Signature v3

Last updated: 2020-09-17 18:03:17

TencentCloud API authenticates every single request, i.e., the request must be signed using the security credentials in the designated steps. Each request has to contain the signature information (Signature) in the common request parameters and be sent in the specified way and format.

Applying for Security Credentials

The security credential used in this document is a key, which includes a SecretId and a SecretKey. Each user can have up to two pairs of keys.

  • SecretId: Used to identify the API caller, which is just like a username.
  • SecretKey: Used to authenticate the API caller, which is just like a password.
  • You must keep your security credentials private and avoid disclosure; otherwise, your assets may be compromised. If they are disclosed, please disable them as soon as possible.

You can apply for the security credentials through the following steps:

  1. Log in to the Tencent Cloud Console.
  2. Go to the TencentCloud API Key console page.
  3. On the TencentCloud API Key page, click Create to create a SecretId/SecretKey pair.

Using the Resources for Developers

TencentCloud API comes with SDKs for seven commonly used programming languages, including Python, Java, PHP, Go, NodeJS and .NET. In addition, it provides API Explorer which enables online call, signature verification, and SDK code generation. If you have any troubles calculating a signature, consult these resources.

TC3-HMAC-SHA256 Signature Algorithm

Compatible with the previous HmacSHA1 and HmacSHA256 signature algorithms, the TC3-HMAC-SHA256 signature algorithm is more secure and supports larger requests and JSON format with better performance. We recommend using TC3-HMAC-SHA256 to calculate the signature.

TencentCloud API supports both GET and POST requests. For the GET method, only the Content-Type: application/x-www-form-urlencoded protocol format is supported. For the POST method, two protocol formats, Content-Type: application/json and Content-Type: multipart/form-data, are supported. The JSON format is supported by default for all business APIs, and the multipart format is supported only for specific business APIs. In this case, the API cannot be called in JSON format. See the specific business API documentation for more information. The POST method is recommended, as there is no difference in the results of both the methods, but the GET method only supports request packets up to 32 KB.

The following uses querying the list of CVM instances in the Guangzhou region as an example to describe the steps of signature splicing. We chose this API because:

  1. CVM is activated by default, and this API is often used;
  2. It is read-only and does not change the status of existing resources;
  3. It covers many types of parameters, which allows it to be used to demonstrate how to use arrays containing data structures.

In the example, we try to choose common parameters and API parameters that are prone to mistakes. When you actually call an API, please use parameters based on the actual conditions. The parameters vary by API. Do not copy the parameters and values in this example.

Assuming that your SecretId and SecretKey are AKIDz8krbsJ5yKBZQpn74WFkmLPx3******* and Gu5t9xGARNpq86cd98joQYCN3*******, respectively, if you want to view the status of the instance in the Guangzhou region whose CVM instance name is "unnamed" and have only one data entry returned, then the request may be:

curl -X POST \
-H "Authorization: TC3-HMAC-SHA256 Credential=AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******/2019-02-25/cvm/tc3_request, SignedHeaders=content-type;host, Signature=c492e8e41437e97a620b728c301bb8d17e7dc0c17eeabce80c20cd70fc3a78ff" \
-H "Content-Type: application/json; charset=utf-8" \
-H "Host:" \
-H "X-TC-Action: DescribeInstances" \
-H "X-TC-Timestamp: 1551113065" \
-H "X-TC-Version: 2017-03-12" \
-H "X-TC-Region: ap-guangzhou" \
-d '{"Limit": 1, "Filters": [{"Values": ["unnamed"], "Name": "instance-name"}]}'

The signature calculation process is explained in detail below.

