What is the difference between this and dropbear ssh?
-passwords are not allowed, only keys
-only a single AEAD cipher is supported, and a single elliptic curve for key exchange
-root cannot be locked out with this server
-the key restrictions available in OpenSSH are not supported
-the server does not use dynamic memory, and has a better security record than dropbear
It is slightly useful to put in smaller devices that don't have much space but I think it still relies on top many linux facilities to be an appropriate fix for that too. Still, cool project.
For initrd you generally prefer static binaries. Not saying that OpenSSHd doesn't build statically, but having less code and dependencies makes it easier to statically compile.
But yes, technically there is no reason to not use OpenSSHd, but in practice having a smaller and more self contained binary helps considering that you would want the bare minimum during initrd.
TinySSH doesnt claim to be compliant, and isnt. Does less in exchange for a reduced attack surface.
tinyssh is small because it only implements a tiny subset of SSH that is needed for secure basic SSH connections. It only includes few crypto primitives excluding even RSA.
There is considerable overlap in the two and you can reach something similar to tinyssh by compiling dropbear with only few select features, but tinyssh aims to be as secure and attack surface minimized as possible out of the box.
Another notable difference:
> no dynamic memory allocation - TinySSH has all memory statically allocated (less than 1MB)