Practical Cryptographic Civil GPS Signal Authentication

Abstract

A practical technique is proposed to authenticate civil GPS signals. The technique combines cryptographic authentication of the GPS navigation message with signal timing authentication based on statistical hypothesis tests to secure civil GPS receivers against spoofing attacks. The notion of GNSS signal authentication is defined in probabilistic terms. Candidate GPS signal authentication schemes are evaluated in terms of effectiveness and practicality leading to a proposal for incorporating digital signatures into the extensible GPS civil navigation (CNAV) message. The proposal is sufficiently detailed to facilitate near‐term implementation of security‐hardened civil GPS. Copyright © 2012 Institute of Navigation.

A blueprint for civil GPS navigation message authentication

“…Thus, civil GPS NMA requires a cryptographic method with asymmetric properties. Two such techniques-ECDSA and TESLAhave emerged in literature as candidate solutions [12]- [14]. According to the U.S. National Institute of Standards and Technology (NIST) guidelines, cryptographic authentication methods are secure beyond 2030 only if their equivalent symmetric-key strength b s is at least 128 bits [17].…”

“…Signal authentication confirms that the underlying GNSS signal is authentic, according to an operational definition in [14]. NMA-enabled receivers can implement a SCER detector and other elements of a signal authentication procedure, provided that the receiver possesses a sufficiently accurate time estimate (e.g., µs-level) [14].…”

“…If periodic digital signatures are inserted in the broadcast navigation data, an NMA-capable receiver can authenticate the received signal upon reception of both the signed data and the signature [14]. An authentication event (AE) occurs at the earliest time that the verification routine can complete.…”

“…More surprisingly, previous work demonstrated that, by modulating unpredictable yet verifiable data onto GNSS signals, NMA allows a receiver to detect a broad category of GNSS spoofing attacks known as security code estimation and replay (SCER) attacks [15]. Signal authentication confirms that the underlying GNSS signal is authentic, according to an operational definition in [14]. NMA-enabled receivers can implement a SCER detector and other elements of a signal authentication procedure, provided that the receiver possesses a sufficiently accurate time estimate (e.g., µs-level) [14].…”

“…These changes are achievable in modernized GPS via the flexible and extensible civil navigation (CNAV) messaging format. Reference [14] argues that the most practical and effective signature generation method for NMA is a public-key authentication protocol known as the Elliptic Curve Digital Signature Algorithm (ECDSA). An alternative to ECDSA, Timed Efficient Stream Loss-Tolerant Authentication (TESLA), is recommended in studies on NMA for space-based augmentation systems and eLORAN [12], [13].…”