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[crypto] Fix conditional subtraction in p384_key_from_seed #28558

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[crypto] Fix conditional subtraction in p384_key_from_seed #28558

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9 changes: 5 additions & 4 deletions sw/otbn/crypto/p384_keygen_from_seed.s
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Original file line number Diff line number Diff line change
@@ -1,3 +1,7 @@
/* Copyright zeroRISC Inc. */
/* Licensed under the Apache License, Version 2.0, see LICENSE for details. */
/* SPDX-License-Identifier: Apache-2.0 */

/* Copyright lowRISC contributors (OpenTitan project). */
/* Licensed under the Apache License, Version 2.0, see LICENSE for details. */
/* SPDX-License-Identifier: Apache-2.0 */
Expand Down Expand Up @@ -98,17 +102,14 @@ p384_key_from_seed:
bn.sub w24, w10, w16
bn.subb w25, w11, w17

/* Clear flags. */
bn.sub w31, w31, w31

/* Compute d1. Because 2^384 < 2 * n, a conditional subtraction is
sufficient to reduce. Similarly to the carry bit, the conditional bit here
is not very sensitive because the shares are large relative to n.
[w6,w5] <= x1 mod n = d1 */
bn.sel w5, w10, w24, FG0.C
bn.sel w6, w11, w25, FG0.C

/* Clear w25 before over writing it with a different share. */
/* Clear w25 before over writing it with a different share and clear flags. */
bn.xor w25, w25, w25

/* Dummy instruction to avoid consecutive share access. */
Expand Down