According to a report by The Intercept on 2015/2/19 local time [1], the NSA and GCHQ had stolen large quantities of keys (Ki) stored on SIMs shipped by Gemalto. As a result, intercepting mobile-phone communications using these SIMs had become trivial. This information was reportedly contained in the files taken by Mr. Snowden.
The SIM card in a mobile phone has an Authentication Key (Ki) written into it by a “personalization company” such as Gemalto. This Authentication Key is used to authenticate the SIM on the network and to generate encryption keys. After it is generated, the key is recorded on the SIM card and made impossible to extract. However, to authenticate the SIM card on the network, the mobile carrier must possess the same key, so it is also sent to the carrier. The problem was how it was sent. SIM cards are personalized in large quantities and delivered to the carrier. At that time, the Authentication Keys written to them were reportedly bundled together in large quantities and sent by FTP or email. Moreover, only weak encryption was used, and in some cases they were sent in plaintext.
GCHQ and the NSA appear to have entered Gemalto’s internal network through the email of Gemalto employees, identified who was performing this critical work, intercepted that person’s communications, and extracted the bulk key-transmission files from them.
I am not very familiar with this field and had no idea that things worked this way. I had naturally assumed that a key pair was generated inside the chip and its Public Key sent to the carrier. I never imagined they were using symmetric keys…
Yesterday I attended a Real World Crypto report meeting. Someone from the floor commented that “rather than cracking cryptographic primitives, it is much easier to crack something elsewhere, so whether the cryptographic primitives themselves are weak is not very relevant.” This is a textbook example of exactly that.
Lessons
- (1) Stop transmitting long-term secret keys (postal mail is no good either)
- (2) Do not let more than 2 person hold a secret key over the long term
- (3) Manage keys properly
I suppose.
[1] https://firstlook.org/theintercept/2015/02/19/great-sim-heist/
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