From 7145e17c9528cbb2dca40895b5493405675a6b49 Mon Sep 17 00:00:00 2001
From: Aditya kumar singh <143548997+Adityakk9031@users.noreply.github.com>
Date: Thu, 16 Jul 2026 01:17:47 +0530
Subject: [PATCH] feat: implement hybrid encryption and compress public key
 URLs (Fixes #13)

---
 js/crypto.js | 128 +++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 119 insertions(+), 9 deletions(-)

diff --git a/js/crypto.js b/js/crypto.js
index 82e1379..1cf5948 100644
--- a/js/crypto.js
+++ b/js/crypto.js
@@ -2,12 +2,44 @@ const ALGO_NAME = 'RSA-OAEP'
 const ALGO_HASH = 'SHA-256'
 const ALGO_KEY_LENGTH = 2048
 
+function bufferToBase64(buffer) {
+    const bytes = new Uint8Array(buffer);
+    let binary = '';
+    const len = bytes.byteLength;
+    for (let i = 0; i < len; i++) {
+        binary += String.fromCharCode(bytes[i]);
+    }
+    return btoa(binary);
+}
+
+function base64ToBuffer(base64) {
+    const binaryString = atob(base64);
+    const len = binaryString.length;
+    const bytes = new Uint8Array(len);
+    for (let i = 0; i < len; i++) {
+        bytes[i] = binaryString.charCodeAt(i);
+    }
+    return bytes.buffer;
+}
+
 function serialize_key(key) {
+    if (key.kty === 'RSA' && key.d === undefined) {
+        return key.n;
+    }
     return btoa(JSON.stringify(key))
 }
 
 function deserialize_key(str) {
-    return JSON.parse(atob(str))
+    if (str.startsWith('ey')) {
+        return JSON.parse(atob(str))
+    }
+    return {
+        kty: "RSA",
+        n: str,
+        e: "AQAB",
+        alg: "RSA-OAEP-256",
+        ext: true
+    };
 }
 
 async function generateKeyPair() {
@@ -69,34 +101,112 @@ async function load_key(serialized_key, usage) {
 }
 
 async function load_private_key(public_key_ser) {
-    return await importJWKey(deserialize_key(localStorage.getItem(pub_key_ser)), 'decrypt')
+    return await importJWKey(deserialize_key(localStorage.getItem(public_key_ser)), 'decrypt')
 }
 
 async function encryptString(publicKey, plaintext) {
+    // 1. Generate ephemeral AES key
+    const aesKey = await window.crypto.subtle.generateKey(
+        {
+            name: 'AES-GCM',
+            length: 256,
+        },
+        true,
+        ['encrypt', 'decrypt']
+    );
+
+    // 2. Encrypt plaintext with AES-GCM
+    const iv = window.crypto.getRandomValues(new Uint8Array(12));
     const textBuffer = new TextEncoder().encode(plaintext);
+    const encryptedSecretBuffer = await window.crypto.subtle.encrypt(
+        {
+            name: 'AES-GCM',
+            iv: iv,
+        },
+        aesKey,
+        textBuffer
+    );
+
+    // 3. Export raw AES key
+    const rawAesKey = await window.crypto.subtle.exportKey('raw', aesKey);
 
-    const encryptedBuffer = await window.crypto.subtle.encrypt(
+    // 4. Encrypt raw AES key with RSA-OAEP public key
+    const encryptedAesKeyBuffer = await window.crypto.subtle.encrypt(
         {
             name: ALGO_NAME,
         },
         publicKey,
-        textBuffer
+        rawAesKey
+    );
+
+    // 5. Pack everything: [RSA Encrypted AES Key (256 bytes) | IV (12 bytes) | AES Ciphertext]
+    const combinedBuffer = new Uint8Array(
+        encryptedAesKeyBuffer.byteLength + iv.length + encryptedSecretBuffer.byteLength
     );
+    combinedBuffer.set(new Uint8Array(encryptedAesKeyBuffer), 0);
+    combinedBuffer.set(iv, encryptedAesKeyBuffer.byteLength);
+    combinedBuffer.set(new Uint8Array(encryptedSecretBuffer), encryptedAesKeyBuffer.byteLength + iv.length);
 
-    return btoa(String.fromCharCode(...new Uint8Array(encryptedBuffer)));
+    // 6. Base64 encode the combined buffer
+    return bufferToBase64(combinedBuffer);
 }
 
 async function decryptString(privateKey, encryptedBase64) {
-    const encryptedBuffer = Uint8Array.from(atob(encryptedBase64), c => c.charCodeAt(0));
+    const combinedBytes = new Uint8Array(base64ToBuffer(encryptedBase64));
+
+    // Backward compatibility check:
+    // Standard 2048-bit RSA-OAEP ciphertext is exactly 256 bytes.
+    if (combinedBytes.length === 256) {
+        const decryptedBuffer = await window.crypto.subtle.decrypt(
+            {
+                name: ALGO_NAME,
+            },
+            privateKey,
+            combinedBytes
+        );
+        return new TextDecoder().decode(decryptedBuffer);
+    }
 
-    const decryptedBuffer = await window.crypto.subtle.decrypt(
+    if (combinedBytes.length < 284) {
+        throw new Error('Invalid encrypted payload size');
+    }
+
+    const rsaKeyLengthBytes = 256;
+    const ivLengthBytes = 12;
+
+    const encryptedAesKey = combinedBytes.slice(0, rsaKeyLengthBytes);
+    const iv = combinedBytes.slice(rsaKeyLengthBytes, rsaKeyLengthBytes + ivLengthBytes);
+    const encryptedSecret = combinedBytes.slice(rsaKeyLengthBytes + ivLengthBytes);
+
+    // 1. Decrypt raw AES key using RSA private key
+    const rawAesKey = await window.crypto.subtle.decrypt(
         {
             name: ALGO_NAME,
         },
         privateKey,
-        encryptedBuffer
+        encryptedAesKey
+    );
+
+    // 2. Import raw AES key
+    const aesKey = await window.crypto.subtle.importKey(
+        'raw',
+        rawAesKey,
+        {
+            name: 'AES-GCM',
+        },
+        true,
+        ['decrypt']
+    );
+
+    // 3. Decrypt the secret payload using AES-GCM
+    const decryptedBuffer = await window.crypto.subtle.decrypt(
+        {
+            name: 'AES-GCM',
+            iv: iv,
+        },
+        aesKey,
+        encryptedSecret
     );
 
-    // Convert the decrypted buffer to a string
     return new TextDecoder().decode(decryptedBuffer);
 }
\ No newline at end of file