Generate a private key:

$ (umask 277 && openssl genrsa -out /nix/etc/nix/signing-key.sec 2048)

The private key should be kept secret (only readable to the Nix daemon
user).


Generate the corresponding public key:

$ openssl rsa -in /nix/etc/nix/signing-key.sec -pubout > /nix/etc/nix/signing-key.pub

The public key should be copied to all machines to which you want to
export store paths.


Signing:

$ nix-hash --type sha256 --flat svn.nar | openssl rsautl -sign -inkey mykey.sec > svn.nar.sign


Verifying a signature:

$ test "$(nix-hash --type sha256 --flat svn.nar)" = "$(openssl rsautl -verify -inkey mykey.pub -pubin -in svn.nar.sign)"