implement SCRAM-SHA-256

vendor/github.com/xdg-go/scram/README.md

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+
+# scram – Go implementation of RFC-5802
+
+## Description
+
+Package scram provides client and server implementations of the Salted
+Challenge Response Authentication Mechanism (SCRAM) described in
+[RFC-5802](https://tools.ietf.org/html/rfc5802) and
+[RFC-7677](https://tools.ietf.org/html/rfc7677).
+
+It includes both client and server side support.
+
+Channel binding and extensions are not (yet) supported.
+
+## Examples
+
+### Client side
+
+    package main
+
+    import "github.com/xdg-go/scram"
+
+    func main() {
+        // Get Client with username, password and (optional) authorization ID.
+        clientSHA1, err := scram.SHA1.NewClient("mulder", "trustno1", "")
+        if err != nil {
+            panic(err)
+        }
+
+        // Prepare the authentication conversation. Use the empty string as the
+        // initial server message argument to start the conversation.
+        conv := clientSHA1.NewConversation()
+        var serverMsg string
+
+        // Get the first message, send it and read the response.
+        firstMsg, err := conv.Step(serverMsg)
+        if err != nil {
+            panic(err)
+        }
+        serverMsg = sendClientMsg(firstMsg)
+
+        // Get the second message, send it, and read the response.
+        secondMsg, err := conv.Step(serverMsg)
+        if err != nil {
+            panic(err)
+        }
+        serverMsg = sendClientMsg(secondMsg)
+
+        // Validate the server's final message.  We have no further message to
+        // send so ignore that return value.
+        _, err = conv.Step(serverMsg)
+        if err != nil {
+            panic(err)
+        }
+
+        return
+    }
+
+    func sendClientMsg(s string) string {
+        // A real implementation would send this to a server and read a reply.
+        return ""
+    }
+
+## Copyright and License
+
+Copyright 2018 by David A. Golden. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License"). You may
+obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0