according to the spec

It makes sense that you decided to go with what most of the users prefer, but it is breaking functionality of my code.
I'm suggesting to add those back beside current ones, so there will be a proof() function and maybe a proof_std() that uses standard implementation of M1 calculation, and also there will be a verify_server_std() beside verify_server() that calculates M2 using this new M1.

Or just add a process_reply_std() that returns a SrpClientVerifier with the other M1 and M2.

I can also send a PR if you need me to.
Thanks in advance

[tarcieri]

For context this is regarding the changes in #79.

A PR sounds good. I think there will be some bikeshedding about naming, but otherwise we can hash out the details there.

[masihyeganeh]

It's up. Please let me know if you like this implementation or I need to try another approach.
Also, the naming is terrible, I know. I'm not good at naming things 😄