SAAR DRIMER AND STEVEN J. MURDOCH COMPUTER LABORATORY, UNIVERSITY OF CAMBRIDGE PUBLISHED: 16TH...
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Transcript of SAAR DRIMER AND STEVEN J. MURDOCH COMPUTER LABORATORY, UNIVERSITY OF CAMBRIDGE PUBLISHED: 16TH...
SAAR DRIMER AND STEVEN J. MURDOCHCOMPUTER LABORATORY, UNIVERSITY OF CAMBRIDGE
PUBLISHED: 16TH USENIX SECURITY SYMPOSIUM
Keep Your Enemies Close:Distance Bounding Against Smartcard Relay Attacks
Presentation by: Scott Conrad
Relay attacks
Cross between a man-in-the-middle and a replay attack
Chess master analogy: how to beat a Grandmaster at chess without knowing the rules to chess Challenge two Grandmasters, G1 and G2, to a game of
chess Forward any moves received from one Grandmaster to the
other G1 and G2 will be playing each other without realizing it In the end you will either beat one Grandmaster or tie
against both
Smart cards
Card with an integrated microchip Can store and process small amounts of data Uses non-volatile memory
http://www.amitbhawani.com/blog/what-are-smart-cards/ http://www.tiresias.org/about/publications/accessibility_visitors/chapter3.htm
Practical use of these attacks
Alice/Carol example Alice (victim) uses her smart card at a terminal in a
restaurant The terminal is actually fake and is connected to a laptop
instead of a bank Waiter Bob (culprit) signals his accomplice Carol
(culprit) that Alice is about to use the terminal Carol inserts her counterfeit card into a legitimate terminal
The data from Carol’s terminal is relayed to Alice’s card When Alice enters her PIN number, the fake terminal
records this and sends it to Carol
When the attack is over, Alice gets a free meal but paid for Carol’s purchase
Another attack
A thief discovers the PIN to a smart card and steals it Normally the card can either be used in the same
place it was stolen, or it can be shipped elsewhere If the card is mailed it drastically shrinks the time
window of when the card will be reported lost/stolen
With this attack, one thief can steal the card and take it to his own fake terminal Another thief in another country can then use his
counterfeit card as if it was the stolen card The time window is virtually unaffected
Why these attacks work
No way to detect a mismatch between the data displayed on the terminal and the data authorized by the card
Existing smartcard systems are tolerant to very high latencies A three second delay was introduced into
transactions, and the attack was still successful
Why these attacks work
Merchants generally do not handle smart cards Customers are often told to not to allow anyone but
themselves to handle the cards
Merchants rarely check the number on the card with the card number of the receipt
Merchants that sell products at a low-margin have little incentive to help prevent attacks The costs of an attack are almost always borne by the
customer and/or bank
EMV protocol
Named after Europay, Mastercard, and Visa
The primary protocol for debit and credit card payments in Europe
UK’s ‘chip & pin’ system
Countermeasures/Defenses
Tamper-resistant terminals
Imposing additional timing constraints
Hardware alterations
Distance bounding
Tamper resistance terminals
Terminals could be produced so that customers would be able to tell if they were tampered with Unrealistic in practice
Tamper-resistant seals have been shown that they can be trivially bypassed
As of May 2007 there were 304 VISA approved terminal designs from 88 vendors Consumers would have to be able to identify all of the
designs as well as the signs of tampering
This also assumes that criminals are unable to fabricate their own terminals
Addition timing restraints
Assuming signals propagate at the speed of light, a 3 second delay gives an attack a 450 000 km range
Terminals could be designed to ensure that a card responds to commands promptly
Problems with this: The terminal’s behavior is very predictable An attacker could preemptively request details from the
genuine card and then buffer them for the counterfeit card Over-clocking the genuine card by 1% can give around a
300 km distance
Hardware alterations
Use of an electronic attorney[2]
A trusted device brought by the consumer that acts as a “man-in-the-middle defense” The user enters the PIN into the attorney, safeguarding
the PIN The attorney then parses thought the data from the
terminal allowing the user to verify the charge
Why this is ineffective The increase complexity may keep banks from
approving such devices Attackers may discourage their use explicitly or by
making their use difficult
Hardware alterations
Integrating a smartcard into a mobile/cell phone Uses a wireless, customer-controlled device
Weaknesses Cell phones are targetable by malware The authors question whether wireless
communication in this manner is secure enough
Distance bounding protocols
It is possible for a terminal to determine the maximum distance a card is from it A signal’s round-trip-time between the terminal and
the smartcard is measured
This distance bounding is based on the Hancke-Kuhn protocol Was originally designed for ultra-wideband radio
(UWB)
Distance bounding protocols
Challenge-response The terminal sends out a challenge bit, and the card
immediately responds with a response bit The response bit must arrive within a certain time
The counterfeit card cannot randomly guess the response bit ahead of time A random guess has a 50% chance of being correct 64 consecutive guesses have about a 1 in 226 of being
correct
Distance bounding protocols
A keyed pseudo-random function is used to created data in two 64-bit registers This is either a symmetric key, which will require an on-line
transaction to verify the result, or a public/private key pair
The terminal uses the data to in these registers to sent a “challenge bit” to the card The card will send back a “response bit”
The timing between the bit exchange will tell the terminal the maximum distance the card is from it The authors determined that a 1.5 meter radius was ideal
Attacks on Distance Bounding
Replay attack By including a nonce (or timestamp) into the
calculation of the pseudo-random function, this can be avoided
Early bit detection and deferred bit signaling With expensive equipment, the attack can read the
challenge bit early and send the response bit late At best this will expand the radius, but only a few meters
at most
Costs to Implement
Few circuitry changes to the smartcard Costs are expected to be minor
More liberal changes to the terminals
Overall, the test case only used 37 flip-flops and 93 lookup tables This was for both the smartcard and the terminal
Criticisms
Authors note that the distance bounding protocol lacks non-repudiation
The paper gives no actual cost estimates
To fully understand the protocol it requires being well versed in computer architecture
Was never actually implemented, just simulated
References
1. Saar Drimer; Steven J. Murdoch. “Keep Your Enemies Close: Distance Bounding Against Smartcard Relay Attacks”. 16th USENIX Security Symposium. 2007
2. Iulia Ion; Boris Dragovic. “Don’t trust POS terminals! Verify in-shop payments with your phone”.
3. “Smart card”. Wikipedia. http://en.wikipedia.org/wiki/Smart_card