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AES Registers
686
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Advance Encryption Standard Accelerator (AES)
9.5.4 AES_C_LENGTH_0 to AES_C_LENGTH_1 Registers (Offset = 0x54 to 0x58) [reset = 0x0]
AES Crypto Data Length 0 (AES_C_LENGTH_0), offset 0x054
AES Crypto Data Length 1 (AES_C_LENGTH_1), offset 0x058
AES Crypto Data Length n (AES_C_LENGTH_n) registers (LSW and MSW) store the cryptographic data
length in bytes for all modes. The AES_C_LENGTH_0 register stores the most significant word and the
AES_C_LENGTH_1 register stores the least significant word. Once processing with this context is started,
this length decrements to zero. Data lengths up to (2
61
– 1) bytes are allowed. For GCM, any value up to
236 – 32 bytes can be used. This is because a 32-bit counter mode is used; the maximum number of 128-
bit blocks is 2
32
– 2, resulting in a maximum number of bytes of 2
36
– 32. A write to this register triggers
the engine to start using this context. This is valid for all modes except GCM and CCM. Note that for the
combined modes, this length does not include the authentication only data; the authentication length is
specified in the AES_AUTH_LENGTH register below. All modes must have a length greater than 0. For
the combined modes, it is permissible to have one of the lengths equal to zero. For the basic encryption
modes (ECB/CBC/CTR/ICM/CFB128) it is permissible to program zero to the length field; in that case the
length is assumed infinite. All data must be byte (8-bit) aligned; bit aligned data streams are not supported
by the AES Engine.
NOTE:
For a Host read operation, these registers return all zeroes.
AES_C_LENGTH_n is shown in
and described in
Return to
Figure 9-17. AES_C_LENGTH_n Register
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
LENGTH
R/W-0x0
Table 9-11. AES_C_LENGTH_n Register Field Descriptions
Bit
Field
Type
Reset
Description
31-0
LENGTH
R/W
0x0
Data Length.
This field stores the cryptographic data length in bytes for all modes.