SEC Lite Execution Units
MPC885 PowerQUICC Family Reference Manual, Rev. 2
48-26
Freescale Semiconductor
NOTE
Restore Decrypt Key: In most networking applications, the decryption of
an AES protected packet will be performed as a single operation. However,
if circumstances dictate that the decryption of a message should be split
across multiple descriptors, the AESU allows the user to save the decrypt
key, and the active AES context, to memory for later re-use. This saves the
internal AESU processing overhead associated with regenerating the
decryption key schedule (~12 AESU clock cycles for the first block of data
to be decrypted.)
The use of RDK is completely optional, as the Input time of the preserved
decrypt key may exceed the ~12 cycles required to restore the decrypt key
for processing the first block.
To use RDK, the following procedure is recommended:
The descriptor type used in decryption of the first portion of the message is
“0100- AESU Key Expand Output”. The AESU mode must be “Decrypt”.
See Chapter 4 “Descriptors” for more information. The descriptor will cause
the SEC Lite to write the contents of the Context registers and the key
registers (containing the expanded decrypt key) to memory.
To process the remainder of the message, use a “common” descriptor type
(0001), and set the "restore decrypt key" mode bit. Load the context
registers and the expanded decrypt key with previously saved key and
context data from the first message. The key size is written as before (16,
24, or 32 bytes).
48.3.3
AESU Key Size Register
The AESU Key Size Register stores the number of bytes in the key (16, 24, 32). Any key data beyond the
number of bytes in the key size register will be ignored. This register is cleared when the AESU is reset or
re-initialized. If a key size other than 16, 24, or 32 bytes is specified, an illegal key size error will be
generated. If the key size register is modified during processing, a context error will be generated.
Summary of Contents for PowerQUICC MPC870
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