1. Concatenating the CanonicalRequest String

Concatenate the canonical request string (CanonicalRequest) in the following pseudocode format:

CanonicalRequest =
    HTTPRequestMethod + '\n' +
    CanonicalURI + '\n' +
    CanonicalQueryString + '\n' +
    CanonicalHeaders + '\n' +
    SignedHeaders + '\n' +
Field Name Explanation
HTTPRequestMethod HTTP request method (GET or POST). This example uses POST.
CanonicalURI URI parameter. Slash ("/") is used for API 3.0.
CanonicalQueryString The query string in the URL of the originating HTTP request. This is always an empty string “” for POST requests, and is the string after the question mark (?) for GET requests. For example: Limit=10&Offset=0。
Note: CanonicalQueryString must be URL-encoded, referencing RFC3986, the UTF8 character set. We recommend using the programming language library. All special characters must be encoded and capitalized.
CanonicalHeaders Header information for signature calculation, including at least two headers of host and content-type. Custom headers can be added to participate in the signature process to improve the uniqueness and security of the request.
Concatenation rules:
  1. Both the key and value of the header should be converted to lowercase with the leading and trailing spaces removed, so they are concatenated in the format of key:value\n format;
  2. If there are multiple headers, they should be sorted in ASCII ascending order by the header keys (lowercase).
The calculation result in this example is content-type:application/json; charset=utf-8\\n.
Note: content-type must match the actually sent content. In some programming languages, a charset value would be added even if it is not specified. In this case, the request sent is different from the one signed, and the sever will return an error indicating that signature verification failed.
SignedHeaders Header information for signature calculation, indicating which headers of the request participate in the signature process (they must each individually correspond to the headers in CanonicalHeaders). Content-type and host are required headers.
Concatenation rules:
  1. Both the key and value of the header should be converted to lowercase;
  2. If there are multiple headers, they should be sorted in ASCII ascending order by the header keys (lowercase) and separated by semicolons (;).
The value in this example is content-type;host
HashedRequestPayload Hash value of the request payload (i.e., the body, such as {"Limit": 1, "Filters": [{"Values": ["unnamed"], "Name": "instance-name"}]} in this example). The pseudocode for calculation is Lowercase(HexEncode(Hash.SHA256(RequestPayload))) by SHA256 hashing the payload of the HTTP request, performing hexadecimal encoding, and finally converting the encoded string to lowercase letters. For GET requests, RequestPayload is always an empty string. The calculation result in this example is 99d58dfbc6745f6747f36bfca17dee5e6881dc0428a0a36f96199342bc5b4907.

According to the rules above, the CanonicalRequest string obtained in the example is as follows:


content-type:application/json; charset=utf-8


2. Concatenating the String to Be Signed

The string to sign is concatenated as follows:

StringToSign =
    Algorithm + \n +
    RequestTimestamp + \n +
    CredentialScope + \n +
Field Name Explanation
Algorithm Signature algorithm, which is currently always TC3-HMAC-SHA256.
RequestTimestamp Request timestamp, i.e., the value of the common parameter X-TC-Timestamp in the request header, which is the UNIX timestamp of the current time in seconds, such as 1551113065 in this example.
CredentialScope Scope of the credential in the format of Date/service/tc3_request, including the date, requested service and termination string (tc3_request). Date is a date in UTC time, whose value should match the UTC date converted by the common parameter X-TC-Timestamp; service is the product name, which should match the domain name of the product called. The calculation result in this example is 2019-02-25/cvm/tc3_request.
HashedCanonicalRequest Hash value of the CanonicalRequest string concatenated in the steps above. The pseudocode for calculation is Lowercase(HexEncode(Hash.SHA256(CanonicalRequest))). The calculation result in this example is 2815843035062fffda5fd6f2a44ea8a34818b0dc46f024b8b3786976a3adda7a.


  1. Date has to be calculated from the timestamp "X-TC-Timestamp" and the time zone is UTC+0. If you add the system's local time zone information (such as UTC+8), calls can succeed both day and night but will definitely fail at 00:00. For example, if the timestamp is 1551113065 and the time in UTC+8 is 2019-02-26 00:44:25, the UTC+0 date in the calculated Date value should be 2019-02-25 instead of 2019-02-26.
  2. Timestamp must be the same as your current system time, and your system time and standard time must be synced; if the difference between Timestamp and your current system time is larger than five minutes, the request will fail. If your system time is out of sync with the standard time for a while, the request will fail and return a signature expiration error.

