feat: load RSA public key via modulus and exponent

Author: siujamo

key-pair-loader/src/main/java/com/onixbyte/security/KeyLoader.java

@@ -17,8 +17,11 @@
 
 package com.onixbyte.security;
 
+import com.onixbyte.security.exception.KeyLoadingException;
+
 import java.security.PrivateKey;
 import java.security.PublicKey;
+import java.security.interfaces.RSAPublicKey;
 
 /**
  * The {@code KeyLoader} class provides utility methods for loading keys pairs from PEM-formatted
@@ -49,6 +52,10 @@ public interface KeyLoader {
      */
     PublicKey loadPublicKey(String pemKeyText);
 
+    default RSAPublicKey loadPublicKey(String modulus, String exponent) {
+        throw new KeyLoadingException("This key loader does not support RSA Public key loading.");
+    }
+
     /**
      * Retrieves the raw content of a PEM formatted key by removing unnecessary headers, footers,
      * and new line characters.

key-pair-loader/src/main/java/com/onixbyte/security/impl/RSAKeyLoader.java

@@ -20,12 +20,14 @@
 import com.onixbyte.security.KeyLoader;
 import com.onixbyte.security.exception.KeyLoadingException;
 
+import java.math.BigInteger;
 import java.security.KeyFactory;
 import java.security.NoSuchAlgorithmException;
 import java.security.interfaces.RSAPrivateKey;
 import java.security.interfaces.RSAPublicKey;
 import java.security.spec.InvalidKeySpecException;
 import java.security.spec.PKCS8EncodedKeySpec;
+import java.security.spec.RSAPublicKeySpec;
 import java.security.spec.X509EncodedKeySpec;
 import java.util.Base64;
 
@@ -47,6 +49,9 @@
 public class RSAKeyLoader implements KeyLoader {
 
     private final Base64.Decoder decoder;
+
+    private final Base64.Decoder urlDecoder;
+
     private final KeyFactory keyFactory;
 
     /**
@@ -58,6 +63,7 @@ public class RSAKeyLoader implements KeyLoader {
     public RSAKeyLoader() {
         try {
             this.decoder = Base64.getDecoder();
+            this.urlDecoder = Base64.getUrlDecoder();
             this.keyFactory = KeyFactory.getInstance("RSA");
         } catch (NoSuchAlgorithmException e) {
             throw new KeyLoadingException(e);
@@ -133,4 +139,22 @@ public RSAPublicKey loadPublicKey(String pemKeyText) {
             throw new KeyLoadingException("Key spec is invalid.", e);
         }
     }
+
+    @Override
+    public RSAPublicKey loadPublicKey(String modulus, String exponent) {
+        try {
+            var _modulus = new BigInteger(1, urlDecoder.decode(modulus));
+            var _exponent = new BigInteger(1, urlDecoder.decode(exponent));
+
+            var keySpec = new RSAPublicKeySpec(_modulus, _exponent);
+            var kf = KeyFactory.getInstance("RSA");
+            if (kf.generatePublic(keySpec) instanceof RSAPublicKey rsaPublicKey) {
+                return rsaPublicKey;
+            } else {
+                throw new KeyLoadingException("Cannot generate RSA public key with given modulus and exponent.");
+            }
+        } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
+            throw new KeyLoadingException("Cannot generate RSA public key with given modulus and exponent.", e);
+        }
+    }
 }