From e8876f418fdb3f9451960f58856cdfc1c27cff84 Mon Sep 17 00:00:00 2001
From: M66B <M66B@users.noreply.github.com>
Date: Thu, 30 Mar 2023 12:04:43 +0200
Subject: [PATCH] DKIM check public key length

---
 .../main/java/eu/faircode/email/MessageHelper.java  | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 49bdfc1cb8..228d3c83f4 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -98,6 +98,7 @@ import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.security.PublicKey;
 import java.security.Signature;
+import java.security.interfaces.RSAPublicKey;
 import java.security.spec.X509EncodedKeySpec;
 import java.text.Normalizer;
 import java.text.ParsePosition;
@@ -191,6 +192,7 @@ public class MessageHelper {
     private static final int FORMAT_FLOWED_LINE_LENGTH = 72; // characters
     private static final int MAX_DIAGNOSTIC = 250; // characters
     private static final int DKIM_MIN_TEXT = 100; // characters
+    private static final int DKIM_MIN_KEY_LENGTH = 1024; //  bits
 
     private static final String DKIM_SIGNATURE = "DKIM-Signature";
     private static final String ARC_SEAL = "ARC-Seal";
@@ -2439,6 +2441,17 @@ public class MessageHelper {
             PublicKey pubKey = keyFactory.generatePublic(pubKeySpec);
             Signature sig = Signature.getInstance(salgo); // a=
 
+            // https://stackoverflow.com/a/43984402/1794097
+            if (pubKey instanceof RSAPublicKey)
+                try {
+                    int keylen = ((RSAPublicKey) pubKey).getModulus().bitLength();
+                    Log.i("DKIM RSA pubkey length=" + keylen);
+                    if (keylen < DKIM_MIN_KEY_LENGTH)
+                        throw new IllegalArgumentException("RSA pubkey length " + keylen + " < " + DKIM_MIN_KEY_LENGTH);
+                } catch (Throwable ex) {
+                    Log.e(ex);
+                }
+
             String hash = kv.get("b");
             if (hash == null)
                 return null;