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Patch set 1:Code-Review +2

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• File src/osmo-bsc/gsm_04_08_rr.c:

  • Patch Set #1, 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.

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