Compress everything. Trust nothing. Encrypt always.

src/zupt_x25519.h

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+/*
+ * Zupt — Backup-oriented compression with AES-256 encryption
+ * Copyright (c) 2026 Cristian Cezar Moisés
+ * SPDX-License-Identifier: MIT
+ *
+ * X25519 Diffie-Hellman key agreement (RFC 7748).
+ * Montgomery ladder — constant-time by construction.
+ */
+#ifndef ZUPT_X25519_H
+#define ZUPT_X25519_H
+
+#include <stdint.h>
+
+/* X25519(scalar, point) → result. All inputs/outputs are 32 bytes.
+ * CT-REQUIRED: Montgomery ladder is inherently constant-time. */
+void zupt_x25519(uint8_t out[32], const uint8_t scalar[32], const uint8_t point[32]);
+
+/* X25519 with the standard basepoint (9).
+ * Used for keygen: public = X25519(private, basepoint). */
+void zupt_x25519_base(uint8_t out[32], const uint8_t scalar[32]);
+
+#endif