Add call to retrieve public key from private key

[vszakats]

No description provided.

[vszakats]

@orlp: Please feel free to reopen these if you get around to dealing with ed25519. They are valid patches, that — I believe — don't have much risk involved and add value to your project. I've been maintaining these patches for years now downstream, they work well.

[vszakats]

This proposed function can also fix or mitigate this vulnerability: https://github.com/MystenLabs/ed25519-unsafe-libs

[vszakats]

Updated patch, which additionally fixes the above vulnerability by ignoring the passed public_key and deriving it from private_key instead. The function remains compatible and automatically fixes the vulnerability for all callers:

diff --git a/src/ed25519.h b/src/ed25519.h
index 8924659..b0d0347 100644
--- a/src/ed25519.h
+++ b/src/ed25519.h
@@ -25,7 +25,8 @@ int ED25519_DECLSPEC ed25519_create_seed(unsigned char *seed);
 #endif
 
 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);
+void ED25519_DECLSPEC ed25519_get_pubkey(unsigned char *public_key, const unsigned char *private_key);
+void ED25519_DECLSPEC ed25519_sign(unsigned char *signature, const unsigned char *message, size_t message_len, const unsigned char *unused, 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 *public_key);
 void ED25519_DECLSPEC ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar);
 void ED25519_DECLSPEC ed25519_key_exchange(unsigned char *shared_secret, const unsigned char *public_key, const unsigned char *private_key);
diff --git a/src/sign.c b/src/sign.c
index 199a839..8251252 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -3,13 +3,23 @@
 #include "ge.h"
 #include "sc.h"
 
+void ed25519_get_pubkey(unsigned char *public_key, const unsigned char *private_key) {
+    ge_p3 A;
 
-void ed25519_sign(unsigned char *signature, const unsigned char *message, size_t message_len, const unsigned char *public_key, const unsigned char *private_key) {
+    ge_scalarmult_base(&A, private_key);
+    ge_p3_tobytes(public_key, &A);
+}
+
+void ed25519_sign(unsigned char *signature, const unsigned char *message, size_t message_len, const unsigned char *unused, const unsigned char *private_key) {
     sha512_context hash;
     unsigned char hram[64];
     unsigned char r[64];
     ge_p3 R;
+    unsigned char public_key[ 32 ];
+
+    (void)unused;
 
+    ed25519_get_pubkey(public_key, private_key);
 
     sha512_init(&hash);
     sha512_update(&hash, private_key + 32, 32);