💡 Firewall & Security August 15, 2026 4

Linux Command: openssl

OpenSSL command line tool for cryptography and SSL/TLS

Terminal — Firewall & Security

Command

$ openssl genrsa -out server.key 4096

Output

Generating RSA private key, 4096 bit long modulus

openssl is a cryptographic toolkit for SSL/TLS operations, certificate management, encryption, hashing, and key generation. It is the Swiss Army knife of cryptography on Linux. openssl is used for generating SSL certificates, creating certificate signing requests (CSRs), testing SSL connections, encrypting/decrypting files, and generating random data. For web servers, openssl is essential for generating private keys, creating CSRs for certificate authorities, and converting between certificate formats (PEM, DER, PFX).

Syntax

openssl COMMAND [OPTION]...

Key Options

genrsa — Generate RSA private key
req — Create certificate signing request
x509 — Create/examine X.509 certificates
s_client — Test SSL/TLS connection
enc — Encrypt/decrypt files
rand — Generate random data

Examples

Generate private key

openssl genrsa -out server.key 4096

Output: Generating RSA private key, 4096 bit long modulus

Generate CSR

openssl req -new -key server.key -out server.csr

Self-signed certificate

openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes

Pro Tips

echo | openssl s_client -connect host:443 2>/dev/null | openssl x509 -text quickly shows any server certificate.
Private keys must be 600 permissions and never shared. chmod 600 server.key immediately after generation.
For production SSL certificates, use certbot (Let's Encrypt) instead of self-signed. openssl is for testing and CSR generation.

Learn more: Full openssl reference →