piv: let user specify pin policy instead of using attestation cert

I'm still seeing issues with my older YubiKey, but it looks like it's
not respecting the pin policy on the generated key. For example,
settings PINPolicyNone results in a "security condition not satisfied"
error.

piv/key.go

@@ -460,6 +460,11 @@ type Key struct {
 	// Algorithm to use when generating the key.
 	Algorithm Algorithm
 	// PINPolicy for the key.
+	//
+	// BUG(ericchiang): some older YubiKeys (third generation) will silently
+	// drop this value. If PINPolicyNever or PINPolicyOnce is supplied but the
+	// key still requires a PIN every time, you may be using a buggy key and
+	// should supply PINPolicyAlways. See https://github.com/go-piv/piv-go/issues/60
 	PINPolicy PINPolicy
 	// TouchPolicy for the key.
 	TouchPolicy TouchPolicy
@@ -536,12 +541,13 @@ type KeyAuth struct {
 	// PINPrompt can be used to interactively request the PIN from the user. The
 	// method is only called when needed. For example, if a key specifies
 	// PINPolicyOnce, PINPrompt will only be called once per YubiKey struct.
-	//
-	// BUG(ericchiang): On YubiKey versions older than 4.3.0 PIN caching isn't
-	// supported and PINPrompt will be called on every signing operation, even if
-	// PINPolicyOnce is specified. This is due to a bug in the VERIFY card
-	// command for older YubiKeys.
 	PINPrompt func() (pin string, err error)
+
+	// PINPolicy can used to specify the PIN caching strategy for the slot. If not
+	// provided, this will be inferred from the attestation certificate.
+	//
+	// This field is only required on older (<4.3.0) YubiKeys.
+	PINPolicy PINPolicy
 }
 
 func isAuthErr(err error) bool {
@@ -624,7 +630,11 @@ func pinPolicy(yk *YubiKey, slot Slot) (PINPolicy, error) {
 //
 func (yk *YubiKey) PrivateKey(slot Slot, public crypto.PublicKey, auth KeyAuth) (crypto.PrivateKey, error) {
 	pp := PINPolicyNever
-	if auth.PIN != "" || auth.PINPrompt != nil {
+	if _, ok := pinPolicyMap[auth.PINPolicy]; ok {
+		// If the PIN policy is manually specified, trust that value instead of
+		// trying to use the attestation certificate.
+		pp = auth.PINPolicy
+	} else if auth.PIN != "" || auth.PINPrompt != nil {
 		// Attempt to determine the key's PIN policy. This helps inform the
 		// strategy for when to prompt for a PIN.
 		policy, err := pinPolicy(yk, slot)

piv/key_test.go

@@ -93,8 +93,6 @@ func TestPINPrompt(t *testing.T) {
 			yk, close := newTestYubiKey(t)
 			defer close()
 
-			testRequiresVersion(t, yk, 4, 3, 0)
-
 			k := Key{
 				Algorithm:   AlgorithmEC256,
 				PINPolicy:   test.policy,
@@ -111,6 +109,11 @@ func TestPINPrompt(t *testing.T) {
 					return DefaultPIN, nil
 				},
 			}
+
+			if !supportsAttestation(yk) {
+				auth.PINPolicy = test.policy
+			}
+
 			priv, err := yk.PrivateKey(SlotAuthentication, pub, auth)
 			if err != nil {
 				t.Fatalf("building private key: %v", err)