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DES Block Diagram
855
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Data Encryption Standard Accelerator (DES)
14.3.3 Register Interface
The Register Interface block performs all address decoding and control; however, not all registers are
available in this block. The context and data input registers are in the DES engine.
14.3.4 DES Engine
The DES buffered engine consists of the following major functional blocks:
•
Cipher core: The DES algorithm
•
Mode control FSM: Manages the data flow to and from the DES buffered engine and starts each
encrypt or decrypt operation
•
DES feedback mode block: The logic that implements the various feedback modes supported by the
DES buffered engine
14.3.4.1 Mode Control FSM
The mode control FSM manages the data flow to and from the DES engine. This block also sends a start
pulse to the encrypt or decrypt core and triggers the core to use the new mode keys when a new context
is needed. This module also controls the 3DES operation, such that the DES core module is triggered
three times (with different keys) before the result data becomes available.
14.3.4.2 DES Feedback Mode Block
The DES feedback mode block buffers the input and output blocks and contains all logic to implement the
3DES and various feedback modes. See the FIPS-81 document for details on the ECB, CBC, and CFB
modes of operation.
By itself, the DES cipher core outputs data compliant for ECB encryption. However, most applications use
DES with feedback. Feedback provides additional security by randomizing repeated patterns in the plain
text, which could otherwise be exploited to attack the cipher text. The DES buffered engine supports ECB,
CBC, and CFB modes of operations for the DES and 3DES algorithm.
3DES mode performs the DES algorithm three times on a single block and uses a different key for each
invocation of the DES algorithm, greatly increasing security of the cipher text but at the cost of a 3x
reduction in throughput. The DES buffered engine implements a 3DES logic wrapper around the 2-round
DES core, enabling seamless 3DES encryption.
14.3.4.3 DES Cipher Core
The DES cipher core implements the DES algorithm as specified in the FIPS 46-3. The core operates on
the input block and performs the required substitution, shift, and mix operations. The core also applies the
correct key-scheduling.
Inherently, considerable parallelism is possible with the DES algorithm. This is exploited in two ways. For
high performance, the 64 bits composing the data block are processed concurrently (4-round
implementation). For low gate count, resources are shared on both the main data and key paths (1-round
implementation).
A fundamental component of the DES algorithm is the substitution box (S-Box). The S-Box provides a
unique 4-bit output for each 6-bit input. The S-Box design is a primary factor for both performance and
gate count. The DES Cipher Core has a standard lookup table S-Box that allows room for the synthesizer
to optimize on timing or gate count.