SSH Key Strength Calculator

Work out ssh key strength instantly with clear inputs, formula shown and shareable results.

About SSH Key Strength Calculator

Ed25519 gives 128-bit security in a 68-byte public key and signs roughly ten times faster than RSA 2048, which is why it is the current recommendation for SSH. RSA 4096 provides only about 140 bits — barely more than Ed25519 — while costing far more per operation, so it is a poor trade for a bastion handling many authentications.

Formula

Key comparison

security level from NIST equivalences; CPU load = authentications per second x signature cost / 1000

Worked example

With the default values below, SSH Key Strength Calculator returns Security level of 128. Change any input above and every figure here is recalculated in your browser.

Inputs used

Key typeone of RSA 2048, RSA 4096, ECDSA P-256, Ed25519: Ed25519
Authentications per secondminimum 0.001: 40
Key rotation intervalaccepts 0.1–20 years: 2 years

Results

Security level: 128 bits
Cost per signature operation: 0.120 ms
CPU load from authentication: 0.48%
Public key size: 68 B

Step by step

1Values used
2Key comparison
3Security level= 128 bits
4Cost per signature operation= 0.120 ms
5CPU load from authentication= 0.48
6Public key size= 68

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Should DSA or RSA 1024 keys still be accepted?

No. DSA is disabled by default in modern OpenSSH and RSA below 2048 bits is deprecated. Both should be removed from authorized_keys.

Is Ed25519 supported everywhere?

It has been in OpenSSH since 6.5 and is supported by all current platforms. Very old appliances and some hardware tokens may still require ECDSA or RSA.