From 561b0e1044efb71edf06f3d27e6a783203238cfa Mon Sep 17 00:00:00 2001
From: Pieter Wuille <sipa@ulyssis.org>
Date: Mon, 27 May 2013 01:18:48 +0200
Subject: [PATCH] Private/public key tweaking

---
 src/secp256k1.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/src/secp256k1.c b/src/secp256k1.c
index 1f9e1f6dc9..9fd7e7f7cf 100644
--- a/src/secp256k1.c
+++ b/src/secp256k1.c
@@ -146,6 +146,54 @@ int secp256k1_ecdsa_pubkey_decompress(unsigned char *pubkey, int *pubkeylen) {
     return 1;
 }
 
+int secp256k1_ecdsa_privkey_tweak(unsigned char *seckey, const unsigned char *tweak) {
+    int ret = 1;
+    secp256k1_num_t term;
+    secp256k1_num_init(&term);
+    secp256k1_num_set_bin(&term, tweak, 32);
+    if (secp256k1_num_cmp(&term, &secp256k1_ge_consts->order) >= 0)
+        ret = 0;
+    secp256k1_num_t sec;
+    secp256k1_num_init(&sec);
+    if (ret) {
+        secp256k1_num_set_bin(&term, seckey, 32);
+        secp256k1_num_add(&sec, &sec, &term);
+        secp256k1_num_mod(&sec, &secp256k1_ge_consts->order);
+        if (secp256k1_num_is_zero(&sec))
+            ret = 0;
+    }
+    if (ret)
+        secp256k1_num_get_bin(seckey, 32, &sec);
+    secp256k1_num_free(&sec);
+    secp256k1_num_free(&term);
+    return ret;
+}
+
+int secp256k1_ecdsa_pubkey_tweak(unsigned char *pubkey, int pubkeylen, const unsigned char *tweak) {
+    int ret = 1;
+    secp256k1_num_t term;
+    secp256k1_num_init(&term);
+    secp256k1_num_set_bin(&term, tweak, 32);
+    if (secp256k1_num_cmp(&term, &secp256k1_ge_consts->order) >= 0)
+        ret = 0;
+    secp256k1_ge_t p;
+    if (ret) {
+        if (!secp256k1_ecdsa_pubkey_parse(&p, pubkey, pubkeylen))
+            ret = 0;
+    }
+    if (ret) {
+        secp256k1_gej_t pt;
+        secp256k1_ecmult_gen(&pt, &term);
+        secp256k1_gej_add_ge(&pt, &pt, &p);
+        secp256k1_ge_set_gej(&p, &pt);
+        int oldlen = pubkeylen;
+        secp256k1_ecdsa_pubkey_serialize(&p, pubkey, &pubkeylen, oldlen <= 33);
+        assert(pubkeylen == oldlen);
+    }
+    secp256k1_num_free(&term);
+    return ret;
+}
+
 int secp256k1_ecdsa_privkey_export(const unsigned char *seckey, unsigned char *privkey, int *privkeylen, int compressed) {
     secp256k1_num_t key;
     secp256k1_num_init(&key);