add PrivateKeyProvider

src/main/kotlin/ltd/hlaeja/exception/KeyProviderException.kt

@@ -0,0 +1,23 @@
+package ltd.hlaeja.exception
+
+@Suppress("unused")
+class KeyProviderException : RuntimeException {
+
+    constructor() : super()
+
+    constructor(message: String) : super(message)
+
+    constructor(cause: Throwable) : super(cause)
+
+    constructor(
+        message: String,
+        cause: Throwable,
+    ) : super(message, cause)
+
+    constructor(
+        message: String,
+        cause: Throwable,
+        enableSuppression: Boolean,
+        writableStackTrace: Boolean,
+    ) : super(message, cause, enableSuppression, writableStackTrace)
+}

src/main/kotlin/ltd/hlaeja/util/PrivateKeyProvider.kt

@@ -0,0 +1,35 @@
+package ltd.hlaeja.util
+
+import java.security.KeyFactory
+import java.security.interfaces.RSAPrivateKey
+import java.security.spec.PKCS8EncodedKeySpec
+import java.util.Base64.getDecoder
+import ltd.hlaeja.exception.KeyProviderException
+
+object PrivateKeyProvider {
+
+    fun load(
+        pemFile: String,
+    ): RSAPrivateKey = readPrivatePemFile(pemFile)
+        .let(::makePrivateKey)
+
+    private fun makePrivateKey(
+        privateKeyBytes: ByteArray,
+    ): RSAPrivateKey = KeyFactory.getInstance("RSA")
+        .generatePrivate(PKCS8EncodedKeySpec(privateKeyBytes)) as RSAPrivateKey
+
+    private fun readPrivatePemFile(
+        privateKey: String,
+    ): ByteArray = javaClass.classLoader
+        .getResource(privateKey)
+        ?.readText()
+        ?.let(::getPrivateKeyByteArray)
+        ?: throw KeyProviderException("Could not load private key")
+
+    private fun getPrivateKeyByteArray(
+        keyText: String,
+    ): ByteArray = keyText.replace(Regex("[\r\n]+"), "")
+        .removePrefix("-----BEGIN PRIVATE KEY-----")
+        .removeSuffix("-----END PRIVATE KEY-----")
+        .let { getDecoder().decode(it) }
+}