Add functions for signing and verifying messages using curve25519 keys

1 files changed, 43 insertions, 0 deletions

diff --git a/lib/ed25519_additions.c b/lib/ed25519_additions.c
new file mode 100644
index 0000000..5fa0c68
--- /dev/null
+++ b/lib/ed25519_additions.c
@@ -0,0 +1,43 @@
+void convert_curve25519_to_ed25519(
+    unsigned char * public_key,
+    unsigned char * signature
+) {
+    fe mont_x, mont_x_minus_one, mont_x_plus_one, inv_mont_x_plus_one;
+    fe one;
+    fe ed_y;
+
+    fe_frombytes(mont_x, public_key);
+    fe_1(one);
+    fe_sub(mont_x_minus_one, mont_x, one);
+    fe_add(mont_x_plus_one, mont_x, one);
+    fe_invert(inv_mont_x_plus_one, mont_x_plus_one);
+    fe_mul(ed_y, mont_x_minus_one, inv_mont_x_plus_one);
+    fe_tobytes(public_key, ed_y);
+
+    public_key[31] &= 0x7F;
+    public_key[31] |= (signature[63] & 0x80);
+    signature[63] &= 0x7F;
+}
+
+
+void convert_ed25519_to_curve25519(
+    unsigned char const * public_key,
+    unsigned char * signature
+) {
+    unsigned char sign_bit = public_key[31] & 0x80;
+    signature[63] &= 0x7F;
+    signature[63] |= sign_bit;
+}
+
+
+void ed25519_keypair(
+    unsigned char * private_key,
+    unsigned char * public_key
+) {
+    ge_p3 A;
+    private_key[0] &= 248;
+    private_key[31] &= 63;
+    private_key[31] |= 64;
+    ge_scalarmult_base(&A, private_key);
+    ge_p3_tobytes(public_key, &A);
+}