Planning a Jailbreak: Use Steganography€¦ · Robert Kunnemann¨ Planning a Jailbreak: Use Steganography. Outline Introduction Information Hiding in History Information Hiding Today

OutlineIntroduction

Information Hiding in HistoryInformation Hiding Today

Kerckhoff’s principleDefinition of steganographic security

Planning a Jailbreak: Use Steganography

Robert Kunnemann

August 11, 2007

Robert Kunnemann Planning a Jailbreak: Use Steganography

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IntroductionThe Prisoners ProblemWhy hiding?Ethical Positions

Information Hiding in HistoryHistoric Stegosystems

Information Hiding TodayTerminologyAttackHiding information in LSBCovert Channels

Kerckhoff’s principleKerckhoff’s principleSecurity by Obscurity

Definition of steganographic securitypublic-key stego-systemsSSA-CA Robert Kunnemann Planning a Jailbreak: Use Steganography

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The Prisoners ProblemWhy hiding?Ethical Positions

The Prisoners Problem

Simmons’ Prisoner’s Problem:

I Alice and Bob in Jail, seperated cells

I plan an escape plan

I send messages via couriers, who are spys set by warden Eve.

I Eve suspicious ⇒ both sent to Solitary confinement

I they shared a secret keyword before going to prison (allthoughthis is not necessary in all cases)


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The Warden

Eve can be

I passive, restricted to eavesdropping or

I active and change messages / emit new ones.


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I Normal” Cryptography does not suffice here! (But may helpat some point.)

I have to somehow embed so called stegotext in harmlesscommunication channel, the covertext.


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I Normal” Cryptography does not suffice here! (But may helpat some point.)

I have to somehow embed so called stegotext in harmlesscommunication channel, the covertext.


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information hiding

steganography, greek for covered writing.


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Why hide, not cipher?


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Ethics

I in cryptography: good guys want to keep their privacy againstevil eavesdroppers

I in steganography: little more complicated

I contact abuse helplines anonymously, vote in private

I plan crimes, pass secrets


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Ethics






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Ethics






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Historic Stegosystems

I Herodotus (486-425 B.C.) tatooed a shaved slave’s head

I Gaspar Schott(1608-1666): hide messages in music scores

I Johannes Trithemus’ (1462-1516) ””Ave Maria”” code:Replace each letter by a word


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The Art in information hiding - The Vexierbild

Figure: Sh: Vexierbild (1530)Robert Kunnemann Planning a Jailbreak: Use Steganography

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Figure: unknown artist: Vexierbild from the 19th century


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TerminologyAttackHiding information in LSBCovert Channels

Terminology

I embedded data: the message you want to send secretly

I this is hidden in cover-text (-image/-audio)

I producing a stego-text (-object)

I stego-key controls hiding process and restricts detection andrecovery


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Terminology






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Terminology






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Terminology






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successful attack := detecting existence of communication


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Least Significant Bit

1 1 10 0 0 0

1232 8 4128 64 16

0

Figure: LSB


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LSB

1 1 10 0 0 0

1232 8 4128 64 16

1

Store information

here

Figure: LSB


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Issues

I BUT: easy to break.

I gets lost/damaged on reencoding


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Covert Channels

Communication paths not designed to transport information


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Examples

I dunno hides information in latencies of the apache webserver

I hide information in video signal for CRTs


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What’s wrong?

we assumed the adversary to be unaware of the method we use tohide the information. . .


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Kerckhoff’s principleSecurity by Obscurity

Kerckhoff’s principle

I set by Auguste Kerckhoff in 1883

I first principles of cryptographic engineering


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I assume the method used to cypher/hide information is knownto the opponent

I security by choice of key


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I assume the method used to cypher/hide information is knownto the opponent

I security by choice of key


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Security by Obscurity

The assumption, that the enemy will ignore the system you use


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Bad idea

I better rely on a key and provable security

I So: we need a definition of security


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Bad idea

I better rely on a key and provable security

I So: we need a definition of security


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public-key stego-systemsSSA-CA

A public-key stego-system

consists of:

I An algorithm SK resulting in a public and a private stegokeyspk, ssk (using some security paramter k)

I An algorithm SE hiding the information in the coverobjectusing spk, it outputs a covertext

I An algorithm SD extracting the embedded data via ssk, or(e.g. if there is no embedded data) an errorsymbol ↓


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consists of:





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consists of:





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Correctness

for all (spk, ssk) output by SK and all messages it should workoften.That means: (Pr[SD(ssk, SE(spk,m))) 6= m] is neglible in asecurity parameter k


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The Game

Security in Cryptography is defined via Games.A challenger with a defined input/output behaviour against anarbitrary adversary.


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Security

The system is secure iff the propability to crack it is negligible forall adversaries.


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spk

Adv. SA1 SO1

Adv. SA2 SO2

sampling

oracle

for C

bR← {0, 1}

c∗ ← SE(spk, m∗)

or c∗R← C

SS-CCA

Challenger

(c∗, s)

b’

ci

SD(ssk, ci)

ci

SD(ssk, ci)

ssk

(m∗, s)

(ci 6= c∗)

Figure: Schematic Sketch


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Steganographic Security against Active Attacks

Pr[SA

SOcca2

2 (spk,m∗, c∗, s) = b;

(spk, ssk)← SK, (m∗, s)← SASO11 , b

R← {0, 1},if b = 0 then c∗ ← SE(spk,m∗)

else c∗R← C

]=

12

+ ε(k)


Planning a Jailbreak: Use Steganography€¦ · Robert Kunnemann¨ Planning a Jailbreak: Use Steganography. Outline Introduction Information Hiding in History Information Hiding Today

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