How to convert a key string back into a pycrypto key object? (Python)

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06-10-2024

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Unlocking the Key: Converting String to PyCrypto Key Object

Python's PyCryptodome library is a powerful tool for cryptography, but sometimes you might find yourself with a key stored as a string and need to transform it back into a usable PyCryptodome key object. This article explains how to perform this essential conversion and provides a practical example.

The Problem:

Imagine you have a cryptographic key securely stored as a string in your application. However, PyCryptodome's encryption and decryption functions require key objects, not simple strings. How do you bridge this gap and safely convert your string key back to a usable key object?

Scenario:

Let's say you have a string representation of a 256-bit AES key stored in a variable key_string:

from Crypto.Cipher import AES
import base64

key_string = 'U2FsdGVkX1+V/wG9u8J/p2wC/yM7ZqI5d4+0r5a686z71Q='

This is a base64-encoded string. To use it with PyCryptodome's AES cipher, you need to convert it back into a bytes object representing the key.

Solution:

1. Decode the Base64 String: First, decode the base64-encoded string using the base64.b64decode() function. This will give you the raw bytes of the key.
2. Create a Key Object: Use the bytes object and the appropriate Crypto.Cipher module to create a key object.

Here's the Python code implementing this solution:

from Crypto.Cipher import AES
import base64

key_string = 'U2FsdGVkX1+V/wG9u8J/p2wC/yM7ZqI5d4+0r5a686z71Q='
key_bytes = base64.b64decode(key_string)
key = AES.new(key_bytes, AES.MODE_CBC)

# Now 'key' is a usable PyCryptodome key object

Explanation:

• base64.b64decode(key_string): This line decodes the base64-encoded string key_string into a bytes object.
• AES.new(key_bytes, AES.MODE_CBC): This line creates a new AES cipher object, using the key_bytes as the key and specifying the cipher mode (in this case, CBC).

Important Considerations:

• Security: Ensure your key storage and handling practices are secure to prevent unauthorized access.
• Key Length: Always check the expected key length for the chosen encryption algorithm.
• Cipher Mode: The cipher mode (e.g., CBC, GCM) should match the mode used during encryption.

Additional Notes:

• If your key string is not base64 encoded, skip the decoding step.
• You can use similar methods to create key objects for other PyCryptodome algorithms like RSA or DES.

Conclusion:

Converting a key string to a PyCryptodome key object is a simple yet critical step for successful encryption and decryption. By following the steps outlined above, you can reliably transform your key string into a usable key object for your cryptographic operations.