Hide and seek in time robust covert timing channels

KOM - Multimedia Communications LabProf. Dr.-Ing. Ralf Steinmetz (Director)

Dept. of Electrical Engineering and Information TechnologyDept. of Computer Science (adjunct Professor)

TUD – Technische Universität Darmstadt Merckstr. 25, D-64283 Darmstadt, Germany

Tel.+49 6151 166150, Fax. +49 6151 166152 www.KOM.tu-darmstadt.de

© author(s) of these slides 2008

Dr.-Ing. Matthias HollickProf. Dr.-Ing. Ralf Steinmetz

[email protected] Tel.+49 6151 166158

18. Mai 2012

Hide and Seek in Time -Robust Covert Timing Channels

Cassius de Oliveira Puodzius

Privacy Enhancing TechnologiesAnonymität im Internet

Vorlesung

KOM – Multimedia Communications Lab MN-00-2

Presentation outline

Mai 2012 | Informatik Fb20 | CASED | Cassius de Oliveira Puodzius

Background Covert Channel Adversaries

Problem Definition Channel Capacity Channel Undetectability

Protocol Robustness Multi-channel (Error Corrector Code)

Modulation/Demodulation Scheme Evaluation Trade-Off Experimental Results Conclusions


Background






Covert channel

Conceal the existence of communication by hiding it into overt communication (legitimate traffic)


Background






Covert channel


Covert storage channels: Data transmission by modifying unused or random bits in the packet header


Background






Covert channel


Covert storage channels: Data transmission by modifying unused or random bits in the packet headerCovert timming channels: Modulation of the message into temporal properties of the traffic


Background






Covert channel



Adversaries

• Passive: Make use of statistical test to distinguish covert from legitimate traffic


Background






Covert channel



Adversaries

• Passive: Make use of statistical test to distinguish covert from legitimate traffic• Active (jammers): Disrupt covert timing channels by adding random delays to individual packets


Problem Definition






Create a covert timing channel which is resilient to passive and active adversaries


Problem Definition (2)






Channel Capacity

Maximum number of (covert) bits which are transmited in each packet, i.e., bits per packet (bpp)








Channel Capacity


Determined by:- Rt: transmission rate- Pe: bit error rate (BER)








Channel Capacity


Determined by:- Rt: transmission rate- Pe: bit error rate (BER)

High channel capacity → High Rt with low Pe








Channel Undetectability

A covert time is undetectable according to some statistical test, if the test cannot distinguish between legitimate and covert traffic










Shape TestP(x)xHs(x)

KS-testHs(x) = supx|F(x) – S(x)|










Shape TestP(x)xHs(x)

KS-testHs(x) = supx|F(x) – S(x)|

Regularity Test

Hr = std(|σi - σj|/σi)


Protocol Robustness






To handle with: Fortuitous delay and/or error during transmission Jammers adversaries


Protocol Robustness







Spreading Codes


Protocol Robustness







Spreading Codes

Sender:1. Choose a orthogonal code words c1, ..., cK ϵ {-1, 1}N


Protocol Robustness







Spreading Codes

Sender:1. Choose a orthogonal code words c1, ..., cK ϵ {-1, 1}N2. Encode k-th bit of the message by c'k = bk ck


Protocol Robustness







Spreading Codes

Sender:1. Choose a orthogonal code words c1, ..., cK ϵ {-1, 1}N2. Encode k-th bit of the message by c'k = bk ck3. Simultaneouly transmit over K parallel channels, s = Σ bk ck


Protocol Robustness







Spreading Codes

Sender:1. Choose a orthogonal code words c1, ..., cK ϵ {-1, 1}N2. Encode k-th bit of the message by c'k = bk ck3. Simultaneouly transmit over K parallel channels, s = Σ bk ckReceiver:Decode k-th bit by 1/N <s, ck> = bk


Protocol Robustness (2)






Example: N = 4, K = 4 msend = (-1, -1, 1, -1)








Example: N = 4, K = 4 msend = (-1, -1, 1, -1)

Vector basis:c1 = (-1, 1, 1, 1)c2 = (1, -1, 1, 1)c3 = (1, 1, -1, 1)c4 = (1, 1, 1, -1)








Example: N = 4, K = 4 msend = (-1, -1, 1, -1)

