SSH protocol supports several public key types for authentication keys. The key type and key size both matter for security. We will dive into every type of SSH key today to check out which one is more secure in Linux.

## Introduction of SSH Key Types

There are four types of SSH key algorithms in the market RSA, DSA, ECDSA, ED25519. The following are the differences between them.

rsa - an old algorithm based on the difficulty of factoring large numbers. A key size of at least 2048 bits is recommended for RSA; 4096 bits is better. RSA is getting old and significant advances are being made in factoring. Choosing a different algorithm may be advisable. It is quite possible the RSA algorithm will become practically breakable in the foreseeable future. All SSH clients support this algorithm.

dsa - an old US government Digital Signature Algorithm. It is based on the difficulty of computing discrete logarithms. A key size of 1024 would normally be used with it. DSA in its original form is no longer recommended.

ecdsa - a new Digital Signature Algorithm standardized by the US government, using elliptic curves. This is probably a good algorithm for current applications. Only three key sizes are supported: 256, 384, and 521 (sic!) bits. We would recommend always using it with 521 bits, since the keys are still small and probably more secure than the smaller keys (even though they should be safe as well). Most SSH clients now support this algorithm.

ed25519 - this is a new algorithm added in OpenSSH. Support for it in clients is not yet universal. Thus its use in general purpose applications may not yet be advisable.

## Recommendation of Generating SSH Key File in Linux

Based on the difference of each SSH key type, we recommend the following ways to generate SSH key file.

```
ssh-keygen -t rsa -b 4096
ssh-keygen -t dsa
ssh-keygen -t ecdsa -b 521
ssh-keygen -t ed25519
```