crypto, crypto/ecies, crypto/secp256k1: libsecp256k1 scalar mult

thanks to Felix Lange (fjl) for help with design & impl

1 files changed, 56 insertions, 0 deletions

diff --git a/crypto/secp256k1/pubkey_scalar_mul.h b/crypto/secp256k1/pubkey_scalar_mul.h
new file mode 100644
index 000000000..0511545ec
--- /dev/null
+++ b/crypto/secp256k1/pubkey_scalar_mul.h
@@ -0,0 +1,56 @@
+// Copyright 2015 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+/** Multiply point by scalar in constant time.
+ *  Returns: 1: multiplication was successful
+ *           0: scalar was invalid (zero or overflow)
+ *  Args:    ctx:      pointer to a context object (cannot be NULL)
+ *  Out:     point:    the multiplied point (usually secret)
+ *  In:      point:    pointer to a 64-byte bytepublic point,
+                       encoded as two 256bit big-endian numbers.
+ *           scalar:   a 32-byte scalar with which to multiply the point
+ */
+int secp256k1_pubkey_scalar_mul(const secp256k1_context* ctx, unsigned char *point, const unsigned char *scalar) {
+    int ret = 0;
+    int overflow = 0;
+    secp256k1_fe feX, feY;
+    secp256k1_gej res;
+    secp256k1_ge ge;
+    secp256k1_scalar s;
+    ARG_CHECK(point != NULL);
+    ARG_CHECK(scalar != NULL);
+    (void)ctx;
+
+    secp256k1_fe_set_b32(&feX, point);
+    secp256k1_fe_set_b32(&feY, point+32);
+    secp256k1_ge_set_xy(&ge, &feX, &feY);
+    secp256k1_scalar_set_b32(&s, scalar, &overflow);
+    if (overflow || secp256k1_scalar_is_zero(&s)) {
+        ret = 0;
+    } else {
+        secp256k1_ecmult_const(&res, &ge, &s);
+        secp256k1_ge_set_gej(&ge, &res);
+        /* Note: can't use secp256k1_pubkey_save here because it is not constant time. */
+        secp256k1_fe_normalize(&ge.x);
+        secp256k1_fe_normalize(&ge.y);
+        secp256k1_fe_get_b32(point, &ge.x);
+        secp256k1_fe_get_b32(point+32, &ge.y);
+        ret = 1;
+    }
+    secp256k1_scalar_clear(&s);
+    return ret;
+}
+