MSC8144E Reference Manual, Rev. 3
26-44
Freescale
Semiconductor
Security Engine (SEC)
26.4.3.9 AESU Context Registers
There are seven 64-bit context data registers that allow the core processor to read/write the
contents of the context used to process the message. The context must be written prior to the key
data. If the Context Registers are written during message processing, a context error is generated.
All Context Registers are cleared when a hard/soft reset or initialization is performed.
The Context Registers must be read when changing context and restored to their original values
to resume processing an interrupted message (CBC, CTR and CCM modes). For CTR and CCM
modes, all seven 64-bit Context Registers must be read to retrieve context, and all seven must be
written back to restore context. Effectively, the user must read the four empty place holder
Context Registers in addition to the three Context Registers holding the Counter and Counter
Modulus Exponent when in CTR mode. The contents of the empty Context Registers need not be
preserved, but when restoring the CTR mode context, the empty registers must be filled with 32
bytes of zeros before writing the saved Counter and Counter Modulus Exponent. Context should
be loaded with the lower bytes in the lowest 64-bit Context Register.
26.4.3.9.1 Context for CBC Mode
The Context Registers include two 64-bit context data registers for use in CBC mode that allow
the core processor to read/write the contents of the initialization vector (IV):
IV1 holds the least significant bytes (LSBs) of the initialization vector (bytes 1–8).
IV2 holds the most significant bytes (MSBs) of the initialization vector (bytes 9–16).
The IV must be written prior to the message data. If the IV registers are written during message
processing, or the CBC mode bit is not set, a context error is generated. The IV registers may
only be read after processing has completed, as indicated by the assertion of a done interrupt (DI)
in the AESU Status Register as shown in Section 26.5.8.5, AESU Status Register (AESUSR), on
page 26-129. If the IV registers are read prior to assertion of Interrupt Done, an early read error is
generated. The IV registers must be read when changing context and restored to resume
processing an interrupted message (CBC mode only).
26.4.3.9.2 Context for Counter Mode
In counter mode, a random 128-bit initial counter value is incremented modulo 2
M
with each
block processed. The running counter is encrypted and XORed with the plaintext to derive the
ciphertext, or with the ciphertext to recover the plaintext. The modulus exponent M can be set
between 8 and 128 in multiples of 8. The value of M is specified by writing to Context Register 3.
26.4.3.9.3 Context for SRT Mode
SRT 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. As with
counter mode, a random 128-bit initial counter value is incremented modulo 2
M
with each block
processed. The running counter is encrypted and XORed with the plaintext to derive the
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 ...