Kotlin conversion: SecretKeyPassphraseProvider and subclasses

This commit also adds a workaround to build.gradle which enables proper Java interop for
Kotlin interfaces with default implementations

pgpainless-core/src/main/kotlin/org/pgpainless/key/protection/passphrase_provider/MapBasedPassphraseProvider.kt

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.key.protection.passphrase_provider
+
+import org.pgpainless.util.Passphrase
+
+/**
+ * Implementation of the [SecretKeyPassphraseProvider] that holds a map of key-IDs and respective [Passphrase].
+ * It will return the right passphrase depending on the key-id.
+ *
+ * Note: This provider might return null!
+ * TODO: Make this null-safe and throw an exception instead?
+ */
+class MapBasedPassphraseProvider(val map: Map<Long, Passphrase>) : SecretKeyPassphraseProvider {
+
+    override fun getPassphraseFor(keyId: Long): Passphrase? = map[keyId]
+
+    override fun hasPassphrase(keyId: Long): Boolean = map.containsKey(keyId)
+}

pgpainless-core/src/main/kotlin/org/pgpainless/key/protection/passphrase_provider/SecretKeyPassphraseProvider.kt

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+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.key.protection.passphrase_provider
+
+import org.bouncycastle.openpgp.PGPSecretKey
+import org.pgpainless.util.Passphrase
+
+/**
+ * Interface to allow the user to provide a [Passphrase] for an encrypted OpenPGP secret key.
+ */
+interface SecretKeyPassphraseProvider {
+
+    /**
+     * Return a passphrase for the given secret key.
+     * If no record is found, return null.
+     * Note: In case of an unprotected secret key, this method must may not return null, but a [Passphrase] with
+     * a content of null.
+     *
+     * @param secretKey secret key
+     * @return passphrase or null, if no passphrase record is found.
+     */
+    fun getPassphraseFor(secretKey: PGPSecretKey): Passphrase? {
+        return getPassphraseFor(secretKey.keyID)
+    }
+
+    /**
+     * Return a passphrase for the given key. If no record has been found, return null.
+     * Note: In case of an unprotected secret key, this method must may not return null, but a [Passphrase] with
+     * a content of null.
+     *
+     * @param keyId if of the secret key
+     * @return passphrase or null, if no passphrase record has been found.
+     */
+    fun getPassphraseFor(keyId: Long): Passphrase?
+
+    fun hasPassphrase(keyId: Long): Boolean
+}

pgpainless-core/src/main/kotlin/org/pgpainless/key/protection/passphrase_provider/SolitaryPassphraseProvider.kt

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+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.key.protection.passphrase_provider
+
+import org.pgpainless.util.Passphrase
+
+/**
+ * Implementation of the [SecretKeyPassphraseProvider] that holds a single [Passphrase].
+ */
+class SolitaryPassphraseProvider(val passphrase: Passphrase?) : SecretKeyPassphraseProvider {
+
+    override fun getPassphraseFor(keyId: Long): Passphrase? = passphrase
+
+    override fun hasPassphrase(keyId: Long): Boolean = true
+}