5G SUPI/SUCI Identity Concealment

Attach Mode

Attach rate 1.0 / s

Live Readouts

Attaches sent

Identity exposed

Captured by rogue BS

ECDH curve

P-256

SUPI (never sent in the open on 5G)imsi-234150999999999

Over the air—

How it works

A subscriber's permanent identity, the SUPI (MCC+MNC+MSIN, the successor to the IMSI), must reach the home network once to attach — but a passive listener between the phone and the tower can be anyone with cheap SDR hardware, i.e. an IMSI-catcher / rogue base station. Pre-5G radio access sent the identity in the clear on this first message, so a catcher could log it instantly and track the phone by that fixed number forever after.

5G fixes this with SUCI (Subscription Concealed Identifier), built with a real ECIES scheme run in this page via the browser's SubtleCrypto (ECDH on curve P-256, standing in for 3GPP's Profile A/B Curve25519/secp256r1):

1. UE generates a fresh ephemeral key pair (e_priv, e_pub) — new every attach
2. shared = ECDH(e_priv, homeNetwork_pub)      // never transmitted
3. key    = SHA-256(shared)                     // KDF stand-in for 3GPP Annex C
4. SUCI   = e_pub || AES-256-GCM(key, SUPI)     // ciphertext + 128-bit auth tag
5. only the home network's SIDF, holding the matching private key,
   can redo step 2 and decrypt back to the SUPI

Mode — 5G runs the ECIES pipeline above; 4G plaintext sends the raw SUPI string as the identity, matching the legacy protocol it stands in for.
Rogue base station — a listener the phone cannot distinguish from a real tower before authentication completes. It intercepts every packet, but under 5G it only ever holds ciphertext: it lacks the home network's private key, so it cannot run step 2.
New ephemeral key pair — forces a fresh (e_priv, e_pub), which also changes the SUCI ciphertext even for the same SUPI — this per-attach freshness is what prevents a passive listener from correlating two attaches to the same subscriber.
Real deployments add a null-scheme fallback for emergency calls and pre-provision the home network's public key on the SIM/eSIM profile — both are simplified away here to keep the mechanic legible.

UE (phone) Real gNB / core Rogue base station

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