Expert: Security Engineer
Objective: Compare openssl and ssh-keygen for generating SSH keys, highlighting their differences and use cases for a blog post primer on SSH keys.
Assumptions: The user wants to understand the pros and cons of openssl versus ssh-keygen in the context of SSH key management, including key formats and practical recommendations for generating keys.

Here's an overview of how ssh-keygen compares to openssl when it comes to SSH key generation:

ssh-keygen Overview

ssh-keygen is a utility specifically designed to create and manage SSH keys. It's part of the OpenSSH suite, making it the default tool for generating SSH keys on most Unix-like systems (Linux, macOS, etc.).

Pros of Using ssh-keygen:

1. Purpose-Built for SSH:

  • ssh-keygen is designed explicitly for SSH, making it the most straightforward tool for creating keys compatible with SSH clients and servers.

2. User-Friendly:

  • The commands are intuitive and well-documented. Generating an SSH key pair is as simple as running ssh-keygen -t rsa -b 2048, which creates a private key (id_rsa) and a public key (id_rsa.pub) in the user's .ssh directory by default.

3. Supports Different Key Types:

  • Easily generate different types of keys: RSA, DSA, ECDSA, and ED25519.
  • Example: ssh-keygen -t ed25519 generates a modern and secure ED25519 key pair.

4. Key Comment and Passphrase Features:

  • Supports adding comments and passphrases to keys easily. This is useful for identifying keys and protecting private keys with an extra layer of security.

5. Automatic Conversion to OpenSSH Format:

  • By default, ssh-keygen generates keys in the OpenSSH format, which is widely supported and preferred for SSH authentication.

Cons of Using ssh-keygen:

1. Limited to SSH-Related Formats:

  • It's not designed to generate keys in formats like PKCS#8 or X.509, commonly used for other cryptographic protocols.
  • If you need non-SSH key formats, use openssl or another tool to convert the key.

2. Less Flexibility with Encryption Algorithms:

  • While ssh-keygen supports basic passphrase encryption for private keys, openssl offers more encryption algorithm options for private key encryption.

openssl Overview

openssl is a versatile tool for working with cryptography, including generating key pairs, certificates, and performing various cryptographic functions.

Pros of Using openssl:

1. Versatile and Flexible:

  • openssl supports a wide range of cryptographic algorithms, key formats, and options, making it ideal for cases where you need more than just SSH.

2. Supports Multiple Key Formats:

  • Can generate keys in various formats like PKCS#1, PKCS#8, and X.509, making it suitable for certificate-related tasks.

3. Customizable Encryption Algorithms:

  • Offers more options for encrypting private keys (e.g., AES-256, DES3). This level of flexibility can be useful for meeting specific compliance requirements.

4. Conversion Capabilities:

  • It's easy to convert keys between formats (e.g., from PKCS#1 to PKCS#8) using openssl, which is useful if you need a different key representation.

Cons of Using openssl:

1. More Complex Commands:

  • openssl commands can be verbose and less intuitive than ssh-keygen. For example, generating an RSA key pair requires multiple command-line options.
  • Example: openssl genrsa -out private_key.pem 2048 followed by openssl rsa -in private_key.pem -pubout -out public_key.pem.

2. Not Purpose-Built for SSH:

  • While you can use openssl to generate keys for SSH, additional steps are required to convert the keys into the right format for OpenSSH compatibility.

3. Compatibility Issues with Default Formats:

  • By default, openssl may output keys in formats not directly compatible with SSH clients, requiring extra steps to convert the keys.

Practical Recommendations for SSH Key Management

1. Use ssh-keygen When Working with SSH Clients and Servers:

  • Since ssh-keygen is purpose-built for SSH, it simplifies generating and using SSH keys.
  • Recommended commands:
# Generate a new RSA key pair with 2048-bit strength
ssh-keygen -t rsa -b 2048 -f ~/.ssh/my_key -C "my_key_comment"

# Generate a modern ED25519 key pair
ssh-keygen -t ed25519 -f ~/.ssh/my_ed25519_key -C "my_ed25519_key_comment"

2. Use openssl for Conversions and Non-SSH Key Formats:

  • If you need to generate or convert keys to PKCS#8 or X.509 formats, openssl is the right choice.
  • Example of converting an existing RSA private key to PKCS#8:
openssl pkcs8 -topk8 -inform PEM -in my_key.pem -out my_key_pkcs8.pem -nocrypt

3. Key Passphrases:

  • Always encrypt private keys with a strong passphrase, especially if storing them in shared environments or using them for sensitive automation tasks.

4. Automating Key Management:

  • Use scripts to automate key generation, conversion, and distribution. Ssh-keygen and openssl can be integrated into shell scripts or configuration management tools.