An Introduction to Cryptography for Homeland Security

1. An Introduction to Cryptography for Homeland Security Jay Ligatti University of South Florida

2. Outline • Communication problems • Cryptographic solutions • Cryptography and homeland security • Limitations of cryptography • Research challenges

3. A Common Scenario • Members of a team need to communicate Hello Bob

4. A Common Scenario • Messages need to be secure Hello Bob Hello Alice, operation X begins in 36 hours

5. A Common Scenario • However, communication needs to go through a medium accessible by an adversary (a non-privileged entity) • Easy to think of transmission medium as the Internet, but could be, e.g., • copper phone lines • radio waves • courier on the ground Hello Bob Hello Alice, operation X begins in 36 hours

6. Problems • Adversary may discover secrets by passivelymonitoring the communication • E.g., operation X begins in 36 hours • E.g., Alice and Bob are communicating, so some operation is likely to begin soon (this is called traffic analysis) • Adversary may prevent communication by destroyinginformation en route • E.g., convince Internet routers not to forward packets • E.g., destroy courier on the ground • Adversary may disrupt knowledge by activelytampering with or forging information en route • E.g., overwrite 36 with 72 to desynchronize Alice from Bob • E.g., send a message to Alice impersonating Bob: “Hello again Alice, cancel operation X”

7. Problems • These are difficult problems! • We don’t have perfect solutions to any of them! • We do have some very convincing solutions, especially for: • Preventing passive discovery of secrets • E.g., operation X begins in 36 hours • Detecting active tampering with/forging information • E.g., overwrite 36 with 72 to desynchronize Alice from Bob • E.g., send a message to Alice impersonating Bob: “Hello again Alice, cancel operation X”

8. Outline • Communication problems • Cryptographic solutions • Cryptography and homeland security • Limitations of cryptography • Research challenges

9. Cryptography • Cryptography is used to prevent passive discovery of, and to detect active tampering with, information en route • Definition: Cryptography is the study of hiding information • Cryptanalysis is the study of finding hidden information • Cryptology = cryptography + cryptanalysis • Basic cryptographic tool is a cipher • Cipher is an algorithm (think recipe) for hiding information in a new message M and retrieving hidden information from M • Here’s how it works...

10. key vD)w’45#... Ciphering Step 1: Key Establishment • Communicating parties need to share a secret key(think of it as a password with about 40 keyboard characters) • How do Alice and Bob obtain a shared, secret key? • No completely satisfactory answer! • Alice (or Bob) could create the key and then travel (or send a trusted courier) to Bob (or Alice) to share the key • More practically, but generally less securely, Alice and Bob could use a special key-exchange protocol (see Wikipedia entry on Diffie-Hellman) key vD)w’45#...

11. ciphertext >W<$%YoPjS s-5eoy5... Ciphering Step 2: Encryption • Cipher begins with plaintext (original message) and the key • Cipher shuffles around the encodings of the plaintext and key in very complicated ways to produce a ciphertext (message with hidden information) • This is called encrypting the plaintext plaintext Operation X begins in 36 hours cipher key vD)w’45#...

12. Ciphering Step 2: Encryption here is no “magic” in the encryption procedure • T • Mostly, complicated repetitions of substituting some characters for other characters in the plaintext and then rearranging (permuting) the characters • Both the substitutions and the rearrangements are guided by the secret key • For low-level details of one cipher, Google “DES” Fixed sequence of steps, precisely defined

13. ciphertext >W<$%YoPjS s-5eoy5... key key vD)w’45#... vD)w’45#... Ciphering Step 3: Message Transmission • One party sends ciphertext to another • Even if ciphertext is intercepted, it would take adversary a very long time to figure out the plaintext (as far as we know) • Best publicly known techniques for modern ciphers require trying every possible key until one “unlocks” the ciphertext • Typically, there are about as many possible keys as there are elementary particles in the observable universe... >W<$%YoPjS s-5eoy5... ?????

14. ciphertext >W<$%YoPjS s-5eoy5... plaintext Operation X begins in 36 hours Ciphering Step 4: Decryption • The key-holding receiver can re-shuffle around the encodings of the ciphertext and key to obtain the original plaintext • This is called decrypting the ciphertext • Low-level details very similar to encryption (substitutions and rearrangements of characters) • Efficient decryption because key is known cipher key vD)w’45#...

15. ciphertext Hello again Alice, cancel operation X plaintext {P#I% TO\s-... key vD)w’45#... Tampering/Forgery Detection • If adversary without the shared key attempts to tamper with or forge a ciphertext, she’ll almost certainly send a ciphertext that decrypts to nonsense cipher ????? This must not be from Bob!

16. Key Properties of Ciphers • Inverse relation between encryption and decryption D(E(p,k),k) = p • Confusion • Relationship between key and ciphertext should be very complex • Makes it difficult to obtain key from ciphertext • Diffusion • Even the most minor changes to the plaintext should cause changes throughout the entire ciphertext • Makes it difficult to infer any part of plaintext from ciphertext alone(even if adversary already knows how other plaintexts get encrypted) • For example...

