Handle empty pinAuth fields.

CTAP2 specifies that an empty pinAuth field is special: it indicates
that the device should block for touch, i.e. it's just a way of letting
a user select from multiple authenticators[1].

This change handles empty pinAuth fields in GetAssertion and
MakeCredential commands.

[1] https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html#using-pinToken-in-authenticatorMakeCredential

fido2/ctap.c

@@ -702,6 +702,14 @@ uint8_t ctap_make_credential(CborEncoder * encoder, uint8_t * request, int lengt
         printf2(TAG_ERR,"error, parse_make_credential failed\n");
         return ret;
     }
+    if (MC.pinAuthEmpty)
+    {
+        if (!device_is_nfc() && !ctap_user_presence_test())
+        {
+                return CTAP2_ERR_OPERATION_DENIED;
+        }
+        return ctap_is_pin_set() == 1 ? CTAP2_ERR_PIN_INVALID : CTAP2_ERR_PIN_NOT_SET;
+    }
     if ((MC.paramsParsed & MC_requiredMask) != MC_requiredMask)
     {
         printf2(TAG_ERR,"error, required parameter(s) for makeCredential are missing\n");
@@ -1133,6 +1141,14 @@ uint8_t ctap_get_assertion(CborEncoder * encoder, uint8_t * request, int length)
         return ret;
     }
 
+    if (GA.pinAuthEmpty)
+    {
+        if (!device_is_nfc() && !ctap_user_presence_test())
+        {
+                return CTAP2_ERR_OPERATION_DENIED;
+        }
+        return ctap_is_pin_set() == 1 ? CTAP2_ERR_PIN_INVALID : CTAP2_ERR_PIN_NOT_SET;
+    }
     if (GA.pinAuthPresent)
     {
         ret = verify_pin_auth(GA.pinAuth, GA.clientDataHash);

fido2/ctap.h

@@ -243,6 +243,11 @@ typedef struct
 
     uint8_t pinAuth[16];
     uint8_t pinAuthPresent;
+    // pinAuthEmpty is true iff an empty bytestring was provided as pinAuth.
+    // This is exclusive with |pinAuthPresent|. It exists because an empty
+    // pinAuth is a special signal to block for touch. See
+    // https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html#using-pinToken-in-authenticatorMakeCredential
+    uint8_t pinAuthEmpty;
     int pinProtocol;
     CTAP_extensions extensions;
 
@@ -266,6 +271,11 @@ typedef struct
 
     uint8_t pinAuth[16];
     uint8_t pinAuthPresent;
+    // pinAuthEmpty is true iff an empty bytestring was provided as pinAuth.
+    // This is exclusive with |pinAuthPresent|. It exists because an empty
+    // pinAuth is a special signal to block for touch. See
+    // https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html#using-pinToken-in-authenticatorGetAssertion
+    uint8_t pinAuthEmpty;
     int pinProtocol;
 
     CTAP_credentialDescriptor creds[ALLOW_LIST_MAX_SIZE];

fido2/ctap_parse.c

@@ -823,14 +823,22 @@ uint8_t ctap_parse_make_credential(CTAP_makeCredential * MC, CborEncoder * encod
                 ret = parse_options(&map, &MC->credInfo.rk, &MC->uv, &MC->up);
                 check_retr(ret);
                 break;
-            case MC_pinAuth:
+            case MC_pinAuth: {
                 printf1(TAG_MC,"CTAP_pinAuth\n");
 
+                size_t pinSize;
+                if (cbor_value_get_type(&map) == CborByteStringType &&
+                    cbor_value_get_string_length(&map, &pinSize) == CborNoError &&
+                    pinSize == 0)
+                {
+                    MC->pinAuthEmpty = 1;
+                    break;
+                }
+
                 ret = parse_fixed_byte_string(&map, MC->pinAuth, 16);
                 if (CTAP1_ERR_INVALID_LENGTH != ret)    // damn microsoft
                 {
                     check_retr(ret);
-
                 }
                 else
                 {
@@ -838,6 +846,7 @@ uint8_t ctap_parse_make_credential(CTAP_makeCredential * MC, CborEncoder * encod
                 }
                 MC->pinAuthPresent = 1;
                 break;
+            }
             case MC_pinProtocol:
                 printf1(TAG_MC,"CTAP_pinProtocol\n");
                 if (cbor_value_get_type(&map) == CborIntegerType)
@@ -1055,9 +1064,18 @@ uint8_t ctap_parse_get_assertion(CTAP_getAssertion * GA, uint8_t * request, int
                 ret = parse_options(&map, &GA->rk, &GA->uv, &GA->up);
                 check_retr(ret);
                 break;
-            case GA_pinAuth:
+            case GA_pinAuth: {
                 printf1(TAG_GA,"CTAP_pinAuth\n");
 
+                size_t pinSize;
+                if (cbor_value_get_type(&map) == CborByteStringType &&
+                    cbor_value_get_string_length(&map, &pinSize) == CborNoError &&
+                    pinSize == 0)
+                {
+                    GA->pinAuthEmpty = 1;
+                    break;
+                }
+
                 ret = parse_fixed_byte_string(&map, GA->pinAuth, 16);
                 if (CTAP1_ERR_INVALID_LENGTH != ret)    // damn microsoft
                 {
@@ -1073,6 +1091,7 @@ uint8_t ctap_parse_get_assertion(CTAP_getAssertion * GA, uint8_t * request, int
                 GA->pinAuthPresent = 1;
 
                 break;
+            }
             case GA_pinProtocol:
                 printf1(TAG_GA,"CTAP_pinProtocol\n");
                 if (cbor_value_get_type(&map) == CborIntegerType)