R 2 Mod N Calculation - Motorola DigitalDNA MPC180E User Manual

Miscellaneous Routines
2
7.5.2 R mod N Calculation
The PKEU has the capability to calculate R
of digits of the modulus vector (Modsize+1, where Modsize is specified independently).
This function is used to assist in placing operands into the Montgomery residue system.
When possible, this value should be pre-computed. If this value is not available, then the
host processor may invoke this function to determine the value before the operation. This
function takes a non-trivial amount of time (see Table 7-26) so if at all possible, this value
should be stored for future use.
Note that this operation primarily exists to support RSA operations since R
not always be known prior to the execution of certain protocols. For ECC applications, the
modulus is a system-wide parameter, which means that the R
pre-computed before any real-time operations by any other system entity and stored for
future use. For this reason, R
the control register bit F
This function operates with a minimum of 4 digits (Modsize = 3) and with the most
significant digit (16-bits) of the modulus being non-zero. The complete set of I/O
conditions is shown below:
2
Computation R mod N, where R = 2
Entry name r2
Entry address 0x00c(r2)
Pre-conditions Modsize = number of digits of the modulus vector - 1
N0-3 = modulus
2
Post-conditions B1 = R mod N
N0-3 = modulus
Unless explicitly noted, all other registers are not guaranteed to be any particular value.
Special —
conditions
7-32
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
2
mod N, where R = 2
2
mod N only supports integer-modulo-n computations (i.e.
M must be 0).
2
Table 7-24. R
16D
and D is the number of digits of the modulus vector
MPC180E Security Processor User's Manual
16D
and D is the number
2
mod N value may be
2
mod N
2
R mod N
2
mod N may