1 files changed, 86 insertions, 1 deletions
diff --git a/handshake.go b/handshake.go
index e47813f..cfe7085 100644
--- a/handshake.go
+++ b/handshake.go
@@ -1,7 +1,9 @@
 package main
 
 import (
+	"fmt"
 	"golang.org/x/sync/errgroup"
+	"io"
 	"net"
 
 	"git.samanthony.xyz/hose/key"
@@ -46,5 +48,88 @@ func handshakeSend(rhost string) error {
 // The user is asked to verify the fingerprint of the key before
 // it is saved to the known hosts file.
 func handshakeRecv(rhost string) error {
-	// TODO
+	// Listen for connection.
+	laddr := net.JoinHostPort("", port)
+	ln, err := net.Listen(network, laddr)
+	if err != nil {
+		return err
+	}
+	defer ln.Close()
+	logf("listening on %s", laddr)
+
+	conn, err := ln.Accept()
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+	logf("accepted connection from %s", conn.RemoteAddr())
+
+	// Receive public key from remote host.
+	var rpubkey [32]byte
+	_, err = io.ReadFull(conn, rpubkey[:])
+	if err != nil {
+		return err
+	}
+	logf("received public key from $s", conn.RemoteAddr())
+
+	// Ask user to verify the fingerprint of the key.
+	ok, err := verifyPublicKey(conn.RemoteAddr(), rpubkey)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		// User rejected the fingerprint.
+		return fmt.Errorf("host key verification failed")
+	}
+
+	return addKnownHost(conn.RemoteAddr(), rpubkey)
+}
+
+// verifyPublicKey asks the user to verify the fingerprint of a public key belonging to a remote host.
+// It returns true if the user accepts the fingerprint, or false if they don't, or a non-nil error.
+func verifyPublicKey(addr net.Addr, pubkey [32]byte) (bool, error) {
+	// Lookup human-friendly name of remote host, or fall back to the address.
+	hostname, err := lookupAddr(addr.String())
+	if err != nil {
+		return false, err
+	}
+
+	// Ask host to verify fingerprint.
+	logf("Fingerprint of host %q: %s\nIs this the correct fingerprint (yes/[no])?",
+		hostname, fingerprint(pubkey))
+	var response string
+	n, err := fmt.Scanln(&response)
+	if err != nil {
+		return false, err
+	}
+	for n > 0 && response != "yes" && response != "no" {
+		logf("Please type 'yes' or 'no'")
+		n, err = fmt.Scanln(&response)
+		if err != nil {
+			return false, err
+		}
+	}
+	if n == 0 {
+		response = "no" // default response (pressed enter without typing anything)
+	}
+	switch response {
+	case "yes":
+		return true, nil
+	case "no":
+		return false, nil
+	}
+	panic("unreachable")
+}
+
+// lookupAddr attempts to resolve the host name of an IP address.
+// If no names are mapped to the address, the address itself is returned.
+func lookupAddr(addr string) (string, error) {
+	hostNames, err := net.LookupAddr(addr)
+	if err != nil {
+		return "", err
+	}
+	if len(hostNames) > 0 {
+		return hostNames[0], nil
+	}
+	return addr, nil
 }