Inspired by Section 1.3 of David Kopec's "Classic Computer Science Problems in Python"
Source Code for this article on GitHub.
Let's build two Azure Functions, both HTTP triggers. One to encrypt a message, the other to decrypt it. Fair warning that if you are looking for a secure encryption solution, you would be better served in the hands of people smarter than me, but I think there is something useful to learn in working through this problem.
As noted above, the logic used in these examples comes from a book by David Kopec. I was working through it last night and wanted to take this section a little further. I reimplemented the work in Section 1.3 "Unbreakable Encryption" as two Azure Functions. I'll walk you through the process so you can too!
Gather Your Materials
Before you begin, you will need the following:
Create a Virtual Environment
Python Azure Functions App requires that you work within a virtual environment. If you are unfamiliar with using venv
, there is a great primer over at RealPython.
Bash (MacOS or Linux)
python3.6 -m venv .env
source .env/bin/activate
PowerShell or Command Prompt (Windows)
py -3.6 -m venv .env
.\.env\Scripts\activate
Initialize Your Azure Function App
Execute the following command to create a new Function App:
func init encryption_and_decryption
There is now a folder called encryption_and_decryption
with the boilerplate code for a Python Azure Function App. If you would like, take some time to open the code in your favorite editor and have a look around.
If you are looking for an editor, I recommend VS Code.
Create the encrypt
Function
Make sure you are in the encryption_and_decryption
directory:
cd encryption_and_decryption
Create the new function using:
func new
At the prompt, choose HTTP trigger
. Then name the function encrypt
.
Encrypting Input
Open the file encrypt/__init__.py
. You should see boilerplate code for an HTTP trigger function. This function will take name
as a parameter and return "Hello name
!". Let's add code at the bottom of this file that will instead encrypt the name
using a random set of token bytes.
from secrets import token_bytes
def random_key(length: int) -> int:
# returns length random bytes
key: bytes = token_bytes(length)
# convert key to bit string
return int.from_bytes(key, "big")
Next, write a function to use this random key to encrypt name
:
from typing import Tuple
def encrypt(name: str) -> Tuple[int, int]:
# returns (encrypted_name, dummy_key), encrypted by one-time pad
# encode the name to bytes, then to a bit string
name_as_bytes: bytes = name.encode()
name_key: int = int.from_bytes(name_as_bytes, "big")
# generate a dummy key for encryption
dummy_key: int = random_key(len(name_as_bytes))
# encrypt by XOR operation
encrypted_name: int = name_key ^ dummy_key
return encrypted_name, dummy_key
Here, you have encoded the name to a number. This is just another way of writing the name
data. The data can be translated back from this form. "Thomas", put though this transform is 92807522771315
. You can see this in the Python REPL:
>>> name: str = "Thomas"
>>> name_as_bytes: bytes = name.encode()
>>> name_key = int.from_bytes(name_as_bytes, "big")
>>> print(f"name: {name}\nname_as_bytes: {name_as_bytes}\nname_key: {name_key}")
name: Thomas
name_as_bytes: b'Thomas'
name_key: 92807522771315
I can transform it back:
>>> name_again = name_key.to_bytes((name_key.bit_length() + 7) // 8, "big").decode()
>>> print(name_again)
Thomas
You take the int
representation of the name, called name_key
and perform an exclusive OR (XOR) operation with the random_key int
. In Python XOR is done using the ^
character. This operation produces an encrypted form of the input name:
name_key ^ random_key = encrypted_name
Performing the XOR operation again with the random_key
will undo the encryption:
encrypted_name ^ random_key = name_key
In a sense, the random_key
locks and unlocks name
.
