Reject data signatures made by non-signing primary key

pgpainless-core/src/main/kotlin/org/pgpainless/signature/consumer/CertificateValidator.kt

@@ -169,6 +169,17 @@ class CertificateValidator {
                         return true
                     }
                 }
+                // Reject sigs by non-signing keys
+                if (userIdSignatures.none { (_, sigs) ->
+                    sigs.any {
+                        SignatureSubpacketsUtil.getKeyFlags(it)?.let { f ->
+                            KeyFlag.hasKeyFlag(f.flags, KeyFlag.SIGN_DATA)
+                        } == true
+                    }
+                }) {
+                    throw SignatureValidationException(
+                        "Signature was generated by non-signing key.")
+                }
             } else { // signing key is subkey
                 val subkeySigs = mutableListOf<PGPSignature>()
                 signingSubkey
@@ -183,7 +194,7 @@ class CertificateValidator {
                             }
                         } catch (e: SignatureValidationException) {
                             rejections[it] = e
-                            LOGGER.debug("REjecting subkey revocation signature: ${e.message}", e)
+                            LOGGER.debug("Rejecting subkey revocation signature: ${e.message}", e)
                         }
                     }
 

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/VerifySignatureByCertificationKeyFailsTest.java

@@ -0,0 +1,228 @@
+// SPDX-FileCopyrightText: 2024 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.decryption_verification;
+
+import org.bouncycastle.openpgp.PGPSecretKeyRing;
+import org.bouncycastle.util.io.Streams;
+import org.junit.jupiter.api.Test;
+import org.pgpainless.PGPainless;
+
+import java.io.ByteArrayInputStream;
+import java.nio.charset.StandardCharsets;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+
+public class VerifySignatureByCertificationKeyFailsTest {
+
+    // Key with non-signing primary key and dedicated signing subkey
+    private static final String KEY = "-----BEGIN PGP PRIVATE KEY BLOCK-----\n" +
+            "Version: PGPainless\n" +
+            "Comment: ACFC 7FFA 02BE C1E8 5002  2D35 AFE4 67B2 9A41 A0CE\n" +
+            "Comment: Complex RSA <test@test.test>\n" +
+            "\n" +
+            "lQcYBGYQBhIBEACVXJstPdymc/0ZAQYWSy/hOpHFV0YBgon4ymgIrN0xlJgW0Lju\n" +
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+            "aD2Ozz0IzDDSMyyZiAibhuoOOL4izinFX9O7AxVEkIf3+IHloxjuT6LSllNOO8ZB\n" +
+            "xKsyeLFgIjQFXjY4sR+JleML+YkCq46NdHVmbb6TryDelkduqCHOk2PGvdv/1zrf\n" +
+            "yTRMWQLx80Urd4rOKxNOnqwblj6yt1eOINXA31GkJSIEOkkLdFOZ/H7zp+UqbGE/\n" +
+            "4nGh/JzBur/jTLIx9ElsrV3lLGEtUa47mgbMYAXIIL2RR+LMlT4JoO3SQXEuvkok\n" +
+            "RT7NpyoVTWHYlQiR0XruCt4bvWaEVCdiNj565yI7pqPzkQgAxZ5tKTPW+8uHOQSh\n" +
+            "NuY9Ok2bUcaNvzboOu7t3c9rETf5/XBAjcmjNSxlDLLiZv9Qp3DhcnXu45ZDSh86\n" +
+            "Sd+o9NlQLTAs/J98rRExDISj42V3q740Hf8jXS3lz4fwlr3YHeV0c1A+YsIoY5nb\n" +
+            "Bs2UHO5nX1PgtE17Bgbt1+Y3jamK/2+W7WDAiIq7G4uRpeORqrXsjWZDuIaKR03C\n" +
+            "lxj1c7LqkFUjVpY47mdlBDuXmra7slcYVdabR5oULpRvloAjp+VX3Yfwzl0/I0o6\n" +
+            "/3WZTRhM4PhkP4imkveykv/BkIDFWfho3pJScru8Aqn3XbccrZO8WY6nONzvYIr+\n" +
+            "COgWpwgA7VALxeELI7nJiWeZUZ5HBHOMKQFTdioKD9BgwvEPDOXq+VqAT2n3basQ\n" +
+            "ShYjCot8gnxe3ei+/KQs/a8DkZVDY7XKMqhZyvidxF6weFAbO4Z+sB6LfyJmz5D1\n" +
+            "nA0iv8laP6at1wgn1MSfpXZm1cXuUHDpyTqWXLxG1qbUpQJS27fl6F0ws1k8Qvno\n" +
+            "PLjGbsbEeujbWU2lqZDs3L72e/arIpHdO94LNOwSKrGjO1KTkGgue+uGm1u17gEZ\n" +
+            "3U+rbCvG2lTq7kHZEQHiGaIygxyDJAyIvroPZ9c5nABLGMoH93Pq9tGX2OPxyWLx\n" +
