Ki
Definition
The 128-bit secret authentication key stored on the SIM and in the operator's Authentication Centre (AuC). Never leaves either party in cleartext under normal use.
- Key length
- 128-bit
- Stored on
- SIM card and operator Authentication Centre (AuC)
- Function
- Root secret for GSM/UMTS authentication
- Transmission
- Never sent in cleartext under normal operation
Common questions
Why can't Ki be read directly from a seized SIM during examination?+
The SIM's security domain is designed to keep Ki inside the card and use it only internally to compute authentication responses, so standard forensic SIM readers extract call records, contacts and messages but cannot read Ki itself without specialised hardware attacks that are rarely feasible or lawful in casework.
What role does Ki play in verifying a subscriber's identity to the network?+
During authentication, the network sends a random challenge, and both the SIM and the AuC independently compute a response using Ki and the same algorithm, so a match confirms the SIM holds the genuine key without Ki ever leaving either party.
Related terms
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- ICCID
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- IMEI
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- IMSI
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- MSC
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