Editing My 3GPP 33.501 notes (section)

=== C.3	Elliptic Curve Integrated Encryption Scheme (ECIES) ===

==== C.3.1 General ====
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Processing on UE side and home network side are described in high level in clauses C.3.2 and C.3.3.

When the SUPI is of type IMSI, the subscription identifier part of the IMSI (i.e., MSIN) that is used to construct the scheme-input shall be coded as hexadecimal digits using packed BCD coding where the order of digits within an octet is same as the order of MSIN digits specified in Figure 9.11.3.4.3a of TS 24.501. If the MSIN is composed of an odd number of digits, then the bits 5 to 8 of final octet shall be coded as "1111".

When the SUPI is of type network specific identifier, the subscription identifier part of the SUPI that is used to construct the scheme-input shall follow the encoding rules specified in Annex B.2.1.2 of TS 33.220.
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See spec for details.