Draft implementation for ed25519 key blinding, as in prop224

This implementation allows somebody to add a blinding factor to a
secret key, and a corresponding blinding factor to the public key.

Robert Ransom came up with this idea, I believe.  Nick Hopper proved a
scheme like this secure.  The bugs are my own.

3 files changed, 83 insertions, 1 deletions

diff --git a/src/ext/ed25519/ref10/blinding.c b/src/ext/ed25519/ref10/blinding.c
new file mode 100644
index 0000000000..a17dbcd3e3
--- /dev/null
+++ b/src/ext/ed25519/ref10/blinding.c
@@ -0,0 +1,75 @@
+/* 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 "ge.h"
+#include "sc.h"
+#include "crypto_hash_sha512.h"
+#include "ed25519_ref10.h"
+
+#include <string.h>
+#include "crypto.h"
+
+static void
+gettweak(unsigned char *out, const unsigned char *param)
+{
+  const char str[] = "Derive temporary signing key";
+  crypto_hash_sha512_2(out, (const unsigned char*)str, strlen(str), param, 32);
+  out[0] &= 248;  /* Necessary ? */
+  out[31] &= 63;
+  out[31] |= 64;
+}
+
+int ed25519_ref10_derive_secret_key(unsigned char *out,
+                              const unsigned char *inp,
+                              const unsigned char *param)
+{
+  const char str[] = "Derive temporary signing key hash input";
+  unsigned char tweak[64];
+  unsigned char zero[32];
+  gettweak(tweak, param);
+
+  memset(zero, 0, 32);
+  sc_muladd(out, inp, tweak, zero);
+
+  crypto_hash_sha512_2(tweak, (const unsigned char *)str, strlen(str),
+                       inp+32, 32);
+  memcpy(out+32, tweak, 32);
+
+  memwipe(tweak, 0, sizeof(tweak));
+
+  return 0;
+}
+
+int ed25519_ref10_derive_public_key(unsigned char *out,
+                              const unsigned char *inp,
+                              const unsigned char *param)
+{
+  unsigned char tweak[64];
+  unsigned char zero[32];
+  unsigned char pkcopy[32];
+  ge_p3 A;
+  ge_p2 Aprime;
+
+  gettweak(tweak, param);
+
+  memset(zero, 0, sizeof(zero));
+  /* Not the greatest implementation of all of this.  I wish I had
+   * better-suited primitives to work with here... (but I don't wish that so
+   * strongly that I'm about to code my own ge_scalarmult_vartime). */
+
+  /* We negate the public key first, so that we can pass it to
+   * frombytes_negate_vartime, which negates it again. */
+  memcpy(pkcopy, inp, 32);
+  pkcopy[31] ^= (1<<7);
+  ge_frombytes_negate_vartime(&A, pkcopy);
+  /* There isn't a regular ge_scalarmult -- we have to do tweak*A + zero*B. */
+  ge_double_scalarmult_vartime(&Aprime, tweak, &A, zero);
+  ge_tobytes(out, &Aprime);
+
+  memwipe(tweak, 0, sizeof(tweak));
+  memwipe(&A, 0, sizeof(A));
+  memwipe(&Aprime, 0, sizeof(Aprime));
+  memwipe(&pkcopy, 0, sizeof(pkcopy));
+
+  return 0;
+}
diff --git a/src/ext/ed25519/ref10/ed25519_ref10.h b/src/ext/ed25519/ref10/ed25519_ref10.h
index da8cea19f0..f4a76e621c 100644
--- a/src/ext/ed25519/ref10/ed25519_ref10.h
+++ b/src/ext/ed25519/ref10/ed25519_ref10.h
@@ -20,5 +20,11 @@ int ed25519_ref10_sign(
 int ed25519_ref10_pubkey_from_curve25519_pubkey(unsigned char *out,
                                                 const unsigned char *inp,
                                                 int signbit);
+int ed25519_ref10_derive_secret_key(unsigned char *out,
+                              const unsigned char *inp,
+                              const unsigned char *param);
+int ed25519_ref10_derive_public_key(unsigned char *out,
+                              const unsigned char *inp,
+                              const unsigned char *param);
 
 #endif
diff --git a/src/ext/include.am b/src/ext/include.am
index 45d7dc565a..69c136b184 100644
--- a/src/ext/include.am
+++ b/src/ext/include.am
@@ -57,7 +57,8 @@ src_ext_ed25519_ref10_libed25519_ref10_a_SOURCES= \
 	src/ext/ed25519/ref10/sc_muladd.c \
 	src/ext/ed25519/ref10/sc_reduce.c \
 	src/ext/ed25519/ref10/sign.c \
-	src/ext/ed25519/ref10/keyconv.c
+	src/ext/ed25519/ref10/keyconv.c \
+	src/ext/ed25519/ref10/blinding.c
 
 ED25519_REF10_HDRS = \
 	src/ext/ed25519/ref10/api.h \