feat(firmware): MeshCore message RX — decrypt DMs addressed to us

- vendor orlp/ed25519 (lib/ed25519, MIT — the exact lib MeshCore uses) for Curve25519 ECDH key exchange; switch identity/advert signing to it (same seed -> same pubkey, so peers still recognise us; sigs are byte-identical) - store pubkeys from heard MeshCore adverts (contacts table) so a DM's 1-byte src_hash can be resolved to a full sender key - decode PAYLOAD_TYPE_TXT_MSG: ECDH shared secret, HMAC-SHA256 MAC check, AES-128-ECB decrypt, parse timestamp+text, drive the message modal - verified end-to-end: real MeshCore node -> "hello from HP Pro Desk" decoded Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
2026-07-01 18:30:20 +01:00
parent 8e5b4c97d7
commit 38c146c477
19 changed files with 5112 additions and 8 deletions
@@ -0,0 +1,69 @@
+#include "ed_25519.h"
+#include "ge.h"
+#include "sc.h"
+#include "sha512.h"
+
+
+/* see http://crypto.stackexchange.com/a/6215/4697 */
+void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar) {
+    const unsigned char SC_1[32] = {1}; /* scalar with value 1 */
+    
+    unsigned char n[32]; 
+    ge_p3 nB;
+    ge_p1p1 A_p1p1;
+    ge_p3 A;
+    ge_p3 public_key_unpacked;
+    ge_cached T;
+
+    sha512_context hash;
+    unsigned char hashbuf[64];
+
+    int i;
+
+    /* copy the scalar and clear highest bit */
+    for (i = 0; i < 31; ++i) {
+        n[i] = scalar[i];
+    }
+    n[31] = scalar[31] & 127;
+
+    /* private key: a = n + t */
+    if (private_key) {
+        sc_muladd(private_key, SC_1, n, private_key);
+
+        // https://github.com/orlp/ed25519/issues/3
+        sha512_init(&hash);
+        sha512_update(&hash, private_key + 32, 32);
+        sha512_update(&hash, scalar, 32);
+        sha512_final(&hash, hashbuf);
+        for (i = 0; i < 32; ++i) {
+            private_key[32 + i] = hashbuf[i];
+        }
+    }
+
+    /* public key: A = nB + T */
+    if (public_key) {
+        /* if we know the private key we don't need a point addition, which is faster */
+        /* using a "timing attack" you could find out wether or not we know the private
+           key, but this information seems rather useless - if this is important pass
+           public_key and private_key seperately in 2 function calls */
+        if (private_key) {
+            ge_scalarmult_base(&A, private_key);
+        } else {
+            /* unpack public key into T */
+            ge_frombytes_negate_vartime(&public_key_unpacked, public_key);
+            fe_neg(public_key_unpacked.X, public_key_unpacked.X); /* undo negate */
+            fe_neg(public_key_unpacked.T, public_key_unpacked.T); /* undo negate */
+            ge_p3_to_cached(&T, &public_key_unpacked);
+
+            /* calculate n*B */
+            ge_scalarmult_base(&nB, n);
+
+            /* A = n*B + T */
+            ge_add(&A_p1p1, &nB, &T);
+            ge_p1p1_to_p3(&A, &A_p1p1);
+        }
+            
+        /* pack public key */
+        ge_p3_tobytes(public_key, &A);
+    }
+}