S/MIME: check if private and public key match

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

@@ -4553,6 +4553,8 @@ public class FragmentCompose extends FragmentBase {
                     if (acertificates != null)
                         for (EntityCertificate acertificate : acertificates) {
                             X509Certificate cert = acertificate.getCertificate();
+                            if (!SmimeHelper.match(privkey, cert))
+                                continue;
                             try {
                                 cert.checkValidity();
                                 certs.add(cert);
@@ -4575,7 +4577,7 @@ public class FragmentCompose extends FragmentBase {
                 }
 
                 // Allow sender to decrypt own message
-                if (own)
+                if (own && SmimeHelper.match(privkey, chain[0]))
                     certs.add(chain[0]);
 
                 // Build signature

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

@@ -21,7 +21,11 @@ package eu.faircode.email;
 
 import android.content.Context;
 
+import java.security.PrivateKey;
+import java.security.PublicKey;
+import java.security.cert.X509Certificate;
 import java.util.List;
+import java.util.Objects;
 
 import javax.mail.Address;
 import javax.mail.internet.InternetAddress;
@@ -42,4 +46,13 @@ public class SmimeHelper {
 
         return (all ? count == recipients.size() : count > 0);
     }
+
+    static boolean match(PrivateKey privkey, X509Certificate cert) {
+        if (privkey == null || cert == null)
+            return false;
+        PublicKey pubkey = cert.getPublicKey();
+        if (pubkey == null)
+            return false;
+        return Objects.equals(privkey.getAlgorithm(), pubkey.getAlgorithm());
+    }
 }