PLCryptoConstantTime.m

/*
 * Copyright (c) 2014 Plausible Labs Cooperative, Inc.
 * All rights reserved.
 */

#import "PLCryptoConstantTime.h"

/**
 * @ingroup plcrypto
 * @{
 */

NSUInteger plcrypto_constant_time_compare_internal (const void *s1, size_t s1len, const void *s2, size_t s2len);

/* Function pointer indirection, used to prevent the compiler from inlining plcrypto_constant_time_compare_internal() */
NSUInteger (* volatile plcrypto_constant_time_compare_internal_ptr)(const void *s1, size_t s1len, const void *s2, size_t s2len) = plcrypto_constant_time_compare_internal;

/**
 * @internal
 *
 * Returns 0 if @a s1 is equal to @a s2, performing the comparison in constant time as to avoid leaking
 * information regarding where the values differ.
 *
 * @param s1 The first array to compare.
 * @param s1len The length of s1.
 * @param s2 The second array to compare.
 * @param s2len The length of s2.
 * @return 0 If the receiver is equal to @a other, otherwise a non-zero value.
 */
NSUInteger plcrypto_constant_time_compare_internal (const void *s1, size_t s1len, const void *s2, size_t s2len) {
    if (s1len != s2len) {
        return 1;
    }
    
    const uint8_t *aPtr = s1;
    const uint8_t *bPtr = s2;
    NSUInteger accumulator = 0;
    for (NSUInteger i = 0; i < s1len; i++) {
        accumulator |= (aPtr[i] ^ bPtr[i]);
    }
    
    /* We don't do a == 0 here, as we don't want a way-too-smart compiler to optimize the above to a memcmp */
    return accumulator;
}


/**
 * Returns a Boolean value that indicates whether the receiver is equal to the given instance,
 * performing the comparison in constant time as to avoid leaking information regarding where
 * the values differ.
 *
 * Note that if the two values do not have the same length, this function returns immediately.
 *
 * @param other The object with which to compare the receiver.
 * @return YES If the receiver is equal to @a other, otherwise NO.
 */
BOOL plcrypto_constant_time_compare (const void *s1, size_t s1len, const void *s2, size_t s2len) {
    return plcrypto_constant_time_compare_internal_ptr(s1, s1len, s2, s2len) == 0;
}

/**
 * @}
 */