[Assignment-7] add rsa_verify

Assignment 7 - SGX Hands-on/rsa/rsa.c

@@ -75,7 +75,7 @@ static int pkcs1(mpz_t message, const u8 *data, const size_t length) {
     // calculate padding size (how many 0xff bytes)
     size_t padding_length = MODULUS_SIZE - length - 3;
 
-    if ((padding_length < 8) || (message == NULL)) {
+    if ((padding_length < 8) || (message == NULL) || (data == NULL)) {
         // message to big
         // or null pointer
         return 0;
@@ -124,13 +124,31 @@ int rsa_sign(u8 *sig, const u8 *sha256, const rsa_key *key) {
     return 1;
 }
 
-// TODO
-int rsa_verify(const u8 *sig, const u8 *sha256, rsa_public_key *pk) {
+
+int rsa_verify(const u8 *sig, const size_t sig_length, u8 *sha256, rsa_public_key *pk) {
     // null pointer handling
     if((sig == NULL) || (sha256 == NULL) || (pk == NULL))
         return 0;
 
+    // initialize bignums
+    mpz_t signature, message; mpz_inits(signature, message, NULL);
 
+    // import signature
+    mpz_import(signature, (sig_length < MODULUS_SIZE) ? sig_length : MODULUS_SIZE, 1, 1, 0, 0, sig);
+
+    // revert rsa signing process
+    mpz_powm(signature, signature, pk->e, pk->n);
+
+    // rebuild signed message
+    if(!pkcs1(message, sha256, 32))
+        return 0;
+
+    // compare signature with expected value
+    if(mpz_cmp(signature, message) != 0)
+        return 0;
+
+    // free bignums and return valid signature
+    mpz_clears(signature, message, NULL);
     return 1;
 }
 
@@ -140,4 +158,4 @@ void rsa_print(rsa_key *key) {
     gmp_printf("%Zu\n", key->n);
     gmp_printf("%Zu\n", key->e);
     gmp_printf("%Zu\n", key->d);
-}
+}

Assignment 7 - SGX Hands-on/rsa/rsa.h

@@ -32,6 +32,6 @@ int rsa_init(rsa_key *key);
 void rsa_free(rsa_key *key);
 
 int rsa_sign(u8 *sig, const u8 *sha256, const rsa_key *key);
-int rsa_verify(const u8 *sig, const u8* sha256, rsa_public_key *pk);
+int rsa_verify(const u8 *sig, const size_t sig_length, u8 *sha256, rsa_public_key *pk);
 
 #endif