// Deal with SASL authentication.

package xmpp

import (

	"crypto/md5"

	"crypto/rand"

	"encoding/base64"

	"encoding/xml"

	"fmt"

	"math/big"

	"regexp"

	"strings"

)

// Server is advertising auth mechanisms it supports. Choose one and

// respond.

// BUG(cjyar): Doesn't implement TLS/SASL EXTERNAL.

func (cl *Client) chooseSasl(fe *Features) {

	for _, m := range fe.Mechanisms.Mechanism {

		switch strings.ToLower(m) {

		case "digest-md5":

			digestMd5 = true

		}

	}

	if digestMd5 {

		auth := &auth{XMLName: xml.Name{Space: NsSASL, Local: "auth"},

			Mechanism: "DIGEST-MD5"}

		cl.sendRaw <- auth

	}

}

// Server is responding to our auth request.

func (cl *Client) handleSasl(srv *auth) {

	switch strings.ToLower(srv.XMLName.Local) {

	case "challenge":

		b64 := base64.StdEncoding

		str, err := b64.DecodeString(srv.Chardata)

		if err != nil {

			cl.setError(fmt.Errorf("SASL: %v", err))

			return

		}

		srvMap := parseSasl(string(str))

		if cl.saslExpected == "" {

			cl.saslDigest1(srvMap)

		} else {

			cl.saslDigest2(srvMap)

		}

	case "failure":

		cl.setError(fmt.Errorf("SASL authentication failed"))

	case "success":

		cl.setStatus(StatusAuthenticated)

		cl.Features = nil

		ss := &stream{To: cl.Jid.Domain(), Version: XMPPVersion}

		cl.sendRaw <- ss

	}

}

func (cl *Client) saslDigest1(srvMap map[string]string) {

	// Make sure it supports qop=auth

	var hasAuth bool

	for _, qop := range strings.Fields(srvMap["qop"]) {

		if qop == "auth" {

			hasAuth = true

		}

	}

	if !hasAuth {

		cl.setError(fmt.Errorf("Server doesn't support SASL auth"))

		return

	}

	// Pick a realm.

	var realm string

	if srvMap["realm"] != "" {

		realm = strings.Fields(srvMap["realm"])[0]

	}

	passwd := cl.password

	nonce := srvMap["nonce"]

	digestUri := "xmpp/" + cl.Jid.Domain()

	nonceCount := int32(1)

	nonceCountStr := fmt.Sprintf("%08x", nonceCount)

	// Begin building the response. Username is

	// user@domain or just domain.

	var username string

	if cl.Jid.Node() == "" {

		username = cl.Jid.Domain()

	} else {

		username = cl.Jid.Node()

	}

	// Generate our own nonce from random data.

	randSize := big.NewInt(0)

	randSize.Lsh(big.NewInt(1), 64)

	cnonce, err := rand.Int(rand.Reader, randSize)

	if err != nil {

		cl.setError(fmt.Errorf("SASL rand: %v", err))

		return

	}

	cnonceStr := fmt.Sprintf("%016x", cnonce)

	/* Now encode the actual password response, as well as the

	 * expected next challenge from the server. */

	response := saslDigestResponse(username, realm, passwd, nonce,

		cnonceStr, "AUTHENTICATE", digestUri, nonceCountStr)

	next := saslDigestResponse(username, realm, passwd, nonce,

		cnonceStr, "", digestUri, nonceCountStr)

	cl.saslExpected = next

	// Build the map which will be encoded.

	clMap := make(map[string]string)

	clMap["realm"] = `"` + realm + `"`

	clMap["username"] = `"` + username + `"`

	clMap["nonce"] = `"` + nonce + `"`

	clMap["cnonce"] = `"` + cnonceStr + `"`

	clMap["nc"] = nonceCountStr

	clMap["qop"] = "auth"

	clMap["digest-uri"] = `"` + digestUri + `"`

	clMap["response"] = response

	if srvMap["charset"] == "utf-8" {

		clMap["charset"] = "utf-8"

	}

	// Encode the map and send it.

	clStr := packSasl(clMap)

	b64 := base64.StdEncoding

	clObj := &auth{XMLName: xml.Name{Space: NsSASL, Local: "response"},

		Chardata: b64.EncodeToString([]byte(clStr))}

	cl.sendRaw <- clObj

}

func (cl *Client) saslDigest2(srvMap map[string]string) {

	if cl.saslExpected == srvMap["rspauth"] {

		clObj := &auth{XMLName: xml.Name{Space: NsSASL, Local: "response"}}

		cl.sendRaw <- clObj

	} else {

		clObj := &auth{XMLName: xml.Name{Space: NsSASL, Local: "failure"}, Any: &Generic{XMLName: xml.Name{Space: NsSASL,

			Local: "abort"}}}

		cl.sendRaw <- clObj

	}

}

// Takes a string like `key1=value1,key2="value2"...` and returns a

// key/value map.

func parseSasl(in string) map[string]string {

	re := regexp.MustCompile(`([^=]+)="?([^",]+)"?,?`)

	strs := re.FindAllStringSubmatch(in, -1)

	m := make(map[string]string)

	for _, pair := range strs {

		key := strings.ToLower(string(pair[1]))

		value := string(pair[2])

		m[key] = value

	}

	return m

}

// Inverse of parseSasl().

func packSasl(m map[string]string) string {

	var terms []string

	for key, value := range m {

		if key == "" || value == "" || value == `""` {

			continue

		}

		terms = append(terms, key+"="+value)

	}

	return strings.Join(terms, ",")

}

// Computes the response string for digest authentication.

func saslDigestResponse(username, realm, passwd, nonce, cnonceStr,

	authenticate, digestUri, nonceCountStr string) string {

	h := func(text string) string {

		h := md5.New()

		h.Write([]byte(text))

		return string(h.Sum(nil))

	}

	hex := func(input string) string {

		return fmt.Sprintf("%x", input)

	}

	kd := func(secret, data string) string {

		return h(secret + ":" + data)

	}

	a1 := h(username+":"+realm+":"+passwd) + ":" +

		nonce + ":" + cnonceStr

	a2 := authenticate + ":" + digestUri

	response := hex(kd(hex(h(a1)), nonce+":"+

		nonceCountStr+":"+cnonceStr+":auth:"+

		hex(h(a2))))

	return response

}