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A E S D A T A E N C R Y P T I O N / D E C R Y P T I O N M O D U L E
Decryption
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The DMA channel does not try a transfer when the F bit is clear. The DMA channel
enters an idle state upon fetching a buffer descriptor with the F bit cleared.
When the F bit is modified by the device driver, the device driver must also write an
‘I’ to the CE bit (in the DMA Control register) to activate the idle channel.
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D e c r y p t i o n
During decryption, the expanded key must be fed to the AES core backwards. The
hardware key expander can handle this, but the input key is different than for
encryption. The key must be expanded and the last words must be written to the
key buffer as shown:
A 128-bit key (K0, K1, K2, K3) is expanded to the following 32-bit word
sequence: K0, K1, ..., K40, K41, K42, K43.
To expand the key backwards, the hardware key expander needs K40-K43.
A 192-bit key (K0, K1, K2, K3, K5, K6) is expanded to the following 32-bit word
sequence: K0, K1, ..., K46, K47, K48, K49, K50, K51.
To expand the key backwards, the hardware key expander core needs
K48-51 followed by K46-47.
A 256-bit key (K0, K1, K2, K3, K5, K6, K7) is expanded to the following 32-bit
word sequence: K0, K1, ..., K52, K53, K54, K55, K56, K57, K58, K59.
To expand the key backwards, the hardware key expander core needs
K56-59 followed by K52-55.
The hardware key expander recreates all the remaining words in backwards order.
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E C B p r o c e s s i n g
ECB mode does not require an initialization vector. Software just needs to set up a
key buffer descriptor, followed by a data buffer descriptor.
Processing flow
diagram
This is the ECB buffer descriptor processing flow:
Summary of Contents for NS9215
Page 1: ...NS9215 Hardware Reference 90000847_C Release date 10 April 2008...
Page 3: ......
Page 4: ......
Page 26: ...26 Hardware Reference NS9215...
Page 44: ...P I N O U T 26 5 System clock 44 Hardware Reference NS9215 System clock drawing...
Page 52: ...P I N O U T 26 5 Power and ground 52 Hardware Reference NS9215...
Page 80: ...I O C O N T ROL M O D U L E Memory Bus Configuration register 80 Hardware Reference NS9215...
Page 136: ...WOR KI N G W I TH T H E C P U Noncachable instruction fetches 136 Hardware Reference NS9215...
Page 202: ...S Y S T E M C O N T RO L M OD U L E RTC Module Control register 202 Hardware Reference NS9215...
Page 354: ...E X T E R N A L D M A DMA Peripheral Chip Select register 354 Hardware Reference NS9215...
Page 472: ...R E A L TI M E C L O C K M O D U L E General Status register 472 Hardware Reference NS9215...
Page 512: ...TI M I NG Clock timing 512 Hardware Reference NS9215...
Page 515: ...PA CKA GING Processor Dimensions www digiembedded com 515...
Page 516: ...PA CKA GING Processor Dimensions 516 Hardware Reference NS9215...