Public key accelerator (PKA) RM0453
704/1454 RM0453 Rev 2
24.4.3 Modular addition
Modular addition operation consists in the computation of A + B mod n. Operation
instructions are summarized in Table 147.
24.4.4 Modular subtraction
Modular subtraction operation consists in the following computations:
• If A ≥ B result equals A - B mod n
• If A < B result equals A + n - B mod n
Operation instructions are summarized in Table 148.
24.4.5 Modular and Montgomery multiplication
To be more efficient when performing a sequence of multiplications the PKA accelerates
multiplication which has at least one input in the Montgomery domain. The two main uses of
this operation are:
• Map a value from natural domain to Montgomery domain and vice-versa
• Perform a modular multiplication A x B mod n
The method to perform above operations are described below. Note that “x” function is this
operation, and A, B, C operands are in the natural domain.
Table 147. Modular addition
Parameters with direction Value (Note) Storage Size
IN
MODE 0x0E PKA_CR 6 bits
Operand length (In bits, not null) RAM@0x404 32 bits
Operand A (0 ≤ A < n) RAM@0x8B4
ROS
Operand B (0 ≤ B < n) RAM@0xA44
Modulus value n (n < 2
3136
)RAM@0xD5C
OUT Result: A+B mod n (0 ≤ result < n) RAM@0xBD0
Table 148. Modular subtraction
Parameters with direction Value (Note) Storage Size
IN
MODE 0x0F PKA_CR 6 bits
Operand length (In bits, not null) RAM@0x404 32 bits
Operand A (0 ≤ A < n) RAM@0x8B4
ROS
Operand B (0 ≤ B < n) RAM@0xA44
Modulus value n (n < 2
3136
)RAM@0xD5C
OUT Result: A-B mod n (0 ≤ result < n) RAM@0xBD0
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