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[crypto] Harden RSA modexp subroutine. #25694

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Dec 19, 2024
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[crypto] Harden RSA modexp subroutine. #25694

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16 changes: 11 additions & 5 deletions sw/otbn/crypto/modexp.s
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Original file line number Diff line number Diff line change
Expand Up @@ -28,23 +28,29 @@
* @param[in] x11: pointer to temp reg, must be set to 2
*
* clobbered registers: x8, x21, w0, w2
* clobbered Flag Groups: none
* clobbered Flag Groups: FG1
*/
sel_sqr_or_sqrmul:
/* iterate over all limbs */
loop x30, 4
loop x30, 8
/* load limb from dmem */
bn.lid x9, 0(x21)

/* load limb from regfile buffer */
bn.movr x11, x8++

/* conditional select: w0 = FG0.C?w[x8+i]:dmem[x21+i] */
bn.sel w0, w2, w3, C
/* randomize dmem with value from URND (one extra dummy call to clear) */
bn.addi w31, w31, 0, FG1 /* dummy */
bn.wsrr w0, URND
bn.sid x0, 0(x21)

/* store selected limb to dmem */
/* select a limb and store to dmem */
bn.sel w0, w2, w3, FG0.C
bn.sid x0, 0(x21++)

/* dummy call to clear */
bn.addi w31, w31, 0, FG1

ret


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