Execution Units
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
26-47
field, it must be generated and stored along with other session specific information for
loading into the AESU Context Register prior to CCM decryption.
CCM decryption processing is the reverse of encryption. With the session specific key and
context, the AESU performs the following operations.
1.
Initializes the IV, and encrypts with the symmetric key. Simultaneously, the counter
(Initial Counter Value) from Context Registers 5–6 is encrypted with the symmetric
key. The result is hashed with the Encrypted MAC (from Context Register 3–4), and the
resulting Original MAC is written to Context Registers 3–4, overwriting the encrypted
MAC.
Note:
Strictly speaking, the Counter is encrypted with the symmetric key, however the
AESU should be set for decrypt to perform the counter and CBC processes in the
correct order.
2.
The IEEE Std. 802.11 frame header is hashed with the encrypted IV. (The AESU
automatically determines the header length.) Simultaneously, the counter is
incremented, and is then encrypted with the symmetric key. The result is then hashed
with the first block of ciphertext to produce the first block of plaintext. The plaintext is
placed in the AESU output FIFO, while simultaneously, in CBC fashion, a copy of the
first block of plaintext is hashed with the output of encryption of the IEEE Std. 802.11
frame header. The output is encrypted with the symmetric key.
3.
As each ciphertext block is converted to plaintext, the plaintext is CBC encrypted. When
the final plaintext block has been processed, the CBC MAC (MAC Tag) is written to
Context Registers 1–2. The first 64 bits of the MAC Tag are compared to the MAC Tag
recovered in step 1.
Note:
For both encrypt and decrypt operations, if the 802.11 frame is being processed as a
whole (not split across multiple descriptors), the Initialize and Final MAC bits should
be set in the AESU Mode Register.
26.4.3.10 AESU Key Registers
The AESU key registers hold from 16, 24, or 32 bytes of key data, with the first 8 bytes of key
data written to key 1. Any key data written to bytes beyond the value written to the Key Size
Register will be ignored. The Key Data Registers are cleared when the AESU is reset or
reinitialized. If these registers are modified during message processing, a context error will be
generated.
The Key Data Registers may be read when changing context in decrypt mode. To resume
processing, the value read must be written back to the key registers and the restore decrypt key bit
must be set in the Mode Register. This eliminates the overhead of expanding the key prior to
starting decryption when switching context.
Summary of Contents for MSC8144E
Page 1: ...MSC8144E Reference Manual Quad Core Media Signal Processor MSC8144ERM Rev 3 July 2009 ...
Page 48: ...MSC8144E Reference Manual Rev 3 xlviii Freescale Semiconductor ...
Page 86: ...MSC8144E Reference Manual Rev 3 1 38 Freescale Semiconductor Overview ...
Page 168: ...MSC8144E Reference Manual Rev 3 3 60 Freescale Semiconductor External Signals ...
Page 242: ...MSC8144E Reference Manual Rev 3 5 26 Freescale Semiconductor Reset ...
Page 314: ...MSC8144E Reference Manual Rev 3 8 24 Freescale Semiconductor General Configuration Registers ...
Page 414: ...MSC8144E Reference Manual Rev 3 10 14 Freescale Semiconductor MSC8144E SC3400 DSP Subsystem ...
Page 452: ...MSC8144E Reference Manual Rev 3 11 38 Freescale Semiconductor Internal Memory Subsystem ...
Page 520: ...MSC8144E Reference Manual Rev 3 12 68 Freescale Semiconductor DDR SDRAM Memory Controller ...
Page 1070: ...MSC8144E Reference Manual Rev 3 21 28 Freescale Semiconductor Timers ...