added scalar addition

readme.md

@@ -83,6 +83,13 @@ must be a readable 64 byte buffer. `message` must have at least `message_len`
 bytes to be read. `public_key` must be a 32 byte public key generated by
 `ed25519_create_keypair`. Returns 1 if the signature matches, 0 otherwise.
 
+```c
+void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key,
+                        const unsigned char *scalar);
+```
+
+Adds `scalar` to the given keypair where scalar is a 32 byte buffer (possibly generated with `ed25519_create_seed`), generating a new keypair. You can calculate the public key sum without knowing the private key and vice versa by passing in NULL. This is useful for enforcing randomness on a keypair while only knowing the public key, among other things. 
+
 Example
 -------
 ```c

src/add_scalar.c

@@ -0,0 +1,46 @@
+#include "ge.h"
+#include "sc.h"
+
+
+/* see http://crypto.stackexchange.com/a/6215/4697 */
+void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar) {
+    const unsigned char SC_1[32] = {1}; /* scalar with value 1 */
+    
+    unsigned char n[32]; 
+    ge_p3 nB;
+    ge_p1p1 A_p1p1;
+    ge_p3 A;
+    ge_p3 public_key_unpacked;
+    ge_cached T;
+
+    int i;
+
+    /* copy the scalar and clear highest bit */
+    for (i = 0; i < 31; ++i) {
+        n[i] = scalar[i];
+    }
+    n[31] = scalar[31] & 127;
+
+    /* private key: a = n + t */
+    if (private_key) {
+        sc_muladd(private_key, SC_1, n, private_key);
+    }
+
+    if (public_key) {
+        /* unpack public key into T */
+        ge_frombytes_negate_vartime(&public_key_unpacked, public_key);
+        fe_neg(public_key_unpacked.X, public_key_unpacked.X); // undo negate
+        fe_neg(public_key_unpacked.T, public_key_unpacked.T); // undo negate
+        ge_p3_to_cached(&T, &public_key_unpacked);
+
+        /* calculate n*B */
+        ge_scalarmult_base(&nB, n);
+
+        /* A = n*B + T */
+        ge_add(&A_p1p1, &nB, &T);
+        ge_p1p1_to_p3(&A, &A_p1p1);
+        
+        /* pack public key */
+        ge_p3_tobytes(public_key, &A);
+    }
+}

src/ed25519.h

@@ -27,6 +27,7 @@ int ED25519_DECLSPEC ed25519_create_seed(unsigned char *seed);
 void ED25519_DECLSPEC ed25519_create_keypair(unsigned char *public_key, unsigned char *private_key, const unsigned char *seed);
 void ED25519_DECLSPEC ed25519_sign(unsigned char *signature, const unsigned char *message, size_t message_len, const unsigned char *public_key, const unsigned char *private_key);
 int ED25519_DECLSPEC ed25519_verify(const unsigned char *signature, const unsigned char *message, size_t message_len, const unsigned char *private_key);
+void ED25519_DECLSPEC ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar);
 
 
 #ifdef __cplusplus

test.c

@@ -3,13 +3,17 @@
 #include <string.h>
 #include <time.h>
 
-#define ED25519_DLL
+//#define ED25519_DLL
 #include "src/ed25519.h"
 
+#include "src/ge.h"
+#include "src/sc.h"
+
 const char message[] = "Hello, world!";
 
+
 int main(int argc, char *argv[]) {
-    unsigned char public_key[32], private_key[64], seed[32];
+    unsigned char public_key[32], private_key[64], seed[32], scalar[32];
     unsigned char signature[64];
 
 	clock_t start;
@@ -30,6 +34,20 @@ int main(int argc, char *argv[]) {
         printf("invalid signature\n");
     }
 
+    /* create scalar and add it to the keypair */
+    ed25519_create_seed(scalar);
+    ed25519_add_scalar(public_key, private_key, scalar);
+
+    /* create signature with the new keypair */
+    ed25519_sign(signature, message, strlen(message), public_key, private_key);
+
+    /* verify the signature with the new keypair */
+    if (ed25519_verify(signature, message, strlen(message), public_key)) {
+        printf("valid signature\n");
+    } else {
+        printf("invalid signature\n");
+    }
+
     /* make a slight adjustment and verify again */
     signature[44] ^= 0x10;
     if (ed25519_verify(signature, message, strlen(message), public_key)) {
@@ -67,5 +85,15 @@ int main(int argc, char *argv[]) {
 
     printf("%fus per signature\n", ((double) ((end - start) * 1000)) / CLOCKS_PER_SEC / i * 1000);
     
+
+    printf("testing scalar addition performance: ");
+    start = clock();
+    for (i = 0; i < 10000; ++i) {
+        ed25519_add_scalar(public_key, private_key, scalar);
+    }
+    end = clock();
+
+    printf("%fus per signature\n", ((double) ((end - start) * 1000)) / CLOCKS_PER_SEC / i * 1000);
+
     return 0;
 }