According to the preceding rules, the string to be signed obtained in the example is as follows:


3. Calculating the Signature

1) Calculate the derived signature key with the following pseudocode:

SecretKey = "Gu5t9xGARNpq86cd98joQYCN3*******"
SecretDate = HMAC_SHA256("TC3" + SecretKey, Date)
SecretService = HMAC_SHA256(SecretDate, Service)
SecretSigning = HMAC_SHA256(SecretService, "tc3_request")
Field Name Explanation
SecretKey The original SecretKey, i.e., Gu5t9xGARNpq86cd98joQYCN3*******.
Date The Date field information in Credential, such as 2019-02-25 in this example.
Service Value in the Service field in Credential, such as cvm in this example.

2) Calculate the signature with the following pseudocode:

Signature = HexEncode(HMAC_SHA256(SecretSigning, StringToSign))

4. Concatenating the Authorization

The Authorization is concatenated as follows:

Authorization =
    Algorithm + ' ' +
    'Credential=' + SecretId + '/' + CredentialScope + ', ' +
    'SignedHeaders=' + SignedHeaders + ', ' +
    'Signature=' + Signature
Field Name Explanation
Algorithm Signature algorithm, which is always TC3-HMAC-SHA256.
SecretId The SecretId in the key pair, i.e., AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******.
CredentialScope Credential scope (see above). The calculation result in this example is 2019-02-25/cvm/tc3_request.
SignedHeaders Header information for signature calculation (see above), such as content-type;host in this example.
Signature Signature value. The calculation result in this example is c492e8e41437e97a620b728c301bb8d17e7dc0c17eeabce80c20cd70fc3a78ff.

According to the rules above, the value obtained in the example is:

TC3-HMAC-SHA256 Credential=AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******/2019-02-25/cvm/tc3_request, SignedHeaders=content-type;host, Signature=c492e8e41437e97a620b728c301bb8d17e7dc0c17eeabce80c20cd70fc3a78ff

The following example shows a finished authorization header:

Authorization: TC3-HMAC-SHA256 Credential=AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******/2019-02-25/cvm/tc3_request, SignedHeaders=content-type;host, Signature=c492e8e41437e97a620b728c301bb8d17e7dc0c17eeabce80c20cd70fc3a78ff
Content-Type: application/json; charset=utf-8
X-TC-Action: DescribeInstances
X-TC-Version: 2017-03-12
X-TC-Timestamp: 1551113065
X-TC-Region: ap-guangzhou

{"Limit": 1, "Filters": [{"Values": ["unnamed"], "Name": "instance-name"}]}

5. Signature Demo

When calling API 3.0, you are recommended to use the corresponding Tencent Cloud SDK 3.0 which encapsulates the signature process, enabling you to focus on only the specific APIs provided by the product when developing. See SDK Center for more information. Currently, the following programming languages are supported:

To further explain the signing process, we will use a programming language to implement the process described above. The request domain name, API and parameter values in the sample are used here. This goal of this example is only to provide additional clarification for the signature process, please see the SDK for actual usage.

The final output URL might be:*******&Signature=EliP9YW3pW28FpsEdkXt%2F%2BWcGeI%3D&Timestamp=1465185768&Version=2017-03-12.

Note: The key in the example is fictitious, and the timestamp is not the current time of the system, so if this URL is opened in the browser or called using commands such as curl, an authentication error will be returned: Signature expired. In order to get a URL that can work properly, you need to replace the SecretId and SecretKey in the example with your real credentials and use the current time of the system as the Timestamp.

Note: In the example below, even if you use the same programming language, the order of the parameters in the URL may be different for each execution. However, the order does not matter, as long as all the parameters are included in the URL and the signature is calculated correctly.

Note: The following code is only applicable to API 3.0. It cannot be directly used in other signature processes. Even with an older API, signature calculation errors may occur due to the differences in details. Please refer to the corresponding documentation.


