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Improve comment justifying a hard-coded limitation

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
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Manuel Pégourié-Gonnard 2020-06-17 12:57:33 +02:00
parent a90a95bcbd
commit 7d7c00412f
1 changed files with 13 additions and 2 deletions
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15
library/ecp.c
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@ -327,8 +327,19 @@ static int ecp_drbg_random( void *p_rng, unsigned char *output, size_t output_le
{ {
uint8_t use_len; uint8_t use_len;
/* We don't need to draw more that 255 blocks, so don't bother with /* This function is only called for coordinate randomisation, which
* carry propagation and just return an error instead. */ * happens only twice in a scalar multiplication. Each time needs a
* random value in the range [2, p-1], and gets it by drawing len(p)
* bytes from this function, and retrying up to 10 times if unlucky.
*
* So for the largest curve, each scalar multiplication draws at most
* 2 * 66 bytes. The minimum block size is 20 bytes (with SHA-1), so
* that means at most 66 blocks.
*
* Since we don't need to draw more that 255 blocks, don't bother
* with carry propagation and just return an error instead. We can
* change that it we even need to draw more blinding values.
*/
ctx->buf[3] += 1; ctx->buf[3] += 1;
if( ctx->buf[3] == 0 ) if( ctx->buf[3] == 0 )
return( MBEDTLS_ERR_ECP_RANDOM_FAILED ); return( MBEDTLS_ERR_ECP_RANDOM_FAILED );