Add scalarmult_ed25519_base, correct is_canonical() test, add clamping helper,

check that the result of scalarmult_ed25519() is not the point at infinity

src/libsodium/crypto_scalarmult/ed25519/ref10/scalarmult_ed25519_ref10.c

@@ -4,30 +4,75 @@
 #include "crypto_scalarmult_ed25519.h"
 #include "private/curve25519_ref10.h"
 
+static int
+_crypto_scalarmult_ed25519_is_inf(const unsigned char s[32])
+{
+    unsigned char c;
+    unsigned int  i;
+
+    c = s[0] ^ 0x01;
+    for (i = 1; i < 31; i++) {
+        c |= s[i];
+    }
+    c |= s[31] & 0x7f;
+
+    return ((((unsigned int) c) - 1U) >> 8) & 1;
+}
+
+static inline void
+_crypto_scalarmult_ed25519_clamp(unsigned char k[32])
+{
+    k[0] &= 248;
+    k[31] &= 63;
+    k[31] |= 64;
+}
+
 int
 crypto_scalarmult_ed25519(unsigned char *q, const unsigned char *n,
                           const unsigned char *p)
 {
-    unsigned char *t = q;
-    ge_p3          Q;
-    ge_p3          P;
+    unsigned char t[32];
+    ge_p3         Q;
+    ge_p3         P;
+    unsigned int  i;
 
-    if (ge_is_canonical(p) != 0 || ge_has_small_order(p) != 0 ||
+    if (ge_is_canonical(p) == 0 || ge_has_small_order(p) != 0 ||
         ge_frombytes_negate_vartime(&P, p) != 0 ||
         ge_is_on_main_subgroup(&P) == 0) {
         return -1;
     }
-    memmove(t, n, 32);
-    t[0] &= 248;
-    t[31] &= 63;
-    t[31] |= 64;
+    for (i = 0; i < 32; ++i) {
+        t[i] = n[i];
+    }
+    _crypto_scalarmult_ed25519_clamp(t);
     ge_scalarmult(&Q, t, &P);
     ge_p3_tobytes(q, &Q);
+    if (_crypto_scalarmult_ed25519_is_inf(q) != 0) {
+        return -1;
+    }
     q[31] ^= 0x80;
 
     return 0;
 }
 
+int
+crypto_scalarmult_ed25519_base(unsigned char *q,
+                               const unsigned char *n)
+{
+    unsigned char t[32];
+    ge_p3         Q;
+    unsigned int  i;
+
+    for (i = 0; i < 32; ++i) {
+        t[i] = n[i];
+    }
+    _crypto_scalarmult_ed25519_clamp(t);
+    ge_scalarmult_base(&Q, t);
+    ge_p3_tobytes(q, &Q);
+
+    return 0;
+}
+
 size_t
 crypto_scalarmult_ed25519_bytes(void)
 {

src/libsodium/include/sodium/crypto_scalarmult_ed25519.h

@@ -23,6 +23,9 @@ int crypto_scalarmult_ed25519(unsigned char *q, const unsigned char *n,
                               const unsigned char *p)
             __attribute__ ((warn_unused_result));
 
+SODIUM_EXPORT
+int crypto_scalarmult_ed25519_base(unsigned char *q, const unsigned char *n);
+
 #ifdef __cplusplus
 }
 #endif