This is as easy and usable by normal people as would be "web login by signing a string with your PGP key". No way. The solution for this kind of thing is still

This is as easy and usable by normal people as would be "web login by signing a string with your PGP key". No way. The solution for this kind of thing is still something like OpenID, however simpler, so everyone can implement everywhere they want, even using custom SSH-OpenID servers, and normal people can use with their Google emails.