code: function getu(params, A, B) {
    assertIsNBuffer(A, params, "A");
    assertIsNBuffer(B, params, "B");
    var u_buf = crypto.createHash(params.hash)
      .update(A).update(B)
      .digest();
    return(new BigInteger(u_buf));
  }
docs: /*
  getu() hashes the two public messages together, to obtain a scrambling
  parameter "u" which cannot be predicted by either party ahead of time.
  This makes it safe to use the message ordering defined in the SRP-6a
  paper, in which the server reveals their "B" value before the client
  commits to their "A" value.
  
  params:
  params (obj)     group parameters, with .N, .g, .hash
  A (Buffer)       client ephemeral public key
  B (Buffer)       server ephemeral public key
  
  returns: u (bignum)      shared scrambling parameter