ssh/knownhosts: improve IPv6 support in Normalize

Correctly converts bracketed IPv6:

- [abcd::abcd:abcd:abcd] => abcd::abcd:abcd:abcd
- [abcd::abcd:abcd:abcd]:22 => abcd::abcd:abcd:abcd
- [abcd::abcd:abcd:abcd]:23 => [abcd::abcd:abcd:abcd]:23

Fixes golang/go#53463

Change-Id: Id0a7460d8448a72e2a8c6d46137245bead9ecf9f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/694575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

ssh/knownhosts/knownhosts.go

@@ -421,20 +421,26 @@ func New(files ...string) (ssh.HostKeyCallback, error) {
 	return certChecker.CheckHostKey, nil
 }
 
-// Normalize normalizes an address into the form used in known_hosts
+// Normalize normalizes an address into the form used in known_hosts. Supports
+// IPv4, hostnames, bracketed IPv6. Any other non-standard formats are returned
+// with minimal transformation.
 func Normalize(address string) string {
+	const defaultSSHPort = "22"
+
 	host, port, err := net.SplitHostPort(address)
 	if err != nil {
 		host = address
-		port = "22"
+		port = defaultSSHPort
 	}
-	entry := host
-	if port != "22" {
-		entry = "[" + entry + "]:" + port
-	} else if strings.Contains(host, ":") && !strings.HasPrefix(host, "[") {
-		entry = "[" + entry + "]"
+
+	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") {
+		host = host[1 : len(host)-1]
 	}
-	return entry
+
+	if port == defaultSSHPort {
+		return host
+	}
+	return "[" + host + "]:" + port
 }
 
 // Line returns a line to add append to the known_hosts files.