CS 111: Program Design I Lecture 5: US Law when others have encryption keys; if, for

Transcription

1 CS 111: Program Desig I Lecture 5: US Law whe others have ecryptio keys; if, for Robert H. Sloa & Richard Warer Uiversity of Illiois at Chicago September 8, 2016

2 Lavabit ad Sowde Lavabit was a ecrypted webmail service betwee owed ad operated by Ladar Leviso. Edward Sowde used the service. I 2013, the Uited States sought to obtai certai iformatio about a target [Sowde] i a crimial ivestigatio. The Govermet obtaied court orders... requirig Lavabit to tur over particular iformatio related to the target. Whe Lavabit ad Leviso failed to comply with those orders, the district court held them i cotempt ad imposed moetary sactios.

3 Our Diagram Documets (icludig digital files) The govermet thiks you may have the documets. Lavabit The govermet thiks someoe else may have the documets. Sowde s s

4 The 4 th Amedmet Does the 4 th Amedmet apply? The 4 th Amedmet: The right of the people to be secure i their persos, houses, papers, ad effects, agaist ureasoable searches ad seizures, shall ot be violated, ad o Warrats shall issue, but upo probable cause, supported by Oath or affirmatio, ad particularly describig the place to be searched, ad the persos or thigs to be seized.

5 What It Meas There is a zoe of privacy secure i their persos, houses, papers, ad effects that caot be ivaded without a warrat. The poit: q to prevet the govermet from seeig too deeply ito your life without a warrat.

6 Iformatio Others Have About Us How much would I kow about you if I examied all the iformatio that you have stored olie? Would you let me look at all of it? What about all of your s? How much would I kow about you if I examied all of them? If the govermet ca see deeply eough ito your life, should t 4 th Amedmet apply?

7 The 3 rd Party Doctrie The Fourth Amedmet does ot prohibit the obtaiig of iformatio revealed to a third party ad coveyed by him to Govermet authorities, eve if the iformatio is revealed o the assumptio that it will be used oly for a limited purpose ad the cofidece placed i the third party will ot be betrayed. q Uited States v. Miller, 425 U.S. 435, 445 (1976).

8 The Cosequece Documets (icludig digital files) The govermet thiks you may have the documets. The govermet thiks someoe else may have the documets. 4 th Amedmet does ot apply

9 Court Orders I the someoe else has the documets situatios, the govermet gets a court order (a subpoea). Gettig a court order: Go to court ad say Uder law so-ad-so, the govermet may request a order compellig X to do Y. The Apple/FBI debate ivolved a court order.

10 iphoe Ecryptio The data o the phoe is ecrypted. Apple does ot have the key. q The user s password determies the key, ad Apple does ot have the password. There is delay betwee password guesses. 1-4 oe 5 1 miute 6 5 miutes miutes 9 1 hour The phoe could be set up to erase the key after too may guesses.

11 But the iphoe Has A Vulerability While the data is ecrypted, the software cotrollig the phoe is ot. This meas that someoe ca create a hacked versio of the software ad istall it o the phoe without the coset of the phoe's ower ad without kowig the ecryptio key. This is what the FBI - ad ow the court - is demadig Apple do: It wats Apple to rewrite the phoe's software to make it possible to guess possible passwords quickly ad automatically. q decryptig_a_i.html

12 Backgroud Facts Keepig vulerabilities secret reduces security. q Systems that have those vulerabilities ca t patch them. Vulerabilities do ot remai secret. q Oce they're discovered, word gets out. Today's topsecret Natioal Security Agecy attack techiques become tomorrow's PhD theses ad the ext day's hacker tools. q Scheier, your_iphoe_just_got.html. So if Apple provides access to Rizwa s phoe, all iphoes get less secure.

13 The Tradeoff How ca oe best meet these coflictig goals? q q Effective law eforcemet, atioal security versus Privacy ad effective limits o govermetal power.

14 A Example You are a jouralist i coutry with a repressive dictatorship that routiely violates huma rights. The govermet discovers that you have this sequece of letters stored o DropBox: q VPLWK, D, JDUFLD, E, MRKQVRQ, F, ZLOOLDPV, G, MRQHV, H, EURZQ, H, GDYLV, I, PLOOHU, J,PDUWLQ, K, WKRPSVRQ, M, PDUWLQHC, N This is data you have ecrypted with your Caesar cypher (key = 3). It is a list of ames ad code ames of govermet whistleblowers.

