Wire the 5G Core
into your head.

In 2G/3G/4G the permanent subscriber identity (IMSI) was sent over the air in the clear whenever the network had no temporary identity for you — which a passive IMSI-catcher ("Stingray") could sniff to identify and track a phone.

5G closes that hole. The permanent identity — the SUPI — is never transmitted in clear over the radio. The UE always sends a SUCI (concealed SUPI) at first contact, or a temporary 5G-GUTI once it has one. "User identity confidentiality" is a named 5G security feature. TS 23.501 §5.10.1 · TS 33.501 §6.12

The SUPI (Subscription Permanent Identifier) lives in the UDM/UDR and identifies the subscription globally. It comes in two shapes §5.9.2 · TS 33.501 §6.12.1:

• IMSI-based — the classic MCC (3) + MNC (2–3) + MSIN. The MSIN is the part that actually names you; MCC+MNC name your home operator.
• NAI-based — a network-specific identifier username@realm (RFC 7542), used for SNPN / non-3GPP / IoT.

The SUPI may also carry the NSI and is provisioned in the USIM's home network. It never leaves the home network in clear — the visited network only ever learns it after the home UDM de-conceals it.

The SUCI (Subscription Concealed Identifier) is "a privacy-preserving identifier containing the concealed SUPI" §5.9.2a. For an IMSI-based SUPI it is built from these fields TS 24.501 §9.11.3.4:

• SUPI Type (3 bits) — 000 IMSI · 001 Network-Specific Identifier · 010 GCI · 011 GLI.
• MCC + MNC — the home PLMN, BCD-coded, in the clear (the routing needs it).
• Routing Indicator — 1–4 BCD digits, assigned by the home operator; 0000+padding if none is configured.
• Protection Scheme Id (4 bits) — 0000 Null · 0001 ECIES Profile A · 0010 ECIES Profile B · 1100–1111 operator-specific.
• Home Network Public Key Id (1 octet) — which home public key concealed this SUCI (0–254); 0 for the null scheme — lets the operator rotate keys.
• Scheme Output — the only encrypted field: for Null it is the MSIN in clear (BCD); for ECIES it is hex = ephemeral public key ‖ ciphertext ‖ MAC.

Concealment uses the Elliptic Curve Integrated Encryption Scheme (ECIES) with the operator's home-network public key provisioned in the USIM. The private key never leaves the home network, so only the home network can reverse it. UE-side steps TS 33.501 Annex C.3.2:

1. Generate a fresh ephemeral ECC key pair (new for every concealment → two SUCIs of the same SUPI look unrelated).
2. ECDH: ephemeral private key × home public key → a shared secret.
3. KDF (ANSI-X9.63, SHA-256) derives an encryption key, an initial counter block and a MAC key.
4. AES-128-CTR encrypts the plaintext block (the MSIN / username) → ciphertext.
5. HMAC-SHA-256 over the ciphertext → truncated MAC tag.
6. Scheme Output = ephemeral public key ‖ ciphertext ‖ MAC tag.

Because the ephemeral key is random each time, the SUCI is non-linkable across registrations — a tracker can't tell two SUCIs belong to one subscriber.

3GPP standardises two ECIES profiles; an operator's key belongs to exactly one TS 33.501 Annex C.3.4:

• Profile A — Curve Curve25519 / X25519 (RFC 7748); 32-byte ephemeral public key; ANSI-X9.63-KDF (SHA-256), AES-128-CTR, HMAC-SHA-256. C.3.4.1
• Profile B — Curve secp256r1 (NIST P-256); 33-byte compressed ephemeral public key; same KDF / AES-CTR / HMAC family. C.3.4.2

Both give the same privacy guarantee; the choice is an operator/regulator preference. The Null scheme (Protection Scheme = 0000) sends the MSIN in clear and is only allowed for unauthenticated emergency calls or where a regulator forbids concealment.

De-concealment happens at exactly one place: the SIDF (Subscription Identifier De-concealing Function), a service of the UDM that holds the home-network private key TS 33.501 §6.12.5. Home-side steps C.3.3:

• The AUSF passes the SUCI to the UDM during authentication (Nudm_UEAuthentication_Get).
• The SIDF does ECDH with its private key × the UE's ephemeral public key → the same shared secret → the same keys.
• It verifies the MAC, AES-CTR-decrypts the ciphertext, recovers the MSIN → the full SUPI, and returns it with the authentication vector.

So the serving/visited network runs the whole authentication without ever seeing the SUPI until the home network chooses to reveal it.

A large operator runs many UDM/AUSF instances, each holding a slice of the subscriber base. The Routing Indicator (carried in the SUCI, assigned by the home operator, stored on the USIM) is the key that lets the network route signalling to the specific AUSF/UDM group that can de-conceal and authenticate this subscriber — via NRF discovery, using Routing Indicator + Home Network Id. TS 23.003 · TS 29.510

Default value 0000 means "no specific routing" — discovery falls back to any suitable UDM. This is why the Routing Indicator stays in the clear inside the SUCI.

Once you're registered, the AMF hands you a 5G-GUTI and you stop sending the SUCI. Verbatim structures TS 23.501 §5.9.4:

• <5G-GUTI> := <GUAMI> <5G-TMSI>
• <GUAMI> := <MCC> <MNC> <AMF Region ID (8b)> <AMF Set ID (10b)> <AMF Pointer (6b)> — routes N2 signalling to the serving AMF.
• <5G-S-TMSI> := <AMF Set ID> <AMF Pointer> <5G-TMSI (32b)> — the shortened form used for paging & Service Request on the radio.

Two spec-exact details worth knowing: the AMF may re-assign a new 5G-GUTI at any time (and does so to defeat tracking — the temporary-identity equivalent of the ephemeral SUCI), and the NG-RAN uses the 10 least-significant bits of the 5G-TMSI to compute paging occasions, so "the AMF shall ensure that the 10 LSBs are evenly distributed." §5.9.4 · TS 33.501 §6.12.3

• PEI (Permanent Equipment Identifier) — identifies the device, not the subscription: an IMEI / IMEISV for any UE with a 3GPP radio (EUI-64 for pure non-3GPP). The AMF checks it against the 5G-EIR over N17 to block stolen/blacklisted handsets. §5.9.3
• GPSI (Generic Public Subscription Identifier) — a public handle for the subscription outside the 3GPP domain: an MSISDN (the phone number) or an External Identifier. The system stores the GPSI↔SUPI association — but "there is no implied 1-to-1 relationship." Used by the NEF/AF for exposure. §5.9.8