Exercises Information Security Course Eric Laermans – Tom Dhaene

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 2 Introduction Password storage in MS Windows old system  LM hash (LAN Manager hash) –untill Windows Me new system  NTLM hash (NT LAN Manager) –since Windows NT 3.1

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 3 Introduction Password storage in MS Windows encoded storage  in SAM (Security Accounts Manager) –non-accessible while OS is active »file locked by OS when Windows is operating (impossible to read, copy or remove) –QUESTION 1: »still possible to access file to test passwords offline?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 4 LM Hash Limitations passwords of at most 14 ANSI-characters  95 possible characters  a.k.a. “printable ASCII”

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 5 LM Hash Operation 1. converting lower case to upper case 2. adding NULL-characters to obtain 14 characters 3. splitting in two sequences of 7 characters 4. each of these sequences is used as a key to encrypt (ECB)  results in two encoded blocks of 8 bytes  the thus obtained 16 bytes are the LM Hash QUESTION 2  possible attacks, weaknesses?  estimated time required for possible attack?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 6 NTLM Hash Operation MD4-hash of password  case-sensitive password  MD4: hash function with 128 bits hash value –predecessor of MD5 –strong collision resistance totally broken –effective strength as a one-way-function (preimage resistance) only 102 bits »rather theoretical weakness, not really practical QUESTION 3  comparie with present password storage in Linux?  reasonable time to crack?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 7 Backward compatibility double password storage using NTLM Hash using LM Hash  if possible, otherwise fake value  default up to Windows XP –can be disabled by registry modification –disabled by default since Windows Vista QUESTION 4  weaknesses of this scheme?  better than LM Hash only?  how can you make sure LM Hash is not stored?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 8 Improved attacks Attacks until now (generally) feasible if LM Hash is available  but still requires quite a lot of compuation time if brute force is used QUESTION 5  suggestions to improve the attack technique? –hint: can part of the job be precomputed?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 9 Improved attacks Precomputed hash chains not feasible to precompute and store all encoded passwords  QUESTION 6: –how much storage would be required for password encoded using LM Hash?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 10 Improved attacks Precomputed hash chains how can we select the password we want to store?  precomputed hash chains –technique using trade-off between required computation time and required storage –for N possible passwords: »storage:O(N 2/3 ) »computation time:O(N 2/3 )

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 11 Improved attacks Precomputed hash chains two functions  hash function H:P  C –transforms password into encoded password –domain: space of possible passwords (P) –range: space of possible hash values (C)  reduction function R:C  P –derives a (pseudorandom) password from hash value »doesn’t need to be a one-way-function »simple choice possible –domain: space of possible hash values (C) –range: space of possible passwords (P)

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 12 Improved attacks Precomputed hash chains choose a (sufficiently large) number (n) of different passwords  p j,0 (with j:0..(n-1)) compute (not too large) a number (k) of links for each chain  p j,i+1 = R(H(p j,i ))(with i:0..(k-1)) only store the start and end points of the chains  p j,0 and p j,k (with j:0..(n-1))

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 13 Improved attacks Precomputed hash chains cracking an encoded password h  compute:p (0) = R(h)  compute:p (i) = R(H(p (i-1) )) –until some p (i) is found which is present in the table of end points p j,k of the hash chains  recompute the chain, starting from p j,0 until the right value p j,k-i-1 is found, such that H(p j, k-i-1 ) = h NOTE:  some chains may overlap  chains may contain loops  false positives are possible

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 14 Improved attacks Precomputed hash chains required improvement upon basic approach  multiple tables –each with different reduction function –reducing impact op overlapping chains –number typically proportional to chain length »drawback: larger search time (proportional to chain length and number of chains)

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 15 Improved attacks Precomputed hash chains possible simplification  “distinguished points” –stop chain computation when easily distinguishable password is reached (instead of fixed length chains) »e.g. starting / ending with 10 null-bits  QUESTION 7: –what is the advantage of this approach?

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 16 Improved attacks Rainbow tables improvement over precomputed hash chains  using different reduction function for each link in the chain –k reduction functions R i needed(with i:0..(k-1)) –p j,i+1 = R i (H(p j,i ))(with i:0..(k-1))  look up encoded password h –compute p (0,0) = R k-1 (h) and lookup in table of end points –if not found, look up p (1,1) = R k-1 (H(R k-2 (h))) –if needed, continue with p (i,i) = R k-1 (H(p (i,i-1) )) »with p (i,j) = R k-i+j-1 (H(p (i,j-1) ))

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 17 Improved attacks Rainbow tables advantages  fewer lookups than with multiple tables for precomputed hash chains –approximately half as many  fewer overlapping chains –and easier to identify which chains merge  no loops in chains  chains of constant length –in opposition to “distinguished points”

Information Security Vakgroep Informatietechnologie – IBCN – Eric Laermans p. 18 Improved attacks Rainbow tables references:  P. Oechslin, “Making a faster cryptanalytic time- memory trade-off,” Advances in Cryptology - CRYPTO 2003, pp  project RainbowCrack