Attention is currently required from: fixeria. neels has posted comments on this change. ( https://gerrit.osmocom.org/c/osmo-bsc/+/27507 ) Change subject: gsm48_make_ho_cmd(): optionally add Cipher Mode Setting IE ...................................................................... Patch Set 1: Code-Review+2 (1 comment) File src/osmo-bsc/gsm_04_08_rr.c: https://gerrit.osmocom.org/c/osmo-bsc/+/27507/comment/7dba7aeb_6fc14bf1 PS1, Line 585: (new_lchan->encr.alg_id - 2) << 1 This looked wrong at first but i figured it out. I think it would be good to explain this, maybe put it in a separate function. Explanation: looking at 44.018 10.5.2.9: if n is the A5/n number, then from the spec i get to this formula: (n - 1) << 1 However, encr.alg_id is (for historical reasons) stored as what seems to be n + 1, which makes this formula correct. Some comments spread around osmo-msc.git explain as: /*! alg_id is in encoded format: * alg_id == 1 means A5/0 i.e. no encryption, alg_id == 4 means A5/3. * alg_id == 0 means no such IE was present. */ The "encoded format" is in fact the Algorithm Identifier as in 3GPP TS 48.008 3.2.2.44 Chosen Encryption Algorithm. If I were to write this from scratch, I'd have rather stored the plain A5/n number in encr.alg_id (as encr.alg_a5). Since we have alg_id around everywhere already, I think it would be nice to have a macro like ALG_A5_NR_TO_RSL() or a seprate function to explicitly show the intended conversion. can also follow later, we'd like to get this fix out for testing. -- To view, visit https://gerrit.osmocom.org/c/osmo-bsc/+/27507 To unsubscribe, or for help writing mail filters, visit https://gerrit.osmocom.org/settings Gerrit-Project: osmo-bsc Gerrit-Branch: master Gerrit-Change-Id: I1d270e82d0a9b12897fc94dae4e8999aa132a22f Gerrit-Change-Number: 27507 Gerrit-PatchSet: 1 Gerrit-Owner: fixeria &lt;vyanitskiy(a)sysmocom.de&gt; Gerrit-Reviewer: Jenkins Builder Gerrit-Reviewer: laforge &lt;laforge(a)osmocom.org&gt; Gerrit-Reviewer: neels &lt;nhofmeyr(a)sysmocom.de&gt; Gerrit-Reviewer: pespin &lt;pespin(a)sysmocom.de&gt; Gerrit-Attention: fixeria &lt;vyanitskiy(a)sysmocom.de&gt; Gerrit-Comment-Date: Mon, 14 Mar 2022 10:53:45 +0000 Gerrit-HasComments: Yes Gerrit-Has-Labels: Yes Gerrit-MessageType: comment