Import ed25519 implementation from https://github.com/orlp/ed25519

Externals/ed25519/key_exchange.c

@@ -0,0 +1,79 @@
+#include "ed25519.h"
+#include "fe.h"
+
+void ed25519_key_exchange(unsigned char *shared_secret, const unsigned char *public_key, const unsigned char *private_key) {
+    unsigned char e[32];
+    unsigned int i;
+    
+    fe x1;
+    fe x2;
+    fe z2;
+    fe x3;
+    fe z3;
+    fe tmp0;
+    fe tmp1;
+
+    int pos;
+    unsigned int swap;
+    unsigned int b;
+
+    /* copy the private key and make sure it's valid */
+    for (i = 0; i < 32; ++i) {
+        e[i] = private_key[i];
+    }
+
+    e[0] &= 248;
+    e[31] &= 63;
+    e[31] |= 64;
+
+    /* unpack the public key and convert edwards to montgomery */
+    /* due to CodesInChaos: montgomeryX = (edwardsY + 1)*inverse(1 - edwardsY) mod p */
+    fe_frombytes(x1, public_key);
+    fe_1(tmp1);
+    fe_add(tmp0, x1, tmp1);
+    fe_sub(tmp1, tmp1, x1);
+    fe_invert(tmp1, tmp1);
+    fe_mul(x1, tmp0, tmp1);
+
+    fe_1(x2);
+    fe_0(z2);
+    fe_copy(x3, x1);
+    fe_1(z3);
+
+    swap = 0;
+    for (pos = 254; pos >= 0; --pos) {
+        b = e[pos / 8] >> (pos & 7);
+        b &= 1;
+        swap ^= b;
+        fe_cswap(x2, x3, swap);
+        fe_cswap(z2, z3, swap);
+        swap = b;
+
+        /* from montgomery.h */
+        fe_sub(tmp0, x3, z3);
+        fe_sub(tmp1, x2, z2);
+        fe_add(x2, x2, z2);
+        fe_add(z2, x3, z3);
+        fe_mul(z3, tmp0, x2);
+        fe_mul(z2, z2, tmp1);
+        fe_sq(tmp0, tmp1);
+        fe_sq(tmp1, x2);
+        fe_add(x3, z3, z2);
+        fe_sub(z2, z3, z2);
+        fe_mul(x2, tmp1, tmp0);
+        fe_sub(tmp1, tmp1, tmp0);
+        fe_sq(z2, z2);
+        fe_mul121666(z3, tmp1);
+        fe_sq(x3, x3);
+        fe_add(tmp0, tmp0, z3);
+        fe_mul(z3, x1, z2);
+        fe_mul(z2, tmp1, tmp0);
+    }
+
+    fe_cswap(x2, x3, swap);
+    fe_cswap(z2, z3, swap);
+
+    fe_invert(z2, z2);
+    fe_mul(x2, x2, z2);
+    fe_tobytes(shared_secret, x2);
+}