add PrivateKeyProvider

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

@@ -0,0 +1,35 @@
+package ltd.hlaeja.jwt.util
+
+import java.security.KeyException
+import java.security.KeyFactory
+import java.security.interfaces.RSAPrivateKey
+import java.security.spec.PKCS8EncodedKeySpec
+import java.util.Base64.getDecoder
+
+object PrivateKeyProvider {
+
+    fun load(
+        pemFile: String,
+    ): RSAPrivateKey = readPrivatePemFile(pemFile)
+        .let(PrivateKeyProvider::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(PrivateKeyProvider::getPrivateKeyByteArray)
+        ?: throw KeyException("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) }
+}