SPAKE2/SPAKE2+

SPAKE2/SPAKE2+ according to https://tools.ietf.org/id/draft-irtf-cfrg-spake2-04.html#rfc.section.2

** SPAKE2 **
A:    w = pwkdf(...)
A:    X = x*G + w*M
A->B: X
   B: Y = y*G + w*N
   B: X' = X - w*M
   B: S = y*X'
   B: K = H(Aid, Bid, X, Y, S, w)
A<-B: Y
A:    Y' = Y - w*N
A:    S = x*Y'
A:    K = H(Aid, Bid, X, Y, S, w)

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** SPAKE2+ **
S has: w0, L = w1*G

C:    w0, w1 = pwkdf(...)
C:    X = x*G + w0*M
C->S: X
   S: Y = y*G + w0*N
   S: X' = X - w0*M
   S: S = y*X'
   S: V = y*L
   S: K = H(Cid, Sid, X, Y, S, V)
C<-S: Y
C:    Y' = Y - w0*N
C:    S = x*Y'
C:    V = w1*Y'
C:    K = H(Cid, Sid, X, Y, S, V)