MSC8144E Reference Manual, Rev. 3
26-54
Freescale
Semiconductor
Security Engine (SEC)
26.4.5.1 AFEU Mode Register
The AFEU Mode Register contains three bits that are used to program the AFEU. The Mode
Register is cleared when the AFEU is reset or reinitialized. Setting a reserved mode bit generates
a data error. If the Mode Register is modified during processing, a context error is generated.
26.4.5.1.1 Core Processor-Provided Context via Prevent Permute
In the default mode of operation, the core processor provides the key and key size to the AFEU.
The initial memory values in the S-Box are permuted with the key to create new S-Box values,
which are used to encrypt the plaintext.
If the ‘Prevent Permute’ mode bit is set, the AFEU will not require a key. Rather, the core
processor will write the context to the AFEU and message processing will occur using the
provided context. This mode is used to resume processing of a message using the already
permuted S-Box. The context may be written through the FIFO if the ‘context source’ mode bit is
set. The AFEU context is 259 bytes long, and must be in the format provided by the Dump
Context function (see Section 26.4.5.1.2, Dump Context).
26.4.5.1.2 Dump Context
This mode may be independently specified in addition to core processor-provided context mode.
In this mode, once message processing is complete and the output data is read, the AFEU will
make the current context data available for reads via the output FIFO. The AFEU context is 259
bytes long.
Note:
After the initial key permute to generate a context for an AFEU encrypted session, all
subsequent messages will re-use that context, such that it is loaded, modified during
the encryption, and unloaded, similar to the use of a CBC initialization vector in DES
operations. A new context is generated (via key permute) according to a re-keying
interval specified by the security protocol. Context should never be loaded to encrypt a
message if a key is loaded and permuted at the same time.
26.4.5.2 AFEU Key Size Register
The AFEU key size value indicates the number of bytes of key memory to use in performing
S-box permutation. Any key data beyond the number of bytes in the Key Size Register is ignored.
This register is cleared when the AFEU is reset or reinitialized. If the key size specified is less
than 1 or greater than 16, a key size error is generated. If the Key Size Register is modified during
processing, a context error is generated. Note: Although the AFEU supports key lengths as short
as 1 byte, a 1-byte key offers little security. Most ARC-4 applications specify keys of 5–16 bytes.
Note:
The device driver creates properly formatted descriptors for situations requiring a key
permute prior to ciphering. When using core processor-controlled access (typically for
debug), the user must set the AFEU Mode Register to perform ‘permute with key’,
Содержание MSC8144E
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