Updated: encrypted string format

web/mkdocs/docs/privacy/encryption.md

@@ -2,6 +2,8 @@
 
 The application supports end-to-end encryption by encrypting message text and recipients' phone numbers before sending them to the API and decrypting them on the device. This ensures that no one – including us as the service provider, the hosting provider, or any third parties – can access the content and recipients of the messages.
 
+Please note that using encryption will increase device battery usage.
+
 ## Requirements
 
 1. Fields `message` and every value in the `phoneNumbers` field must be encrypted.
@@ -12,9 +14,9 @@ The application supports end-to-end encryption by encrypting message text and re
 
 1. Select a passphrase that will be used for encryption and specify it on the device.
 2. Generate a random salt, with 16 bytes being the recommended size.
-3. Create a secret key using the PBKDF2 algorithm with SHA1 hash function, key size of 256 bits, and an iteration count of 300,000.
+3. Create a secret key using the PBKDF2 algorithm with SHA1 hash function, key size of 256 bits, and recommended iteration count of 75,000.
 4. Encrypt the message text and recipients' phone numbers using the AES-256-CBC algorithm and encode the result as Base64.
-5. Add the Base64-encoded salt to the front of the encrypted data, separated by a dot.
+5. Format result as `$aes-256-cbc/pbkdf2-sha1$i=<iteration count>$<base64 encoded salt>$<base 64 encoded encrypted data>`. The format is inspired by [PHC](https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md).
 
 Or use one of the following realization:
 
@@ -23,25 +25,47 @@ Or use one of the following realization:
 ```php
 class Encryptor {
     protected string $passphrase;
+    protected int $iterationCount;
 
+    /**
+     * Encryptor constructor.
+     * @param string $passphrase Passphrase to use for encryption
+     * @param int $iterationCount Iteration count
+     */
     public function __construct(
-        string $passphrase
+        string $passphrase,
+        int $iterationCount = 75000
     ) {
         $this->passphrase = $passphrase;
+        $this->iterationCount = $iterationCount;
     }
 
     public function Encrypt(string $data): string {
         $salt = $this->generateSalt();
-        $secretKey = $this->generateSecretKeyFromPassphrase($this->passphrase, $salt);
+        $secretKey = $this->generateSecretKeyFromPassphrase($this->passphrase, $salt, 32, $this->iterationCount);
 
-        return base64_encode($salt) . '.' . openssl_encrypt($data, 'aes-256-cbc', $secretKey, 0, $salt);
+        return sprintf(
+            '$aes-256-cbc/pbkdf2-sha1$i=%d$%s$%s',
+            $this->iterationCount,
+            base64_encode($salt),
+            openssl_encrypt($data, 'aes-256-cbc', $secretKey, 0, $salt)
+        );
     }
 
     public function Decrypt(string $data): string {
-        list($saltBase64, $encryptedBase64) = explode('.', $data, 2);
+        list($_, $algo, $paramsStr, $saltBase64, $encryptedBase64) = explode('$', $data);
+
+        if ($algo !== 'aes-256-cbc/pbkdf2-sha1') {
+            throw new \RuntimeException('Unsupported algorithm');
+        }
+
+        $params = $this->parseParams($paramsStr);
+        if (empty($params['i'])) {
+            throw new \RuntimeException('Missing iteration count');
+        }
 
         $salt = base64_decode($saltBase64);
-        $secretKey = $this->generateSecretKeyFromPassphrase($this->passphrase, $salt);
+        $secretKey = $this->generateSecretKeyFromPassphrase($this->passphrase, $salt, 32, intval($params['i']));
 
         return openssl_decrypt($encryptedBase64, 'aes-256-cbc', $secretKey, 0, $salt);
     }
@@ -58,5 +82,18 @@ class Encryptor {
     ): string {
         return hash_pbkdf2('sha1', $passphrase, $salt, $iterationCount, $keyLength, true);
     }
+
+    /**
+     * @return array<string, string>
+     */
+    protected function parseParams(string $params): array {
+        $keyValuePairs = explode(',', $params);
+        $result = [];
+        foreach ($keyValuePairs as $pair) {
+            list($key, $value) = explode('=', $pair, 2);
+            $result[$key] = $value;
+        }
+        return $result;
+    }
 }
 ```