genrsa.1s

openssl genrsa [options] [numbits]
The numbits argument specifies the desired key length in bits, typically 2048 or 4096. If omitted, 2048 bits is the default.

-help
    Displays a usage message and exits.

-out filename
    Specifies the output file for the generated key. If omitted, the key is written to standard output (stdout).

-passout arg
    Specifies the source for the passphrase used to encrypt the output key. Common formats include pass:password, env:VAR, file:path, or fd:num.

-des, -des3, -aes128, -aes192, -aes256
    Encrypts the private key using the specified symmetric cipher (DES, Triple DES, or AES with various key sizes). If no passphrase is provided via -passout, the user will be prompted for one.

-f4 or -3
    Sets the public exponent for the RSA key. -f4 uses 65537 (the default and recommended value), while -3 uses 3.

-vulnerable
    Generates a key that is intentionally vulnerable to the ROCA (Return Of Coppersmith's Attack) vulnerability, found in some hardware implementations. Use with extreme caution and only for specific testing purposes, never in production.

-noout
    Prevents the output of the encoded version of the key. This is useful when only side effects (like passphrase prompting) are desired.

numbits
    The desired length of the RSA key in bits. Common values are 2048 (default) or 4096. A minimum of 512 bits is usually enforced, but for security, at least 2048 bits is highly recommended.

genrsa is an OpenSSL command-line utility used for generating RSA (Rivest-Shamir-Adleman) private keys. RSA is a widely used public-key cryptographic algorithm fundamental for secure data transmission and authentication.

The command produces a new private key, typically in PEM (Privacy-Enhanced Mail) format, which can be further processed or used by other OpenSSL commands, for instance, to generate a Certificate Signing Request (CSR) or self-signed certificates. Users can specify the key's bit length (e.g., 2048, 4096 bits) to determine its strength, with larger bit lengths providing greater security but requiring more computational resources.

While `genrsa` generates the private key, its corresponding public key can be derived from it. The public key is then shared widely, enabling others to encrypt data for the private key holder or verify digital signatures made by them, ensuring secure and authenticated communication. It's an essential tool for setting up secure servers, client certificates, and various cryptographic applications.

Security Considerations: Choosing a sufficiently large numbits (e.g., 2048 or 4096) is crucial for key strength. Using the -vulnerable option should be strictly avoided in production environments as it deliberately creates insecure keys. Poorly chosen passphrases for encrypted keys can also compromise security.

Randomness: The quality of the generated key heavily relies on the randomness source provided by the operating system. Insufficient entropy can lead to weaker, predictable keys.

The numbits parameter determines the cryptographic strength of the RSA key. While 2048 bits is the current standard, 4096 bits offers greater future-proofing against advancements in cryptanalysis. The public exponent, typically 65537 (F4), is a small prime number used in the RSA algorithm. While technically modifiable, using 65537 is highly recommended for security and compatibility reasons.

By default, genrsa outputs the private key in PEM format, which is a Base64-encoded representation of the DER (Distinguished Encoding Rules) key, typically enclosed between "BEGIN RSA PRIVATE KEY" and "END RSA PRIVATE KEY" markers. This human-readable format is widely compatible with other OpenSSL commands and cryptographic libraries. Encrypted keys will have markers like "BEGIN ENCRYPTED PRIVATE KEY" or "BEGIN DES3 PRIVATE KEY" depending on the encryption cipher used.

genrsa is a foundational component of the OpenSSL project, which began in 1998 as a fork of SSLeay. As a core utility for generating RSA private keys, it has been an integral part of OpenSSL's cryptographic toolkit since its early days, supporting the widespread adoption of RSA for secure communication over the internet. Its functionality has remained largely consistent, with updates primarily focusing on security enhancements, support for stronger ciphers, and integration with evolving cryptographic standards.

rsa(1), pkcs8(1), x509(1), req(1), dsa(1), ec(1), openssl(1)