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) }
+}

src/test/kotlin/ltd/hlaeja/jwt/util/PrivateKeyProviderTest.kt

@@ -0,0 +1,51 @@
+package ltd.hlaeja.jwt.util
+
+import java.security.KeyException
+import java.security.interfaces.RSAPrivateKey
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+
+class PrivateKeyProviderTest {
+
+    @Test
+    fun `load private key - success`() {
+        // given
+        val pemFilePath = "cert/valid-private-key.pem"
+
+        // when
+        val privateKey: RSAPrivateKey = PrivateKeyProvider.load(pemFilePath)
+
+        // then
+        assertThat(privateKey).isNotNull
+        assertThat(privateKey.algorithm).isEqualTo("RSA")
+    }
+
+    @Test
+    fun `load private key - file does not exist`() {
+        // given
+        val nonExistentPemFilePath = "cert/non-existent.pem"
+
+        // when exception
+        val exception = assertThrows<KeyException> {
+            PrivateKeyProvider.load(nonExistentPemFilePath)
+        }
+
+        // then
+        assertThat(exception.message).isEqualTo("Could not load private key")
+    }
+
+    @Test
+    fun `load private key - file is invalid`() {
+        // given
+        val invalidPemFilePath = "cert/invalid-private-key.pem"
+
+        // when exception
+        val exception = assertThrows<IllegalArgumentException> {
+            PrivateKeyProvider.load(invalidPemFilePath)
+        }
+
+        // then
+        assertThat(exception.message).contains("Input byte array has wrong 4-byte ending unit")
+    }
+}