Smart-ID · e-ID mobile eID
02 · SplitKey technology

Two halves. One standard RSA signature.

SplitKey was built and patented by Estonia's Cybernetica. It is a modified 2-of-2 threshold RSA: the private key never exists whole in one place, yet the result is an ordinary RSA signature, so any standard library can verify it.

Diagram

How a signature is made

Phone
Key share 1Derived from the PIN + a random value. Only on this phone.
PIN entryThe PIN never leaves the phone. A wrong PIN produces a random-looking share.
partial signature
+ one-time value(clone detection)
Server
Key share 2Inside an HSM / encrypted by an HSM. Cannot sign on its own.
Combine and verifyCounts wrong PINs and locks after too many. Result: a standard RSA signature.
Security properties

What it protects against

If the phone is stolen

The phone's share alone cannot produce a signature. The PIN cannot be guessed offline — every wrong PIN is counted on the server.

If the app is cloned

Each signature carries a one-time value from the previous one. If a stale value arrives, the server detects the clone and deactivates the account.

If the server is breached

The server's share alone cannot produce a signature. Smart-ID's server side is certified to Common Criteria EAL4+ (AVA_VAN.5).

Founding paper: Buldas, Kalu, Laud, Oruaas, “Server-Supported RSA Signatures for Mobile Devices”, ESORICS 2017. The production key size is not publicly disclosed.

PINs and codes

What the user sees

Smart-IDe-ID Mongolia
PIN1 (login)4–12 digits, user-chosen4 digits
PIN2 (signature)5–12 digits6 digits
Verification code4 digits (in push flow)5 digits
Lockout3 wrong → temporary, then permanent if it continuesServer counts and locks (threshold not published)
Registration

Ways to sign up

  • Bank branch (registration authority)
  • National ID card — with a reader, or from 2026 via the phone's NFC
  • Mobile-ID or an existing Smart-ID
  • Biometric: passport/card NFC chip + selfie (iProov, Inverid)
  • Bank login — Smart-ID Basic only (Latvia, Lithuania)

Certificate levels

ADVANCED, QUALIFIED (default), QSCD. Basic registration works for banking only; Qualified covers all services and QES signatures.