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Correct some statements about the ordering of A and B

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2021-06-21 18:58:39 +02:00
parent b798b35374
commit 37d690c45b
1 changed files with 9 additions and 6 deletions
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15
library/bignum.c
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@ -2428,12 +2428,15 @@ int mbedtls_mpi_gcd( mbedtls_mpi *G, const mbedtls_mpi *A, const mbedtls_mpi *B
* minor differences:
* - Sequences of multiplications or divisions by 2 are grouped into a
* single shift operation.
* - The procedure in HAC assumes that 0 < A <= B.
* - The condition A <= B is not actually necessary for correctness;
* the first round through the loop results in TA < TB.
* - If A = 0, the loop goes through 0 iterations and the result is
* correctly B.
* - The case B=0 was short-circuited above.
* - The procedure in HAC assumes that 0 < TB <= TA.
* - The condition TB <= TA is not actually necessary for correctness.
* TA and TB have symmetric roles except for the loop termination
* condition, and the shifts at the beginning of the loop body
* remove any significance from the ordering of TA vs TB before
* the shifts.
* - If TA = 0, the loop goes through 0 iterations and the result is
* correctly TB.
* - The case TB = 0 was short-circuited above.
*
* For the correctness proof below, decompose the original values of
* A and B as