Comparison of the SSH Key Algorithms

This story is about comparing the main algorithms use to generate an SSH key, describe their weaknesses and place them on the Moore Law.

How far the main algorithms are, cryptologically speaking, from each other ?

DSA vs RSA vs ECDSA vs Ed25519

For years now, advances have been made in solving the complex problem of the DSA, and it is now mathematically broken, especially with a standard key length. Moreover, the attack may be possible to extend to RSA as well.

I’m not saying that you shouldn’t use DSA or RSA, but the key length has to be really long. Of course, there is an impact during the login.

I suggest you to use elliptic curve cryptography instead. The ECC algorithms supported by OpenSSH are ECDSA and, since OpenSSH 6.5, Ed25519.

With OpenSSH, NIST curves are used for ECDSA (generally NIST P-256), and according to the article Security dangers of the NIST curves, it is very likely that an NSA backdoor is hidden there. If the NSA can already crack it, then it won’t be as hard to crack for somebody else…

Ed25519 is quite the same, but with a better curve (Curve25519). compares elliptic curves, there is a big difference between NIST P-256 and Curve25519 !

Also, DSA and ECDSA have a nasty property: they require a parameter usually called k to be completely random, secret, and unique. In practice, that means that if you connect to your server from a machine with a poor random number generator and e.g. the same k happens to be used twice, an observer of the traffic can figure out your private key.

Moore Law

How long do you want to be protected ?

ECRYPT provides rather conservative guiding principles, based on current state-of-the-art research, addressing the construction of new systems with a long life cycle.

Following their recommendation, we can divide in three categories the protection you want.

Legacy standard level

Should not be used in new systems

RSA key length : 1024 bits
ECDSA / Ed25519 : 160 bits

Near term protection

Security for at least ten years (2018–2028)

RSA key length : 3072 bits
ECDSA / Ed25519 : 256 bits

Long-term protection

Security for thirty to fifty years (2018–2068)

RSA key length : 15360 bits
ECDSA / Ed25519 : 512 bits


Never use DSA or ECDSA.

  • Ed25519 is probably the strongest mathematically (and also the fastest), but not yet widely supported. At least 256 bits long.
  • RSA is the best bet if you can’t use Ed25519. At least 3072 bits long.

Now you are aware of the different algorithms, you can upgrade your SSH Key.

Why not use SSH Certificate ?

In addition, you can use an SSH Certificate to be much more secure. With this method, you don’t have to worry about the algorithm, but you will have to sign your public key regularly.

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Nicolas Béguier

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