1 files changed, 37 insertions, 0 deletions
diff --git a/src/ext/ed25519/ref10/keyconv.c b/src/ext/ed25519/ref10/keyconv.c
new file mode 100644
index 0000000000..854b150d69
--- /dev/null
+++ b/src/ext/ed25519/ref10/keyconv.c
@@ -0,0 +1,37 @@
+/* Added to ref10 for Tor. We place this in the public domain.  Alternatively,
+ * you may have it under the Creative Commons 0 "CC0" license. */
+#include "fe.h"
+#include "ed25519_ref10.h"
+
+int ed25519_ref10_pubkey_from_curve25519_pubkey(unsigned char *out,
+                                                const unsigned char *inp,
+                                                int signbit)
+{
+  fe u;
+  fe one;
+  fe y;
+  fe uplus1;
+  fe uminus1;
+  fe inv_uplus1;
+
+  /* From prop228:
+
+   Given a curve25519 x-coordinate (u), we can get the y coordinate
+   of the ed25519 key using
+
+         y = (u-1)/(u+1)
+  */
+  fe_frombytes(u, inp);
+  fe_1(one);
+  fe_sub(uminus1, u, one);
+  fe_add(uplus1, u, one);
+  fe_invert(inv_uplus1, uplus1);
+  fe_mul(y, uminus1, inv_uplus1);
+
+  fe_tobytes(out, y);
+
+  /* propagate sign. */
+  out[31] |= (!!signbit) << 7;
+
+  return 0;
+}