How to use the command 'openssl genrsa' (with examples)

OpenSSL is a powerful toolkit for the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols, and it is also used as a general-purpose cryptography library. The openssl genrsa command is specifically used to generate RSA (Rivest-Shamir-Adleman) private keys. RSA is one of the first public-key cryptosystems and is widely used for secure data transmission. The versatility of OpenSSL makes it a favorite among systems administrators and security professionals alike, and generating RSA keys is a common task for securing communications and systems.

Use case 1: Generate an RSA private key of 2048 bits to stdout

Code:

openssl genrsa

Motivation:
This use case is an entry-level task for anyone looking to start with encryption using RSA. By default, when you simply execute openssl genrsa, it generates a 2048-bit RSA key. This is a common key length that provides a reasonable level of security for general-purpose encryption. Generating a key to stdout allows you to quickly verify the command’s function or redirect the output to another command or file.

Explanation:

• openssl: This is the command-line tool for OpenSSL functionalities.
• genrsa: This specific command within OpenSSL is used to generate RSA keys.
• Without additional arguments, the command uses default settings: it generates a 2048-bit key and outputs it to stdout, which can be redirected if needed.

Example Output:

Generating RSA private key, 2048 bit long modulus
....................................................+++
.................................+++
e is 65537 (0x10001)
-----BEGIN RSA PRIVATE KEY-----
MIIEpAIBAAKCAQEA7v7bFt9lWJq1HVu/bpULNr9hpgcDXYZ8Lpj8/waithIuGN2M
...
-----END RSA PRIVATE KEY-----

Use case 2: Save an RSA private key of an arbitrary number of bits to the output file

Code:

openssl genrsa -out output_file.key 1234

Motivation:
In certain situations, you may require an RSA key of a specific length that does not conform to standard key lengths like 2048, 3072, or 4096 bits. This command allows you to specify any arbitrary length (though it should be a sensible number for practical purposes). This can be useful for experimental purposes or non-standard encryption setups. Exporting the generated key to a file is crucial for persistent storage and later use in security configurations.

Explanation:

• openssl: This indicates the OpenSSL tool.
• genrsa: This relates to RSA key generation.
• -out: This flag specifies that the generated key will be written to the file rather than printed to the screen.
• output_file.key: This is the filename where the RSA private key will be saved.
• 1234: This number specifies the desired bit length of the RSA key being generated. Despite being non-standard, such flexibility might be needed for specific use cases or experiments.

Example Output:

The command will not directly display the key, as it is saved to 'output_file.key'.
Success message or indications about writing to the file will appear in the terminal.

Use case 3: Generate an RSA private key and encrypt it with AES256

Code:

openssl genrsa -aes256

Motivation:
Encrypting your private key is a recommended practice to add an extra layer of security. Even if someone gains access to your key file, they would need a passphrase to decrypt and use it. AES256 is a robust encryption standard providing a high level of security, suitable for protecting sensitive data and communications. Prompting the user for a passphrase ensures that the key is protected by encryption and can only be decoded by someone who knows the passphrase.

Explanation:

• openssl: Indicates we’re using the OpenSSL tool.
• genrsa: Pertains to RSA key generation.
• -aes256: This option encrypts the private key using the AES (Advanced Encryption Standard) with a 256-bit key. Users will be prompted to enter a passphrase, which will be required to access the private key in the future.

Example Output:

Generating RSA private key, 2048 bit long modulus
...........................................................+++
..........+++
e is 65537 (0x10001)
Enter pass phrase for [stdout]:
Verifying - Enter pass phrase for [stdout]:
-----BEGIN ENCRYPTED PRIVATE KEY-----
MIIE6TAbBgkqhkiG9w0BBQMwDgQImAHiwz92yawCAggABIIEyNRFJbsij2yVFm173
...
-----END ENCRYPTED PRIVATE KEY-----

Conclusion

The openssl genrsa command is a versatile tool for generating RSA private keys, a cornerstone for secure communications. Whether you’re generating a default 2048-bit key, specifying an arbitrary length, or adding a security layer with AES256 encryption, understanding how to wield this command effectively is crucial for any tasks involving cryptography. By employing these examples, users can adapt OpenSSL to varied scenarios ranging from simple key generation to encrypted private key management, ensuring flexibility and security in their cryptographic operations.