1 files changed, 45 insertions, 0 deletions
diff --git a/libs/libaxolotl/src/curve25519/ed25519/additions/open_modified.c b/libs/libaxolotl/src/curve25519/ed25519/additions/open_modified.c
new file mode 100644
index 0000000000..a156098191
--- /dev/null
+++ b/libs/libaxolotl/src/curve25519/ed25519/additions/open_modified.c
@@ -0,0 +1,45 @@
+#include <string.h>
+#include "crypto_sign.h"
+#include "crypto_hash_sha512.h"
+#include "crypto_verify_32.h"
+#include "ge.h"
+#include "sc.h"
+#include "crypto_additions.h"
+
+int crypto_sign_open_modified(
+  unsigned char *m,
+  const unsigned char *sm,unsigned long long smlen,
+  const unsigned char *pk
+)
+{
+  unsigned char pkcopy[32];
+  unsigned char rcopy[32];
+  unsigned char scopy[32];
+  unsigned char h[64];
+  unsigned char rcheck[32];
+  ge_p3 A;
+  ge_p2 R;
+
+  if (smlen < 64) goto badsig;
+  if (sm[63] & 224) goto badsig; /* strict parsing of s */
+  if (ge_frombytes_negate_vartime(&A,pk) != 0) goto badsig;
+
+  memmove(pkcopy,pk,32);
+  memmove(rcopy,sm,32);
+  memmove(scopy,sm + 32,32);
+
+  memmove(m,sm,smlen);
+  memmove(m + 32,pkcopy,32);
+  crypto_hash_sha512(h,m,smlen);
+  sc_reduce(h);
+
+  ge_double_scalarmult_vartime(&R,h,&A,scopy);
+  ge_tobytes(rcheck,&R);
+
+  if (crypto_verify_32(rcheck,rcopy) == 0) {
+    return 0;
+  }
+
+badsig:
+  return -1;
+}