For decades, the standard procedure for generating SSH keys began with a specific muscle-memory command: ssh-keygen -t rsa. RSA has been the gold standard of public-key cryptography since the dawn of the internet, serving as the default choice for securing server access. However, the cryptographic landscape has shifted significantly.
While RSA remains relevant, the industry has largely pivoted toward Elliptic Curve Cryptography (ECC), specifically the Ed25519 algorithm. In modern DevOps and infrastructure management, SSH key hygiene is not just a matter of access; it is a foundational component of security posture. Using outdated or inefficient encryption methods introduces unnecessary friction and potential attack vectors.
This guide compares the two algorithms to help you decide which flag to pass into your next key generation command.
The Contenders: RSA vs. Ed25519
RSA (Rivest–Shamir–Adleman)
RSA is the veteran of the cryptographic world. Published in 1977, its security relies on the practical difficulty of factoring the product of two large prime numbers. Because it has been around for nearly half a century, RSA enjoys massive compatibility. Every SSH client, every server, and even the most ancient networking hardware knows how to speak RSA.
However, this longevity comes with baggage. To maintain security against modern computing power, RSA key sizes have had to balloon significantly, leading to computational overhead.
Ed25519 (Edwards-curve Digital Signature Algorithm)
Ed25519 is the modern challenger. It is an implementation of the Edwards-curve Digital Signature Algorithm (EdDSA) using Twisted Edwards curves. Unlike RSA, which relies on prime factorization, Ed25519 relies on the difficulty of the discrete logarithm problem for elliptic curves.
Designed by a team led by Daniel J. Bernstein, Ed25519 was engineered specifically for high performance and security without the bloat. It eliminates many of the complex, error-prone configuration steps required by older algorithms.
Security Strength and Key Sizes
When evaluating security protocols in 2026, it is helpful to visualize the trade-offs between key size, speed, and security strength.
// Security Comparison: RSA vs Ed25519
const cryptoComparison = {
rsa_4096: {
type: "Integer Factorization",
keySize: "4096 bits",
securityStrength: "~140 bits",
performance: "Slow generation, moderate verification",
vulnerabilities: ["Side-channel attacks", "RNG dependence"]
},
ed25519: {
type: "Elliptic Curve (Twisted Edwards)",
keySize: "256 bits",
securityStrength: "~128 bits (equivalent to RSA 3072)",
performance: "Instant generation, fast verification",
features: ["Deterministic", "Collision resilience"]
}
};Bit-for-Bit Efficiency
The most glaring difference between the two algorithms is efficiency. In cryptography, bigger is not always better; it is often just slower. An RSA key requires a massive number of bits to ensure security because the factorization problem is becoming easier for modern hardware to attack. Today, an RSA key must be at least 3072 bits (and preferably 4096 bits) to be considered secure.
In contrast, Ed25519 offers equivalent—and arguably superior—security with a key size of just 256 bits. A 256-bit Ed25519 key provides approximately 128 bits of security strength, which is comparable to a 3072-bit RSA key. This massive reduction in size allows for faster transmission and processing without compromising the mathematical hardness that protects your data.
Math Vulnerabilities and Implementation Risks
Beyond raw strength, implementation safety is critical. RSA is notoriously difficult to implement securely. It is susceptible to side-channel attacks, such as timing attacks, where an attacker can deduce the private key by analyzing how long the CPU takes to perform decryption. Furthermore, RSA signatures require a random number generator (RNG) during the signing process. If the RNG is weak or compromised, the private key can be exposed.
Ed25519 addresses these flaws architecturally. It is designed to be deterministic. When you sign a message with Ed25519, the signature is derived from the private key and the message content itself. It does not rely on a random number generator during the signing phase. This property renders Ed25519 immune to whole classes of attacks that target RNG failures, making it significantly safer against implementation flaws.
Performance Metrics: Speed and User Experience
Generation and Handshake Speed
Performance discrepancies become obvious when operating at scale. Generating a strong 4096-bit RSA key can take several seconds, or even minutes on low-power IoT devices. Ed25519 keys generate almost instantly, regardless of the hardware.
More importantly, Ed25519 is much faster at verifying signatures. While a single developer logging into a single server might not notice a 20-millisecond difference, this latency aggregates in automated environments. For DevOps engineers running Ansible playbooks or Terraform scripts that establish thousands of SSH connections, the switch to Ed25519 can result in measurable reductions in deployment times and CPU overhead.
Key String Length
The user experience improvement is visible in the terminal window. A 4096-bit RSA public key is a massive block of text that spans multiple lines, making it cumbersome to copy-paste into authorized_keys files or YAML configurations. It clutters logs and configuration management files.
An Ed25519 public key, by comparison, is incredibly compact—usually just 68 characters long. It fits easily on a single line, reducing the likelihood of copy-paste errors and keeping configuration files clean.
Compatibility: When to Use Which?
The Case for RSA
Is RSA dead? Not quite. The primary argument for retaining RSA is legacy support. If your infrastructure involves accessing decade-old routers, switches with outdated firmware, or legacy enterprise Linux distributions (older than RHEL 7 or CentOS 7), you may be forced to use RSA. Hardware that does not support OpenSSH 6.5 or higher will not recognize Ed25519 keys.
The Case for Ed25519
For virtually every other scenario, Ed25519 is the answer. It has been supported in OpenSSH since version 6.5 (released in 2014). It is fully supported by all major cloud providers (AWS, Google Cloud, Azure), modern Linux distributions (Ubuntu, Debian, Fedora), and Git hosting platforms (GitHub, GitLab, Bitbucket). If you are building on modern infrastructure, compatibility is no longer a concern.
Conclusion
The cryptographic wars are effectively over regarding SSH keys. RSA remains a reliable workhorse for legacy systems, but it is fundamentally bloated and slower compared to modern alternatives. Ed25519 is compact, lightning-fast, and architecturally secure against the implementation pitfalls that plague older algorithms.
Unless you are managing hardware that is over a decade old, Ed25519 should be your default choice for all new SSH keys. It provides better security with a fraction of the overhead.
Update your credentials today using the following command:
ssh-keygen -t ed25519 -C "email@example.com"Building secure tools requires secure foundations. At ToolShelf, we believe in privacy by design. All our developer tools, from formatters to hash generators, process data locally in your browser to ensure your sensitive information never leaves your machine.
Stay secure & happy coding,
— ToolShelf Team