17. Example of Diffusion in the Popular AES (Advanced Encryption Standard) Cipher • Every ciphertext character depends on every plaintext character • Diffusion prevents adversary from easily inferring parts of the plaintext from the ciphertext(because every plaintext change may alter the entire ciphertext) Key 0000000000000000000000000000000000000000000000000000000000000000 Plaintext 1 Ciphertext 1 00000000000000000000000000000000 dc95c078a2408989ad48a21492842087 AES cipher Plaintext 2 Ciphertext 2 00000000000000000000000000000001 530f8afbc74536b9a963b4f1c4cb738b

18. Outline • Communication problems • Cryptographic solutions • Cryptography and homeland security • Limitations of cryptography • Research challenges

19. The Dual Role of Defense • Members of defense and response teams may need to communicate securely in the presence of adversaries • E.g., groups may want to discuss targets of investigations without adversaries learning of (and tipping off) those targets • E.g., rescue teams controlling remote robots may not want adversaries tampering with the human-robot communications

20. The Dual Role of Defense • To monitor threats, defense teams will also generally want to take on adversarial roles in suspicious, potentially encrypted communications • Hot topic of debate: When, and to what extent, should this occur? • Cryptography can both help and hinder homeland security • Another big debate: Is cryptography a net benefit or net hindrance for homeland security (now and in the future)?

21. Outline • Communication problems • Cryptographic solutions • Cryptography and homeland security • Limitations of cryptography • Research challenges

22. Limitation 1 • Adversaries can circumvent cryptography by monitoring plaintext inputs and outputs of communication channels Intercept Alice’s and/or Bob’s plaintext plaintext plaintext decrypt encrypt ciphertext ciphertext

23. Limitation 1 • Example: Adversary installs a keystroke logger (keylogger) on Alice/Bob’s computer • Keyloggers capture and transmit all keyboard activity before what’s being typed gets encrypted • Adversary can install a keylogging program on Alice/Bob’s machine by: • CD/disk/download (with direct access to target machine) • Remotely connecting to and exploiting a flaw on target machine (to install keylogger without Alice/Bob’s knowledge) • Packaging keylogger program as something benign and convincing target to execute it • E.g., send keylogger as email attachment and entice target to open it

24. Limitation 1 • Alternatively, adversary can quietly install keylogging hardware on Alice/Bob’s machine • E.g., small device inserted between a computer’s keyboard port and the keyboard cable [photo source: www.alibaba.com]

25. Limitation 1 • Alternatively, adversary can acoustically record Alice/Bob’s typing! • Each keyboard key has slightly different acoustic signature • Adversary records typing and analyzes recording • Or adversary can videotape Alice/Bob’s screen ! • Or adversary can videotape and analyze the reflections of the screen images on Alice/Bob’s face! • Or adversary can analyze the radiation emanating from Alice/Bob’s monitor to determine what it is showing! • ... • Point: A powerful adversary has many avenues for monitoring communications of specific targets, even in the presence of strong cryptography

26. Limitation 2 • Installing and using cryptography software takes some effort and knowledge • Easier not to worry about it • Sensitive unencrypted data does get stolen • In May 2006, the Department of Veterans Affairs lost a laptop containing plaintext personal data on more than 26 million U.S. military members • Led to a major overhaul in VA security procedures, including encrypting data on laptops[http://www.usa.gov/veteransinfo.shtml]

27. Limitation 3 • Cryptography is a communications technology, with applications in computer security • Cryptography does not “solve” computer security • Most computer-security problems arise from unrelated issues • Programs are shipped with insecure default settings • Programs contain errors that attackers can exploit • Users execute malicious software because it seems benign

28. Outline • Communication problems • Cryptographic solutions • Cryptography and homeland security • Limitations of cryptography • Research challenges

29. Challenge 1 • Cryptography is a large research field • Just to mention a few topics of interest... • Formal analysis of cryptographic protocols • Adversaries can sometimes gain a surprising amount of information by monitoring and disrupting communications • Can we formally prove that an entire communication session will be secure? • Application: Voting protocols • Team members may vote on best of several alternatives • How to design system to guarantee accurate vote counting with privacy?

30. Challenges 2-4 • Tractability of cryptanalysis • Can we prove the difficulty of obtaining keys from ciphertexts? • What is the simplest cipher that makes cryptanalysis impractical? • Key exchange • Is there a secure way to exchange secret keys in the presence of powerful adversaries? What’s the best we can do? • Multiparty communications • In what ways do standard (two-party) cryptographic solutions apply to communications between more than two parties?

31. Summary • Cryptography’s goal: make it very difficult for adversaries to access and modify information en route • Exactly how difficult is an open question • In any case, powerful adversaries can sometimes use sophisticated surveillance and computer-attack techniques to circumvent cryptography

32. End Thanks / Questions? Acknowledgment: This educational work was supported by NSF CAREER award CNS-0742736. Any opinions expressed are those of the author and do not necessarily reflect the views of the NSF.