rand: implement an unbiassed random integer from a range

Refer: swiftlang/swift#39143 for a description
of the algorithm.

It is optimal in the sense of having:

* no divisions
* minimal number of blocks of random bits from the generator

Author: paulidale

crypto/rand/build.info

@@ -1,7 +1,8 @@
 LIBS=../../libcrypto
 
 $COMMON=rand_lib.c
-$CRYPTO=randfile.c rand_err.c rand_deprecated.c prov_seed.c rand_pool.c
+$CRYPTO=randfile.c rand_err.c rand_deprecated.c prov_seed.c rand_pool.c \
+        rand_uniform.c
 
 IF[{- !$disabled{'egd'} -}]
   $CRYPTO=$CRYPTO rand_egd.c

crypto/rand/rand_uniform.c

@@ -0,0 +1,59 @@
+/*
+ * 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 n = 10;   /* Maxiumum number of followup iterations */
+    int j;
+
+    /* 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 <= -upper))
+        return i;
+
+    for(j = 0; j < n; 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 * n + 32 bits with no firm decision,
+     * this gives a bias with probability < 2^(32*n), likely acceptable.
+     */
+    return i;
+}

include/crypto/rand.h

@@ -140,4 +140,7 @@ EVP_RAND_CTX *ossl_rand_get0_private_noncreating(OSSL_LIB_CTX *ctx);
 # else
 EVP_RAND_CTX *ossl_rand_get0_seed_noncreating(OSSL_LIB_CTX *ctx);
 # endif
+
+uint32_t ossl_rand_uniform_uint32(OSSL_LIB_CTX *ctx, uint32_t upper, int *err);
+
 #endif