Sketch out yubikey-based public key decryption.

Note: Requires a modified copy of Bouncy Castle with the following changes:
* BcPublicKeyDataDecryptorFactory.unwrapSessionData(): make public
* org.bouncycastle.openpgp.operator.bc.RFC6637KDFCalculator: Make class public

pgpainless-yubikey/src/main/kotlin/org/pgpainless/yubikey/YubikeyDataDecryptorFactory.kt

@@ -1,24 +1,47 @@
+// SPDX-FileCopyrightText: 2025 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
 package org.pgpainless.yubikey
 
 import com.yubico.yubikit.core.smartcard.SmartCardConnection
 import com.yubico.yubikit.openpgp.KeyRef
 import com.yubico.yubikit.openpgp.OpenPgpSession
+import org.bouncycastle.bcpg.ECDHPublicBCPGKey
 import org.bouncycastle.bcpg.KeyIdentifier
+import org.bouncycastle.bcpg.PublicKeyAlgorithmTags
+import org.bouncycastle.crypto.params.KeyParameter
+import org.bouncycastle.openpgp.PGPPublicKey
+import org.bouncycastle.openpgp.operator.PGPPad
+import org.bouncycastle.openpgp.operator.RFC6637Utils
+import org.bouncycastle.openpgp.operator.bc.BcKeyFingerprintCalculator
+import org.bouncycastle.openpgp.operator.bc.BcPGPDigestCalculatorProvider
+import org.bouncycastle.openpgp.operator.bc.BcPublicKeyDataDecryptorFactory
+import org.bouncycastle.openpgp.operator.bc.RFC6637KDFCalculator
 import org.pgpainless.decryption_verification.HardwareSecurity
 
 class YubikeyDataDecryptorFactory(
-    smartCardConnection: SmartCardConnection,
     callback: HardwareSecurity.DecryptionCallback,
     keyIdentifier: KeyIdentifier,
 ) : HardwareSecurity.HardwareDataDecryptorFactory(keyIdentifier, callback) {
 
     companion object {
         @JvmStatic
-        fun createDecryptorFromConnection(smartCardConnection: SmartCardConnection): HardwareSecurity.HardwareDataDecryptorFactory {
+        fun createDecryptorFromConnection(
+            smartCardConnection: SmartCardConnection,
+            pubkey: PGPPublicKey
+        ): HardwareSecurity.HardwareDataDecryptorFactory {
             val openpgpSession = OpenPgpSession(smartCardConnection)
             val fingerprintBytes = openpgpSession.getData(KeyRef.DEC.fingerprint)
             val decKeyIdentifier = KeyIdentifier(fingerprintBytes)
-            val rsa = true
+
+            if (!decKeyIdentifier.matches(pubkey.keyIdentifier)) {
+                throw IllegalArgumentException("Fingerprint mismatch.")
+            }
+
+            val isRSAKey = pubkey.algorithm == PublicKeyAlgorithmTags.RSA_GENERAL
+                || pubkey.algorithm == PublicKeyAlgorithmTags.RSA_SIGN
+                || pubkey.algorithm == PublicKeyAlgorithmTags.RSA_ENCRYPT
 
             val callback = object : HardwareSecurity.DecryptionCallback {
                 override fun decryptSessionKey(
@@ -27,25 +50,49 @@ class YubikeyDataDecryptorFactory(
                     sessionKeyData: ByteArray,
                     pkeskVersion: Int
                 ): ByteArray {
+                    // TODO: Move user pin verification somewhere else
                     openpgpSession.verifyUserPin("asdasd".toCharArray(), true)
-                    if(rsa) {
-                        val decryptedSessionKey = openpgpSession.decrypt(sessionKeyData)
 
+                    if(isRSAKey) {
+                        // easy
+                        val decryptedSessionKey = openpgpSession.decrypt(sessionKeyData)
                         return decryptedSessionKey
                     } else {
-                        /*
+                        // meh...
-                        val secret = openpgpSession.decrypt(sessionKeyData)
+                        val ecPubKey: ECDHPublicBCPGKey = pubkey.publicKeyPacket.key as ECDHPublicBCPGKey
-                        val hashAlgorithm: Int = ecPubKey.getHashAlgorithm().toInt()
+                        // split session data into peer key and encrypted session key
-                        val symmetricKeyAlgorithm: Int = ecPubKey.getSymmetricKeyAlgorithm().toInt()
+
+                        // peer key
+                        val pLen =
+                            ((((sessionKeyData[0].toInt() and 0xff) shl 8) + (sessionKeyData[1].toInt() and 0xff)) + 7) / 8
+                        checkRange(2 + pLen + 1, sessionKeyData)
+                        val pEnc = ByteArray(pLen)
+                        System.arraycopy(sessionKeyData, 2, pEnc, 0, pLen)
+
+                        // encrypted session key
+                        val keyLen = sessionKeyData[pLen + 2].toInt() and 0xff
+                        checkRange(2 + pLen + 1 + keyLen, sessionKeyData)
+                        val keyEnc = ByteArray(keyLen)
+                        System.arraycopy(sessionKeyData, 2 + pLen + 1, keyEnc, 0, keyLen)
+
+                        // perform ECDH key agreement via the Yubikey
+                        val secret = openpgpSession.decrypt(pEnc)
+
+                        // Use the shared key to decrypt the session key
+                        val hashAlgorithm: Int = ecPubKey.hashAlgorithm.toInt()
+                        val symmetricKeyAlgorithm: Int = ecPubKey.symmetricKeyAlgorithm.toInt()
                         val userKeyingMaterial = RFC6637Utils.createUserKeyingMaterial(
-                            this.pgpPrivKey.getPublicKeyPacket(),
+                            pubkey.publicKeyPacket,
                             BcKeyFingerprintCalculator(),
                         )
-                        val rfc6637KDFCalculator = RFC6637KDFCalculator(
+                        val rfc6637KDFCalculator =
-                            BcPGPDigestCalculatorProvider()[hashAlgorithm], symmetricKeyAlgorithm,
+                            RFC6637KDFCalculator(
+                                BcPGPDigestCalculatorProvider()[hashAlgorithm],
+                                symmetricKeyAlgorithm,
                             )
                         val key =
                             KeyParameter(rfc6637KDFCalculator.createKey(secret, userKeyingMaterial))
+
                         return PGPPad.unpadSessionData(
                             BcPublicKeyDataDecryptorFactory.unwrapSessionData(
                                 keyEnc,
@@ -53,14 +100,11 @@ class YubikeyDataDecryptorFactory(
                                 key,
                             ),
                         )
-                         */
+                    }
-                        throw UnsupportedOperationException("ECDH decryption is not yet implemented.")
                 }
             }
 
-            }
+            return YubikeyDataDecryptorFactory(callback, decKeyIdentifier)
-
-            return YubikeyDataDecryptorFactory(smartCardConnection, callback, decKeyIdentifier)
         }
     }