ST STM32L4x6 Reference Manual page 748

Advanced encryption standard hardware accelerator (AES)
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Bits 31:0 KEYR7[31:0]: Data output register (MSB key [255:224])
Same description as AES_KEYR0 for the key[255:224].
Note: The key registers from 4 to 7 are used only when 256-bit key length is selected. These
registers have no effect when 128-bit key length is selected (only key registers from 0 to 3
are used).
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KEYR731:16]
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DocID024597 Rev 3
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KEYR7[15:0]
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RM0351
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