Support cert authorities

Author: Javier-varez

knownhosts.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net"
 	"sort"
 	"strings"
@@ -34,12 +35,17 @@ func (hkcb HostKeyCallback) HostKeyCallback() ssh.HostKeyCallback {
 	return ssh.HostKeyCallback(hkcb)
 }
 
+type PublicKey struct {
+	ssh.PublicKey
+	cert bool
+}
+
 // HostKeys returns a slice of known host public keys for the supplied host:port
 // found in the known_hosts file(s), or an empty slice if the host is not
 // already known. For hosts that have multiple known_hosts entries (for
 // different key types), the result will be sorted by known_hosts filename and
 // line number.
-func (hkcb HostKeyCallback) HostKeys(hostWithPort string) (keys []ssh.PublicKey) {
+func (hkcb HostKeyCallback) HostKeys(hostWithPort string) (keys []PublicKey) {
 	var keyErr *xknownhosts.KeyError
 	placeholderAddr := &net.TCPAddr{IP: []byte{0, 0, 0, 0}}
 	placeholderPubKey := &fakePublicKey{}
@@ -53,14 +59,47 @@ func (hkcb HostKeyCallback) HostKeys(hostWithPort string) (keys []ssh.PublicKey)
 			return (kkeys[i].Filename == kkeys[j].Filename && kkeys[i].Line < kkeys[j].Line)
 		}
 		sort.Slice(kkeys, knownKeyLess)
-		keys = make([]ssh.PublicKey, len(kkeys))
-		for n := range kkeys {
-			keys[n] = kkeys[n].Key
+		keys = make([]PublicKey, len(kkeys))
+		for n, k := range kkeys {
+			content, err := ioutil.ReadFile(k.Filename)
+			if err != nil {
+				continue
+			}
+			lines := strings.Split(string(content), "\n")
+			line := lines[k.Line-1]
+			isCert := strings.HasPrefix(line, "@cert-authority")
+
+			keys[n] = PublicKey{
+				PublicKey: k.Key,
+				cert:      isCert,
+			}
 		}
 	}
 	return keys
 }
 
+func keyTypeToCertType(keyType string) string {
+	switch keyType {
+	case ssh.KeyAlgoRSA:
+		return ssh.CertAlgoRSAv01
+	case ssh.KeyAlgoDSA:
+		return ssh.CertAlgoDSAv01
+	case ssh.KeyAlgoECDSA256:
+		return ssh.CertAlgoECDSA256v01
+	case ssh.KeyAlgoSKECDSA256:
+		return ssh.CertAlgoSKECDSA256v01
+	case ssh.KeyAlgoECDSA384:
+		return ssh.CertAlgoECDSA384v01
+	case ssh.KeyAlgoECDSA521:
+		return ssh.CertAlgoECDSA521v01
+	case ssh.KeyAlgoED25519:
+		return ssh.CertAlgoED25519v01
+	case ssh.KeyAlgoSKED25519:
+		return ssh.CertAlgoSKED25519v01
+	}
+	return ""
+}
+
 // HostKeyAlgorithms returns a slice of host key algorithms for the supplied
 // host:port found in the known_hosts file(s), or an empty slice if the host
 // is not already known. The result may be used in ssh.ClientConfig's
@@ -84,14 +123,27 @@ func (hkcb HostKeyCallback) HostKeyAlgorithms(hostWithPort string) (algos []stri
 	}
 	for _, key := range hostKeys {
 		typ := key.Type()
-		if typ == ssh.KeyAlgoRSA {
-			// KeyAlgoRSASHA256 and KeyAlgoRSASHA512 are only public key algorithms,
-			// not public key formats, so they can't appear as a PublicKey.Type.
-			// The corresponding PublicKey.Type is KeyAlgoRSA. See RFC 8332, Section 2.
-			addAlgo(ssh.KeyAlgoRSASHA512)
-			addAlgo(ssh.KeyAlgoRSASHA256)
+		if key.cert {
+			certType := keyTypeToCertType(typ)
+			if certType == ssh.CertAlgoRSAv01 {
+
+				// CertAlgoRSASHA256v01 and CertAlgoRSASHA512v01 can't appear as a
+				// Certificate.Type (or PublicKey.Type), but only in
+				// ClientConfig.HostKeyAlgorithms.
+				addAlgo(ssh.CertAlgoRSASHA256v01)
+				addAlgo(ssh.CertAlgoRSASHA512v01)
+			}
+			addAlgo(certType)
+		} else {
+			if typ == ssh.KeyAlgoRSA {
+				// KeyAlgoRSASHA256 and KeyAlgoRSASHA512 are only public key algorithms,
+				// not public key formats, so they can't appear as a PublicKey.Type.
+				// The corresponding PublicKey.Type is KeyAlgoRSA. See RFC 8332, Section 2.
+				addAlgo(ssh.KeyAlgoRSASHA512)
+				addAlgo(ssh.KeyAlgoRSASHA256)
+			}
+			addAlgo(typ)
 		}
-		addAlgo(typ)
 	}
 	return algos
 }