Datasheet

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Option Explanation
PUKCL_EXPMOD_REGULARRSA Performs a Regular computation, slower than the Fast version, but
using regular calculus methods.
For this service, four window sizes for the Modular Exponentiation Steps are possible. The window size in
bits is those of the windowing method used for the exponent.
The choice of the window size is a balance between the size of the parameters and the computation time:
• Increasing the window size increases the precomputation workspace.
• Increasing the window size reduces the computation time (may not be relevant for very small
exponents). The length of the Rval and Precomp area depends on the window size W and
u2ModLength.
The Rval and Precomp area length is:
RandPrecompLen = 4 * (u2ModLength + 4) + max(64 , 2
(W-1)
* (u2ModLength + 4)) + 8
Important: Please calculate precisely the length RandPrecompLen with the formula and the
max() macro, which takes the maximum of two values.
The following table shows the size of the Rval and Precomp area, depending on the chosen window size
option.
Table 43-64. CRT Service Window Size Options and Rval and Precomp Area Size
Option Specified Size of the Rval and Precomp Area
(bytes)
Precomputation Values
PUKCL_EXPMOD_WINDOWSIZE_1 4*(u2ModLength + 4) + max(64 ,
(u2ModLength + 4)) + 8
x
PUKCL_EXPMOD_WINDOWSIZE_2 4*(u2ModLength + 4) + max(64 ,
2*(u2ModLength + 4)) + 8
x x
3
PUKCL_EXPMOD_WINDOWSIZE_3 4*(u2ModLength + 4) + max(64 ,
4*(u2ModLength + 4)) + 8
x x
3
x
5
x
7
PUKCL_EXPMOD_WINDOWSIZE_4 10*(u2ModLength + 4) + max(64 ,
8*(u2ModLength + 4)) + 8
x x
3
x
5
x
7
x
9
x
11
x
13
x
15
The exponent area can be located in RAM or in the data space. If one part of the exponent area is in
Crypto RAM this must be mandatory signaled by using the PUKCL_EXPMOD_EXPINPUKCCRAM
option.
The following table describes this option.
SAM D5x/E5x Family Data Sheet
Public Key Cryptography Controller (PUKCC)
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 1504