Secure multimedia communication
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Secure Multimedia Communication. Curtsey of Professor Min Wu Electrical & Computer Engineering Univ. of Maryland, College Park. Evolving Multimedia and Comm. Technologies. Well-developed multimedia standards @ Source compression has matured: MPEG-1 Layer 3, JPEG-2000, MPEG-4

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Secure multimedia communication l.jpg

Secure Multimedia Communication

Curtsey of Professor Min Wu Electrical & Computer EngineeringUniv. of Maryland, College Park

Evolving multimedia and comm technologies l.jpg
Evolving Multimedia and Comm. Technologies

  • Well-developed multimedia standards @

    • Source compression has matured: MPEG-1 Layer 3, JPEG-2000, MPEG-4

    • Mature standards have created many devices and applications: MP3, DVD, Streaming video

  • Rapid development of communication technologies

    • Broadband: DSL, Cable Modems, Satellite

    • HDTV will convey data and media

    • Wireless for any-where any-time connections: 3G, 802.11A/B

  • Advances in networking technologies

    • Promise of ubiquitous, heterogeneous networks.

Min Wu @ U. Maryland 2002

Compression l.jpg



  • Color image of 600x800 pixels

    • 600*800 * 24 bits/pixel = 1.44M bytes

    • After JPEG compression (popularly used on web)

      • only 89K bytes

      • compression ratio ~ 16:1

  • Movie

    • 720x480 per frame, 30 frames/sec, 24 bits/pixel ~ 243M bits/sec

    • DVD ~ about 5M bits/sec

      • Compression ratio ~ 48:1

  • Audio

    • 44.1KHz * 16bit * 2 ch. = 1.4 Mbps

    • MP3 ~ about 64K – 128 Kbps

“Library of Congress” by M.Wu (600x800)

Min Wu @ U. Maryland 2002

Mm data comm effective mm comm l.jpg
MM + Data Comm. = Effective MM Comm.?

  • Multimedia vs. Generic Data

    • Perceptual no-difference vs. Bit-by-bit accuracy

    • Unequal importance within multimedia data

    • High data volume and real-time requirements

  • Need consider the interplay between source coding and transmission and make use of MM specific properties

  • E.g. wireless video need “good” compression algo. to:

    • Support scalable video compression rate ( from 10 to several hundred kbps)

    • Be robust to the transmission errors and channel impairments

    • Minimize end-to-end delay

    • Handle missing frames intelligently

Min Wu @ U. Maryland 2002

Example error concealment l.jpg

(a) original lenna image

(b) corrupted lenna image

(c) concealed lenna image

25% blocks in a checkerboard pattern are corrupted

corrupted blocks are concealed via edge-directed interpolation

Example: Error Concealment

  • Multimedia-specific ways of error recovery

Examples were generated using the source codes provided by W.Zeng.

Min Wu @ U. Maryland 2002

Error resilient coding with localized synch marker l.jpg

H.263 encoder

H.263 decoder

Output sequence



Error concealment

MB detection


Random noise

H.263 with FRM

H.263 with LRM

Error-Resilient Coding with Localized Synch Marker

  • Reduce error propagation

(From D. Lun @ HK PolyUniv. Short Course 6/01)

Min Wu @ U. Maryland 2002

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Demands on Info. Security and Protection

  • Intellectual property management for digital media

    • Promising electronic marketplace for digital music and movies

    • Napster controversy

  • Conventional encryption alone still leaves many problems unsolved

    • Directly apply conventional encryption to compressed MM bitstream?

      • May lose error resilience and scalability

      • Require much computation power

      • Exploring MM property in encryption is desired

    • How to distinguish changes introduced by compression vs. malicious tampering?