import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;
import java.util.TreeMap;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import javax.xml.bind.DatatypeConverter;

public class TencentCloudAPITC3Demo {
    private final static Charset UTF8 = StandardCharsets.UTF_8;
    private final static String SECRET_ID = "AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******";
    private final static String SECRET_KEY = "Gu5t9xGARNpq86cd98joQYCN3*******";
    private final static String CT_JSON = "application/json; charset=utf-8";

    public static byte[] hmac256(byte[] key, String msg) throws Exception {
        Mac mac = Mac.getInstance("HmacSHA256");
        SecretKeySpec secretKeySpec = new SecretKeySpec(key, mac.getAlgorithm());
        return mac.doFinal(msg.getBytes(UTF8));

    public static String sha256Hex(String s) throws Exception {
        MessageDigest md = MessageDigest.getInstance("SHA-256");
        byte[] d = md.digest(s.getBytes(UTF8));
        return DatatypeConverter.printHexBinary(d).toLowerCase();

    public static void main(String[] args) throws Exception {
        String service = "cvm";
        String host = "";
        String region = "ap-guangzhou";
        String action = "DescribeInstances";
        String version = "2017-03-12";
        String algorithm = "TC3-HMAC-SHA256";
        String timestamp = "1551113065";
        //String timestamp = String.valueOf(System.currentTimeMillis() / 1000);
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        // Pay attention to the time zone; otherwise, errors may occur
        String date = sdf.format(new Date(Long.valueOf(timestamp + "000")));

        // ************* Step 1: Concatenate the CanonicalRequest string *************
        String httpRequestMethod = "POST";
        String canonicalUri = "/";
        String canonicalQueryString = "";
        String canonicalHeaders = "content-type:application/json; charset=utf-8\n" + "host:" + host + "\n";
        String signedHeaders = "content-type;host";

        String payload = "{\"Limit\": 1, \"Filters\": [{\"Values\": [\"unnamed\"], \"Name\": \"instance-name\"}]}";
        String hashedRequestPayload = sha256Hex(payload);
        String canonicalRequest = httpRequestMethod + "\n" + canonicalUri + "\n" + canonicalQueryString + "\n"
                + canonicalHeaders + "\n" + signedHeaders + "\n" + hashedRequestPayload;

        // ************* Step 2: Concatenate the string to sign *************
        String credentialScope = date + "/" + service + "/" + "tc3_request";
        String hashedCanonicalRequest = sha256Hex(canonicalRequest);
        String stringToSign = algorithm + "\n" + timestamp + "\n" + credentialScope + "\n" + hashedCanonicalRequest;

        // ************* Step 3: Calculate the signature *************
        byte[] secretDate = hmac256(("TC3" + SECRET_KEY).getBytes(UTF8), date);
        byte[] secretService = hmac256(secretDate, service);
        byte[] secretSigning = hmac256(secretService, "tc3_request");
        String signature = DatatypeConverter.printHexBinary(hmac256(secretSigning, stringToSign)).toLowerCase();

        // ************* Step 4: Concatenate the Authorization *************
        String authorization = algorithm + " " + "Credential=" + SECRET_ID + "/" + credentialScope + ", "
                + "SignedHeaders=" + signedHeaders + ", " + "Signature=" + signature;

        TreeMap<String, String> headers = new TreeMap<String, String>();
        headers.put("Authorization", authorization);
        headers.put("Content-Type", CT_JSON);
        headers.put("Host", host);
        headers.put("X-TC-Action", action);
        headers.put("X-TC-Timestamp", timestamp);
        headers.put("X-TC-Version", version);
        headers.put("X-TC-Region", region);

        StringBuilder sb = new StringBuilder();
        sb.append("curl -X POST https://").append(host)
        .append(" -H \"Authorization: ").append(authorization).append("\"")
        .append(" -H \"Content-Type: application/json; charset=utf-8\"")
        .append(" -H \"Host: ").append(host).append("\"")
        .append(" -H \"X-TC-Action: ").append(action).append("\"")
        .append(" -H \"X-TC-Timestamp: ").append(timestamp).append("\"")
        .append(" -H \"X-TC-Version: ").append(version).append("\"")
        .append(" -H \"X-TC-Region: ").append(region).append("\"")
        .append(" -d '").append(payload).append("'");


# -*- coding: utf-8 -*-
import hashlib, hmac, json, os, sys, time
from datetime import datetime

# Key Parameters
secret_id = "AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******"
secret_key = "Gu5t9xGARNpq86cd98joQYCN3*******"

service = "cvm"
host = ""
endpoint = "https://" + host
region = "ap-guangzhou"
action = "DescribeInstances"
version = "2017-03-12"
algorithm = "TC3-HMAC-SHA256"
#timestamp = int(time.time())
timestamp = 1551113065
date = datetime.utcfromtimestamp(timestamp).strftime("%Y-%m-%d")
params = {"Limit": 1, "Filters": [{"Name": "instance-name", "Values": ["unnamed"]}]}

