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select_best_cipher(): Prefer A5/1 over A5/2 

We cannot simply use the highest 'x' in A5/x codecs.

For A5/7 through A5/3, larger 'x' means better.
But: A5/1 is better than A5/2, so we need to prefer the former
over the latter.

Change-Id: I399fff8d07d1bfcbc6b385e90914ff6d9e00eb73
Closes: OS#4975
This commit is contained in:
Harald Welte 2021-01-25 17:59:06 +01:00 committed by laforge
parent 6d32c92e4f
commit 5e01039d79
1 changed files with 7 additions and 3 deletions
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src/osmo-bsc/osmo_bsc_bssap.c
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@ -402,12 +402,16 @@ int bsc_paging_start(struct bsc_paging_params *params)
/* select the best cipher permitted by the intersection of both masks */
static int select_best_cipher(uint8_t msc_mask, uint8_t bsc_mask)
{
/* A5/7 ... A5/3: We assume higher is better,
* but: A5/1 is better than A5/2, which is better than A5/0 */
const uint8_t codec_strength[8] = { 7, 6, 5, 4, 3, 1, 2, 0 };
uint8_t intersection = msc_mask & bsc_mask;
int i;
for (i = 7; i >= 0; i--) {
if (intersection & (1 << i))
return i;
for (i = 0; i < ARRAY_SIZE(codec_strength); i++) {
uint8_t codec = codec_strength[i];
if (intersection & (1 << codec))
return codec;
}
return -1;
}