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ergo/vendor/github.com/xdg-go/scram/server_conv.go

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2021-07-30 18:20:13 +02:00
// Copyright 2018 by David A. Golden. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
package scram
import (
"crypto/hmac"
"encoding/base64"
"errors"
"fmt"
)
type serverState int
const (
serverFirst serverState = iota
serverFinal
serverDone
)
// ServerConversation implements the server-side of an authentication
// conversation with a client. A new conversation must be created for
// each authentication attempt.
type ServerConversation struct {
nonceGen NonceGeneratorFcn
hashGen HashGeneratorFcn
credentialCB CredentialLookup
state serverState
credential StoredCredentials
valid bool
gs2Header string
username string
authzID string
nonce string
c1b string
s1 string
}
// Step takes a string provided from a client and attempts to move the
// authentication conversation forward. It returns a string to be sent to the
// client or an error if the client message is invalid. Calling Step after a
// conversation completes is also an error.
func (sc *ServerConversation) Step(challenge string) (response string, err error) {
switch sc.state {
case serverFirst:
sc.state = serverFinal
response, err = sc.firstMsg(challenge)
case serverFinal:
sc.state = serverDone
response, err = sc.finalMsg(challenge)
default:
response, err = "", errors.New("Conversation already completed")
}
return
}
// Done returns true if the conversation is completed or has errored.
func (sc *ServerConversation) Done() bool {
return sc.state == serverDone
}
// Valid returns true if the conversation successfully authenticated the
// client.
func (sc *ServerConversation) Valid() bool {
return sc.valid
}
// Username returns the client-provided username. This is valid to call
// if the first conversation Step() is successful.
func (sc *ServerConversation) Username() string {
return sc.username
}
// AuthzID returns the (optional) client-provided authorization identity, if
// any. If one was not provided, it returns the empty string. This is valid
// to call if the first conversation Step() is successful.
func (sc *ServerConversation) AuthzID() string {
return sc.authzID
}
func (sc *ServerConversation) firstMsg(c1 string) (string, error) {
msg, err := parseClientFirst(c1)
if err != nil {
sc.state = serverDone
return "", err
}
sc.gs2Header = msg.gs2Header
sc.username = msg.username
sc.authzID = msg.authzID
sc.credential, err = sc.credentialCB(msg.username)
if err != nil {
sc.state = serverDone
return "e=unknown-user", err
}
sc.nonce = msg.nonce + sc.nonceGen()
sc.c1b = msg.c1b
sc.s1 = fmt.Sprintf("r=%s,s=%s,i=%d",
sc.nonce,
base64.StdEncoding.EncodeToString([]byte(sc.credential.Salt)),
sc.credential.Iters,
)
return sc.s1, nil
}
// For errors, returns server error message as well as non-nil error. Callers
// can choose whether to send server error or not.
func (sc *ServerConversation) finalMsg(c2 string) (string, error) {
msg, err := parseClientFinal(c2)
if err != nil {
return "", err
}
// Check channel binding matches what we expect; in this case, we expect
// just the gs2 header we received as we don't support channel binding
// with a data payload. If we add binding, we need to independently
// compute the header to match here.
if string(msg.cbind) != sc.gs2Header {
return "e=channel-bindings-dont-match", fmt.Errorf("channel binding received '%s' doesn't match expected '%s'", msg.cbind, sc.gs2Header)
}
// Check nonce received matches what we sent
if msg.nonce != sc.nonce {
return "e=other-error", errors.New("nonce received did not match nonce sent")
}
// Create auth message
authMsg := sc.c1b + "," + sc.s1 + "," + msg.c2wop
// Retrieve ClientKey from proof and verify it
clientSignature := computeHMAC(sc.hashGen, sc.credential.StoredKey, []byte(authMsg))
clientKey := xorBytes([]byte(msg.proof), clientSignature)
storedKey := computeHash(sc.hashGen, clientKey)
// Compare with constant-time function
if !hmac.Equal(storedKey, sc.credential.StoredKey) {
return "e=invalid-proof", errors.New("challenge proof invalid")
}
sc.valid = true
// Compute and return server verifier
serverSignature := computeHMAC(sc.hashGen, sc.credential.ServerKey, []byte(authMsg))
return "v=" + base64.StdEncoding.EncodeToString(serverSignature), nil
}