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- An in-depth discussion of authentication and public key infrastructure is beyond the scope of this book. But now that you know how public key cryptography provides confidentiality, let’s look at how to generate keys for the public key cipher. Steps for Generating Public and Private Keys. Each key in the public key scheme is made of two numbers.
- An example of symmetric encryption in python using a single known secret key - utilizes AES from PyCrypto library - AESexample.py.
(CkPython) RSA Encrypt/Decrypt AES Key. Demonstrates how to use RSA to protect a key for AES encryption. It can be used in this scenario: You will provide your RSA public key to any number of counterparts. Your counterpart will generate an AES key, encrypt data (or a file) using it, then encrypt the AES key using your RSA public key. Generating a secure AES key? Ask Question Asked 3 years, 5 months ago. Active 3 years, 5 months ago. Viewed 7k times 3. I asked this question a while ago about IVs in AES, and I got a very nice and helpful answer (thanks!) so I was thinking maybe you guys could help me out again, this time with the actual key generation. TL;DR - See bottom. Sep 20, 2017 A pure-Python implementation of the AES block cipher algorithm and the common modes of operation (CBC, CFB, CTR, ECB and OFB). Supports all AES key sizes; Supports all AES common modes; Pure-Python (no external dependencies) BlockFeeder API allows streams to easily be encrypted and decrypted. Generating Keys for Encryption and Decryption.; 3 minutes to read +7; In this article. Creating and managing keys is an important part of the cryptographic process. Symmetric algorithms require the creation of a key and an initialization vector (IV). The key must be kept secret from anyone who should not decrypt your data.
Contents
- 6. File Encryption with AES
- Conclusion
1. Introduction
Pycrypto is a python module that provides cryptographic services. Pycrypto is somewhat similar to JCE (Java Cryptography Extension) for Java. In our experience JCE is more extensive and complete, and the documentation for JCE is also more complete. That being said, pycrypto is a pretty good module covering many aspects of cryptography.
In this article, we investigate using pycrypto’s implementation of AES for file encryption and decryption.
[Note: We have also covered AES file encryption and decryption in java previously.]
2. Generating a Key
AES encryption needs a strong key. The stronger the key, the stronger your encryption. This is probably the weakest link in the chain. By strong, we mean not easily guessed and has sufficient entropy (or secure randomness).
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That being said, for the sake of demonstration of AES encryption, we generate a random key using a rather simple scheme. Do not copy and use this key generation scheme in production code.
AES encryption needs a 16-byte key.
3. Initialization Vector
In addition to the key, AES also needs an initialization vector. This initialization vector is generated with every encryption, and its purpose is to produce different encrypted data so that an attacker cannot use cryptanalysis to infer key data or message data.
A 16-byte initialization vector is required which is generated as follows.
The initialization vector must be transmitted to the receiver for proper decryption, but it need not be kept secret. It is packed into the output file at the beginning (after 8 bytes of the original file size), so the receiver can read it before decrypting the actual data.
4. Encrypting with AES
![Python Generating An Aes Key Python Generating An Aes Key](/uploads/1/2/6/9/126944471/633290186.jpg)
We now create the AES cipher and use it for encrypting a string (or a set of bytes; the data need not be text only).
The AES cipher is created with CBC Mode wherein each block is “chained” to the previous block in the stream. (You do not need to know the exact details unless you are interested. All you need to know is – use CBC mode).
Also, for AES encryption using pycrypto, you need to ensure that the data is a multiple of 16-bytes in length. Pad the buffer if it is not and include the size of the data at the beginning of the output, so the receiver can decrypt properly.
5. Decrypting with AES
Decryption requires the key that the data was encrypted with. You need to send the key to the receiver using a secure channel (not covered here).
In addition to the key, the receiver also needs the initialization vector. This can be communicated as plain text, no need for encryption here. One way to send this is to include it in the encrypted file, at the start, in plaintext form. We demonstrate this technique below (under File Encryption with AES). For now, we assume that the IV is available.
And that is how simple it is. Now read on to know how to encrypt files properly.
6. File Encryption with AES
We have three issues to consider when encrypting files using AES. We explain them in detail below.
First step is to create the encryption cipher.
6.1. Write the Size of the File
First we have to write the size of the file being encrypted to the output. This is required to remove any padding applied to the data while encrypting (check code below).
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Open the output file and write the size of the file. We use the struct package for the purpose.
6.2. Save the Initialization Vector
As explained above, the receiver needs the initialization vector. Write the initialization vector to the output, again in clear text.
6.3. Adjust Last Block
The third issue is that AES encryption requires that each block being written be a multiple of 16 bytes in size. So we read, encrypt and write the data in chunks. The chunk size is required to be a multiple of 16.
This means the last block written might require some padding applied to it. This is the reason why the file size needs to be stored in the output.
Here is the complete write code.
7. Decrypting File Using AES
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Now we need to reverse the above process to decrypt the file using AES.
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First, open the encrypted file and read the file size and the initialization vector. The IV is required for creating the cipher.
Next create the cipher using the key and the IV. We assume the key has been communicated using some other secure channel.
We also write the decrypted data to a “verification file”, so we can check the results of the encryption and decryption by comparing with the original file.
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Note that when the last block is read and decrypted, we need to remove the padding (if any has been applied). This is where we need the original file size.
Conclusion
And that is all there is to encrypting and decrypting a file using AES in python. We need to generate or obtain a key, create the initialization vector and write the original file size followed by the IV into the output file. This is followed by the encrypted data. Finally decryption does the same process in reverse.
Warning
Please do not mistake this article for anything more than what it is: myfeeble attempt at learning how to use PyCrypto. If you need to useencryption in your project, do not rely on this code. It is bad. It willhaunt you. And some cute creature somewhere will surely die a painfuldeath. Don't let that happen.
If you want encryption in Python, you may be interested in these libraries:
I spent a little bit of time last night and this morning trying to find someexamples for AES encryption using Python and PyCrypto. To my surprise, I had quite adifficult time finding an example of how to do it! I posted a message onTwitter asking for any solid examples, but people mostly just responded withthings I had seen before--the libraries that do the encryption, not examplesfor how to use the libraries.
It wasn't long after that when I just decided to tackle the problem myself. Mysolution ended up being pretty simple (which is probably why there weren't anysolid examples for me to find). However, out of respect for those out therewho might still be looking for a solid example, here is my solution:
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Edit: thanks to John and Kaso for their suggestions, though John's didn'tseem to work for me (?) Keytool generate self signed certificate with private key.
Edit 2015.12.14: thanks to Stephen for pointing out that the block size for AES is always 16, and the key size can be 16, 24, or 32. See FIPS-197 for more details.
If you plan to use this script, you'll need to have PyCrypto installed on yourcomputer. I have had a difficult time finding this for Windows in the past, soI will mirror the installer that I found overhere:http://jintoreedwine.wordpress.com/2008/07/20/python-25-and-encryption-pycrypto-under-windows/.I haven't tried it on Mac OS X yet, but it should be fairly simple to installit. Same goes for Linux.
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The output of the script should always change with each execution thanks to therandom secret key. Here's some sample output:
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If the comments in the script aren't explanatory enough, please comment and askfor clarification. I will offer any that I am capable of, and I invite othersto do the same.