equal.c

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//
// equal.c  Memory comparison routine that is safe against side channels.
//
// Copyright (c) Microsoft Corporation. Licensed under the MIT license.
//
 
#include "precomp.h"
 
BOOLEAN
SYMCRYPT_CALL
SymCryptEqual(  _In_reads_( cbBytes )  PCBYTE pbSrc1,
                _In_reads_( cbBytes )  PCBYTE pbSrc2,
                                       SIZE_T cbBytes )
{
    UINT32 neq = 0;
    BYTE b;
    volatile BYTE * p1 = (volatile BYTE *) pbSrc1;
    volatile BYTE * p2 = (volatile BYTE *) pbSrc2;
 
    //
    // We use forced-access memory reads to ensure that the compiler doesn't get
    // smart and implement an early-out solution.
    //
 
    while( cbBytes >= 4 )
    {
        neq |= SYMCRYPT_FORCE_READ32( (volatile UINT32 *) p1 ) ^ SYMCRYPT_FORCE_READ32( (volatile UINT32 *) p2 );
        p1 += 4;
        p2 += 4;
        cbBytes -= 4;
    }
 
    // We have to deal with the remaining bytes using a separate accumulator to work around an issue in the ARM64 compiler.
    if( cbBytes > 0 )
    {
        b = 0;
        while( cbBytes > 0 )
        {
            b |= SYMCRYPT_FORCE_READ8( p1 ) ^ SYMCRYPT_FORCE_READ8( p2 );
            p1++;
            p2++;
            cbBytes--;
        }
        neq |= b;
    }
 
    return neq == 0;
}