Actually add `Configuring SSH'

docs/ssh-config.md

@@ -0,0 +1,45 @@
+# Configuring SSH
+
+## Setting a Default Username
+
+If your username is different on your tilde than on your personal computer,
+you may want to specify a default username in `~/.ssh/config`.
+That can be done by adding the following entries:
+
+    Host tilde.black
+        User user
+
+## Using a Tilde-Specific key
+
+### Step 1: Generate Your Key
+
+[Generate a key](ed25519.md), except outputting to a Tilde-specific location.
+
+    mkdir -p ~/.ssh/tilde.black
+    ssh-keygen -a100 -ted25519 -f ~/.ssh/tilde.black/id_ed25519 -C ''
+
+### Step 2: Authorize Your Key
+
+If you have yet to [sign up](sign-up.md) to your desired Tilde,
+send the *public* key (`~/.ssh/tilde.black/id_ed25519.pub`) to the admin.
+
+If you already have an account, you can authorize your new key by adding it to,
+or replacing your old key in, `~/.ssh/authorized_keys`.
+
+    $ scp ~/.ssh/tilde.black/id_ed25519.pub user@tilde.black:key.pub
+    $ ssh user@tilde.black
+    tilde$ cat key.pub >> .ssh/authorized_keys
+    tilde$ rm key.pub
+
+### Step 3: Configure SSH
+
+To make `ssh(1)` *only* use that key for that tilde, you can add the following
+lines to `~/.ssh/config`:
+
+    Host tilde.black
+        IdentitiesOnly yes
+        IdentityFile ~/.ssh/tilde.black/id_ed25519
+
+## See Also
+
+[SSH over Tor](ssh-over-tor.md)