# ************* Step 1: Concatenate the CanonicalRequest string *************
http_request_method = "POST"
canonical_uri = "/"
canonical_querystring = ""
ct = "application/json; charset=utf-8"
payload = json.dumps(params)
canonical_headers = "content-type:%s\nhost:%s\n" % (ct, host)
signed_headers = "content-type;host"
hashed_request_payload = hashlib.sha256(payload.encode("utf-8")).hexdigest()
canonical_request = (http_request_method + "\n" +
                     canonical_uri + "\n" +
                     canonical_querystring + "\n" +
                     canonical_headers + "\n" +
                     signed_headers + "\n" +

# ************* Step 2: Concatenate the string to sign *************
credential_scope = date + "/" + service + "/" + "tc3_request"
hashed_canonical_request = hashlib.sha256(canonical_request.encode("utf-8")).hexdigest()
string_to_sign = (algorithm + "\n" +
                  str(timestamp) + "\n" +
                  credential_scope + "\n" +

# ************* Step 3: Calculate the Signature *************
# Function for computing signature digest
def sign(key, msg):
    return, msg.encode("utf-8"), hashlib.sha256).digest()
secret_date = sign(("TC3" + secret_key).encode("utf-8"), date)
secret_service = sign(secret_date, service)
secret_signing = sign(secret_service, "tc3_request")
signature =, string_to_sign.encode("utf-8"), hashlib.sha256).hexdigest()

# ************* Step 4: Concatenate the Authorization *************
authorization = (algorithm + " " +
                 "Credential=" + secret_id + "/" + credential_scope + ", " +
                 "SignedHeaders=" + signed_headers + ", " +
                 "Signature=" + signature)

print('curl -X POST ' + endpoint
      + ' -H "Authorization: ' + authorization + '"'
      + ' -H "Content-Type: application/json; charset=utf-8"'
      + ' -H "Host: ' + host + '"'
      + ' -H "X-TC-Action: ' + action + '"'
      + ' -H "X-TC-Timestamp: ' + str(timestamp) + '"'
      + ' -H "X-TC-Version: ' + version + '"'
      + ' -H "X-TC-Region: ' + region + '"'
      + " -d '" + payload + "'")


package main

import (

func sha256hex(s string) string {
    b := sha256.Sum256([]byte(s))
    return hex.EncodeToString(b[:])

func hmacsha256(s, key string) string {
    hashed := hmac.New(sha256.New, []byte(key))
    return string(hashed.Sum(nil))

func main() {
    secretId := "AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******"
    secretKey := "Gu5t9xGARNpq86cd98joQYCN3*******"
    host := ""
    algorithm := "TC3-HMAC-SHA256"
    service := "cvm"
    version := "2017-03-12"
    action := "DescribeInstances"
    region := "ap-guangzhou"
    //var timestamp int64 = time.Now().Unix()
    var timestamp int64 = 1551113065

    // step 1: build canonical request string
    httpRequestMethod := "POST"
    canonicalURI := "/"
    canonicalQueryString := ""
    canonicalHeaders := "content-type:application/json; charset=utf-8\n" + "host:" + host + "\n"
    signedHeaders := "content-type;host"
    payload := `{"Limit": 1, "Filters": [{"Values": ["unnamed"], "Name": "instance-name"}]}`
    hashedRequestPayload := sha256hex(payload)
    canonicalRequest := fmt.Sprintf("%s\n%s\n%s\n%s\n%s\n%s",

    // step 2: build string to sign
    date := time.Unix(timestamp, 0).UTC().Format("2006-01-02")
    credentialScope := fmt.Sprintf("%s/%s/tc3_request", date, service)
    hashedCanonicalRequest := sha256hex(canonicalRequest)
    string2sign := fmt.Sprintf("%s\n%d\n%s\n%s",

