ssh: fix error message on unsupported cipher

Until now, when ssh keys using one of these[1] ciphers were passed, we were
giving a parse error "ssh: parse error in message type 0".

With this fix, we parse it successfully and return the correct error message.

[1] aes{128,256}-gcm@openssh.com and chacha20-poly1305@openssh.com

Fixes golang/go#52135

Change-Id: I3010fff43c48f29f21edb8d63f44e167861a054e
GitHub-Last-Rev: 14ac7e97306d41cba48053b9c60f2ffc7caded45
GitHub-Pull-Request: golang/crypto#324
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/709275
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/keys_test.go

@@ -271,6 +271,21 @@ func TestParseEncryptedPrivateKeysWithPassphrase(t *testing.T) {
 	}
 }
 
+func TestParseEncryptedPrivateKeysWithUnsupportedCiphers(t *testing.T) {
+    for _, tt := range testdata.UnsupportedCipherData {
+        t.Run(tt.Name, func(t *testing.T){
+            _, err := ParsePrivateKeyWithPassphrase(tt.PEMBytes, []byte(tt.EncryptionKey))
+            if err == nil {
+                t.Fatalf("expected 'unknown cipher' error for %q, got nil", tt.Name)
+                // If this cipher is now supported, remove it from testdata.UnsupportedCipherData
+            }
+            if !strings.Contains(err.Error(), "unknown cipher") {
+                t.Errorf("wanted 'unknown cipher' error, got %v", err.Error())
+            }
+        })
+    }
+}
+
 func TestParseEncryptedPrivateKeysWithIncorrectPassphrase(t *testing.T) {
 	pem := testdata.PEMEncryptedKeys[0].PEMBytes
 	for i := 0; i < 4096; i++ {