Generate Aes Key Python.
“Believe in your infinite potential. Your only limitations are those you set upon yourself.” ― Roy T. Bennett, The Light in the Heart
Using the cryptography module in Python, this post will look into methods of generating keys, storing keys and using the asymmetric encryption method RSA to encrypt and decrypt messages and files. We will be using cryptography.hazmat.primitives.asymmetric.rsa to generate keys. Installing cryptography. Implementing AES in Python Fortunately, we don’t have to implement AES from scratch, but you can give it a try if you’re feeling spicy. In order to avoid doing so, we first need to install the.
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).
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
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).
Determine the size of the file.
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
Now we need to reverse the above process to decrypt the file using AES.
![Aes Key Generation Python Aes Key Generation Python](/uploads/1/1/8/0/118023074/518977463.png)
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.
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.
Aes Key Generation Python Programming
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.
Update 2: 30-May-2016
There have been a number of comments and questions about whether this code is compliant or interoperable with other implementations. It is not. I wrote it as a non-crypto expert to teach myself more about AES. It was never intended to perform well or to be compatible with other AES implementations.
Update: 04-Apr-2009
I fixed two bugs in my AES implementation pointed out to me by Josiah Carlson. First, I was failing to pad properly files whose length was an even multiple of the block size. In those cases, bytes would be lost upon decrypting the file. Josiah also pointed out that I was using a static IV, which leaks information about messages which share common prefixes. This is a serious security bug and I was glad to have it pointed out.
Feel free to check out the changes I made or simply download the updated script.
I’ve put together a series of slides as well as a Python implementation of AES, the symmetric-key cryptosystem.
Source:pyAES.py
Sample Usage: (color added for clarity)
[brandon@zodiac pyAES]$ cat > testfile.txt
The sky was the color of television tuned to a dead channel.
[brandon@zodiac pyAES]$ ./pyAES.py -e testfile.txt -o testfile_encrypted.txt
Password:
Encrypting file: testfile.txt
Encryption complete.
[brandon@zodiac pyAES]$ ./pyAES.py -d testfile_encrypted.txt -o testfile_decrypted.txt
Password:
Decrypting file: testfile_encrypted.txt
Decryption complete.
[brandon@zodiac pyAES]$ cat testfile_decrypted.txt
The sky was the color of television tuned to a dead channel.
[brandon@zodiac pyAES]$ md5sum *
19725cef7495fd55540728759a6262c8 pyAES.py
2fffc9072a7c09f4f97862c0bceb6021 testfile_decrypted.txt
3e57070eaf1b4adf7f43b38e1c5ee631 testfile_encrypted.txt
2fffc9072a7c09f4f97862c0bceb6021 testfile.txt
The sky was the color of television tuned to a dead channel.
[brandon@zodiac pyAES]$ ./pyAES.py -e testfile.txt -o testfile_encrypted.txt
Password:
Encrypting file: testfile.txt
Encryption complete.
[brandon@zodiac pyAES]$ ./pyAES.py -d testfile_encrypted.txt -o testfile_decrypted.txt
Password:
Decrypting file: testfile_encrypted.txt
Decryption complete.
[brandon@zodiac pyAES]$ cat testfile_decrypted.txt
The sky was the color of television tuned to a dead channel.
