--- /dev/null
+/*
+ * Copyright 2023 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License"). You may not use
+ * this file except in compliance with the License. You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+#include "crypto/rand.h"
+#include "internal/common.h"
+
+/*
+ * Implementation an optimal random integer in a range function.
+ * Refer: https://github.com/apple/swift/pull/39143 for a description
+ * of the algorithm.
+ */
+uint32_t ossl_rand_uniform_uint32(OSSL_LIB_CTX *ctx, uint32_t upper, int *err)
+{
+ uint32_t i, f; /* integer and fractional parts */
+ uint32_t f2, rand; /* extra fractional part and random material */
+ uint64_t prod; /* temporary holding double width product */
+ const int max_followup_iterations = 10;
+ int j;
+
+ if (!ossl_assert(upper > 0)) {
+ *err = 0;
+ return 0;
+ }
+ if (unlikely(upper == 1))
+ return 0;
+ /* Get 32 bits of entropy */
+ if (RAND_bytes_ex(ctx, (unsigned char *)&rand, sizeof(rand), 0) <= 0) {
+ *err = 1;
+ return 0;
+ }
+ prod = (uint64_t)upper * rand;
+ i = prod >> 32;
+ f = prod & 0xffffffff;
+ if (likely(f <= 1 + ~upper)) /* 1+~upper == -upper but compilers whine */
+ return i;
+
+ for (j = 0; j < max_followup_iterations; j++) {
+ if (RAND_bytes_ex(ctx, (unsigned char *)&rand, sizeof(rand), 0) <= 0) {
+ *err = 1;
+ return 0;
+ }
+ prod = (uint64_t)upper * rand;
+ f2 = prod >> 32;
+ f += f2;
+ /* On overflow, add the carry to our result */
+ if (f < f2)
+ return i + 1;
+ /* For not all 1 bits, there is no carry so return the result */
+ if (unlikely(f != 0xffffffff))
+ return i;
+ /* setup for the next word of randomness */
+ f = prod & 0xffffffff;
+ }
+ /*
+ * If we get here, we've consumed 32 * max_followup_iterations + 32 bits
+ * with no firm decision, this gives a bias with probability < 2^(32*n),
+ * likely acceptable.
+ */
+ return i;
+}
+
+uint32_t ossl_rand_range_uint32(OSSL_LIB_CTX *ctx, uint32_t lower, uint32_t upper,
+ int *err)
+{
+ if (!ossl_assert(lower < upper)) {
+ *err = 1;
+ return 0;
+ }
+ return lower + ossl_rand_uniform_uint32(ctx, upper - lower, err);
+}
# else
EVP_RAND_CTX *ossl_rand_get0_seed_noncreating(OSSL_LIB_CTX *ctx);
# endif
+
+/* Generate a uniformly distributed random integer in the interval [0, upper) */
+uint32_t ossl_rand_uniform_uint32(OSSL_LIB_CTX *ctx, uint32_t upper, int *err);
+
+/*
+ * Generate a uniformly distributed random integer in the interval
+ * [lower, upper).
+ */
+uint32_t ossl_rand_range_uint32(OSSL_LIB_CTX *ctx, uint32_t lower, uint32_t upper,
+ int *err);
+
#endif