Comparing SSH Keys: RSA, ECDSA, Ed25519

SSH (Secure Shell) authentication relies on public-key cryptography to provide secure remote access. Over the years, different algorithms have emerged, each with distinct characteristics. This guide compares RSA, ECDSA, and Ed25519 to help you choose the right key type for your needs.

Understanding SSH Key Algorithms

SSH keys consist of two parts: a public key (shared with servers) and a private key (kept secret). When authenticating, the server verifies you possess the private key corresponding to the authorized public key, without transmitting the private key itself.

Three major algorithm families dominate SSH authentication:

1. RSA (Rivest-Shamir-Adleman): The oldest and most widely supported algorithm
2. ECDSA (Elliptic Curve Digital Signature Algorithm): Based on elliptic curve cryptography
3. Ed25519: Modern elliptic curve algorithm using Curve25519

RSA Keys

RSA has been the default SSH key type since the protocol’s inception. It relies on the computational difficulty of factoring large numbers.

Key Generation

# Generate 4096-bit RSA key (recommended size)
ssh-keygen -t rsa -b 4096 -C "your.email@example.com"

# Generate 2048-bit RSA key (minimum acceptable)
ssh-keygen -t rsa -b 2048 -C "your.email@example.com"

Characteristics

Key Sizes: 1024, 2048, 3072, or 4096 bits

• 1024-bit: Deprecated and insecure
• 2048-bit: Minimum acceptable, widely supported
• 3072-bit: Enhanced security margin
• 4096-bit: Maximum security, slower performance

Security: RSA security depends entirely on key size. 2048-bit keys are currently considered secure, but 3072-bit or 4096-bit keys provide better long-term security against advances in computing power and quantum computing threats.

Performance: Larger keys mean slower signature generation and verification. A 4096-bit key is noticeably slower than 2048-bit, especially on resource-constrained systems.

Compatibility: Universal support across all SSH implementations, servers, and clients. The safest choice for legacy systems.

ECDSA Keys

ECDSA uses elliptic curve mathematics to provide equivalent security to RSA with smaller key sizes, resulting in faster operations and reduced bandwidth.

Key Generation

# Generate 256-bit ECDSA key (default)
ssh-keygen -t ecdsa -b 256 -C "your.email@example.com"

# Generate 384-bit ECDSA key
ssh-keygen -t ecdsa -b 384 -C "your.email@example.com"

# Generate 521-bit ECDSA key (maximum)
ssh-keygen -t ecdsa -b 521 -C "your.email@example.com"

Characteristics

Key Sizes: 256, 384, or 521 bits

• 256-bit: Equivalent to ~3072-bit RSA
• 384-bit: Equivalent to ~7680-bit RSA
• 521-bit: Equivalent to ~15360-bit RSA

Security: ECDSA provides strong security with smaller key sizes. However, implementation quality matters significantly. Poor random number generation during signature creation can compromise the entire key.

Performance: Significantly faster than RSA for equivalent security levels. Signature generation and verification are both quicker, and smaller keys reduce bandwidth usage.

Compatibility: Widely supported on modern systems (OpenSSH 5.7+, released 2011). May not work with very old SSH implementations.

Important Considerations

ECDSA uses NIST curves (P-256, P-384, P-521) which have faced scrutiny due to potential NSA influence in their selection. While no practical vulnerabilities have been demonstrated, some security-conscious users prefer alternatives.

Ed25519 Keys

Ed25519 is a modern elliptic curve algorithm designed for high performance and security. It uses the Curve25519 elliptic curve developed by Daniel J. Bernstein, specifically optimized to avoid implementation pitfalls and side-channel attacks.

Key Generation

# Generate Ed25519 key (fixed 256-bit size)
ssh-keygen -t ed25519 -C "your.email@example.com"

Characteristics

Key Size: Fixed at 256 bits (equivalent to ~3072-bit RSA)

Security: Designed with security best practices from the ground up. Resistant to timing attacks and other side-channel vulnerabilities. The algorithm is deterministic, eliminating risks from poor random number generation during signing. Widely trusted by the cryptographic community.

Performance: Fastest of all three algorithms. Both signature generation and verification are significantly quicker than RSA or ECDSA. Smaller key sizes reduce storage and transmission overhead.

Compatibility: Supported in OpenSSH 6.5+ (released 2014). Most modern systems support it, but some older enterprise systems and embedded devices may not.

Why Ed25519 is Recommended

Ed25519 represents the current best practice for SSH key generation:

1. Secure by design: Immune to many implementation vulnerabilities
2. Fast: Significantly faster than RSA and ECDSA
3. Small: 256-bit keys are easier to manage and transmit
4. Deterministic: No risk from poor random number generators during signing
5. Modern: Designed with lessons learned from decades of cryptographic research

Comprehensive Comparison

Security Comparison

Performance Comparison

Based on typical modern hardware (approximate relative speeds):

Compatibility Comparison

Practical Recommendations

For Most Users: Ed25519

Ed25519 is the best choice for most modern use cases:

ssh-keygen -t ed25519 -C "your.email@example.com" -f ~/.ssh/id_ed25519

Use when:

• Working with modern systems (2015+)
• Security and performance are priorities
• You control both client and server environments

For Legacy Compatibility: RSA-4096

Use RSA when compatibility with older systems is required:

ssh-keygen -t rsa -b 4096 -C "your.email@example.com" -f ~/.ssh/id_rsa

Use when:

• Connecting to legacy systems
• Working in enterprise environments with old infrastructure
• Maximum compatibility is essential

ECDSA: Middle Ground

ECDSA offers a compromise between RSA and Ed25519:

ssh-keygen -t ecdsa -b 256 -C "your.email@example.com" -f ~/.ssh/id_ecdsa

Use when:

• You need better performance than RSA
• Ed25519 is not supported
• You trust NIST curve selection

Multiple Keys Strategy

Many users maintain multiple key types for different purposes:

# Primary key for modern systems
ssh-keygen -t ed25519 -C "primary@example.com" -f ~/.ssh/id_ed25519

# Fallback key for legacy systems
ssh-keygen -t rsa -b 4096 -C "legacy@example.com" -f ~/.ssh/id_rsa_legacy

Configure ~/.ssh/config to use appropriate keys per host:

# Modern servers
Host modern-server
    HostName server.example.com
    IdentityFile ~/.ssh/id_ed25519

# Legacy systems
Host legacy-server
    HostName old.example.com
    IdentityFile ~/.ssh/id_rsa_legacy

Conclusion

For new SSH key generation in 2025, Ed25519 is the recommended choice for its superior security design, excellent performance, and small key size. It represents the current best practice in SSH authentication.

Use RSA-4096 only when compatibility with legacy systems is required, as it provides adequate security at the cost of larger keys and slower performance.

ECDSA serves as a middle ground but is generally superseded by Ed25519 for modern deployments, with the added concern about NIST curve selection.

Quick Decision Guide

• Default choice: Ed25519
• Legacy compatibility needed: RSA-4096
• Old system without Ed25519 support: ECDSA-256 or RSA-4096
• Maximum compatibility: RSA-2048 (minimum) or RSA-4096 (recommended)

Choose based on your specific requirements, but prioritize security and maintainability for long-term deployments.