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Proper Use of Cryptography Never write your own crypto functions if you have any choice Another favorite piece of advice from industry Never, ever, design your own encryption algorithm Unless thats your area of expertise

  1. Proper Use of Cryptography • Never write your own crypto functions if you have any choice – Another favorite piece of advice from industry • Never, ever, design your own encryption algorithm – Unless that’s your area of expertise • Generally, rely on tried and true stuff – Both algorithms and implementations Lecture 14 Page 1 CS 236 Online

  2. Proper Use of Crypto • Even with good crypto algorithms (and code), problems are possible • Proper use of crypto is quite subtle • Bugs possible in: – Choice of keys – Key management – Application of cryptographic ops Lecture 14 Page 2 CS 236 Online

  3. An Example • An application where RSA was used to distribute a triple-DES key • Seemed to work fine • Someone noticed that part of the RSA key exchange was always the same – That’s odd . . . Lecture 14 Page 3 CS 236 Online

  4. What Was Happening? • Bad parameters were handed to the RSA encryption code • It failed and returned an error • Which wasn’t checked for – Since it “couldn’t fail” • As a result, RSA encryption wasn’t applied at all • The session key was sent in plaintext . . . Lecture 14 Page 4 CS 236 Online

  5. Another Example • Many pieces of malware use cryptography • RC4 is a frequent choice of cipher – Seems easy and fast • So the hackers implement it themselves • Which often gives the defenders advantages • Because the hackers screw it up • Being evil doesn’t necessarily make you smart Lecture 14 Page 5 CS 236 Online

  6. Trust Management • Don’t trust anything you don’t need to • Don’t trust other programs • Don’t trust other components of your program • Don’t trust users • Don’t trust the data users provide you Lecture 14 Page 6 CS 236 Online

  7. Trust • Some trust required to get most jobs done • But determine how much you must trust the other – Don’t trust things you can independently verify • Limit the scope of your trust – Compartmentalization helps • Be careful who you trust Lecture 14 Page 7 CS 236 Online

  8. Two Important Lessons 1. Many security problems arise because of unverified assumptions – You think someone is going to do something he actually isn’t 2. Trusting someone doesn’t just mean trusting their honesty – It means trusting their caution, too Lecture 14 Page 8 CS 236 Online

  9. Input Verification • Never assume users followed any rules in providing you input • They can provide you with anything • Unless you check it, assume they’ve given you garbage – Or worse • Just because the last input was good doesn’t mean the next one will be Lecture 14 Page 9 CS 236 Online

  10. Treat Input as Hostile • If it comes from outside your control and reasonable area of trust • Probably even if it doesn’t • There may be code paths you haven’t considered • New code paths might be added • Input might come from new sources Lecture 14 Page 10 CS 236 Online

  11. For Example • Shopping cart exploits • Web shopping carts sometimes handled as a cookie delivered to the user • Some of these weren’t encrypted • So users could alter them • The shopping cart cookie included the price of the goods . . . Lecture 14 Page 11 CS 236 Online

  12. What Was the Problem? • The system trusted the shopping cart cookie when it was returned – When there was no reason to trust it • Either encrypt the cookie – Making the input more trusted – Can you see any problem with this approach? • Or scan the input before taking action on it – To find refrigerators being sold for 3 cents Lecture 14 Page 12 CS 236 Online

  13. Variable Synchronization • Often, two or more program variables have related values • Common example is a pointer to a buffer and a length variable • Are the two variables always synchronized? • If not, bad input can cause trouble Lecture 14 Page 13 CS 236 Online

  14. An Example • From Apache web server • cdata is a pointer to a buffer • len is an integer containing the length of that buffer • Programmer wanted to get rid of leading and trailing white spaces Lecture 14 Page 14 CS 236 Online

  15. The Problematic Code while (apr_isspace(*cdata)) ++cdata; while (len-- >0 && apr_isspace(cdata[len])) continue; cdata[len+1] = ‘/0’; • len is not decremented when leading white spaces are removed • So trailing white space removal can overwrite end of buffer with nulls • May or may not be serious security problem, depending on what’s stored in overwritten area Lecture 14 Page 15 CS 236 Online

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