diff --git a/README.md b/README.md
index 8221932..40254e2 100644
--- a/README.md
+++ b/README.md
@@ -21,20 +21,50 @@ $ python setup.py install # install py-common
 
 ## Usage
 
-**Note:** Before using the py-common in your operating system, you need to make a two-step preparation:
+**Note:** Before using the py-common in your application, you need to make the following preparations:
 
-1. Build executables with sdk-go-common cryption tools. To build these tools, you may need to install **golang** package **sdk-go-common**. For more details please refer to [sdk-go-common](https://github.com/arxanchain/sdk-go-common/tree/master/crypto/tools/README.md)
+### 1.Configure your encryption and signing libraries
 
-2. Copy executables **crypto-util** and **sign-util** into your py-common installation path `cryption/utils`.
+1. Build executables with sdk-go-common encryption tools. To build these executables, you need to install **golang** and download **sdk-go-common**. For more details please refer to [sdk-go-common](https://github.com/arxanchain/sdk-go-common/tree/master/crypto/tools/README.md).
 
-If you have no idea where your py-common is installed, use the following command to check out.
+2. Copy the executables **crypto-util** and **sign-util** into your py-common installation path `cryption/utils`.
+
+If you have no idea where your py-common is installed, use the following command to check it out (you need to leave the py-common code repo before running this command).
 
 ```sh
 $ python -c 'import imp;print imp.find_module("cryption")[1]'
 /usr/local/lib/python2.7/site-packages/py_common-1.5.0-py2.7.egg/cryption
 ```
 
-In this case, you should copy executables into path `/usr/local/lib/python2.7/site-packages/py_common-1.5.0-py2.7.egg/cryption/utils/`.
+In this case, you should create directory `/usr/local/lib/python2.7/site-packages/py_common-1.5.0-py2.7.egg/cryption/utils/`, and copy the executables into this path.
+
+### 2. Configure you certificates
+
+To communicate with the server, you need to download a TLS certificate, register api-key and download the corresponding private key file from your ArxanChain BaaS Chainconsole. Refer to [API cert management](http://www.arxanfintech.com/infocenter/html/chainconsole/manual.html#api) for more details.
+
+After downloading the two files, use the following command to convert your private key file into PEM format.
+
+```sh
+$ openssl ec -in apikey.key -outform PEM -out apikey.key
+```
+
+Then copy (rename as follows) your TLS certificate and PEM private key file into your py-common installation path as follows. Please pay special attention to the absolute path of your certificate `./py_common-1.5.0-py2.7.egg/cryption/ecc/certs`, which will be used to create a wallet client.
+
+```
+.
+├── py_common-1.5.0-py2.7.egg
+|   └── cryption
+|       ├── ecc
+|       |   └── certs
+|       |       ├── tls
+|       |       |   └── tls.cert
+|       |       └── users
+|       |           └── pWEzB4yMM1518346407
+|       |               └── pWEzB4yMM1518346407.key
+|       └── utils
+|           ├── sign-util
+|           └── crypto-util
+```
 
 ### Run unit test
 
diff --git a/rest/api/api.py b/rest/api/api.py
index 775245b..6bdad9a 100644
--- a/rest/api/api.py
+++ b/rest/api/api.py
@@ -34,12 +34,15 @@ APIKEY = "pWEzB4yMM1518346407"
 def set_body(body, apikey, cert_path):
     """Set body encdypted.
 
-    :param body: body dictionary to be encrypted
+    :param body: body dictionary or string to be encrypted
     :param apikey: api key generated from server
     :param cert_path: path of private key file and cert file
     :Returns: crypted cipher text
     """
-    return sign_and_encrypt(json.dumps(body), apikey, cert_path)
+    if isinstance(body, dict):
+        body = json.dumps(body)
+
+    return sign_and_encrypt(body, apikey, cert_path)
 
 def set_sign_body(body, secret_key, did, nonce, apikey, cert_path):
     """Set body signed.
@@ -69,7 +72,12 @@ def do_post(url, headers, body, files=None):
     :param body: body dictionary
     :Returns: response
     """
-    return requests.post(url, headers=headers, data=body, files=files)
+    return requests.post(
+            url,
+            headers=headers,
+            data=body,
+            files=files
+            )
 
 def do_put(url, headers, body):
     """Start POST request.
@@ -91,7 +99,9 @@ def require_ok(resp, apikey, cert_path):
     """
     client_err_msg = ""
     if resp.status_code != STATUS_CODE_OK:
-        logging.error("Status code: {}, Client Error, body: {}".format(resp.status_code, resp.text))
+        logging.error("Status code: {}, Client Error, body: {}".format(
+                resp.status_code,
+                resp.text))
 
     if len(resp.text) <= 0:
         client_err_msg = "Respond error: Body empty"
@@ -101,11 +111,16 @@ def require_ok(resp, apikey, cert_path):
     result = {}
     plain_body = ""
     try:
-        plain_body = decrypt_and_verify(resp.text, apikey, cert_path)
+        plain_body = decrypt_and_verify(
+                resp.text,
+                apikey,
+                cert_path
+                )
         result = json.loads(plain_body)
     except Exception:
         logging.error("cannot decrypt_and_verify response body: {}".format(resp.text))
         client_err_msg = resp.text
+
     result["ClientErrMsg"] = client_err_msg
     
