feat(#5) : PublicKey 생성

src/main/java/trothly/trothcam/feign/PublicKeyGenerator.java

@@ -0,0 +1,48 @@
+package trothly.trothcam.feign;
+
+import org.springframework.stereotype.Component;
+import org.springframework.util.Base64Utils;
+import trothly.trothcam.dto.auth.apple.ApplePublicKey;
+import trothly.trothcam.dto.auth.apple.ApplePublicKeys;
+
+import java.math.BigInteger;
+import java.security.KeyFactory;
+import java.security.NoSuchAlgorithmException;
+import java.security.PublicKey;
+import java.security.spec.InvalidKeySpecException;
+import java.security.spec.RSAPublicKeySpec;
+import java.util.Map;
+
+@Component
+public class PublicKeyGenerator {
+
+    private static final String SIGN_ALGORITHM_HEADER_KEY = "alg";
+    private static final String KEY_ID_HEADER_KEY = "kid";
+    private static final int POSITIVE_SIGN_NUMBER = 1;
+
+    // Public Key 생성
+    public PublicKey generatePublicKey(Map<String, String> headers, ApplePublicKeys applePublicKeys) {
+        ApplePublicKey applePublicKey =
+                applePublicKeys.getMatchesKey(headers.get(SIGN_ALGORITHM_HEADER_KEY), headers.get(KEY_ID_HEADER_KEY));
+
+        return generatePublicKeyWithApplePublicKey(applePublicKey);
+    }
+
+    // Public Key 속 정보로 Public Key 생성
+    private PublicKey generatePublicKeyWithApplePublicKey(ApplePublicKey publicKey) {
+        byte[] nBytes = Base64Utils.decodeFromUrlSafeString(publicKey.getN());
+        byte[] eBytes = Base64Utils.decodeFromUrlSafeString(publicKey.getE());
+
+        BigInteger n = new BigInteger(POSITIVE_SIGN_NUMBER, nBytes);
+        BigInteger e = new BigInteger(POSITIVE_SIGN_NUMBER, eBytes);
+
+        RSAPublicKeySpec publicKeySpec = new RSAPublicKeySpec(n, e);
+
+        try {
+            KeyFactory keyFactory = KeyFactory.getInstance(publicKey.getKty());
+            return keyFactory.generatePublic(publicKeySpec);
+        } catch (NoSuchAlgorithmException | InvalidKeySpecException exception) {
+            throw new IllegalStateException("Apple OAuth 로그인 중 public key 생성에 문제가 발생했습니다.");
+        }
+    }
+}