CIT 380: Securing Computer Systems

1. CIT 380: Securing Computer Systems Modern Cryptography CIT 380: Securing Computer Systems

2. Steganography Hiding messages in another text (the covertext) so that no one except intended recipient knows a message has been sent. • Wax Tablets: In ancient times, messages were written in wax poured on top of a stone or wood tablet. Messages were hidden by engraving them in the stone then pouring wax over them. • Invisible Ink: Write message using lemon juice on paper. Write covertext in regular ink after dries. Heat to view hidden message. • Null Cipher: Hide message in ordinary text, using nth letter of each word, or every nth word of the message. CIT 380: Securing Computer Systems

3. Digital Steganography • Choose a cover medium file. • JPEG, MP3, etc. • Identify redundant bits in cover medium. • Low order bits in image and audio files. • Replace subset of redundant data with secret message. • Send steganographic file to recipient. CIT 380: Securing Computer Systems

4. JSteg: JPEG Steganography JPEG image format • For each color component, a discrete cosine transform (DCT) transforms successive 8x8 pixel blocks into 64 DCT coefficients. • Quantize DCT coefficients. Derek Upham’s JSteg algorithm • LSBs of DCT coefficients are redundancy. • Modification of a single DCT coef affects all 64 pixels. • Frequency domain changes are not visually observable. CIT 380: Securing Computer Systems

5. Wikipedia • http://en.wikipedia.org/wiki/Steganography CIT 380: Securing Computer Systems

6. Steganalysis Compare steganographic file with original. • 100% effective at identifying presence. • Original file is “secret key” of steganography. Statistical analysis • Inserting high entropy changes histogram of color frequencies in predictable ways. • Reduces frequency difference between adjacent colors. Countermeasures • Insert less information to reduce impact. • Choose DCT coefficients to modify at random. • Alternate +/- DCT coefficient value to encode bits. • Use parity of groups of DCT LSBs to encode a message. CIT 380: Securing Computer Systems

7. Key Points • Two types of cryptosystems: • classical (symmetric) • public key (asymmetric) • Cryptographic checksums provide integrity check. • One-way functions. • Keyed hash functions. • Public Key Cryptography • One-way trapdoor functions. • Confidentiality: encipher with public, deciper with private • Integrity: encipher with private, decipher with public • Steganography • Hiding existence of message inside other data. CIT 380: Securing Computer Systems

8. References • Matt Bishop, Introduction to Computer Security, Addison-Wesley, 2005. • Cryptography Research, “Hash Collision FAQ,” http://www.cryptography.com/cnews/hash.html, 2005. • Paul Garrett, Making, Breaking Codes: An Introduction to Cryptology, Prentice Hall, 2001. • Steven Levy, Crypto, Penguin Putnam, 2002. • Wenbo Mao, Modern Cryptography: Theory and Practice, Prentice Hall, 2004. • Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone, Handbook of Applied Cryptography, http://www.cacr.math.uwaterloo.ca/hac/, CRC Press, 1996. • Bruce Schneier, Applied Cryptography, 2nd edition, Wiley, 1996. • NIST, FIPS-198a, “The Keyed-Hash Message Authentication Code (HMAC)”,http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf • Niels Provos and Peter Honeyman, “Hide and Seek: An Introduction to Steganography,” IEEE Security & Privacy, May/June 2003. • John Viega and Gary McGraw, Building Secure Software, Addison-Wesley, 2002. CIT 380: Securing Computer Systems