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G. Avoine et al.
spread type for contactless payments. In their work, Chothia et al. also introduced a
countermeasure to mitigate their own relay attack. Their so-called PaySafe protocol
is put forward as a slight variant of the contactless version of PayWave, i.e., the
EMV protocol used by Visa. In PaySafe, a new command is introduced into the
EMV contactless protocol such that a calculation of the round trip times becomes
possible for EMV readers. Namely, the reader sends a nonce to the card and expects
that the latter will respond with a pre-generated nonce; the reader measures the
time taken by the whole exchange and if it is beyond a pre-established bound, then
the reader aborts the protocol. In PaySafe, the nonces used in this timed phase are
encompassed in some other messages, included in a MAC issued by the card and
keyed on a key the card shares only with the bank.
It is worth noting that PaySafe did not aim to be a full distance-bounding protocol
(i.e., it did not mean to protect against the distance-bounding frauds presented in
Sect. 7.4)
EMVCo—is the consortium behind EMV—give the EMV contactless payments’
specifications in [199] (current version is 2.7, April 2018). Since 2016, these
specifications include the possibility for a relay-resistance mechanism, which is
inspired by PaySafe [141]. A friendly introduction to this protocol is provided
in [563]. As of today, there are unfortunately no public figures about the number
of MasterCard/Visa readers that benefit from this feature.
7.6 Current Challenges in Distance Bounding
7.6.1 Theory vs. Practice
Provable-security/formal-methods models for DB (see Sect. 7.4) generally do not
capture accurately the DB threats shown in practice. For instance, one major
assumption that most DB formal models make is that the computation on the
prover’s side, during the timed exchanges, is instantaneous or constant. In practice,
as [141] showed, different cards have significantly distinct response-times, leading
to practical attacks which cannot be easily found via theoretical tools.
Besides such coarse abstractions, other approximations are made by provablesecurity models for cryptographic-proofs to become possible. For instance, in some
variants of the model in [193], no communication is allowed between colluding
attacking parties during the timed phase (i.e., the coalition has to be active outside
the timed phase). Or, in the formalism in [110], the time taken to compute over
bits equal to 0 is always considered the same as that to compute over bits equal
to 1, which—as Sect. 7.2 explained—is not always factually true. These two
approximations entail that the respective models are too weak. But also there is
the possibility that some formal security definition is too strong, i.e., that it would
classify a protocol as insecure when in practice the protocol is secure (see [216]).
G. Avoine et al.
spread type for contactless payments. In their work, Chothia et al. also introduced a
countermeasure to mitigate their own relay attack. Their so-called PaySafe protocol
is put forward as a slight variant of the contactless version of PayWave, i.e., the
EMV protocol used by Visa. In PaySafe, a new command is introduced into the
EMV contactless protocol such that a calculation of the round trip times becomes
possible for EMV readers. Namely, the reader sends a nonce to the card and expects
that the latter will respond with a pre-generated nonce; the reader measures the
time taken by the whole exchange and if it is beyond a pre-established bound, then
the reader aborts the protocol. In PaySafe, the nonces used in this timed phase are
encompassed in some other messages, included in a MAC issued by the card and
keyed on a key the card shares only with the bank.
It is worth noting that PaySafe did not aim to be a full distance-bounding protocol
(i.e., it did not mean to protect against the distance-bounding frauds presented in
Sect. 7.4)
EMVCo—is the consortium behind EMV—give the EMV contactless payments’
specifications in [199] (current version is 2.7, April 2018). Since 2016, these
specifications include the possibility for a relay-resistance mechanism, which is
inspired by PaySafe [141]. A friendly introduction to this protocol is provided
in [563]. As of today, there are unfortunately no public figures about the number
of MasterCard/Visa readers that benefit from this feature.
7.6 Current Challenges in Distance Bounding
7.6.1 Theory vs. Practice
Provable-security/formal-methods models for DB (see Sect. 7.4) generally do not
capture accurately the DB threats shown in practice. For instance, one major
assumption that most DB formal models make is that the computation on the
prover’s side, during the timed exchanges, is instantaneous or constant. In practice,
as [141] showed, different cards have significantly distinct response-times, leading
to practical attacks which cannot be easily found via theoretical tools.
Besides such coarse abstractions, other approximations are made by provablesecurity models for cryptographic-proofs to become possible. For instance, in some
variants of the model in [193], no communication is allowed between colluding
attacking parties during the timed phase (i.e., the coalition has to be active outside
the timed phase). Or, in the formalism in [110], the time taken to compute over
bits equal to 0 is always considered the same as that to compute over bits equal
to 1, which—as Sect. 7.2 explained—is not always factually true. These two
approximations entail that the respective models are too weak. But also there is
the possibility that some formal security definition is too strong, i.e., that it would
classify a protocol as insecure when in practice the protocol is secure (see [216]).