Merge pull request #7578 from daverodgman/safer-ct5

Improve constant-time interface

library/bignum.c

@@ -54,6 +54,132 @@
 #define MPI_VALIDATE(cond)                                           \
     MBEDTLS_INTERNAL_VALIDATE(cond)
 
+/*
+ * Compare signed values in constant time
+ */
+int mbedtls_mpi_lt_mpi_ct(const mbedtls_mpi *X,
+                          const mbedtls_mpi *Y,
+                          unsigned *ret)
+{
+    mbedtls_ct_condition_t different_sign, X_is_negative, Y_is_negative, result;
+
+    MPI_VALIDATE_RET(X != NULL);
+    MPI_VALIDATE_RET(Y != NULL);
+    MPI_VALIDATE_RET(ret != NULL);
+
+    if (X->n != Y->n) {
+        return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+    }
+
+    /*
+     * Set sign_N to 1 if N >= 0, 0 if N < 0.
+     * We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
+     */
+    X_is_negative = mbedtls_ct_bool((X->s & 2) >> 1);
+    Y_is_negative = mbedtls_ct_bool((Y->s & 2) >> 1);
+
+    /*
+     * If the signs are different, then the positive operand is the bigger.
+     * That is if X is negative (X_is_negative == 1), then X < Y is true and it
+     * is false if X is positive (X_is_negative == 0).
+     */
+    different_sign = mbedtls_ct_bool_xor(X_is_negative, Y_is_negative); // non-zero if different sign
+    result = mbedtls_ct_bool_and(different_sign, X_is_negative);
+
+    /*
+     * Assuming signs are the same, compare X and Y. We switch the comparison
+     * order if they are negative so that we get the right result, regardles of
+     * sign.
+     */
+
+    /* This array is used to conditionally swap the pointers in const time */
+    void * const p[2] = { X->p, Y->p };
+    size_t i = mbedtls_ct_size_if_else_0(X_is_negative, 1);
+    mbedtls_ct_condition_t lt = mbedtls_mpi_core_lt_ct(p[i], p[i ^ 1], X->n);
+
+    /*
+     * Store in result iff the signs are the same (i.e., iff different_sign == false). If
+     * the signs differ, result has already been set, so we don't change it.
+     */
+    result = mbedtls_ct_bool_or(result,
+                                mbedtls_ct_bool_and(mbedtls_ct_bool_not(different_sign), lt));
+
+    *ret = mbedtls_ct_uint_if_else_0(result, 1);
+
+    return 0;
+}
+
+/*
+ * Conditionally assign X = Y, without leaking information
+ * about whether the assignment was made or not.
+ * (Leaking information about the respective sizes of X and Y is ok however.)
+ */
+#if defined(_MSC_VER) && defined(_M_ARM64) && (_MSC_FULL_VER < 193131103)
+/*
+ * MSVC miscompiles this function if it's inlined prior to Visual Studio 2022 version 17.1. See:
+ * https://developercommunity.visualstudio.com/t/c-compiler-miscompiles-part-of-mbedtls-library-on/1646989
+ */
+__declspec(noinline)
+#endif
+int mbedtls_mpi_safe_cond_assign(mbedtls_mpi *X,
+                                 const mbedtls_mpi *Y,
+                                 unsigned char assign)
+{
+    int ret = 0;
+    MPI_VALIDATE_RET(X != NULL);
+    MPI_VALIDATE_RET(Y != NULL);
+
+    MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
+
+    mbedtls_ct_condition_t do_assign = mbedtls_ct_bool(assign);
+
+    X->s = (int) mbedtls_ct_uint_if(do_assign, Y->s, X->s);
+
+    mbedtls_mpi_core_cond_assign(X->p, Y->p, Y->n, do_assign);
+
+    mbedtls_ct_condition_t do_not_assign = mbedtls_ct_bool_not(do_assign);
+    for (size_t i = Y->n; i < X->n; i++) {
+        X->p[i] = mbedtls_ct_mpi_uint_if_else_0(do_not_assign, X->p[i]);
+    }
+
+cleanup:
+    return ret;
+}
+
+/*
+ * Conditionally swap X and Y, without leaking information
+ * about whether the swap was made or not.
+ * Here it is not ok to simply swap the pointers, which would lead to
+ * different memory access patterns when X and Y are used afterwards.
+ */
+int mbedtls_mpi_safe_cond_swap(mbedtls_mpi *X,
+                               mbedtls_mpi *Y,
+                               unsigned char swap)
+{
+    int ret = 0;
+    int s;
+    MPI_VALIDATE_RET(X != NULL);
+    MPI_VALIDATE_RET(Y != NULL);
+
+    if (X == Y) {
+        return 0;
+    }
+
+    mbedtls_ct_condition_t do_swap = mbedtls_ct_bool(swap);
+
+    MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_grow(Y, X->n));
+
+    s = X->s;
+    X->s = (int) mbedtls_ct_uint_if(do_swap, Y->s, X->s);
+    Y->s = (int) mbedtls_ct_uint_if(do_swap, s, Y->s);
+
+    mbedtls_mpi_core_cond_swap(X->p, Y->p, X->n, do_swap);
+
+cleanup:
+    return ret;
+}
+
 /* Implementation that should never be optimized out by the compiler */
 #define mbedtls_mpi_zeroize_and_free(v, n) mbedtls_zeroize_and_free(v, ciL * (n))
 
@@ -1624,10 +1750,8 @@ static int mpi_select(mbedtls_mpi *R, const mbedtls_mpi *T, size_t T_size, size_
 
     for (size_t i = 0; i < T_size; i++) {
         MBEDTLS_MPI_CHK(mbedtls_mpi_safe_cond_assign(R, &T[i],
-                                                     (unsigned char) mbedtls_ct_size_bool_eq(i,
-                                                                                             idx)));
+                                                     (unsigned char) mbedtls_ct_uint_eq(i, idx)));
     }
-
 cleanup:
     return ret;
 }