mirror of
https://github.com/openssl/openssl.git
synced 2024-12-15 06:01:37 +08:00
f1e0c94545
The macro was introduced in commit ed6dfd1e36 without an
openssl-specific prefix as mandated by the coding style.
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/22603)
110 lines
4.1 KiB
C
110 lines
4.1 KiB
C
/*
|
|
* 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.
|
|
*
|
|
* Essentially it boils down to incrementally generating a fixed point
|
|
* number on the interval [0, 1) and multiplying this number by the upper
|
|
* range limit. Once it is certain what the fractional part contributes to
|
|
* the integral part of the product, the algorithm has produced a definitive
|
|
* result.
|
|
*
|
|
* Refer: https://github.com/apple/swift/pull/39143 for a fuller 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 (ossl_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;
|
|
}
|
|
|
|
/*
|
|
* We are generating a fixed point number on the interval [0, 1).
|
|
* Multiplying this by the range gives us a number on [0, upper).
|
|
* The high word of the multiplication result represents the integral
|
|
* part we want. The lower word is the fractional part. We can early exit if
|
|
* if the fractional part is small enough that no carry from the next lower
|
|
* word can cause an overflow and carry into the integer part. This
|
|
* happens when the fractional part is bounded by 2^32 - upper which
|
|
* can be simplified to just -upper (as an unsigned integer).
|
|
*/
|
|
prod = (uint64_t)upper * rand;
|
|
i = prod >> 32;
|
|
f = prod & 0xffffffff;
|
|
if (ossl_likely(f <= 1 + ~upper)) /* 1+~upper == -upper but compilers whine */
|
|
return i;
|
|
|
|
/*
|
|
* We're in the position where the carry from the next word *might* cause
|
|
* a carry to the integral part. The process here is to generate the next
|
|
* word, multiply it by the range and add that to the current word. If
|
|
* it overflows, the carry propagates to the integer part (return i+1).
|
|
* If it can no longer overflow regardless of further lower order bits,
|
|
* we are done (return i). If there is still a chance of overflow, we
|
|
* repeat the process with the next lower word.
|
|
*
|
|
* Each *bit* of randomness has a probability of one half of terminating
|
|
* this process, so each each word beyond the first has a probability
|
|
* of 2^-32 of not terminating the process. That is, we're extremely
|
|
* likely to stop very rapidly.
|
|
*/
|
|
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 (ossl_likely(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),
|
|
* which is 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);
|
|
}
|