Change in osmo-bsc[master]: select_best_cipher(): Prefer A5/1 over A5/2

[laforge]

laforge has submitted this change. ( https://gerrit.osmocom.org/c/osmo-bsc/+/22432 )

Change subject: select_best_cipher(): Prefer A5/1 over A5/2
......................................................................

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
---
M src/osmo-bsc/osmo_bsc_bssap.c
1 file changed, 7 insertions(+), 3 deletions(-)

Approvals:
  fixeria: Looks good to me, approved
  pespin: Looks good to me, but someone else must approve
  Jenkins Builder: Verified



diff --git a/src/osmo-bsc/osmo_bsc_bssap.c b/src/osmo-bsc/osmo_bsc_bssap.c
index 63dee9e..9d78153 100644
--- a/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/src/osmo-bsc/osmo_bsc_bssap.c
@@ -402,12 +402,16 @@
 /* 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;
 }

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Gerrit-Project: osmo-bsc
Gerrit-Branch: master
Gerrit-Change-Id: I399fff8d07d1bfcbc6b385e90914ff6d9e00eb73
Gerrit-Change-Number: 22432
Gerrit-PatchSet: 2
Gerrit-Owner: laforge <laforge at osmocom.org>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: fixeria <vyanitskiy at sysmocom.de>
Gerrit-Reviewer: laforge <laforge at osmocom.org>
Gerrit-Reviewer: pespin <pespin at sysmocom.de>
Gerrit-MessageType: merged
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