SSH key types (RSA / ED25519 / ECDSA) — what actually differs, and which one to pick
If you maintain WordPress sites over SSH — running wp-cli remotely, checking logs, transferring files with rsync — you're relying on SSH key authentication as the foundation. What rarely gets explained clearly is what you're actually choosing when ssh-keygen -t asks for an algorithm. This post walks through what RSA, ECDSA, and ED25519 actually rest on mathematically, and which one makes sense to…
SSH key authentication is a fundamental method for accessing servers remotely using tools like WordPress' wp-cli and rsync. When generating keys with ssh-keygen -t, three algorithms are commonly presented: RSA, ECDSA, and ED25519. Each type relies on different mathematical principles and results in keys with varying sizes and performance characteristics.
RSA keys, the most familiar, depend on the difficulty of factoring large numbers. Keys typically range from 2048 to 4096 bits in size and have been used since the late 1990s. In contrast, ECDSA and ED25519 utilize elliptic curve cryptography, with ECDSA relying on discrete logarithm problems and ED25519 using a fixed elliptic curve design. ED25519 keys are significantly shorter, generally around 256 bits, compared to the longer RSA keys.
While all three algorithms provide equivalent security, ED25519 offers faster processing times for key generation, signing, and verification. This speed advantage is noticeable during routine maintenance tasks involving SSH. Additionally, ED25519 keys require less computational power, leading to a lighter workload during every operation. However, older servers or embedded devices might lack support for ED25519, in which case RSA keys are the suitable fallback option.
ED25519's design minimizes vulnerability risks associated with implementation bugs, as its elliptic curve parameters are not tied to a single government standard. This distinction matters more in theoretical terms but contributes to ED25519's reputation for being a robust choice for new keys. For compatibility or policy requirements, RSA remains an acceptable alternative, particularly if a key pair already exists and has been validated.
To generate an ED25519 key using ssh-keygen, the command is: ssh-keygen -t ed25519 -C "example-comment". A passphrase for encrypting the private key is recommended for enhanced security. The public key, ending in .pub, is added to the authorized_keys file on the server. In contrast, RSA keys can be created with the command: ssh-keygen -t rsa -b 4096 -C "example-comment", with a similar approach to SSH key configuration.
Understanding the rationale behind choosing ED25519 as the default key type simplifies decision-making amidst outdated documentation favoring RSA. By recognizing the advantages ED25519 offers in terms of security, performance, and reduced implementation risks, managing SSH access across WordPress environments becomes more efficient and secure.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
