Fix: Do not set IssuerKeyId on v6 key-signatures

pgpainless-core/src/main/kotlin/org/pgpainless/key/generation/KeyRingBuilder.kt

@@ -97,7 +97,7 @@ class KeyRingBuilder(
         val signatureGenerator = PGPSignatureGenerator(signer, certKey.publicKey)
 
         val hashedSubPacketGenerator = primaryKeySpec!!.subpacketGenerator
-        hashedSubPacketGenerator.setIssuerFingerprintAndKeyId(certKey.publicKey)
+        hashedSubPacketGenerator.setAppropriateIssuerInfo(certKey.publicKey, version)
         expirationDate?.let { hashedSubPacketGenerator.setKeyExpirationTime(certKey.publicKey, it) }
         if (userIds.isNotEmpty()) {
             hashedSubPacketGenerator.setPrimaryUserId()

pgpainless-core/src/main/kotlin/org/pgpainless/signature/subpackets/BaseSignatureSubpackets.kt

@@ -11,12 +11,25 @@ import org.bouncycastle.bcpg.sig.*
 import org.bouncycastle.openpgp.PGPPublicKey
 import org.bouncycastle.openpgp.PGPSignature
 import org.pgpainless.algorithm.HashAlgorithm
+import org.pgpainless.algorithm.OpenPGPKeyVersion
 import org.pgpainless.algorithm.PublicKeyAlgorithm
 
 interface BaseSignatureSubpackets {
 
     interface Callback : SignatureSubpacketCallback<BaseSignatureSubpackets>
 
+    fun setAppropriateIssuerInfo(key: PGPPublicKey): BaseSignatureSubpackets
+
+    /**
+     * Depending on the given [version], use the appropriate means of setting issuer information.
+     * V6 signatures for example MUST NOT contain an [IssuerKeyID] packet.
+     *
+     * @param key issuer key
+     * @param version signature version
+     * @return this
+     */
+    fun setAppropriateIssuerInfo(key: PGPPublicKey, version: OpenPGPKeyVersion): BaseSignatureSubpackets
+
     /**
      * Add both an [IssuerKeyID] and [IssuerFingerprint] subpacket pointing to the given key.
      *

pgpainless-core/src/main/kotlin/org/pgpainless/signature/subpackets/SignatureSubpackets.kt

@@ -72,7 +72,7 @@ class SignatureSubpackets :
             issuer: PGPPublicKey,
             base: PGPSignatureSubpacketVector
         ): SignatureSubpackets {
-            return createSubpacketsFrom(base).apply { setIssuerFingerprintAndKeyId(issuer) }
+            return createSubpacketsFrom(base).apply { setAppropriateIssuerInfo(issuer) }
         }
 
         @JvmStatic
@@ -82,7 +82,7 @@ class SignatureSubpackets :
 
         @JvmStatic
         fun createHashedSubpackets(issuer: PGPPublicKey): SignatureSubpackets {
-            return createEmptySubpackets().setIssuerFingerprintAndKeyId(issuer)
+            return createEmptySubpackets().setAppropriateIssuerInfo(issuer)
         }
 
         @JvmStatic
@@ -352,6 +352,18 @@ class SignatureSubpackets :
         this.featuresSubpacket = features
     }
 
+    override fun setAppropriateIssuerInfo(key: PGPPublicKey) = apply {
+        setAppropriateIssuerInfo(key, OpenPGPKeyVersion.from(key.version))
+    }
+
+    override fun setAppropriateIssuerInfo(key: PGPPublicKey, version: OpenPGPKeyVersion) = apply {
+        when (version) {
+            OpenPGPKeyVersion.v3 -> setIssuerKeyId(key.keyID)
+            OpenPGPKeyVersion.v4 -> setIssuerFingerprintAndKeyId(key)
+            OpenPGPKeyVersion.librePgp, OpenPGPKeyVersion.v6 -> setIssuerFingerprint(key)
+        }
+    }
+
     override fun setIssuerFingerprintAndKeyId(key: PGPPublicKey): SignatureSubpackets = apply {
         setIssuerKeyId(key.keyID)
         setIssuerFingerprint(key)