add PrivateKeyProvider

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

@@ -0,0 +1,51 @@
+package ltd.hlaeja.util
+
+import java.security.interfaces.RSAPrivateKey
+import ltd.hlaeja.exception.KeyProviderException
+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 = "keys/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 = "keys/non-existent.pem"
+
+        // when exception
+        val exception = assertThrows<KeyProviderException> {
+            PrivateKeyProvider.load(nonExistentPemFilePath)
+        }
+
+        // then
+        assertThat(exception.message).isEqualTo("Could not load private key")
+    }
+
+    @Test
+    fun `load private key - file is invalid`() {
+        // given
+        val invalidPemFilePath = "keys/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")
+    }
+}