1 files changed, 94 insertions, 0 deletions

diff --git a/src/btfchan.cpp b/src/btfchan.cpp
new file mode 100644
index 0000000..09aaaad
--- /dev/null
+++ b/src/btfchan.cpp
@@ -0,0 +1,94 @@
+// Copyright (c) 2026 Bitflash developers
+// Distributed under the MIT/X11 software license.
+//
+// Bitflash end-to-end encrypted channel. Ported from Itzal echan.rs.
+// Uses libsodium crypto_box_curve25519xchacha20poly1305 (X25519 + XChaCha20-Poly1305),
+// the same primitive as Rust's crypto_box ChaChaBox.
+
+#include "btfchan.h"
+#include <sodium.h>
+#include <cstring>
+
+namespace btf
+{
+
+// Compile-time sanity: our constants match libsodium's.
+static_assert(CHAN_NONCEBYTES == crypto_box_curve25519xchacha20poly1305_NONCEBYTES, "nonce");
+static_assert(CHAN_MACBYTES   == crypto_box_curve25519xchacha20poly1305_MACBYTES,   "mac");
+static_assert(CHAN_KEYBYTES   == crypto_box_curve25519xchacha20poly1305_BEFORENMBYTES, "key");
+
+bool ChanInit()
+{
+    return sodium_init() >= 0;
+}
+
+void ChanKeypair(unsigned char pk[32], unsigned char sk[32])
+{
+    crypto_box_curve25519xchacha20poly1305_keypair(pk, sk);
+}
+
+void ChanPublicFromSecret(unsigned char pk[32], const unsigned char sk[32])
+{
+    // X25519 public = scalarmult of the secret with the curve25519 base point.
+    crypto_scalarmult_base(pk, sk);
+}
+
+bool ChanClientHandshake(const unsigned char endpoint_static_pub[32],
+                         unsigned char eph_pub_out[32], unsigned char shared_out[32])
+{
+    unsigned char eph_sk[32];
+    crypto_box_curve25519xchacha20poly1305_keypair(eph_pub_out, eph_sk);
+    // shared = X25519(eph_sk, endpoint_static_pub) -> hashed to a box key
+    int r = crypto_box_curve25519xchacha20poly1305_beforenm(shared_out, endpoint_static_pub, eph_sk);
+    sodium_memzero(eph_sk, sizeof(eph_sk));
+    return r == 0;
+}
+
+bool ChanEndpointHandshake(const unsigned char client_eph_pub[32],
+                           const unsigned char my_static_sk[32], unsigned char shared_out[32])
+{
+    // shared = X25519(my_static_sk, client_eph_pub) -> same box key as the client computed
+    return crypto_box_curve25519xchacha20poly1305_beforenm(shared_out, client_eph_pub, my_static_sk) == 0;
+}
+
+std::vector<unsigned char> ChanEncryptFrame(const unsigned char shared[32],
+                                            const unsigned char* pt, size_t n)
+{
+    unsigned char nonce[CHAN_NONCEBYTES];
+    randombytes_buf(nonce, sizeof(nonce));
+
+    size_t ctlen = n + CHAN_MACBYTES;
+    size_t bodylen = CHAN_NONCEBYTES + ctlen;
+
+    std::vector<unsigned char> out(4 + bodylen);
+    out[0] = (unsigned char)((bodylen >> 24) & 0xff);
+    out[1] = (unsigned char)((bodylen >> 16) & 0xff);
+    out[2] = (unsigned char)((bodylen >> 8) & 0xff);
+    out[3] = (unsigned char)(bodylen & 0xff);
+    memcpy(&out[4], nonce, CHAN_NONCEBYTES);
+
+    crypto_box_curve25519xchacha20poly1305_easy_afternm(
+        &out[4 + CHAN_NONCEBYTES], pt, n, nonce, shared);
+    return out;
+}
+
+bool ChanDecryptBody(const unsigned char shared[32],
+                     const unsigned char* body, size_t n, std::vector<unsigned char>& out)
+{
+    if (n < (size_t)(CHAN_NONCEBYTES + CHAN_MACBYTES))
+        return false;
+    const unsigned char* nonce = body;
+    const unsigned char* ct = body + CHAN_NONCEBYTES;
+    size_t ctlen = n - CHAN_NONCEBYTES;
+    out.resize(ctlen - CHAN_MACBYTES);
+    int r = crypto_box_curve25519xchacha20poly1305_open_easy_afternm(
+        out.empty() ? NULL : &out[0], ct, ctlen, nonce, shared);
+    if (r != 0)
+    {
+        out.clear();
+        return false;
+    }
+    return true;
+}
+
+} // namespace btf