AN2492 Freescale Semiconductor / Motorola, AN2492 Datasheet - Page 16

• Manufacturer Part Number: AN2492
• Description: MPC184 Descriptor Programmers Guide--PCI View
• Manufacturer: Freescale Semiconductor / Motorola

Descriptor Type Field
Descriptor Type Field
For RNG operations, there is no key, context, or data to send in to the MPC184, so the only relevant pointer
is the one which causes random data to be written from the RNG output FIFO to memory.
For HMAC only operations, the HMAC key should be loaded, followed by the data. The HMAC itself is
written out via L/P 6. If an HMAC calculation is spread across multiple descriptors, all descriptors after the
first would need to load the MDEU context registers via L/P 2. This requires the first descriptor to output
the MDEU context or message digest, rather than an HMAC, with L/P 6.
Certain protocols do not rely on the HMAC function provided by the MDEU to generate MACs, or message
integrity check values.
5.2 Snoop Type Bit
As mentioned in Table 1, bit 1 controls the type of ‘snooping’ which must occur between the primary and
secondary EU. The rationale for ‘in-snooping’ vs. ‘out-snooping’ is found in security protocols which
perform both encryption and integrity checking, such IPSec. When transmitting an IPSec ESP packet, the
encapsulator must encrypt the packet payload, then calculate an HMAC over the header plus encrypted
payload. Because the MDEU cannot generate the HMAC without the output of the primary EU (the one
performing encryption, typically the DEU or AESU), the MDEU must ‘out-snoop.’
When receiving an IPSec packet, the decapsulator must calculate the HMAC over the encrypted portion of
the packet prior to decryption. This allows the MDEU to source its data from the input FIFO of the primary
EU, without waiting for the primary EU to finish its task.
Note that slightly different portions of an IPSec packet would pass through the primary and secondary EUs,
in both the in-snooping and out-snooping cases. These offsets are dealt with by providing different starting
pointers and byte lengths to the channel in the body of the descriptor.
An overview of the snooping concept is shown in Figure 11.
16
Descriptor
Type
0001
0001
0001
0001
Null
Null
Null
Null
L/P 1
Table 11. Descriptor Type 0001 Length/Pointer Mapping
Null
Ctx-in
(opt)
Ctx-in
(opt)
IV
MPC184 Descriptor Programmer’s Guide— PCI View
L/P 2
Freescale Semiconductor, Inc.
For More Information On This Product,
Null
Null
HMAC
Key
Key
L/P 3
Go to: www.freescale.com
Data in
Null
Data in
Data in
L/P 4
Data out
Data out
Null
Null
L/P 5
Null
Hash out
HMAC out Null
IV out
L/P 6
Null
Null
MAC out
L/P 7
RNG only
Hash only
HMAC only
Self integrity
checking
operations
MOTOROLA
Use