What’s worse than an unsafe private key? An unsafe public key.
The “secure” in secure shell comes from the combination of hashing, symmetric encryption, and asymmetric encryption. Together, SSH uses cryptographic primitives to safely connect clients and servers. In the 25 years since its founding, computing power and speeds in accordance with Moore’s Law have necessitated increasingly complicated low-level algorithms.
As of 2020, the most widely adopted asymmetric crypto algorithms in the PKI world are RSA, DSA, ECDSA, and EdDSA. So which one is best? Well, it depends.