initial prototype based on nat-as-server

lib/encrypted_payload.php

@@ -0,0 +1,60 @@
+<?php
+/**
+ * Application-layer encryption for sensitive mobile API payloads.
+ * AES-256-GCM (IV prepended) + HKDF-SHA256 session key from the Bearer token (info: qdb-aes).
+ * Legacy CBC envelopes are accepted for compatibility with older app builds.
+ */
+
+function qdb_sensitive_envelope(string $plainJson, string $tokenHex): array {
+    $key = hkdf_session_key_from_token($tokenHex);
+    return [
+        'encrypted' => true,
+        'alg'       => 'A256GCM',
+        'payload'   => base64_encode(qdb_aes256_gcm_encrypt_bytes($plainJson, $key)),
+    ];
+}
+
+function qdb_decrypt_sensitive_envelope(array $envelope, string $tokenHex): string {
+    if (empty($envelope['encrypted']) || !isset($envelope['payload']) || !is_string($envelope['payload'])) {
+        throw new InvalidArgumentException('Invalid encrypted envelope');
+    }
+    $raw = base64_decode($envelope['payload'], true);
+    if ($raw === false) {
+        throw new InvalidArgumentException('Invalid base64 payload');
+    }
+    $key = hkdf_session_key_from_token($tokenHex);
+    $alg = isset($envelope['alg']) && is_string($envelope['alg']) ? $envelope['alg'] : 'A256CBC';
+    return match ($alg) {
+        'A256GCM' => qdb_aes256_gcm_decrypt_bytes($raw, $key),
+        'A256CBC' => aes256_cbc_decrypt_bytes($raw, $key),
+        default => throw new InvalidArgumentException('Unsupported encrypted envelope algorithm'),
+    };
+}
+
+function qdb_aes256_gcm_encrypt_bytes(string $plain, string $key): string {
+    $key = str_pad(substr($key, 0, 32), 32, "\0");
+    $iv = random_bytes(12);
+    $tag = '';
+    $cipher = openssl_encrypt($plain, 'aes-256-gcm', $key, OPENSSL_RAW_DATA, $iv, $tag, '');
+    if ($cipher === false || strlen($tag) !== 16) {
+        throw new RuntimeException('openssl_encrypt failed');
+    }
+    return $iv . $cipher . $tag;
+}
+
+function qdb_aes256_gcm_decrypt_bytes(string $data, string $key): string {
+    $ivLen = 12;
+    $tagLen = 16;
+    if (strlen($data) <= $ivLen + $tagLen) {
+        throw new InvalidArgumentException('cipher too short');
+    }
+    $key = str_pad(substr($key, 0, 32), 32, "\0");
+    $iv = substr($data, 0, $ivLen);
+    $tag = substr($data, -$tagLen);
+    $ct = substr($data, $ivLen, -$tagLen);
+    $plain = openssl_decrypt($ct, 'aes-256-gcm', $key, OPENSSL_RAW_DATA, $iv, $tag, '');
+    if ($plain === false) {
+        throw new InvalidArgumentException('openssl_decrypt failed');
+    }
+    return $plain;
+}