add PublicKeyProvider

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

@@ -0,0 +1,35 @@
+package ltd.hlaeja.jwt.util
+
+import java.security.KeyException
+import java.security.KeyFactory
+import java.security.interfaces.RSAPublicKey
+import java.security.spec.X509EncodedKeySpec
+import java.util.Base64.getDecoder
+
+object PublicKeyProvider {
+
+    fun load(
+        pemFile: String,
+    ): RSAPublicKey = readPublicPemFile(pemFile)
+        .let(PublicKeyProvider::makePublicKey)
+
+    private fun makePublicKey(
+        publicKeyBytes: ByteArray,
+    ): RSAPublicKey = KeyFactory.getInstance("RSA")
+        .generatePublic(X509EncodedKeySpec(publicKeyBytes)) as RSAPublicKey
+
+    private fun readPublicPemFile(
+        publicKey: String,
+    ): ByteArray = javaClass.classLoader
+        ?.getResource(publicKey)
+        ?.readText()
+        ?.let(PublicKeyProvider::getPublicKeyByteArray)
+        ?: throw KeyException("Could not load public key")
+
+    private fun getPublicKeyByteArray(
+        keyText: String,
+    ): ByteArray = keyText.replace(Regex("[\r\n]+"), "")
+        .removePrefix("-----BEGIN PUBLIC KEY-----")
+        .removeSuffix("-----END PUBLIC KEY-----")
+        .let { getDecoder().decode(it) }
+}