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In cryptanalysis and computer security, password cracking is the process of recovering passwords from data that have been stored in or transmitted by a computer system. A common approach (brute-force attack) is to try guesses repeatedly for the password and check them against an available cryptographic hash of the password. The purpose of password cracking might be to help a user recover a forgotten password (installing an entirely new password is less of a security risk, but it involves System Administration privileges), to gain unauthorized access to a system, or as a preventive measure by System Administrators to check for easily crackable passwords. On a file-by-file basis, password cracking is utilized to gain access to digital evidence for which a judge has allowed access but the particular file's access is restricted. ==Time needed for password searches== The time to crack a password is related to bit strength (''see'' password strength), which is a measure of the password's information entropy, and the details of how the password is stored. Most methods of password cracking require the computer to produce many candidate passwords, each of which is checked. One example is brute-force cracking, in which a computer tries ''every'' possible key or password until it succeeds. More common methods of password cracking, such as dictionary attacks, pattern checking, word list substitution, etc. attempt to reduce the number of trials required and will usually be attempted before brute force. Higher password bit strength exponentially increases the number of candidate passwords that must be checked, on average, to recover the password and reduces the likelihood that the password will be found in any cracking dictionary. The ability to crack passwords using computer programs is also a function of the number of possible passwords per second which can be checked. If a hash of the target password is available to the attacker, this number can be quite large. If not, the rate depends on whether the authentication software limits how often a password can be tried, either by time delays, CAPTCHAs, or forced lockouts after some number of failed attempts. Another situation where quick guessing is possible is when the password is used to form a cryptographic key. In such cases, an attacker can quickly check to see if a guessed password successfully decodes encrypted data. For example, one commercial product claims to test 103,000 WPA PSK passwords per second.〔(Elcomsoft Wireless Security Auditor, HD5970 GPU ), accessed 2011-2-11〕 For some kinds of password hash, ordinary desktop computers can test over a hundred million passwords per second using password cracking tools running on a general purpose CPU and billions of passwords per second using GPU-based password cracking tools.〔(oclHashcat-lite – advanced password recovery ). Hashcat.net. Retrieved on 2013-01-31.〕〔Alexander, Steven. (June 20, 2012) (The Bug Charmer: How long should passwords be? ). Bugcharmer.blogspot.com. Retrieved on 2013-01-31.〕〔(Cryptohaze Blog: 154 Billion NTLM/sec on 10 hashes ). Blog.cryptohaze.com (2012-07-15). Retrieved on 2013-01-31.〕 See: John the Ripper benchmarks.〔(John the Ripper benchmarks ). openwall.info (2010-03-30). Retrieved on 2013-01-31.〕 The rate of password guessing depends heavily on the cryptographic function used by the system to generate password hashes. A suitable password hashing function, such as bcrypt, is many orders of magnitude better than a naive function like simple MD5 or SHA. A user-selected eight-character password with numbers, mixed case, and symbols, with commonly selected passwords and other dictionary matches filtered out, reaches an estimated 30-bit strength, according to NIST. 230 is only one billion permutations and would be cracked in seconds if the hashing function is naive. When ordinary desktop computers are combined in a cracking effort, as can be done with botnets, the capabilities of password cracking are considerably extended. In 2002, distributed.net successfully found a 64-bit RC5 key in four years, in an effort which included over 300,000 different computers at various times, and which generated an average of over 12 billion keys per second.〔(【引用サイトリンク】 64-bit key project status )〕 Graphics processors can speed up password cracking by a factor of 50 to 100 over general purpose computers. As of 2011, available commercial products claim the ability to test up to 2,800,000,000 passwords a second on a standard desktop computer using a high-end graphics processor.〔(ElcomSoft Password Recovery Speed table ), NTLM passwords, Nvidia Tesla S1070 GPU, accessed 2011-2-1〕 Such a device can crack a 10 letter single-case password in one day. Note that the work can be distributed over many computers for an additional speedup proportional to the number of available computers with comparable GPUs. Despite their capabilities, desktop CPUs are slower at cracking passwords than purpose-built password breaking machines. In 1998, the Electronic Frontier Foundation (EFF) built a dedicated password cracker using ASICs, as opposed to general purpose CPUs. Their machine, Deep Crack, broke a DES 56-bit key in 56 hours, testing over 90 billion keys per second.〔(【引用サイトリンク】 EFF DES Cracker machine brings honesty to crypto debate )〕 In 2010, the Georgia Tech Research Institute developed a method of using GPGPU to crack passwords, coming up with a minimum secure password length of 12 characters.〔(【引用サイトリンク】 title=Teraflop Troubles: The Power of Graphics Processing Units May Threaten the World's Password Security System )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Password cracking」の詳細全文を読む スポンサード リンク
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