Decrypt Symmetric Key with Asymmetric

TL;DR

Someone has encrypted a symmetric key using your public asymmetric key. This guide shows you how to decrypt it and then use the recovered symmetric key to decrypt further data.

Steps

1. Understand the Scenario: Someone used your public key (e.g., RSA) to encrypt a secret symmetric key (e.g., AES). They’ve sent you the encrypted symmetric key. You need to use your private key to decrypt it.
2. Ensure You Have Your Private Key: This is crucial! The private key is what unlocks the encryption. It should be stored securely, ideally in a dedicated hardware security module (HSM) or protected by a strong passphrase. Know its location and format (e.g., PEM, PKCS#8).
3. Choose Your Decryption Tool: Several tools can perform asymmetric decryption. Common options include:
   • OpenSSL: A versatile command-line tool available on most platforms.
   • GnuPG (GPG): Another powerful command-line option, often used for email encryption but also capable of asymmetric key operations.
   • Programming Languages (Python, Java, etc.): Libraries like PyCryptodome (Python) or Bouncy Castle (Java) provide programmatic access to decryption functions.
4. Decrypt the Symmetric Key using OpenSSL: This is a common approach.

   openssl rsadecrypt -in encrypted_key.bin -out symmetric_key.bin -pkey private.pem

   • Replace encrypted_key.bin with the actual filename of the file containing the encrypted symmetric key.
   • Replace symmetric_key.bin with the desired filename for the decrypted symmetric key.
   • Replace private.pem with the path to your private key file.
5. Decrypt the Symmetric Key using GPG:

   gpg --decrypt -o symmetric_key.bin encrypted_key.bin

   • GPG will likely prompt you for your private key passphrase if it’s protected.
   • Replace symmetric_key.bin with the desired filename for the decrypted symmetric key.
   • Replace encrypted_key.bin with the actual filename of the file containing the encrypted symmetric key.
6. Decrypt Data using the Symmetric Key: Now that you have the decrypted symmetric key, use it to decrypt the data that was originally encrypted with it.

   openssl aes-256-cbc -d -in encrypted_data.bin -out decrypted_data.bin -k 

   • Replace encrypted_data.bin with the filename of the data encrypted using AES (or whatever symmetric algorithm was used).
   • Replace decrypted_data.bin with the desired output filename for the decrypted data.
   • Replace <symmetric_key> with the actual content of your symmetric_key.bin file. Be careful not to expose this key!
7. Verify Decryption: Check that the decrypted data is what you expect. If it’s a text file, open it in a text editor. If it’s another type of file (image, document), try opening it with the appropriate application.

Important Security Considerations

• Private Key Protection: Never share your private key! Store it securely and protect it with a strong passphrase if necessary.
• Key Management: Implement proper key rotation practices to minimize the impact of potential compromises.
• Algorithm Strength: Ensure you’re using strong encryption algorithms (e.g., AES-256, RSA 2048 or higher). Avoid outdated or weak algorithms.
• cyber security Best Practices: Regularly audit your systems and processes to identify and mitigate potential vulnerabilities.