    // step 3: sign string
    secretDate := hmacsha256(date, "TC3"+secretKey)
    secretService := hmacsha256(service, secretDate)
    secretSigning := hmacsha256("tc3_request", secretService)
    signature := hex.EncodeToString([]byte(hmacsha256(string2sign, secretSigning)))

    // step 4: build authorization
    authorization := fmt.Sprintf("%s Credential=%s/%s, SignedHeaders=%s, Signature=%s",

    curl := fmt.Sprintf(`curl -X POST https://%s\
 -H "Authorization: %s"\
 -H "Content-Type: application/json; charset=utf-8"\
 -H "Host: %s" -H "X-TC-Action: %s"\
 -H "X-TC-Timestamp: %d"\
 -H "X-TC-Version: %s"\
 -H "X-TC-Region: %s"\
 -d '%s'`, host, authorization, host, action, timestamp, version, region, payload)


$secretId = "AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******";
$secretKey = "Gu5t9xGARNpq86cd98joQYCN3*******";
$host = "";
$service = "cvm";
$version = "2017-03-12";
$action = "DescribeInstances";
$region = "ap-guangzhou";
// $timestamp = time();
$timestamp = 1551113065;
$algorithm = "TC3-HMAC-SHA256";

// step 1: build canonical request string
$httpRequestMethod = "POST";
$canonicalUri = "/";
$canonicalQueryString = "";
$canonicalHeaders = "content-type:application/json; charset=utf-8\n"."host:".$host."\n";
$signedHeaders = "content-type;host";
$payload = '{"Limit": 1, "Filters": [{"Values": ["unnamed"], "Name": "instance-name"}]}';
$hashedRequestPayload = hash("SHA256", $payload);
$canonicalRequest = $httpRequestMethod."\n"
echo $canonicalRequest.PHP_EOL;

// step 2: build string to sign
$date = gmdate("Y-m-d", $timestamp);
$credentialScope = $date."/".$service."/tc3_request";
$hashedCanonicalRequest = hash("SHA256", $canonicalRequest);
$stringToSign = $algorithm."\n"
echo $stringToSign.PHP_EOL;

// step 3: sign string
$secretDate = hash_hmac("SHA256", $date, "TC3".$secretKey, true);
$secretService = hash_hmac("SHA256", $service, $secretDate, true);
$secretSigning = hash_hmac("SHA256", "tc3_request", $secretService, true);
$signature = hash_hmac("SHA256", $stringToSign, $secretSigning);
echo $signature.PHP_EOL;

// step 4: build authorization
$authorization = $algorithm
    ." Credential=".$secretId."/".$credentialScope
    .", SignedHeaders=content-type;host, Signature=".$signature;
echo $authorization.PHP_EOL;

$curl = "curl -X POST https://".$host
    .' -H "Authorization: '.$authorization.'"'
    .' -H "Content-Type: application/json; charset=utf-8"'
    .' -H "Host: '.$host.'"'
    .' -H "X-TC-Action: '.$action.'"'
    .' -H "X-TC-Timestamp: '.$timestamp.'"'
    .' -H "X-TC-Version: '.$version.'"'
    .' -H "X-TC-Region: '.$region.'"'
    ." -d '".$payload."'";
echo $curl.PHP_EOL;


# -*- coding: UTF-8 -*-
# require ruby>=2.3.0
require 'digest'
require 'json'
require 'time'
require 'openssl'

# Key Parameters
secret_id = 'AKIDz8krbsJ5yKBZQpn74WFkmLPx3*******'
secret_key = 'Gu5t9xGARNpq86cd98joQYCN3*******'

service = 'cvm'
host = ''
endpoint = 'https://' + host
region = 'ap-guangzhou'
action = 'DescribeInstances'
version = '2017-03-12'
algorithm = 'TC3-HMAC-SHA256'
# timestamp =
timestamp = 1551113065
date ='%Y-%m-%d')

# ************* Step 1: Concatenate the CanonicalRequest string *************
http_request_method = 'POST'
canonical_uri = '/'
canonical_querystring = ''
canonical_headers = "content-type:application/json; charset=utf-8\nhost:#{host}\n"
signed_headers = 'content-type;host'
# params