Added option to check S/MIME key usage on sending

app/src/main/java/eu/faircode/email/FragmentCompose.java

@@ -4388,6 +4388,7 @@ public class FragmentCompose extends FragmentBase {
 
                 SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
                 boolean check_certificate = prefs.getBoolean("check_certificate", true);
+                boolean check_key_usage = prefs.getBoolean("check_key_usage", false);
 
                 File tmp = Helper.ensureExists(context, "encryption");
 
@@ -4455,8 +4456,30 @@ public class FragmentCompose extends FragmentBase {
                     // Check public key validity
                     try {
                         chain[0].checkValidity();
-                        // TODO: check digitalSignature/nonRepudiation key usage
+
-                        // https://datatracker.ietf.org/doc/html/rfc3850#section-4.4.2
+                        if (check_key_usage) {
+                            // Signing Key: Key Usage: Digital Signature, Non-Repudiation
+                            // Encrypting Key: Key Usage: Key Encipherment, Data Encipherment
+
+                            boolean[] usage = chain[0].getKeyUsage();
+                            if (usage != null && usage.length > 3) {
+                                // https://datatracker.ietf.org/doc/html/rfc3280#section-4.2.1.3
+                                // https://datatracker.ietf.org/doc/html/rfc3850#section-4.4.2
+                                boolean digitalSignature = usage[0];
+                                boolean keyEncipherment = usage[2];
+
+                                if (EntityMessage.SMIME_SIGNONLY.equals(type)) {
+                                    if (!digitalSignature)
+                                        throw new IllegalAccessException("Invalid key usage:" +
+                                                " digitalSignature=" + digitalSignature);
+                                } else if (EntityMessage.SMIME_SIGNENCRYPT.equals(type)) {
+                                    if (!digitalSignature || !keyEncipherment)
+                                        throw new IllegalAccessException("Invalid key usage:" +
+                                                " digitalSignature=" + digitalSignature +
+                                                " keyEncipherment=" + keyEncipherment);
+                                }
+                            }
+                        }
                     } catch (CertificateException ex) {
                         String msg = ex.getMessage();
                         throw new IllegalArgumentException(

app/src/main/java/eu/faircode/email/FragmentOptionsEncryption.java

@@ -105,6 +105,7 @@ public class FragmentOptionsEncryption extends FragmentBase
     private Spinner spSignAlgoSmime;
     private Spinner spEncryptAlgoSmime;
     private SwitchCompat swCheckCertificate;
+    private SwitchCompat swCheckKeyUsage;
     private Button btnManageCertificates;
     private Button btnImportKey;
     private Button btnManageKeys;
@@ -123,7 +124,7 @@ public class FragmentOptionsEncryption extends FragmentBase
             "sign_default", "encrypt_default", "encrypt_auto", "encrypt_reply",
             "auto_verify", "auto_decrypt", "auto_undecrypt",
             "openpgp_provider", "autocrypt", "autocrypt_mutual", "encrypt_subject",
-            "sign_algo_smime", "encrypt_algo_smime", "check_certificate"
+            "sign_algo_smime", "encrypt_algo_smime", "check_certificate", "check_key_usage"
     ));
 
     @Override
@@ -159,6 +160,7 @@ public class FragmentOptionsEncryption extends FragmentBase
         spSignAlgoSmime = view.findViewById(R.id.spSignAlgoSmime);
         spEncryptAlgoSmime = view.findViewById(R.id.spEncryptAlgoSmime);
         swCheckCertificate = view.findViewById(R.id.swCheckCertificate);
+        swCheckKeyUsage = view.findViewById(R.id.swCheckKeyUsage);
         btnManageCertificates = view.findViewById(R.id.btnManageCertificates);
         btnImportKey = view.findViewById(R.id.btnImportKey);
         btnManageKeys = view.findViewById(R.id.btnManageKeys);
@@ -395,6 +397,14 @@ public class FragmentOptionsEncryption extends FragmentBase
             @Override
             public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
                 prefs.edit().putBoolean("check_certificate", checked).apply();
+                swCheckKeyUsage.setEnabled(checked);
+            }
+        });
+
+        swCheckKeyUsage.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
+            @Override
+            public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
+                prefs.edit().putBoolean("check_key_usage", checked).apply();
             }
         });
 
@@ -709,6 +719,8 @@ public class FragmentOptionsEncryption extends FragmentBase
                 }
 
             swCheckCertificate.setChecked(prefs.getBoolean("check_certificate", true));
+            swCheckKeyUsage.setChecked(prefs.getBoolean("check_key_usage", false));
+            swCheckKeyUsage.setEnabled(swCheckCertificate.isChecked());
         } catch (Throwable ex) {
             Log.e(ex);
         }

app/src/main/res/layout/fragment_options_encryption.xml

@@ -421,6 +421,19 @@
                     app:layout_constraintTop_toBottomOf="@id/tvAlgoHint"
                     app:switchPadding="12dp" />
 
+                <androidx.appcompat.widget.SwitchCompat
+                    android:id="@+id/swCheckKeyUsage"
+                    android:layout_width="0dp"
+                    android:layout_height="wrap_content"
+                    android:layout_marginStart="12dp"
+                    android:layout_marginTop="12dp"
+                    android:checked="true"
+                    android:text="@string/title_advanced_check_key_usage"
+                    app:layout_constraintEnd_toEndOf="parent"
+                    app:layout_constraintStart_toStartOf="parent"
+                    app:layout_constraintTop_toBottomOf="@id/swCheckCertificate"
+                    app:switchPadding="12dp" />
+
                 <Button
                     android:id="@+id/btnManageCertificates"
                     style="?android:attr/buttonStyleSmall"
@@ -431,7 +444,7 @@
                     android:drawablePadding="6dp"
                     android:text="@string/title_advanced_manage_certificates"
                     app:layout_constraintStart_toStartOf="parent"
-                    app:layout_constraintTop_toBottomOf="@id/swCheckCertificate" />
+                    app:layout_constraintTop_toBottomOf="@id/swCheckKeyUsage" />
 
                 <Button
                     android:id="@+id/btnImportKey"

app/src/main/res/values/strings.xml

@@ -869,6 +869,7 @@
         The default algorithms SHA256 and AES128 are commonly supported.
     </string>
     <string name="title_advanced_check_certificate">Check public key on sending</string>
+    <string name="title_advanced_check_key_usage">Check key usage on sending</string>
     <string name="title_advanced_manage_certificates">Manage public keys</string>
     <string name="title_advanced_import_key">Import private key</string>
     <string name="title_advanced_manage_keys">Manage private keys</string>