Datasheet

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Parameter Type Direction Location Data Length Before
Executing the
Service
After
Executing
the Service
u1Blinding (see
Note 3)
u4 I – – Exponent
unblinding value
Exponent
unblinding
value
Note:
1. This zone contains the number to be exponentiated (u2ModLength bytes) and is used during the
computations as a workspace (four 32-bit words longer than the number to be exponentiated). At
the end of the computation, it contains the correct result of the operation.
2. If the PUKCL_EXPMOD_EXPINPUKCCRAM option is not set, the location of the exponent MUST
NOT be placed in the Crypto RAM, even partially.
3. It is possible to mask the exponent in memory using a 32-bit XOR mask value. Be aware that not
only the exponent, but also the supplemental spill word has to be masked. If masking is not
desired, the parameter should be set to 0.
43.3.5.4.5 Options
Most of the CRT options configure the Modular Exponentiation steps of the CRT and so are very similar
to the Fast Modular Exponentiation options.
The options are set by the u2Options input parameter, which is composed of:
• the mandatory Calculus Mode Option described in Table 43-63
• the mandatory Window Size Option described in Table 43-64
• the indication of the presence of the exponent in Crypto RAM
Important: Please check precisely if one part of the exponent area (containing EP and EQ) is
in Crypto RAM. If this is the case, the PUKCL_EXPMOD_EXPINPUKCCRAM option must be
used.
The u2Options number is calculated by an “Inclusive OR” of the options. Some Examples in C language
are:
• Operation: CRT using the Fast Modular Exponentiation with the window size equal to 1 and with no
part of the Exponent area in the Crypto RAM
PUKCL(u2Options) = PUKCL_EXPMOD_FASTRSA | PUKCL_EXPMOD_WINDOWSIZE_1;
• Operation:CRT using the Regular Modular Exponentiation with the window size equal to 2 and with
one part the Exponent area in the Crypto RAM
PUKCL(u2Options) = PUKCL_EXPMOD_REGULARRSA | PUKCL_EXPMOD_WINDOWSIZE_2 |
PUKCL_EXPMOD_EXPINPUKCCRAM;
For this service, two exclusive Calculus Modes for the Modular Exponentiation steps of the CRT are
possible. The following table describes the Calculus Mode Options.
Table 43-63. CRT Service Calculus Mode Options
Option Explanation
PUKCL_EXPMOD_FASTRSA Perform a Fast computation.
SAM D5x/E5x Family Data Sheet
Public Key Cryptography Controller (PUKCC)
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 1503