Using encrypt
Now that you have the encrypt
function written, edit the boilerplate code to take the name
input, encrypt it and return the encrypted name with the key. Altogether, the code in __init__.py
should like like this:
import logging
from secrets import token_bytes
from typing import Tuple
import azure.functions as func
def main(req: func.HttpRequest) -> func.HttpResponse:
logging.info("func: encrypt: request received")
name = req.params.get("name")
if not name:
try:
req_body = req.get_json()
except ValueError:
pass
else:
name = req_body.get("name")
if name:
# encrypt name
encrypted_name, dummy_key = encrypt(name)
response = str({"encrypted_name": encrypted_name, "dummy_key": dummy_key})
return func.HttpResponse(response)
else:
# ask for input if none provided
return func.HttpResponse(
"Please pass a name on the query string or in the request body",
status_code=400,
)
def random_key(length: int) -> int:
# returns length random bytes
key: bytes = token_bytes(length)
# convert key to bit string
return int.from_bytes(key, "big")
def encrypt(name: str) -> Tuple[int, int]:
# return (encrypted_name, dummy_key); encrypted by one-time pad
# encode the name to bytes, then to a bit string
name_as_bytes: bytes = name.encode()
name_key: int = int.from_bytes(name_as_bytes, "big")
# generate a dummy key for encryption
dummy_key: int = random_key(len(name_as_bytes))
# encrypt by XOR operation
encrypted_name: int = name_key ^ dummy_key
return encrypted_name, dummy_key
Testing encrypt
Make sure you are in the encryption_and_decryption
directory and run the Function App with the command:
func host start
The output of this command includes a link to the HTTP endpoint for this function: http://localhost:7071/api/encrypt
.
Visiting this URL in a browser will call the Azure Function and you will get the response "Please pass a name on the query string or in the request body". Pass the Function a name
parameter by appending ?name=Sophia
to the URL so that it reads:
http://localhost:7071/api/encrypt?name=Sophia
You will get a response back that resembles JSON with the encrypted name and dummy key:
{
"encrypted_name": 191667540651033,
"dummy_key": 278440108655992
}
Try it out with different names!
Create the decrypt
function
As you did with the encrypt
function, from the command line, execute:
func new
At the prompt, choose HTTP trigger
. Then name the function decrypt
. You will pass both the encrypted_name
and the dummy_key
to this Azure Function. It will perform the XOR operation that undoes the encryption.
Decrypting Input
Open the file decrypt/__init__.py
. At the bottom of this file, add a function to decrypt
def decrypt(encrypted_name: int, dummy_key: int) -> str:
# returns XOR of encrypted_name and dummy_key converted to bytes and decoded
name_key: int = encrypted_name ^ dummy_key
return name_key.to_bytes((name_key.bit_length() + 7) // 8, "big").decode()
As shown above, performing the XOR operation on the encrypted_name
and dummy_key
will output the same value as name_key
. Then, the function undoes the encoding from int
to str
.
Note:
In Python 3+, the//
operator divides numerator by denominator and returns the quotient as anint
, dropping the digits after the decimal.e.g.
5 // 2 == 2
Using decrypt
Edit the boilerplate code to accept two arguments encrypted_name
and dummy_key
. It should look like this:
import logging
import azure.functions as func
def main(req: func.HttpRequest) -> func.HttpResponse:
logging.info("func: decrypt: request received")
encrypted_name = req.params.get("encrypted_name")
dummy_key = req.params.get("dummy_key")
if not encrypted_name or not dummy_key:
try:
req_body = req.get_json()
except ValueError:
pass
else:
encrypted_name = req_body.get("encrypted_name")
dummy_key = req_body.get("dummy_key")
if encrypted_name and dummy_key:
name = decrypt(int(encrypted_name), int(dummy_key))
return func.HttpResponse(f"Hello {name}!")
else:
return func.HttpResponse(
"Please pass a name on the query string or in the request body",
status_code=400,
)
def decrypt(encrypted_name: int, dummy_key: int) -> str:
# returns XOR of encrypted_name and dummy_key converted to bytes and decoded
name_key: int = encrypted_name ^ dummy_key
return name_key.to_bytes((name_key.bit_length() + 7) // 8, "big").decode()
Testing decrypt
Start the Azure Function App with
func host start
Using the output from the encrypt
Azure Function, test the decrypt
Function by visiting:
http://localhost:7071/api/decrypt?encrypted_name=133673440856088&dummy_key=47262719615353
This should return "Hello Sophia!".
Try it with your own encrypted values by sending a name to the encrypt
endpoint and replacing the values for encrypted_name
and dummy_key
with the values returned by the Azure Function.
Going Further
Python Azure Functions:
- Blog post: Taking a closer look at Python support for Azure Functions
- GitHub Repo: Azure Functions Python Worker
The encryption and decryption code comes from David Kopec's Classic Computer Science Problems in Python with the permission of the author.
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