Openssl

OpenSSL

Swiss Army knife for practical crypto plumbing. Also a terrific way to learn that syntax is a form of weather.

When to use OpenSSL

Use OpenSSL when you need to:

• encrypt or decrypt data with a known algorithm and secret
• hash or HMAC files quickly from the terminal
• inspect X.509 certificates or PEM/DER material
• check private-key structure or convert key formats

Quick Start

# SHA-256 digest
openssl dgst -sha256 file.bin

# Inspect a certificate
openssl x509 -in cert.pem -text -noout

# Check an RSA private key
openssl rsa -in key.pem -check

High-Value Workflows

Symmetric decryption

openssl enc -aes-256-cbc -d -in cipher.bin -out plain.txt -pbkdf2 -pass pass:secret
openssl enc -aes-128-cbc -in plain.txt -out cipher.bin -pbkdf2 -pass pass:secret

Certificates and key material

openssl x509 -in cert.pem -text -noout
openssl pkey -in key.pem -text -noout
openssl rsa -pubin -in pub.pem -text -noout

Format conversion

openssl x509 -in cert.pem -outform der -out cert.der
openssl x509 -inform der -in cert.der -out cert.pem

Practical Notes

• Prefer explicit algorithms and modern flags like -pbkdf2 instead of trusting old defaults.
• dgst, enc, x509, pkey, and rsa cover most day-to-day shell work.
• Pair with cyberchef, sagemath, or application-specific tooling when the task moves beyond basic transforms.

Caveats

• OpenSSL syntax varies across versions and legacy tutorials age badly.
• Bad assumptions about IVs, salts, padding, or input format break otherwise correct commands.
• The CLI can mutate key material or output formats in ways that are easy to miss; keep originals.

Resources

No bundled scripts/, references/, or assets/. Use openssl help and the version-matched man pages for command-specific flags.