Vector basis:c1 = (-1, 1, 1, 1) c1' = b1 c1 = (1, -1, -1, -1)c2 = (1, -1, 1, 1) c2' = b2 c2 = (-1, 1, -1, -1)c3 = (1, 1, -1, 1) c3' = b3 c3 = (1, 1, -1, 1)c4 = (1, 1, 1, -1) c4' = b4 c4 = (-1, -1, -1, 1)








Example: N = 4, K = 4 msend = (-1, -1, 1, -1)


s = Σ bk ck = (0, 0, -4, 0)








Example: N = 4, K = 4 msend = (-1, -1, 1, -1)


s = Σ bk ck = (0, 0, -4, 0)

b1 = 1/N <s, c1> = ¼ -4 = -1b2 = 1/N <s, c2> = ¼ -4 = -1b3 = 1/N <s, c3> = ¼ 4 = 1

b4 = 1/N <s, c4> = ¼ -4 = -1

mreceived = (-1, -1, 1, -1)








Example: N = 4, K = 4 msend = (-1, -1, 1, -1)


s = Σ bk ck = (1, 2, -4, 0)

b1 = 1/N <s, c1> = ¼ -3 ≈ -1b2 = 1/N <s, c2> = ¼ -5 ≈ -1b3 = 1/N <s, c3> = ¼ 7 ≈ 1

b4 = 1/N <s, c4> = ¼ -1 ≈ -1

mreceived = (-1, -1, 1, -1)


Modulation/Demodulation Scheme






Prior knowledge: channel characteristics (delay, jitter, ...)









Modulation Modulation group: Group of K bits which are encoded each time on K parallel channels










Linear modulation of inter-packet delay

tn := α + β sn (n = 1, ..., N)

α: shift parameter β: pseudo-random parameter











tn := α + β sn (n = 1, ..., N)


Remark: α is transmited on the fly through one of the channels











tn := α + β sn (n = 1, ..., N)



Demodulation Receive a modulation group t, which might be changed to t' due to some additive channel noise x











tn := α + β sn (n = 1, ..., N)



Demodulation Receive a modulation group t, which might be changed to t' due to some additive channel noise x

To decode the k-th bit, one computebk' = 1/N < 1/β t', ck>Hence one get bk' = bk + 1/(N β) <x , ck>


Modulation/Demodulation Scheme (2)






Removing Regularity

Variance of each modulation group g: σg2 = β2 σs2 such that β and σs are determined by K and T








Removing Regularity

Variance of each modulation group g: σg2 = β2 σs2 such that β and σs are determined by K and TThe correlation coefficient of the modulated inter-packet delay t is given by








Removing Regularity


The correlation of the inter-packet delays can dynamically change by appropriately controlling the generation of α and β








Removing Regularity



As long as T is the parameter which controls the system robustness and undetectability, it remains fixed, while K is uniformly chosen in [1, Kmax]








Removing Regularity



As long as T is the parameter which controls the system robustness and undetectability, it remains fixed, while K is uniformly chosen in [1, Kmax]Remark: A seed of a pseudo-random generator can be priorly shared between the sender and the receiver, so that they are able to get β without any further communication


Evaluation Trade-Off






Transmission Rate

Rt = K / N








Transmission Rate

Rt = K / N• At least one channel to transmit α








Transmission Rate

Rt = K / N• At least one channel to transmit αMaximum transmission rate is N – 1/ N


Evaluation Trade-Off (2)






Robustness

Robustness gainG = β2 N








Robustness

Robustness gainG = β2 Nβ = TB

T is fixed








Robustness

Robustness gain The relation between B and KG = β2 N β = TB T is fixed








Robustness

Robustness gain The relation between B and KG = β2 N β = TB T is fixed

Hence, one can achive higher robustness by Decreasing K

Increasing N and T








Undetectability

Accurate shape approximation achieved with a smaller T








Undetectability

Accurate shape approximation While better regularity achievedachieved with a smaller T with bigger T or Kmax


Experimental Results






Experimental scenarios:1. LAN environment in a medium-size campus network








Experimental scenarios:1. LAN environment in a medium-size campus network2. WAN environment with sender and receiver located in USA and Germany, respectively









Network conditions for each scenario









Network conditions for each scenario

Carrier applications


Experimental Results (2)






Robustness


Experimental Results (3)






Undetectability


Thank you!






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Hide and seek in time robust covert timing channels