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Transmisia datelor multimedia in retele de calculatoare
Steganografia
Milas Iulian Constantin<[email protected]>
UNIVERSITY POLITEHNICA of BUCHARESTDEPARTMENT OF COMPUTER SCIENCE
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Introducere
Scopul steganorafiei;
Diferenta intre criptografie si steganografie;
steganography_medium = hidden_message + carrier + steganography_key
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Clasificarea Thenicilor de Steganografie
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Null Ciphers
Pe Internet spam-urile sunt potentiale purtatoare de mesaje:
Dear Friend , This letter was specially selected to be sent to you ! We will comply with all removal requests ! This mail is being sent in compliance with Senate bill 1621 ; Title 5 ; Section 303 ! Do NOT confuse us with Internet scam artists . Why work for somebody else when you can become rich within 38 days ! Have you ever noticed the baby boomers are more demanding than their parents & more people than ever are surfing the web ! etc.
Meet at Main and Willard at 8:30
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Metode folosite in purtatorii digitali:The Embedding Process
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Metode folosite in purtatorii digitali: Fingerprinting si Watermarking
Ce este fingerprinting?
Ce este Watermarking?
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Metode folosite in purtatorii digitali: Least Significant Bit Insertion #1
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Metode folosite in purtatorii digitali: Least Significant Bit Insertion #2Sa ascundem litera “G” care are codul ASCII
binar 01000111Fie secventa de octeti dintr-o imagine :
Codarea lui “G” pe LSB:
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10010101 00001101 11001001 1001011000001111 11001011 10011111 00010000
10010100 00001101 11001000 1001011000001110 11001011 10011111 00010001
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Metode folosite in purtatorii digitali: Public Key Steganography
Presupunem ca measajul este codat cu o cheie privata;
Dar unul dintre cei doi nu are decat cheia publica;
Ce face primul?
Dar al doilea?9
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Metode folosite in purtatorii digitali: Transform Domain Based Steganography
Algoritmii de extractie se impart in doua grupuri:
- domeniu spatiu/timp;- domeniu transformare;
Algoritmul Patchwork ;
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Metode folosite in purtatorii digitali:Diverse #1Spread-spectrum steganography – se bazeaza
pe portiunile de “energie” ridicata;
Ascundere a mesajelor in portiunile neutilizate ale headerelor fisierelor;
Folosirea partitiilor secrete de pe hard;
Mici modificari ale sunetului: siftare faza, cadenta vorbirii;
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Metode folosite in purtatorii digitali:Diverse #2Fisierele audio sunt foarte bune pentru
ascuns mesaje deoarece sunt mari, iar informatia ascunsa se va gasii greu;
Steganografia nu este limitata doar la imagini si fisiere audio:
- executabilele – Ex: redundanta instructiunilor i386 ==> se pastreaza si dimensiunea originala a fisierului;
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Detectia Steganografiei #1
Steganaliza :- detecteaza sau estimeaza informatia ascunsa pe baza observarii unor transferuri de date, fara sa faca vreo presupunere asupra algoritmului de steganografie;
O prima metoda este urmarirea “purtatorului”;
O alta metoda este urmarirea modificarilor structurale ale “purtatorului”;
Tehnicile steganografice altereaza de obicei statisticile despre un “purtator”;
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Detectia Steganografiei #2
Steganaliza este in general signature-based;
Steganaliza Anomaly-based – (blind steganalysis);
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Exemple de Steganografie
Vom ascunde un GIF de 11,067 bytes intr-un GIF, JPEG si WAV;
Fig 1:
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Imagine ascunsa intr-un GIF #1
Fig urm este o imagine a Washington DC ce confine Fig1. Imaginea initiala este 632,778 bytes si foloseste 249 culori unice; la final 677,733 bytes si 256 culori unice; marimea este mai mare din cauza folosirii extensiei de culori.
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Imagine ascunsa intr-un GIF #2
Paleta de culori de la img Washington (purtatorul) inainte (stanga) si dupa (dreapta) ce Fig1 a fost ascunsa:
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Imagine ascunsa intr-un JPEG
Initial 207,244 bytes si 224,274 culori unice;Dupa steganografiere: 207,275 bytes si 227,870 culori
unice;Nu putem vedea vreo paleta de culori deoarece JPEG
foloseste codarea 24 bit-color si DCT;
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Imagine ascunsa intr-un WAV Initial 178,544 bytes, iar WAV-ul steganografiat 178,298
bytes :
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Concluzii
Teroristii si eBay;
Dupa 11 Septembrie 2001 s-au efectuat multe studii pe continutul Internetului:
- s-au verificat milioane de JPEG-uri pe eBay ==> 1-2% din imagini erau suspecte – niciun mesaj gasit;- alt studiu a examinat catvea sute de mii de imagini de pe diverse pagini Web alese random ==> rezultate similare;
Deci nimic concludent – doar JPEG-uri s-au verificat; numar scazut de site-uri; unele rapoarte pot fi tinute secrete;
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Q&A
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