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Server remembers last pwd p and when p is oered

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Unformatted text preview: the same as 968 •  Used by GNU Mailman mailing list so|ware (?) 30 User Selec'on •  Problem: people pick easy- to- guess passwords –  Based on account names, user names, computer names, place names –  Dic'onary words (also reversed, odd capitaliza'ons, control characters, “l33t- speak”, conjuga'ons or declensions, Torah/Bible/Koran/… words) –  Too short, digits only, leCers only –  License plates, acronyms, social security numbers –  Personal characteris'cs or foibles (pet names, nicknames, etc.) –  Using the same password in mul'ple accounts 31 User Password Educa'on •  Use the first leCer of each word in a phrase –  “My dog’s first name is Rex.” becomes “MdfniR” Reac've Password Checking •  Have a password cracking program running in the background –  Shut down account of passwords it can crack –  CPU intensive –  Shu~ng down ac've accounts is likely to annoy someone important eventually. Proac've password checking •  Don’t let them pick a “bad” password in the first place •  Need to have a fairly fast test of the “goodness” of a password Bloom Filter Space efficient probabilis'c data structure to tell whether a given element is a member of a set •  No false nega'ves –  If an element is not a member, the BF will not report that it is a member •  False posi'ves are possible Applica'on – determine whether a password given at crea'on is one of a large list of easily cracked passwords Bloom Filter •  Create N bit array •  Use k independent hash func'ons which hash into a space of 0 to N- 1 •  For each bad password bp, –  For every hash func'on h compute h(bp) in [0,N- 1] and set the corresponding bit in the hash table •  Each word marks up to k bits Bloom Filter •  To check a password – Computer every version of the hash, and check the corresponding bits in the array – If all bits are 1, then the password is bad •  What about false posi'ves? Bloom Filter Bloom Filter Using this expression you can answer ques'ons such as •  For a given N (output width of a hash func'on) and B (size of bad password dic'onary) how many hash func'ons (k) do I need to achieve a false posi've probability less than p? Token Based Authen'ca'on •  Memory Cards –  Stores data like an ATM card –  Does no computa'on –  General...
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