go.crypto/ssh: some cleanup

Simplify MarshalAuthorizedKey by using the algoName func.
Make the algoName func be very specific about supported key types in openssh certs.
Generalize some of the commentary that previously mentioned specific key types.

R=agl, dave
CC=golang-dev
https://golang.org/cl/6938067

ssh/common_test.go

@@ -5,6 +5,11 @@
 package ssh
 
 import (
+	"crypto/dsa"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rsa"
+	"errors"
 	"testing"
 )
 
@@ -24,3 +29,49 @@ func TestSafeString(t *testing.T) {
 		}
 	}
 }
+
+func TestAlgoNameSupported(t *testing.T) {
+	supported := map[string]interface{}{
+		KeyAlgoRSA:          new(rsa.PublicKey),
+		KeyAlgoDSA:          new(dsa.PublicKey),
+		KeyAlgoECDSA256:     &ecdsa.PublicKey{Curve: elliptic.P256()},
+		KeyAlgoECDSA384:     &ecdsa.PublicKey{Curve: elliptic.P384()},
+		KeyAlgoECDSA521:     &ecdsa.PublicKey{Curve: elliptic.P521()},
+		CertAlgoRSAv01:      &OpenSSHCertV01{Key: new(rsa.PublicKey)},
+		CertAlgoDSAv01:      &OpenSSHCertV01{Key: new(dsa.PublicKey)},
+		CertAlgoECDSA256v01: &OpenSSHCertV01{Key: &ecdsa.PublicKey{Curve: elliptic.P256()}},
+		CertAlgoECDSA384v01: &OpenSSHCertV01{Key: &ecdsa.PublicKey{Curve: elliptic.P384()}},
+		CertAlgoECDSA521v01: &OpenSSHCertV01{Key: &ecdsa.PublicKey{Curve: elliptic.P521()}},
+	}
+
+	for expected, key := range supported {
+		actual := algoName(key)
+		if expected != actual {
+			t.Errorf("expected: %s, actual: %s", expected, actual)
+		}
+	}
+
+}
+
+func TestAlgoNameNotSupported(t *testing.T) {
+	notSupported := []interface{}{
+		&ecdsa.PublicKey{Curve: elliptic.P224()},
+		&OpenSSHCertV01{Key: &ecdsa.PublicKey{Curve: elliptic.P224()}},
+	}
+
+	panicTest := func(key interface{}) (algo string, err error) {
+		defer func() {
+			if r := recover(); r != nil {
+				err = errors.New(r.(string))
+			}
+		}()
+		algo = algoName(key)
+		return
+	}
+
+	for _, unsupportedKey := range notSupported {
+		if algo, err := panicTest(unsupportedKey); err == nil {
+			t.Errorf("Expected a panic, Got: %s (for type %T)", algo, unsupportedKey)
+		}
+	}
+}