When setting up secure shell (SSH) authentication, developers often face the choice between two primary tools for key generation: the dedicated ssh-keygen utility and the more general-purpose openssl command. While both can produce functional key pairs, their approaches and outputs differ in meaningful ways.

ssh-keygen is specifically designed for SSH protocol needs:

# Standard ssh-keygen command
ssh-keygen -t ed25519 -C "user@example.com" -f ~/.ssh/id_ed25519

Whereas OpenSSL offers more cryptographic flexibility:

# Equivalent OpenSSL commands
openssl genpkey -algorithm ED25519 -out private.key
openssl pkey -in private.key -pubout -out public.key

The native ssh-keygen format is immediately compatible with SSH servers:

• Private keys in PEM format (BEGIN OPENSSH PRIVATE KEY)
• Public keys in authorized_keys format

OpenSSL generates more generic cryptographic keys that may require conversion:

# Converting OpenSSL key to SSH format
ssh-keygen -y -f private.key > public_key.pub

For most SSH use cases, ssh-keygen is preferable because:

1. Direct compatibility with SSH servers and clients
2. Simplified key management with passphrase protection
3. Native support for modern algorithms like Ed25519

OpenSSL becomes valuable when you need:

• Cross-protocol key usage (TLS, SSH, etc.)
• Advanced cryptographic options
• Integration with PKI systems

Regardless of tool choice:

# Always use strong key types
ssh-keygen -t ed25519    # Preferred
ssh-keygen -t ecdsa -b 521   # Alternative
ssh-keygen -t rsa -b 4096    # Legacy

Avoid weak algorithms like DSA and short RSA keys (less than 2048 bits).

When working with SSH authentication, you essentially have two main approaches for generating key pairs:

# Using ssh-keygen (most common)
ssh-keygen -t rsa -b 4096 -C "your_email@example.com"

# Using OpenSSL (less common for SSH)
openssl genpkey -algorithm RSA -out private_key.pem -aes256
openssl rsa -pubout -in private_key.pem -out public_key.pem

The primary distinction lies in the output formats:

• ssh-keygen produces keys in OpenSSH format by default
• OpenSSL generates keys in PEM format (though conversion is possible)

For most SSH use cases, ssh-keygen is preferable because:

1. It's specifically designed for SSH protocol requirements
2. The output is immediately compatible with SSH clients/servers
3. Simpler command syntax for common operations

Consider OpenSSL when you need:

# Generating ECDSA keys with specific curves
openssl ecparam -name secp521r1 -genkey -noout -out ec_key.pem

# Working with PKCS#8 formatted keys
openssl pkcs8 -topk8 -in private.pem -out encrypted.pem

You can convert OpenSSL-generated keys to SSH format:

# Convert PEM private key to SSH format
ssh-keygen -f private_key.pem -i

# Extract public key in SSH format
ssh-keygen -f private_key.pem -y > ssh_public_key.pub

Both methods can produce equally secure keys when using:

• Sufficient key length (≥2048 bits for RSA)
• Modern algorithms (Ed25519 preferred over RSA)
• Proper key protection (passphrases, file permissions)

Benchmarking key generation times (on i7-1185G7):

For most developers:

# Recommended ssh-keygen usage
ssh-keygen -t ed25519 -a 100 -f ~/.ssh/id_ed25519

# With OpenSSL only when necessary
openssl genpkey -algorithm ED25519 -out ed25519.key