+            "P1db87EzKRQjFGmZSeeKENRvLYAVcwgAqZTCPD9CkUG6O//rZI/1pPArhN8CY/+x\n" +
+            "8L2jksvaTLimdBzXyVXlNAPIynMih/QaxNq+WNOmnihVFAw7ur/nmN9Oea33Ue7v\n" +
+            "6Ryrz9nzi6GonLz0/grzB5XWAiO3+i9WUdVpeTgjlFPXSJsbOjEVRAB2W0heioeM\n" +
+            "M6weKP8i2O+GF3ULoWzmam4EgsanaSCXgDq1RYXyJhfUYTikVqrD+qJYT0asxU0H\n" +
+            "S92KmdZ6UKLOTPZdrIL5X0Cj/ejBX/94xHLM0GfXT9CIHTVIVSa2poSbWhN0O+yT\n" +
+            "NMySr7OHGDUjEUqBL2O5Wm3oyMK/5EoSQ9YJBCrkbkymvoahjrooXZSeiQIzBBgB\n" +
+            "CgAdBQJmEAYYAp4BApsMBRYCAwEABAsJCAcFFQoJCAsACgkQr+RnsppBoM7gBQ//\n" +
+            "YGQ6suANXV0C7TvofcMwubahLriBiWGAkI+pNQ7Tmzgql4ALcf4BFsDffcTCy3ue\n" +
+            "dLAZkEEwT86Ip1qW7mKCN1tScj0g8uU8U78oVShqoyEq0ebemaVmY7gvFWCpGXgr\n" +
+            "JgNjL4QzRbwzKUdvIkTMmUDiPqhmGuCFV76pazzTmhI/RAzVfnroG0kVDVwODlCI\n" +
+            "CcswoEOI32v+B8SJbbsXXQ4E6jrDwPIUV/z54VERbi+JcKaes0a/rOLF0FOnoTFs\n" +
+            "2b2aqt+Knj/RQCP9r3UNl06dONBMaydkhFmMjRjenQb6p+91JyNkXJ6tgD4CDbps\n" +
+            "2kqryCpZf39E0QU75Rvuofc8vhSRbRvSl7D+NhOdk9c6EEOJPbtozaG+/l8g1K42\n" +
+            "aMT+TvccBPK0b4EcZtAyCLjDg0eA/GN5+DIhBQCQA4y4KdEf3IMoS+BeEPBNfNyG\n" +
+            "isVD/f/R/68uGg6S9sKFXOCAXO0rnVIu1Oe123l09FsXVZpFDV3PMbpk7Sxh/20L\n" +
+            "ERTp5jBV+2J2szmYYxcSfVl6h+P3k8Y8l4evEoPPL6Skz9uZI5C3UB1c6dnWBcRm\n" +
+            "G759QEMpJuvyuwFZkcoGxfVvtneZsTGajEnNOB4o46hb9a5DLJ9tc06zG+j+eUeR\n" +
+            "C6mwvn/JYRaGr9uG4zpdxNeFmQ80yitmGGllHenjfaU=\n" +
+            "=hhYV\n" +
+            "-----END PGP PRIVATE KEY BLOCK-----\n";
+
+    private static byte[] DATA = "Hello, World!".getBytes(StandardCharsets.UTF_8);
+
+    // Signature by primary key (shall be rejected, as the primary key is not signing capable.
+    //  We instead expect the signing subkey to sign).
+    private static final String SIG = "-----BEGIN PGP SIGNATURE-----\n" +
+            "Version: PGPainless\n" +
+            "\n" +
+            "iQIcBAABCAAGBQJmEAYYAAoJEK/kZ7KaQaDOH4MP/2kaK8lQaBU+jChpWPLR2R2+\n" +
+            "dB7j29tFPRAqbzbazxaF+jZQxuuHWtM3bwd9Vdta9zirDc27b7XyufFLBza4Bn+R\n" +
+            "7fT7uHTQQts/zaX8YGxJ90rb06toFXiv/rlm531kLaGXxlACU6SpI8maqpP4im+G\n" +
+            "W0LgBDZiT9udFs3eeJZ8O3yDLP29Rdw8sHPa6pOyyhkkhsvo0bNaBaSt6GDW5UK9\n" +
+            "f5Gz+XF9ZLJgsNqQwWQM55+4ZhdkfEszRcJgAhuSCamk+ZLfvIPhEu21/7weNq3c\n" +
+            "Yp0hvaz27gaW7IkjEgI1FqkPrmJmyk5SVMMvaev9p0WXDgUIeDLI6CwvoXaoMCAX\n" +
+            "pg0Q754ccHu2pELwNb5YIxGSPSMXRVH8xUDqicZpl/50ucy3g348s5HekcVnzBtX\n" +
+            "UKVX3tU6r5HrkVAX7bDGht3WXE1jRE98W3uKpFWzrJBK+uQIyOtOEXeKT+z1BbNy\n" +
+            "CvYeDq4xjpGYB2tJY2LKXrC4+IzJ56e3XU2t75KhO0SBV+Ax1bJ0MBnmAedg0pg5\n" +
+            "0r+mknBqoYu+OHNwMS/N+YH1iZEV0QxP+ldLp+ff2QiIvtiDcOIQ0oNEJy0bqh0p\n" +
+            "TrS9PKMl+kH+FaAPUVb0ruviTd/zPljjiJ2P396bu1JBXdoncn2y4KklQOWHJSVI\n" +
+            "F5ZjatEBixl6pdW7I5Cr\n" +
+            "=vPG/\n" +
+            "-----END PGP SIGNATURE-----";
+
+    @Test
+    public void testSignatureByNonSigningPrimaryKeyIsRejected() throws Exception {
+        PGPSecretKeyRing key = PGPainless.readKeyRing().secretKeyRing(KEY);
+
+        DecryptionStream verifier = PGPainless.decryptAndOrVerify()
+                .onInputStream(new ByteArrayInputStream(DATA))
+                .withOptions(ConsumerOptions.get()
+                        .addVerificationCert(PGPainless.extractCertificate(key))
+                        .addVerificationOfDetachedSignatures(new ByteArrayInputStream(SIG.getBytes(StandardCharsets.UTF_8))));
+
+        Streams.drain(verifier);
+        verifier.close();
+
+        MessageMetadata result = verifier.getMetadata();
+        assertFalse(result.isVerifiedSigned());
+    }
+}