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EAP-SIM/EAP-AKA and EAP-AKA' are EAP methods that allow a supplicant to gain access to a resource by using a SIM (Subscriber Identity Module) card. These EAP methods are usually deployed by MNOs (Mobile Network Operators), where the MNO or a partner or the MNO also operate a large scale WiFi network, and the MNO wishes to offload subscribers onto this WiFi network where available.
Interest in EAP-SIM/EAP-AKA/EAP-AKA' has been increasing in recent years, as with the standardization of "WiFi calling" and the release of the HotSpot 2.0 standards, it has become significantly easier to enable WiFi offload in a way that is seamless to the end user.
Authentication vectors are sets of cryptographically generated keys and challenges. The authentication vectors themselves do not get sent over the wire, only the RAND (Random challenge) component of the vectors is sent between parties.
The authentication vector is the output of an algorithm running on an AuC (Authentication Centre), and on the SIM of the subscriber. The AuC and SIM have knowledge of a secret subscriber key (Ki) and in the case of USIMs a key unique to the Operator and SIM (OPc).
The fact that the AuC and SIM have knowledge of these keys, but the keys are never sent over the wire or revealed in some other manner, is what allows the supplicant to authenticate the AS, and the AS to authenticate the supplicant.
When a supplicant wishes to authenticate, it signals the AS with its IMSI (subscriber ID), and the AS queries an AuC for an authentication vector. These authentication vectors can be seen as temporary credentials, and should only be used once.
The type of authentication vector generated depends on the algorithm running on the AuC and SIM card.
The COMP128-1,2,3 algorithm sets output GSM triplets consisting of:
Note: Only the EAP-SIM method can function with GSM triplets, and even then each authentication attempt requires 2-3 sets of triplets as single triplet does not provide enough keying material.
The Milenage algorithm set outputs UMTS quintuplets consisting of:
UMTS quintuplets can be folded into GSM triplets, and so can be used with EAP-SIM, EAP-AKA and EAP-AKA[']. The algorithm used for this is known as COMP128-4.
EAP-SIM, EAP-AKA and EAP-AKA' are very similar. The TLV and packet format are virtually identical. The major differences are the Pseudo-Random Function (PRF) and Key Derivation Function (KDF) used to generate sessions keys, and the type of the authentication vectors used.
All the SIM base EAP-Methods function in a similar way. In each case, an AS (Authentication server - like FreeRADIUS), requests an authentication vector from an AuC with knowledge of a SIM's Ki. The AuC generates a random challenge (RAND), feeds it and the Ki into a vector generation algorithm (COMP128-, Milenage). The vector generation algorithm produces multiple keys, which are passed back to the AS along with the challenge.
The AS then runs a KDF (Key derivation function), which performs additional cryptographic operations on the keys received from the AuC. The purpose of this KDF is usually to increase the key size, to modify the keys in such a way that they can't be reused, and in the case of AKA' to bind the use of the keys to a particular network.
After the KDF has run, the AS will generate an EAP-[SIM|AKA] Challenge packet, encapsulate it in an EAP-Request, and send it to the supplicant. The contents of the challenge packet vary, but it always contains one or more random challenges (
AT_RAND), and a HMAC (
AT_MAC). The HMAC is the digest of the contents of the entire EAP-Request (and in the case of SIM some extra data), with the
AT_MAC attribute zeroed, and keyed with
K_aut, an authentication key output from the KDF.
When the supplicant receives the Challenge packet, it feeds RAND into the same algorithm as used on the AuC, and then feeds to results of that algorithm into the method specific KDF. The supplicant uses the
K_aut value from the KDF it ran locally to generate its own HMAC digest and compares that to the one received from the AS. If the MACs match, it proves to the supplicant that the challenge packet was not tampered with and that the AS has an identical authentication vector to the one generated locally.
The supplicant then constructs its challenge response, encapsulates it in an EAP-Response, and generates another HMAC over the packet, the supplicant then sends the challenge response to the AS.
The AS validates the response and sends an EAP-Success/EAP-Failure depending on the outcome of the validation.
EAP-SIM, EAP-AKA, EAP-AKA', methods also generate keying material used to encrypt the network session. With RADIUS/802.1X these keys are distributed to the NAS by the AS in the RADIUS MPPE Key attributes sent with the EAP-Success, and derived by the supplicant locally.
In terms of security (key sizes, the strength of authentication, MITM attack prevention), EAP-SIM is the weakest, and EAP-AKA' is the strongest.
EAP-SIM should only be used with legacy 2G AuC that can only generate GSM authentication vectors. EAP-AKA should only be used where the supplicant does not support EAP-AKA'.
In practical terms, the best ways to develop an EAP-SIM/EAP-AKA['] service and perform testing, is to get eapol_test working with a smart card reader or to use eapol_test's virtual usim functionality. This lets you run the entire EAP-SIM/EAP-AKA protocol against a smart card, with no phones or access points needed.
If you're set on using physical hardware or have particular handsets you want to use for testing you may want to consider purchasing a sysmocom sim, which allows you to set the Ki, and algorithms used. The SCM SCR3310 CCID reader and the mechanical adapter sysmocom sell both work very nicely out of the box (at least on OSX).
Smart card interfaces are remarkably well standardized. The PS/CS (Personal Computer/Smart Card) API is present on Windows by default and bundled OSX (as a framework). On Linux, PSCS lite can be installed to provide the API.
This API should provide an abstraction over all PS/CS compatible devices. eapol_test in fact, just links to a PS/CS library and calls a single initialization function, for all Smart Card readers, on all operating systems. There's very little OS-specific boilerplate.
In addition to a C interface, there's also
psyscard, which is a python wrapper around the PSCS API. Its used by the project osmocom utilities and would provide a good basis for building your own SIM utilities.
Despite being able to theoretically use EAP-SIM with any SIM card in a local environment, it's not recommended.
Yes you COULD extract multiple authentication vectors from a SIM as some sort of onboarding process, and you COULD store those vectors, and then authenticate the SIM card locally on your wireless network. But that's going to be quite a small pool of vectors, and you really shouldn't be repeating vectors as it can compromise the protocol.
If you're intent on using EAP-SIM/EAP-AKA to provide local authentication, you should get some programmable SIM cards, for which you know (or can set) the Ki, and distribute those to your users. On the GSM/UMTS side, they'll be useless (unless you also happen to be running a cell service), but it's conceivable that you could put them in company iPads (if the user doesn't want to use the 3G/4G radio).
Using EAP-SIM/EAP-AKA with an HLR either via M3UA/SUA (Sigtran) or Diameter is the way to go if you want to perform wireless handoff. In that scenario instead of storing vectors, you call out to the HLR with the SIM's IMSI number, the HLR passes this to the AuC, which generates authentication vectors, which it passes back to the HLR, which passes them back to the AAA server.
But that is outside the scope of this page (for now).
Should work out of the box
Last edited by Arran Cudbard-Bell (arr2036), 2017-12-13 14:33:24
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