      • Bit-by-bit accuracy is not always desired authenticity criterion for MM

    • Protection from encryption vanishes once data is decrypted

      • Still want establish ownership and restrict illegal re-distributions

Min Wu @ U. Maryland 2002

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Visible Digital Watermarks

from IBM Watson web page“Vatican Digital Library”

Min Wu @ U. Maryland 2002

Invisible watermark l.jpg
Invisible Watermark

  • human visual model for imperceptibility: protect smooth areas and sharp edges

  • 1st & 30th Mpeg4.5Mbps frame of original, marked, and their luminance difference

Min Wu @ U. Maryland 2002

Data hiding for annotating binary line drawings l.jpg
Data Hiding for Annotating Binary Line Drawings

pixel-wise difference

marked w/ “01/01/2000”


Min Wu @ U. Maryland 2002

Multimedia fingerprinting trace traitors l.jpg

original media

Customer: Eve

Sell Content

= Fingerprint

101101 …



Fingerprint Tracing:



= Suspicious




101101 …

Customer: Eve

Multimedia Fingerprinting: Trace Traitors

Min Wu @ U. Maryland 2002

16 bit anti collusion code acc example for detecting 3 colluders l.jpg

( -1, 1, 1, 1, 1, 1, …, -1, 1, 1, 1 ) User#4

User#1 ( -1,-1, -1, -1, 1, 1, 1, 1, …, 1 )

Collude by Averaging

Uniquely Identify User 1 & 4

Extracted fingerprint code ( -1, 0, 0, 0, 1, …, 0, 0, 0, 1, 1, 1 )

16-bit Anti-Collusion Code (ACC) Example for Detecting 3 Colluders

Min Wu @ U. Maryland 2002

Conveying one bit through noisy channel l.jpg
Conveying One-bit Through Noisy Channel

  • Optimal detection ~ minimize prob. of error

    MAP ~ maximize posterior probability

    => ML ~ maximum likelihood detector [for equal prior]

    => Minimum distance detector [for iid Gaussian noise]

    => Maximum correlation detector [for equal-energy sig.]

  • Detection statistics

    • [correlator] i yi si

      • Prob. distribution under each hypothesis ~ N( ||s||2 , ||s||2d 2)

    • [correlator with unit-variance] i yi si/ [(i si 2) d 2]1/2 ~ N( ||s||/d ,1)

Min Wu @ U. Maryland 2002

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Performance of Optimal Detector

  • Probability of detection error = Q (||s||/d )

    • Q (x) is monotonically decreasing for non-negative x

    • Signal-to-noise ratio (SNR) ~ (||s||2/n) / d 2

  • Communications under very low SNR

    • Choose large n

      • collect info. (energy) from many signal components

      • a basic idea behind “spread spectrum communications”

  • Useful in invisible watermarking (data hiding)

    • Adding or subtracting a weak signal to convey one-bit hidden info.

    • Will go into more details next time

  • Extension for non-i.i.d. Gaussian noise

Min Wu @ U. Maryland 2002

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Add Security Layers to Communications

  • Confidentiality =>

    • Messages for “your eyes” only

  • Integrity

    • Message is what sender intended to deliver at this moment

  • Threats and Attacks on information

    (1) Use limited info. to find out ways to decipher confidential msg.

    • Prefer a system s.t. the best attack strategy is guessing and exhaustive search

      => unbreakable within reasonable time period

      (2) Altering a message s.t. authentication system still regard it as unaltered

      (3) Replaying an old message as if it is being sent by sender right now

Min Wu @ U. Maryland 2002

Useful crypto tools building blocks l.jpg
Useful Crypto Tools/Building-Blocks

  • Crypto’ly strong one-way function f(x)

    • Easy to compute f(x) given x, but difficult to find x when given f(x)

    • Given a set of (xi, f(xi)) and f(x), difficult to find x

    • SHA (Secure Hash Algorithm) and DES are popular choice for one-way function

  • “Low-cost” crypto’ly strong random number generator

    • Generating truly random seq. via natural randomness ~ flip coins, etc.

