# AESDEC256KL — Perform 14 Rounds of AES Decryption Flow With Key Locker Using 256-Bit Key

Opcode/Instruction | Op/En | 64/32-bit Mode | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
F3 0F 38 DF !(11):rrr:bbb AESDEC256KL xmm, m512 | A | V/V | AESKLE | Decrypt xmm using 256-bit AES key indicated by handle at m512 and store result in xmm.

## Instruction Operand Encoding ¶

Op/En | Tuple | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | --- | ---
A | N/A | ModRM:reg (r, w) | ModRM:r/m (r) | N/A | N/A

## Description ¶

The AESDEC256KL1 instruction performs 14 rounds of AES to decrypt the first operand using the 256-bit key indicated by the handle from the second operand. It stores the result in the first operand if the operation succeeds (e.g., does not run into a handle violation failure).

## Operation ¶

#### AESDEC256KL ¶

    Handle := UnalignedLoad of 512 bit (SRC); // Load is not guaranteed to be atomic.
    Illegal Handle = (HandleReservedBitSet (Handle) ||
                    (Handle[0] AND (CPL > 0)) ||
                    Handle [2] ||
                    HandleKeyType (Handle) != HANDLE_KEY_TYPE_AES256);
    IF (Illegal Handle)
        THEN RFLAGS.ZF := 1;
        ELSE
            (UnwrappedKey, Authentic) := UnwrapKeyAndAuthenticate512 (Handle[511:0], IWKey);
            IF (Authentic == 0)
                THEN RFLAGS.ZF := 1;
                ELSE
                        DEST := AES256Decrypt (DEST, UnwrappedKey) ;
                        RFLAGS.ZF := 0;
            FI;
    FI;
    RFLAGS.OF, SF, AF, PF, CF := 0;

## Flags Affected ¶

ZF is set to 0 if the operation succeeded and set to 1 if the operation failed due to a handle violation. The other arithmetic flags (OF, SF, AF, PF, CF) are cleared to 0.

## Intel C/C++ Compiler Intrinsic Equivalent ¶

    AESDEC256KL unsigned char _mm_aesdec256kl_u8(__m128i* odata, __m128i idata, const void* h);

    1. Further details on Key Locker and usage of this instruction can be found here:

### https://software.intel.com/content/www/us/en/develop/download/intel-key-locker-specification.html. ¶

## Exceptions (All Operating Modes) ¶

#UD If the LOCK prefix is used.

If CPUID.07H:ECX.KL[bit 23] = 0.

If CR4.KL = 0.

If CPUID.19H:EBX.AESKLE[bit 0] = 0.

If CR0.EM = 1.

If CR4.OSFXSR = 0.

#NM If CR0.TS = 1.

#PF If a page fault occurs.

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.

If the DS, ES, FS, or GS register is used to access memory and it contains a NULL segment selector.

If the memory address is in a non-canonical form.

#SS(0) If a memory operand effective address is outside the SS segment limit.

If a memory address referencing the SS segment is in a non-canonical form.