     return result
@@ -120,18 +135,49 @@ def do_request(req_params, apikey, cert_path, request_func):
     :param request_func: request function to be used
     :Returns: time duration, response
     """
+
     if len(cert_path) <= 0:
         cert_path = CERT_PATH
     if len(apikey) <= 0:
         apikey = APIKEY
     beg_time = time.time()
+
     if request_func == do_get and "body" in req_params:
         del req_params["body"]
     else:
-        req_body = set_body(req_params["body"], apikey, cert_path)
+        req_body = set_body(
+                req_params["body"],
+                apikey,
+                cert_path
+                )
         req_params["body"] = req_body
-    resp = require_ok(request_func(**req_params),
+
+    resp = require_ok(
+            request_func(**req_params),
             apikey, cert_path)
+
     time_dur = time.time() - beg_time
 
     return time_dur, resp
+
+def do_prepare(prepared, apikey, cert_path):
+    """ Do requst with the given request object.
+        And calculate total time used.
+
+    :param requests.PreparedRequest object used to do the request
+    :param apikey: the api key authorized by the server
+    :param cert_path: path of private key file and cert file
+    :Returns: time duration, response
+    """
+    prepared.body = set_body(prepared.body, apikey, cert_path)
+    prepared.headers['Content-Length'] = str(len(prepared.body))
+    beg_time = time.time()
+    resp = require_ok(
+            requests.session().send(prepared),
+            apikey,
+            cert_path
+            )
+    time_dur = time.time() - beg_time
+
+    return time_dur, resp
+

diff --git a/test/test_api.py b/test/test_api.py
index 60eace4..57328fe 100644
--- a/test/test_api.py
+++ b/test/test_api.py
@@ -20,12 +20,13 @@ import json
 import sys
 import httpretty
 import mock
+import requests
 ROOTPATH = os.path.join(
     os.path.dirname(__file__),
     "../"
     )
 sys.path.append(ROOTPATH)
-from rest.api.api import set_body, set_sign_body, do_get, do_post, do_put, require_ok, do_request
+from rest.api.api import set_body, set_sign_body, do_get, do_post, do_put, require_ok, do_request, do_prepare
 
 class Response(object):
     def __init__(self, status_code, text):
@@ -38,6 +39,7 @@ class ApiTest(unittest.TestCase):
     def setUp(self):
         # Every test needs access to the request factory.
         self.header = {}
+        self.url = "http://127.0.0.1"
         self.status_not_found = 404
         self.resp_not_found = "404 Not Found"
         self.nonce = "nonce"
@@ -152,4 +154,53 @@ class ApiTest(unittest.TestCase):
                     )
 
                 self.assertEqual(self.resp_not_found, result["ClientErrMsg"])
-        
+
+    def test_do_prepare_succ(self):
+        mock_send = mock.Mock(return_value=Response(self.status_ok, json.dumps(self.resp)))
+        mock_run_cmd = mock.Mock(side_effect=[self.cipher, json.dumps(self.resp)])
+        request_func = do_post
+        with mock.patch('cryption.crypto.run_cmd', mock_run_cmd):
+            with mock.patch('requests.Session.send', mock_send):
+                poeid_filepart = (
+                        "",
+                        "poe id",
+                        )
+                files = {"poe_id": poeid_filepart}
+
+                _, result = do_prepare(
+                        requests.Request(
+                            "POST",
+                            url=self.url,
+                            files=files
+                            ).prepare(),
+                        self.apikey,
+                        self.cert_path
+                        )
+
+                self.assertEqual(0, result["ErrCode"])
+
+    def test_do_prepare_fail(self):
+        mock_send = mock.Mock(return_value=Response(self.status_not_found, self.resp_not_found))
+        mock_run_cmd = mock.Mock(side_effect=[self.cipher, {}])
+        with mock.patch('cryption.crypto.run_cmd', mock_run_cmd):
+            with mock.patch('requests.Session.send', mock_send):
+                poeid_filepart = (
+                        "",
+                        "poe id",
+                        )
+                files = {
+                        "poe_id": poeid_filepart,
+                        }
+
+                _, result = do_prepare(
+                        requests.Request(
+                            "POST",
+                            url=self.url,
+                            files=files
+                            ).prepare(),
+                        self.apikey,
+                        self.cert_path
+                        )
+
+                self.assertEqual(self.resp_not_found, result["ClientErrMsg"])
+

README add certs tree description
```
|           └── certs
|               ├── tls
|               |    └── tls.cert
|               └── users
|                   └── pWEzB4yMM1518346407
|                       └── pWEzB4yMM1518346407.key
```
test/test_api.py::ApiTest::test_do_prepare_fail test/test_api.py::ApiTest::test_do_prepare_succ