      • slow and difficult to store/transmit efficiently

      • prefer low cost in both computation and storage/delivery

    • Use “pseudo-random” generator that can

      • Given a subset of output bits, the rest are unpredictable

      • Produce output using a small secret ~ say, a small set of parameters

      • Produce output fast and be easily implementation, say, in software

    • Use one-way function to generate unpredictable bits Xj = f( s + j )

      • seed “s”, one-way function “f( )”

Min Wu @ U. Maryland 2002

Useful crypto tools building blocks17 l.jpg
Useful Crypto Tools/Building-Blocks

  • Crypto’ly strong hash or digest function H( )

    • One-way “compression” function

      • M-bit input to N-bit output often with fixed N and M >> N

      • Often used to produce a short ID for identifying the input

    • Properties to be satisfied:

      1) Given a message m, H(m) can be calculated very quickly

      2) Given a digest y, it is computationally infeasible to find a message m s.t. H(m) = y (i.e., H is one-way)

      3) It is computationally infeasible to find messages m1 & m2 s.t. H(m1) = H(m2) (i.e. H is strongly collision-free)

    • Keyed Hash:

      • H( k, m ) = Hash( concatenated string derived from k & m )

    • Commonly used crypto hash

      • 160-bit SHA (Secure Hash Algorithm) by NIST

      • 128-bit MD4 and MD5 by Rivest

Min Wu @ U. Maryland 2002

Encryption ciphers l.jpg
Encryption / Ciphers

  • Examples <=

    • Shift cipher: e.g. “plaintext” => “sodlqwhaw” (shift by +3)

    • Substitution cipher ~ equiv. to apply a permutation of alphabet to plaintext

    • Stream cipher using XOR ~ Xi Ki = Yi

      • one-time pad with key size as large as the message

    • Block cipher

      • encrypt a large block of data at a time to make freq. attack difficult

      • many modern ciphers are block ciphers

  • Attacks

    • A small number of searches/guesses

    • Cipher-text and Plaintext attack

      • use some knowns to find/guess unknowns ~ solving equation arrays

    • Frequency analysis (esp. when plaintext is natural language)

Min Wu @ U. Maryland 2002

Encryption keys l.jpg
Encryption Keys

  • Symmetric

    • Encryption and decryption share the same key

    • Key establishment and update are often non-trivial

  • Asymmetric (public-key crypto)

    • Different keys for encryption and decryption

    • Difficult to derive one key from the other key

    • Useful for confidentiality, identity verification, key establishment, etc.

    • Message for Bob’s eye

      • Alice encrypts a msg using Bob’s public key

      • only private key holder can decrypt a ciphertext encrypted by the corresponding public key

    • Message only Bob can produce

      • Bob encrypts a msg using his private key

      • only private key holder can produce a ciphertext decryptable by the corresponding public key

Min Wu @ U. Maryland 2002

A few widely used ciphers l.jpg





A Few Widely Used Ciphers

  • DES and new AES

    • A building block (“Feistel”) scrambles the input

    • Apply a given number of rounds of Feistel blocks

    • Extensive cryptanalysis

      • A good crypto system should not rely on the secrecy of the algorithm

  • RSA (public-key encryption):

    • Security strength based on discrete log problem

      • Fix a large prime p, let nonzero integer a and b (mod p) s.t. b = a x=> difficult to find x

    • Encryption and Decryption perform exponential modulo operation with different exponents

      • slow

Min Wu @ U. Maryland 2002

Data integrity verification data authentication l.jpg
Data Integrity Verification (data authentication)

  • Authentication is always “relative”

    • with respect to a reference

  • How to establish and use a reference

    [Method-1] Give a “genuine” copy to a trusted 3rd party

    [Method-2] Append “check bits”

    • Want hard to find a different meaningful msg. with same “check bits”=> use crypto’ly strong hash

    • Want tamper-proof if hash func. is public

      • Encrypt concatenated version of message and hash

      • Keyed Hash (Message Authentication Code) ~ no extra encryption needed

  • Digital signature algo. (using public-key crypto)

    • Signed Msg|Hash ~ i.e., encrypt by private key s.t. others can’t forge

Min Wu @ U. Maryland 2002