[brandon@zodiac pyAES]$ md5sum *
19725cef7495fd55540728759a6262c8 pyAES.py
2fffc9072a7c09f4f97862c0bceb6021 testfile_decrypted.txt
3e57070eaf1b4adf7f43b38e1c5ee631 testfile_encrypted.txt
2fffc9072a7c09f4f97862c0bceb6021 testfile.txt
Symmetric Key Cryptography
- Identical keys used to encrypt/decrypt messages
- Can be implemented as block ciphers or stream ciphers
Strengths:
- Speed
- Much less computationally intensive than public-key crypto
- Easy to implement in hardware as well as software
Weaknesses:
- Key Management
- n users require n(n-1)/2 keys for all to communicate
- secure key distribution is a challenge
- Cannot be used (directly) for authentication or non-repudiation
AES – The Advanced Encryption Standard
- Rijndael algorithm invented by Joan Daemen and Vincent Rijmen and selected as AES winner by NIST in 2001
- AES uses fixed block size of 128-bits and key sizes of 128, 192 or 256 bits (though Rijndael specification allows for variable block and key sizes)
- Most of the calculations in AES are performed within a finite field
- There are a finite number of elements within the field and all operations on those elements result in an element also contained in the field
AES Operations
- AES operates on a 4×4 matrix referred to as the state
- 16 bytes 128 bits block size
- All operations in a round of AES are invertible
- AddRoundKey – each byte of the round key is combined with the corresponding byte in the state using XOR
- SubBytes – each byte in the state is replaced with a different byte according to the S-Box lookup table
- ShiftRows – each row in the state table is shifted by a varying number of bytes
- MixColumns – each column in the state table is multiplied with a fixed polynomial
AES Operation – AddRoundKey
- Each byte of the round key is XORed with the corresponding byte in the state table
- Inverse operation is identical since XOR a second time returns the original values
AES Operation – SubBytes
- Each byte of the state table is substituted with the value in the S-Box whose index is the value of the state table byte
- Provides non-linearity (algorithm not equal to the sum of its parts)
- Inverse operation is performed using the inverted S-Box
AES Operation – ShiftRows
- Each row in the state table is shifted left by the number of bytes represented by the row number
- Inverse operation simply shifts each row to the right by the number of bytes as the row number
AES Operation – MixColumns
- MixColumns is performed by multiplying each column (within the Galois finite field) by the following matrix:
- The inverse operation is performed by multiplying each column by the following inverse matrix:
Python Aes 128
AES – Pulling It All Together
The AES Cipher operates using a varying number of rounds, based on the size of the cipher key.
- A round of AES consists of the four operations performed in succession: AddRoundKey, SubBytes, ShiftRows, and MixColumns (MixColumns is omitted in the final round)
- 128-bit key → rounds, 192-bit key → 12 rounds, 256-bit key → 14 rounds
- The AES cipher key is expanded according to the Rijndael key schedule and a different part of the expanded key is used for each round of AES
- The expanded key will be of length (block size * num rounds+1)
- 128-bit cipher key expands to 176-byte key
- 192-bit cipher key expands to 208-byte key
- 256-bit cipher key expands to 240-byte key
AES – Key Expansion Operations
AES key expansion consists of several primitive operations:
- Rotate – takes a 4-byte word and rotates everything one byte to the left, e.g. rotate([1,2,3,4]) → [2, 3, 4, 1]
- SubBytes – each byte of a word is substituted with the value in the S-Box whose index is the value of the original byte
- Rcon – the first byte of a word is XORed with the round constant. Each value of the Rcon table is a member of the Rinjdael finite field.
AES – Key Expansion Algorithm (256-bit)
Pseudo-code for AES Key Expansion:
- expandedKey[0:32] → cipherKey[0:32] # copy first 32 bytes of cipher key to expanded key
- i → 1 # Rcon iterator
- temp = byte[4] # 4-byte container for temp storage
- while size(expandedKey) < 240
temp → last 4 bytes of expandedKey# every 32 bytes apply core schedule to temp
if size(expandedKey)%32 0
temp = keyScheduleCore(temp, i)
i → i + 1
# since 256-bit key -> add an extra sbox transformation to each new byte
for j in range(4):
temp[j] = sbox[temp[j]]
# XOR temp with the 4-byte block 32 bytes before the end of the current expanded key.
# These 4 bytes become the next bytes in the expanded key
expandedKey.append( temp XOR expandedKey[size(expandedKey)-32:size(expandedKey)-28]
Another function to note…
Aes Key Generation Python Compiler
AES – Encrypting a Single Block
- state → block of plaintext # 16 bytes of plaintext are copied into the state
- expandedKey = expandKey(cipherKey) # create 240-bytes of key material to be used as round keys
- roundNum → 0 # counter for which round number we are in
- roundKey → createRoundKey(expandedKey, roundNum)
- addRoundKey(state, roundKey) # each byte of state is XORed with the present roundKey
- while roundNum < 14 # 14 rounds in AES-256
roundKey → createRoundKey(expandedKey, roundNum)
# round of AES consists of 1. subBytes, 2. shiftRows, 3. mixColumns, and 4. addRoundKey
aesRound(state, roundKey)
roundNum → roundNum + 1 - # for the last round leave out the mixColumns operation
roundKey = createRoundKey(expandedKey, roundNum)
subBytes(state)
shiftRows(state)
addRoundKey(state) - return state as block of ciphertext