AN2492 Freescale Semiconductor / Motorola, AN2492 Datasheet - Page 26

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

Additional Examples
Additional Examples
The descriptor header encodes the information required to select the DEU for Op_0, and the MDEU for
Op_1. The Op_0 mode data configured the DEU to operate in 3DES, CBC, encrypt mode. The Op_1 mode
data configured the MDEU to operate in HMAC-SHA-1 mode. Because all the data necessary to calculate
the HMAC in a single dynamic descriptor is available, initialize, and autopad are set, while continue is off.
The descriptor header also encodes the descriptor type 0010, which defines the input and output ordering
for ‘hmac_snoop_no_afeu.’ The HMAC key is loaded first, followed by the length and pointer to the data
over which the HMAC will be calculated. The 3DES key is loaded next, followed by the 3DES IV. The
number of bytes to be encrypted and starting address will be an offset of the number of bytes being
HMAC’d. The data to be encrypted and HMAC’d is only brought into the MPC184 a single time, with the
DEU and MDEU only reading the portion that matches the starting address and byte length in the
length/pointer fields corresponding to their data of interest.
Plaintext is brought into the DEU input FIFO, with the MDEU out-snooping the portion of the data it has
been told to process. As the encryption continues, the ciphertext fills the DEU output FIFO, and this data is
written back to system memory as needed. When the final byte of data to be HMAC’d has been processed
through the MDEU, the descriptor will cause the MDEU to write the HMAC to the indicated area in PCI
memory. The MPC184 will write the entire 20 bytes HMAC-SHA-1 to PCI memory, and the host will
append the most significant 12 bytes of the HMAC generated by the MPC184 to the packet as the
authentication trailer. Common practice in IPSec ESP with 3DES-CBC is to use the last 8 bytes of the
ciphertext as the IV for the next packet. If this is the case, the host should copy the last 8 bytes of the
ciphertext to the Security Association database entry for this particular session before transmitting the
packet.
The next descriptor pointer is optional, and if a next descriptor is indicated, that descriptor may be
completely unrelated to the operation performed on the descriptor shown in Table 20.
9.3 Dynamically Assigned HMAC-MD-5 (Inbound/Outbound
Table 21 shows a dynamic descriptor example of an inbound/outbound IPSec AH transform.
26
LEN_4
PTR_4
LEN_5
PTR_5
LEN_6
PTR_6
LEN_7
PTR_7
PTR_NEXT
IPSec AH)
Field
0010_3DES_CBC_HMAC_SHA-1_Decrypt (continued)
MPC184 Descriptor Programmer’s Guide— PCI View
Freescale Semiconductor, Inc.
Table 20. Representative Descriptor DPD_Type
Value/Type
For More Information On This Product,
Pointer
Pointer
Pointer
Pointer
Length
Length
Length
Length
Pointer
Go to: www.freescale.com
Number of bytes of IV to be written to DEU IV register (always 8)
PCI address of IV
Number of bytes of plaintext to be encrypted
PCI address of plaintext to be encrypted
Number of bytes of ciphertext to be written out to memory (should be
equal to length of data-in)
PCI address where ciphertext is to be written
Number of bytes of HMAC to be written to PCI memory space
(always 20)
PCI address where HMAC is to be written
Pointer to next descriptor
Description
MOTOROLA