Freescale Semiconductor MPC8313E PowerQUICC II Pro Family Reference Manual page 684

Security Engine (SEC) 2.2
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Note on Restore decrypt key (RDK)—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_0- AESU Key Expand Output'. The AESU mode must be 'Decrypt'.
See Table 14-7
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_0), 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).
14.4.3.2 AESU Key Size Register (AESUKSR)
The AESU key size register (AESUKSR), shown in
(16, 24, 32). Any key data beyond the number of bytes in the key size register will be ignored. The
AESUKSR 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.
MPC8313E PowerQUICC™ II Pro Integrated Processor Family Reference Manual, Rev. 2
14-42
Table 14-26. AES Cipher Modes (continued)
Mode
XOR
Reserved
SRT is not a new AES mode, it is an AESU method of performing AES-CTR
mode with reduced context loading overhead specifically for performing
SRTP. It should be used with descriptor type 0010_0 'srtp'. See
Section 14.4.3.9.3, "Context for SRT Mode,"
SRT mode reduces context loading overhead.
for more information. The descriptor will cause the SEC to
ECM (56–57) CM (61–62)
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all others
for more information on how
NOTE
Figure 14-27, stores the number of bytes in the key
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