RSA vs OpenSSH Private Key: What’s the Difference and Which Should You Use?
In the ever-evolving landscape of cybersecurity, secure communication is critical. One of the most relied-upon technologies for ensuring secure access to remote systems is SSH (Secure Shell). At the heart of SSH authentication is the private key — a component that must remain protected to maintain system integrity. Two prominent formats for SSH private keys are the RSA private key and the OpenSSH private key. Although they can sometimes be confused, each has distinct features, implications, and recommended use cases.
What Is a Private Key in the Context of SSH?
Before we delve into the differences, it’s essential to understand what a private key does in an SSH environment. SSH uses cryptographic key pairs for authentication. These key pairs consist of:
- Public key: Shared with the server and used to confirm the identity of the connecting client.
- Private key: Remains on the client machine and must be kept secure. It is used to create a signature that the server can validate using the corresponding public key.
The private key you use for SSH can be stored in different formats, with RSA and OpenSSH being two of the most common. Though often used interchangeably, they represent different encryption techniques and standards.
What Is an RSA Private Key?
RSA (Rivest–Shamir–Adleman) is one of the first public-key cryptosystems and is still widely used. RSA keys are based on the mathematical difficulty of factoring large integers.
When people talk about an “RSA private key” in the context of SSH, they typically mean a key encoded using the PEM (Privacy-Enhanced Mail) format, usually beginning with:
-----BEGIN RSA PRIVATE KEY-----
This format has been the default for many years, especially in OpenSSH implementations prior to version 7.8.
Advantages of RSA private keys:
- Supported by virtually every SSH and SSL implementation.
- Proven security model with widespread adoption.
- Works seamlessly for backward compatibility in older systems.
Downsides:
- RSA keys tend to be significantly larger than their elliptic-curve counterparts for equivalent strength.
- Processing time is longer due to complex mathematical operations.
What Is an OpenSSH Private Key?
With OpenSSH version 7.8, released in August 2018, a new private key format was introduced. This format is called the “OpenSSH private key format” and may start with:
-----BEGIN OPENSSH PRIVATE KEY-----
This format is not limited to RSA. It is actually part of a more flexible framework designed by the OpenSSH developers and supports different key types like:
- RSA
- ECDSA (Elliptic Curve Digital Signature Algorithm)
- ED25519
The OpenSSH private key format is binary-based and encoded in Base64 when stored as a file. It can hold multiple keys, include metadata, and optionally provide better defense against side-channel attacks by using advanced encryption techniques when the key is encrypted.
Comparing RSA and OpenSSH Private Key Formats
It’s important to recognize that these two are not mutually exclusive. RSA refers to the cryptographic algorithm, while OpenSSH file format refers to how the key is stored. However, confusion typically arises because older RSA keys use a different format than newer OpenSSH keys.
Key differences include:
| Feature | RSA Private Key | OpenSSH Private Key |
|---|---|---|
| Header | -----BEGIN RSA PRIVATE KEY----- |
-----BEGIN OPENSSH PRIVATE KEY----- |
| Supports Other Algorithms | No, RSA only | Yes, supports RSA, ECDSA, ED25519 |
| Backward Compatibility | High (works with old systems) | Moderate (some older systems might not support) |
| Encryption Scheme | Typically uses PKCS#1 | Uses bcrypt for improved security |
Which Should You Use?
The choice between using an RSA private key and an OpenSSH private key format largely depends on your specific environment, security requirements, and compatibility needs.
Use RSA Private Keys If:
- You need maximum compatibility with older systems or third-party SSH clients.
- Your organization relies heavily on infrastructure that predates 2018.
- You want to maintain consistent key formats across legacy systems and newer ones.
Use OpenSSH Private Keys If:
- You are working in a modern environment that exclusively uses up-to-date OpenSSH.
- You require advanced security features such as stronger encryption for password-protected keys.
- You want to take advantage of multiple algorithms and metadata storage within keys.
Security Considerations
It is generally accepted that using the latest standards is a better practice in terms of security. While RSA still offers a secure option, especially with 2048-bit or 4096-bit keys, newer algorithms supported by OpenSSH, such as ED25519, offer similar or better security with smaller key sizes and better performance.
Key security tips:
- Always protect private keys with a passphrase.
- Keep keys in secure locations with appropriate file permissions (e.g.,
chmod 600 your-private-key). - Periodically rotate keys to reduce the risk of compromise.
- Avoid sharing your private key between multiple systems or users.
How to Identify Your Key Type
If you’re unsure which key format you are using, simply open the key file and inspect the first line. It will either say BEGIN RSA PRIVATE KEY or BEGIN OPENSSH PRIVATE KEY. Moreover, you can run the following command to get more information about an OpenSSH key:
ssh-keygen -lf your-key-file
This will give you details about the key type, bits, and fingerprint. It’s a useful diagnostic tool for checking key integrity and compatibility.
How to Convert Between Formats
Sometimes you may need to convert between the RSA and OpenSSH formats to accommodate different systems. The tool ssh-keygen provides the capability to do this:
To convert an older RSA key to the new OpenSSH format:
ssh-keygen -p -m PEM -f ~/.ssh/id_rsa
To convert a new OpenSSH key to the older RSA format:
ssh-keygen -p -m PEM -f ~/.ssh/id_rsa
In each case, -m PEM tells ssh-keygen to use the legacy format. This is particularly useful for ensuring compatibility with certain systems or software that expect a specific format.
Conclusion
Choosing between RSA and OpenSSH private key formats is ultimately a matter of context. If you are operating in a modern, updated environment, the OpenSSH private key format provides better flexibility, security, and support for new cryptographic algorithms. On the other hand, RSA keys ensure compatibility and stability in mixed or legacy environments.
Security professionals and system administrators should weigh their organization’s specific needs, compatibility requirements, and security posture when choosing between the two. Regardless of the format you choose, maintaining key security through strong passphrases, restricted access, and routine audits is paramount in safeguarding sensitive systems.