From d2dd0f161a7ebdd3236e5d75d763f5ada69d5429 Mon Sep 17 00:00:00 2001
From: Pol Henarejos <pol.henarejos@cttc.es>
Date: Sun, 5 Jul 2026 01:21:21 +0200
Subject: [PATCH] Fix hmac-secret-mc check.

Signed-off-by: Pol Henarejos <pol.henarejos@cttc.es>
---
 src/fido/cbor_make_credential.c         | 28 ++++++++++++-------------
 tests/pico-fido/test_035_hmac_secret.py | 20 ++++++++++++++++++
 2 files changed, 34 insertions(+), 14 deletions(-)

diff --git a/src/fido/cbor_make_credential.c b/src/fido/cbor_make_credential.c
index 4ddc89e..c3be020 100644
--- a/src/fido/cbor_make_credential.c
+++ b/src/fido/cbor_make_credential.c
@@ -212,6 +212,20 @@ int cbor_make_credential(const uint8_t *data, size_t len) {
         }
     }
     CBOR_PARSE_MAP_END(map, 1);
+    if (hmac_secret_mc && extensions.hmac_secret != ptrue) {
+        CBOR_ERROR(CTAP2_ERR_MISSING_PARAMETER);
+    }
+    if (hmac_secret_mc) {
+        if (kax.present == false || kay.present == false || crv == 0 || hmac_alg == 0 ||
+            salt_enc.present == false || salt_enc.len == 0 ||
+            salt_auth.present == false || salt_auth.len == 0) {
+            CBOR_ERROR(CTAP2_ERR_MISSING_PARAMETER);
+        }
+        if (salt_enc.len != 32 + (hmacSecretPinUvAuthProtocol - 1) * IV_SIZE &&
+            salt_enc.len != 64 + (hmacSecretPinUvAuthProtocol - 1) * IV_SIZE) {
+            CBOR_ERROR(CTAP1_ERR_INVALID_PARAMETER);
+        }
+    }
     rp_id = rp.id.data;
     user_name = user.parent.name.data;
     display_name = user.displayName.data;
@@ -427,20 +441,6 @@ int cbor_make_credential(const uint8_t *data, size_t len) {
         CBOR_ERROR(CTAP2_ERR_INVALID_OPTION);
     }
 
-    if (hmac_secret_mc && extensions.hmac_secret != ptrue) {
-        CBOR_ERROR(CTAP2_ERR_MISSING_PARAMETER);
-    }
-    if (hmac_secret_mc) {
-        if (kax.present == false || kay.present == false || crv == 0 || hmac_alg == 0 ||
-            salt_enc.present == false || salt_auth.present == false) {
-            CBOR_ERROR(CTAP2_ERR_MISSING_PARAMETER);
-        }
-        if (salt_enc.len != 32 + (hmacSecretPinUvAuthProtocol - 1) * IV_SIZE &&
-            salt_enc.len != 64 + (hmacSecretPinUvAuthProtocol - 1) * IV_SIZE) {
-            CBOR_ERROR(CTAP1_ERR_INVALID_PARAMETER);
-        }
-    }
-
     if (options.up == ptrue || options.up == NULL) { //14.1
         if (pinUvAuthParam.present == true) {
             if (getUserPresentFlagValue() == false) {
diff --git a/tests/pico-fido/test_035_hmac_secret.py b/tests/pico-fido/test_035_hmac_secret.py
index ccd276c..7c40202 100644
--- a/tests/pico-fido/test_035_hmac_secret.py
+++ b/tests/pico-fido/test_035_hmac_secret.py
@@ -121,6 +121,26 @@ def test_missing_saltEnc(device,):
         device.GA(extensions={"hmac-secret": { 3: b'1234'}})
     assert e.value.code == CtapError.ERR.MISSING_PARAMETER
 
+def test_make_credential_hmac_secret_mc_empty_salt(device):
+    key_agreement = {
+        1: 2,
+        3: -25,
+        -1: 1,
+        -2: b'\x00' * 32,
+        -3: b'\x00' * 32,
+    }
+    with pytest.raises(CtapError) as e:
+        device.MC(extensions={
+            "hmac-secret": True,
+            "hmac-secret-mc": {
+                1: key_agreement,
+                2: b'',
+                3: b'\x00' * 32,
+                4: 2,
+            },
+        })
+    assert e.value.code == CtapError.ERR.MISSING_PARAMETER
+
 def test_bad_auth(device,  MCHmacSecret):
 
     key_agreement = {