15 Questios The govermet asks DropBox to decrypt the iformatio for them. Should DropBox do so? q q A: Yes B: No Suppose istead govermet foud this strig: q L ORYH SBWKRQ This is I love Pytho, key = 3. Should DropBox help govermet decrypt? q q A: Yes B: No

16 THE ZEN OF LEARNING PYTHON & PROGRAMMING

17 Supreme Court of Pytho rules: Official Documetatio Do't believe ay old authority, e.g., professor who leared Pytho 2.3 i 2004 Tur to Pytho 3 Library Referece You ca lear, e.g., about slice goig past last idex of strig: q "The slice of s from i to j is defied as the sequece of items with idex k such that i <= k < j. If i or j is greater tha le(s), use le(s). If "

18 Learig programmig 1. Expect it to be differet! 2. Do't feel you eed to memorize it 3. Immersio == Experimetatio q How Prof. Sloa leared that i Pytho 3.5 slices edig i idex after ed of strig gracefully treat ed idex as if it were le of the strig

19 Fidig strigs i strigs Suppose we wat to fid secod occurrece of substrig target i strig s

20 Fidig strigs i strigs Suppose we wat to fid secod occurrece of substrig target i strig s positio s.fid(target) gives positio of first occurrece i s

21 Fidig strigs i strigs Suppose we wat to fid secod occurrece of substrig target i strig s s.fid(target) gives positio of first occurrece i s s.fid(target, start) gives positio of first occurrece of target but ow i slice s[start:]

22 Problem: Shiftig a whole strig Last lecture: saw how to shift/rotate oe character: wrote shift_letter fuctio So, we kow how to shift strigs of legth 1 Ad we saw == operator, so we ca tell if strig s has legth oe: q le(s) == 1 Notice: shift_letter eded with retur, ot prit

23 retur vs. prit Must choose the oe that does what we eed o case-by-case basis to match specificatios of job we eed to do But: retur is ofte the right choice, to make our work usable later i ew/bigger/related/ etc. problem

24 retur vs. prit example def trip(x): retur 3 * x def times3(x): prit (3 * x) def works(iput): retur trip(iput) def whoops(iput): retur times3(iput)

25 CONDITIONAL STATEMENTS RUN CODE ONLY IF CERTAIN CONDITION IS MET

26 if statemets if <coditio>: <body> <coditio> is Boolea expressio q E.g., le(s) == 1 Code i <body> rus oly if <coditio> is True Colo ad idetatio of body madatory!

27 Example x = "ATTACK" if le(x) > 1: prit("x is loger tha 1 letter!")

28 What will this prit? x = 5 if x == 3: prit("hi!") prit("bye!") A. Nothig B. "Hi!" C. "Bye!" D. "Hi!" ad "Bye!"

29 What will the value of z be after this code rus? def foo(x): if x!= 3: retur 1 retur 2 A. 1 B. 2 C. This will cause a error z = foo(-1)

30 What will the value of z be after this code rus? def foo(x): if x!= 3: prit(1) retur 2 A. 1 B. 2 C. This will cause a error z = foo(-1)

31 shiftig by k for very short strigs def shift(s, k): if le(s) == 1: retur shift_letter(s, k) if le(s) == 2: as = (shift_letter(s[0], k) + retur as shift_letter(s[1], k)) prit("sorry, ca't help you")

32 From legth 2 to 3 def shift(s, k): if le(s) == 1: retur shift_letter(s, k) if le(s) == 2: as = (shift_letter(s[0], k) + shift_letter(s[1], k)) retur as if le(s) == 3: retur (shift_letter(s[0], k) + shift_letter(s[1], k) + shift_letter(s[2], k)) prit("sorry, ca't help you")

33 Or eve 4 def shift(s, k): if le(s) == 1: retur shift_letter(s, k) if le(s) == 2: retur shift_letter(s[0], k) + shift_letter(s[1], k) if le(s) == 3: retur (shift_letter(s[0], k) + shift_letter(s[1], k) + shift_letter(s[2], k)) if le(s) == 4: retur (shift_letter(s[0], k) + shift_letter(s[1], k) + shift_letter(s[2], k) + shift_letter(s[3], k)) prit("sorry, ca't help you")

34 But This will be a real drag eve for 140 character tweets Imagie the 5-page report with 15,000 characters.... We eed for loops q for loops allow us to do same thig for every item i a sequece!

35 FIRST LOOK AT FOR LOOPS

36 for loop basic idea for c i st: <body that refers to c> execute body le(st) times, oce each with c beig each character of strig st i order (I geeral, st could be ay sequece, e.g., a list ad c is each item of st oce)

37 Example: d detector >>> d("i would like a dodecarchy i the US") 3 >>> d("i am agaist the letter after c") 0 def d(iput): couter = 0 for symbol i iput:

38 Example: d detector >>> d("i would like a dodecarchy i the US") 3 >>> d("i am agaist the letter after c") 0 def d(iput): couter = 0 We choose the ame of a variable for symbol i iput: if symbol == 'd': couter = couter + 1 retur couter ad we provide a sequece

39 This will prit? for x i "0123": prit(x) A. 0 D. 0 1 B C. 0 1 E. This will 2 ru 3 forever