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How to implement an app application to limit users' time use?

2022-07-25 22:03:00 【Empty bad uncle】

author ： Empty bad uncle
Blog ：https://xuhss.com

The breakfast shop won't be open till night , The people who want to eat have already come ！

How to achieve a App Applications , Limit user time usage ？

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One 、 The scene that

Suppose there's a scenario like this , You took a private job , Make a software for others , The software has no networking function . When things are done , The customer hasn't paid yet , Said to try it first . You chose to trust the customer , Sent him the software . Then he pulled you black .

To avoid that , The first thing you think of , It must be writing the expiration time into the code , time out App Automatically destroy . After the other party pays , You can extend or remove the dead time . Then recompile and send it to the customer .

But the problem is , It's very troublesome to recompile the code and send it to the user every time , Is there an easier way to ？ Can software always be a software , But give the user a registration code , The registration code is marked with the effective time . Wait until it expires , Just give the user a new registration code , You can continue to use .

See here , Some students will think , How to specify the validity period in the registration code ？ First of all, this time must not be in clear text , Otherwise, the user will change it , Can't it be extended by itself .

But if encrypted , We must put the decryption algorithm into the software , Once the user preliminarily decompiles the program , You can get the decryption algorithm or symmetric encryption key .

therefore , We can only use asymmetric encryption . And asymmetric encryption , Encryption by public key , Decrypt with private key . If we want the software to decode the effective time from the registration code , Do you want to put the private key in the software ？

The private key cannot be disclosed , Therefore, what can only be put into the software is the public key . But can we use private key encryption , Decryption with public key ？

actually , You can really do this , But this is not called private key encryption, public key decryption , This is called private key signature (sign), Public key verification (verify). also , There is one advantage to using this method , That is, the effective time can be stored directly in plaintext , Not afraid of user modification . Because once modified , The signature doesn't match .

Two 、PyCryptodome Libraries installed

Suppose we have a string message, Use private key , You can sign this string , Get a signature string signature. And we use the public key , Can verify message Whether the signature string can be generated signature. If message Changes have taken place , perhaps signature Changes have taken place , perhaps message and signature At the same time, changes have taken place , Public key verification will fail .

Each language has a third-party library related to asymmetric encryption . We use it Python Medium PyCryptodome To demonstrate .

First , We are macOS below , Generate a pair of public and private keys ：

ssh-keygen -t rsa

Just enter the storage path of the key according to the prompt , As shown in the figure below ：

In the current folder , Generated private key sign And public key sign.pub.

Next , Use pip install PyCryptodome:

pip install pycryptodome

3、 ... and 、 The actual demonstration limits the user's use time

Next , Import public and private keys ：

>>> from Crypto.PublicKey import RSA
>>> with open('sign') as f:
...  private = f.read()
...
>>> with open('sign.pub') as f:
...  public = f.read()
...
>>> private_key = RSA.import_key(private)
>>> public_key = RSA.import_key(public)

Because the key we generated before is SHA256 Algorithm , So we need to use SHA256 The algorithm generates a summary of the data that needs to be signed . This step needs to be done when signing and verifying the signature .

>>> from Crypto.Hash import SHA256
>>> digest = SHA256.new()
>>> message = 'expire: 2022-03-01'
>>> digest.update(message.encode())

Next , Sign this data ：

>>> import base64
>>> from Crypto.Signature import PKCS1_v1_5
>>> signer = PKCS1_v1_5.new(private_key)
>>> code = signer.sign(digest)
>>> signature = base64.b64encode(code)
>>> print(signature.decode())

The operation effect is shown in the figure below ：

Now? , You just need to put the string expire: 2022-03-01 And signature string xbelbTNpq8M... A long string ... Just send it to the customer .

The customer inputs the expiration time string and signature string into the software , The software uses the public key to verify whether the string is signed by its corresponding private key ：

>>> message = 'expire: 2022-03-01'
>>> signature = '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'
>>> digest = SHA256.new()
>>> digest.update(message.encode())
>>> reader = PKCS1_v1_5.new(public_key)
>>> reader.verify(digest, base64.b64decode(signature.encode()))
True

But if you tamper message The content of , Then the verification will fail , As shown in the figure below ：

After the first verification of the software , You can save the expiration time string and the signature string to the hard disk in the form of a file , Check it every time you start the software .

It runs normally when it is found to be legal and not expired , If it is found that it is expired or illegal, the dialog box for entering the registration code will pop up again .

Four 、 summary

In this paper, complete MFC The program only runs a single instance Simple example .
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