add check password hook

its main use case is to allow to easily support things like password+OTP for
protocols without keyboard interactive support such as FTP and WebDAV

examples/OTP/authy/checkpwd/main.go

@@ -0,0 +1,106 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"os"
+	"time"
+)
+
+type userMapping struct {
+	SFTPGoUsername string
+	AuthyID        int64
+	AuthyAPIKey    string
+}
+
+type checkPasswordResponse struct {
+	// 0 KO, 1 OK, 2 partial success
+	Status int `json:"status"`
+	// for status == 2 this is the password that SFTPGo will check against the one stored
+	// inside the data provider
+	ToVerify string `json:"to_verify"`
+}
+
+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 printResponse(status int, toVerify string) {
+	r := checkPasswordResponse{
+		Status:   status,
+		ToVerify: toVerify,
+	}
+	resp, _ := json.Marshal(r)
+	fmt.Printf("%v\n", string(resp))
+	if status > 0 {
+		os.Exit(0)
+	} else {
+		os.Exit(1)
+	}
+}
+
+func main() {
+	// get credentials from env vars
+	username := os.Getenv("SFTPGO_AUTHD_USERNAME")
+	password := os.Getenv("SFTPGO_AUTHD_PASSWORD")
+
+	for _, m := range mapping {
+		if m.SFTPGoUsername == username {
+			// Authy token len is 7, we assume that we have the password followed by the token
+			pwdLen := len(password)
+			if pwdLen <= 7 {
+				printResponse(0, "")
+			}
+			pwd := password[:pwdLen-7]
+			authyToken := password[pwdLen-7:]
+			// now verify the authy token and instruct SFTPGo to check the password if the token is OK
+			url := fmt.Sprintf("https://api.authy.com/protected/json/verify/%v/%v", authyToken, m.AuthyID)
+			req, err := http.NewRequest(http.MethodGet, url, nil)
+			if err != nil {
+				log.Fatal(err)
+			}
+			req.Header.Set("X-Authy-API-Key", m.AuthyAPIKey)
+			httpClient := &http.Client{
+				Timeout: 10 * time.Second,
+			}
+			resp, err := httpClient.Do(req)
+			if err != nil {
+				printResponse(0, "")
+			}
+			defer resp.Body.Close()
+			if resp.StatusCode != http.StatusOK {
+				// status code 200 is expected
+				printResponse(0, "")
+			}
+			var authyResponse map[string]interface{}
+			respBody, err := ioutil.ReadAll(resp.Body)
+			if err != nil {
+				printResponse(0, "")
+			}
+			err = json.Unmarshal(respBody, &authyResponse)
+			if err != nil {
+				printResponse(0, "")
+			}
+			if authyResponse["success"].(string) == "true" {
+				printResponse(2, pwd)
+			}
+			printResponse(0, "")
+			break
+		}
+	}
+
+	// no mapping found
+	printResponse(0, "")
+}