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![Defining secrecy (take 1)
♦ Even an adversary running for an unbounded
amount of time learns nothing about the message
from the ciphertext
♦ Perfect secrecy
♦ Formally, for all distributions over the message
space, all m, and all c:
Pr[M=m | C=c] = Pr[M=m]](https://image.slidesharecdn.com/lecture2-160712051129/75/Computer-and-Network-Security-29-2048.jpg)
Uploaded byMuhammad Yousuf Abdul Qadir
Computer and Network Security
This document provides an overview of a lecture on computer and network security. It discusses an upcoming JCE tutorial and homework assignment. It also lists assigned readings and asks for comments. The rest of the document summarizes a high-level survey of cryptography, covering goals like confidentiality and integrity. It discusses private-key settings where parties share a secret key, and provides examples of simple encryption schemes like shift ciphers and their limitations. It emphasizes using standardized cryptographic algorithms.
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Computer and Network Security
- 1. Computer and NetworkSecurity Lecture 6
- 2. JCE tutorial ♦ Inclass next Wednesday ♦ HW1 will use it
- 3. Assigned readings fromlecture 1 ♦ “Inside the Twisted Mind of the Security Professional” ♦ “We are All Security Customers” ♦ “Information Security and Externalities” ♦ Comments?
- 4. A high-level surveyof cryptography
- 5. Caveats ♦ Everything Ipresent will be (relatively) informal – I may simplify, but I will not say anything that is an outright lie… ♦ Cryptography offers formal definitions and rigorous proofs of security (neither of which we will cover here) – For more details, take CMSC 456 in the Fall (or read my book)! ♦ If you think you already know cryptography from somewhere else (CMSC456, CISSP, your job, the news), you are probably mistaken
- 6. Goals of cryptography ♦Crypto deals primarily with three goals: – Confidentiality – Integrity (of data) – Authentication (of resources, people, systems) ♦ Other goals also considered – E.g., non-repudiation – E-cash (e.g., double spending) – General secure multi-party computation – Anonymity – …
- 7. Private- vs. public-keysettings ♦ For the basic goals, there are two settings: – Private-key / shared-key / symmetric-key / secret-key – Public-key ♦ The private-key setting is the “classical” one (thousands of years old) ♦ The public-key setting dates to the 1970s
- 8. Private-key cryptography ♦ Thecommunicating parties share some information that is random and secret – This shared information is called a key – Key is not known to an attacker – This key must be shared (somehow) in advance of their communication
- 9. To emphasize ♦ Aliceand Bob share a key K – Must be shared securely – Must be completely random – Must be kept completely secret from attacker ♦ We don’t discuss (for now) how they do this – You can imagine they meet on a dark street corner and Alice hands a USB device (with a key on it) to Bob
- 10. Private-key cryptography ♦ Forconfidentiality: – Private-key (symmetric-key) encryption ♦ For data integrity: – Message authentication codes
- 11. Canonical applications ♦ Two(or more) distinct parties communicating over an insecure network – E.g., secure communication ♦ A single party who is communicating “with itself” over time – E.g., secure storage
- 12. Alice Bob shared infoKK Alice K Bob K
- 13.
- 14. Security? ♦ We willspecify the exact threat model being addressed ♦ We will also specify the security guarantees that are ensured, within this threat model – Here: informally; CMSC 456: formally ♦ Crucial to understand these issues before crypto can be successfully deployed! – Make sure the stated threat model matches your application – Make sure the security guarantees are what you need
- 15. Security through obscurity? ♦Always assume that the full details of crypto protocols and algorithms are public – Known as Kerckhoffs’ principle – The only secret information is a key ♦ “Security through obscurity” is a bad idea… – True in general; even more true in the case of cryptography – Home-brewed solutions are BAD! – Standardized, widely-accepted solutions are GOOD!
- 16. Security through obscurity? ♦Why not? ♦ Easier to maintain secrecy of a key than an algorithm – Reverse engineering – Social engineering – Insider attacks ♦ Easier to change the key than the algorithm ♦ In general setting, much easier to share an algorithm than for everyone to use their own
- 17.
- 18. Functional definition ♦ Encryptionalgorithm: – Takes a key and a message (plaintext), and outputs a ciphertext – c ← EK(m) possibly randomized! ♦ Decryption algorithm: – Takes a key and a ciphertext, and outputs a message (or perhaps an error) – m = DK(c) ♦ Correctness: for all K, we have DK(EK(m)) = m ♦ We have not yet said anything about security…
- 19. Alice Bob shared infoKK Alice K Bob K c c←EK(m) m=DK(c)
- 20. A classic example:shift cipher ♦ Assume the English uppercase alphabet (no lowercase, punctuation, etc.) – View letters as numbers in {0, …, 25} ♦ The key is a random letter of the alphabet ♦ Encryption done by addition modulo 26 ♦ Is this secure? – Exhaustive key search – Automated determination of the key
- 21. Another example: substitutioncipher ♦ The key is a random permutation of the alphabet – Note: key space is huge! ♦ Encryption done in the natural way ♦ Is this secure? – Frequency analysis ♦ A large key space is necessary, but not sufficient, for security
- 22. Another example: Vigenerecipher ♦ More complicated version of shift cipher ♦ Believed to be secure for over 100 years ♦ Is it secure?
- 23. Attacking the Vigenerecipher ♦ Let pi (for i=0, …, 25) denote the frequency of letter i in English-language text – Known that Σ pi 2 ≈ 0.065 ♦ For each candidate period t, compute frequencies {qi} of letters in the sequence c0, ct, c2t, … ♦ For the correct value of t, we expect Σ qi 2 ≈ 0.065 – For incorrect values of t, we expect Σ qi 2 ≈ 1/26 ♦ Once we have the period, can use frequency analysis as in the case of the shift cipher
- 24. Moral of thestory? ♦ Don’t use “simple” schemes ♦ Don’t use schemes that you design yourself – Use schemes that other people have already designed and analyzed…
- 25. A fundamental problem ♦A fundamental problem with “classical” cryptography is that no definition of security was ever specified – It was not even clear what it meant for a scheme to be “secure” ♦ As a consequence, proving security was not even an option – So how can you know when something is secure? – (Or is at least based on well-studied, widely-believed assumptions)
- 26. Defining security? ♦ Whatis a good definition? ♦ Why is a good definition important?
- 27. Security goals? ♦ Adversaryunable to recover the key – Necessary, but meaningless on its own… ♦ Adversary unable to recover entire plaintext – Good, but is it enough? ♦ Adversary unable to determine any information at all about the plaintext – Formalize? – Sounds great! – Can we achieve it?
- 28. Note ♦ Even givenour definition, we need to consider the threat model – Multiple messages or a single message? – Passive/active adversary? – Chosen-plaintext attacks? ♦ The threat model matters! – The classical ciphers we have seen are immediately broken by a known-plaintext attack
- 29. Defining secrecy (take1) ♦ Even an adversary running for an unbounded amount of time learns nothing about the message from the ciphertext ♦ Perfect secrecy ♦ Formally, for all distributions over the message space, all m, and all c: Pr[M=m | C=c] = Pr[M=m]
- 30. Next time: theone-time pad; its limitations; overcoming these limitations