add some examples hooks for one time password logins

The examples use Twillo Authy since I use it for my GitHub account.

You can easily use other multi factor authentication software in a
similar way.

examples/OTP/authy/keyint/main.go

@@ -0,0 +1,137 @@
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"os"
+	"time"
+)
+
+type userMapping struct {
+	SFTPGoUsername string
+	AuthyID        int64
+	AuthyAPIKey    string
+}
+
+type keyboardAuthHookResponse struct {
+	Instruction string   `json:"instruction,omitempty"`
+	Questions   []string `json:"questions,omitempty"`
+	Echos       []bool   `json:"echos,omitempty"`
+	AuthResult  int      `json:"auth_result"`
+	CheckPwd    int      `json:"check_password,omitempty"`
+}
+
+var (
+	mapping []userMapping
+)
+
+func init() {
+	// this is for demo only, you probably want to get this mapping dynamically, for example using a database query
+	mapping = append(mapping, userMapping{
+		SFTPGoUsername: "<SFTPGo username>",
+		AuthyID:        1234567,
+		AuthyAPIKey:    "<your api key>",
+	})
+}
+
+func printAuthResponse(result int) {
+	resp, _ := json.Marshal(keyboardAuthHookResponse{
+		AuthResult: result,
+	})
+	fmt.Printf("%v\n", string(resp))
+	if result == 1 {
+		os.Exit(0)
+	} else {
+		os.Exit(1)
+	}
+}
+
+func main() {
+	// get credentials from env vars
+	username := os.Getenv("SFTPGO_AUTHD_USERNAME")
+	var userMap userMapping
+	for _, m := range mapping {
+		if m.SFTPGoUsername == username {
+			userMap = m
+			break
+		}
+	}
+
+	if userMap.SFTPGoUsername != username {
+		// no mapping found
+		os.Exit(1)
+	}
+
+	checkPwdQuestion := keyboardAuthHookResponse{
+		Instruction: "This is a sample keyboard authentication program that ask for your password + Authy token",
+		Questions:   []string{"Your password: "},
+		Echos:       []bool{false},
+		CheckPwd:    1,
+		AuthResult:  0,
+	}
+
+	q, _ := json.Marshal(checkPwdQuestion)
+	fmt.Printf("%v\n", string(q))
+
+	// in a real world app you probably want to use a read timeout
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Scan()
+	if scanner.Err() != nil {
+		printAuthResponse(-1)
+	}
+	response := scanner.Text()
+	if response != "OK" {
+		printAuthResponse(-1)
+	}
+
+	checkTokenQuestion := keyboardAuthHookResponse{
+		Instruction: "",
+		Questions:   []string{"Authy token: "},
+		Echos:       []bool{false},
+		CheckPwd:    0,
+		AuthResult:  0,
+	}
+
+	q, _ = json.Marshal(checkTokenQuestion)
+	fmt.Printf("%v\n", string(q))
+	scanner.Scan()
+	if scanner.Err() != nil {
+		printAuthResponse(-1)
+	}
+	authyToken := scanner.Text()
+
+	url := fmt.Sprintf("https://api.authy.com/protected/json/verify/%v/%v", authyToken, userMap.AuthyID)
+	req, err := http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		printAuthResponse(-1)
+	}
+	req.Header.Set("X-Authy-API-Key", userMap.AuthyAPIKey)
+	httpClient := &http.Client{
+		Timeout: 10 * time.Second,
+	}
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		printAuthResponse(-1)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		// status code 200 is expected
+		printAuthResponse(-1)
+	}
+	var authyResponse map[string]interface{}
+	respBody, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		printAuthResponse(-1)
+	}
+	err = json.Unmarshal(respBody, &authyResponse)
+	if err != nil {
+		printAuthResponse(-1)
+	}
+	if authyResponse["success"].(string) == "true" {
+		printAuthResponse(1)
+	}
+	printAuthResponse(-1)
+}