[URL]http://world.std.com/~reinhold/dicewarefaq.html[/URL]\n\nHow long should my passphrase be?\n\nI recommend five words for most users.\n\nIn their February 1996 report, "Minimal Key Lengths for Symmetric\nCiphers to Provide Adequate Commercial Security" a group of\ncryptography and computer security experts -\- Matt Blaze, Whitfield\nDiffie, Ronald Rivest, Bruce Schneier, Tsutomo Shimomura, Eric\nThompson, and Michael Weiner -\- stated:\n\n "To provide adequate protection against the most serious threats...\nkeys used to protect data today should be at least 75 bits long. To\nprotect information adequately for the next 20 years ... keys in newly-\ndeployed systems should be at least 90 bits long."\n\nA five-word Diceware passphrase has an entropy of at least 64.6 bits;\nsix words have 77.5 bits, seven words 90.4 bits, eight words 103 bits,\nfour words 51.6 bits. Inserting an extra letter at random adds about 10\nbits of entropy. Here is a rough idea of how much protection various\nlengths provide, based on updated estimates by A.K. Lenstra (See\n[URL="http://www.kelength.com"]www.kelength.com[/URL]). Needless to say, projections for the far future have\nthe most uncertainty.\n\n * Four words are breakable with a hundred or so PCs.\n * Five words are only breakable by an organization with a large\nbudget.\n * Six words appear unbreakable for the near future, but may be\nwithin the range of large organizations by around 2014.\n * Seven words and longer are unbreakable with any known technology,\nbut may be within the range of large organizations by around 2030.\n * Eight words should be completely secure through 2050.\n\nPick your passphrase size based on the level of security you want.\n\nAnother way to think about passphrase length is to consider what\nsecurity precautions you take to physically protect your computer and\ndata. Here is a list of possible passphrase lengths and commensurate\nsecurity precautions. The list of precautions is not intended to be\ncomplete. I am not trying to discourage anyone from using longer\npassphrases if they feel up to it, but the added strength without\ncomparable physical security for your computer is of limited value.\n\n4 words\n * You would be content to keep paper copies of the encrypted\ndocuments in an ordinary desk or filing cabinet in an un-secured office.\n\n5 words\n * You need or want strong security, but take no special precautions\nto protect your computer from unauthorized physical access, beyond\nlocking the front door of your house or office.\n\n6 words\n * Your computer is protected from unauthorized access at all times\nwhen not in your personal possession by being locked in a room or\ncabinet in a building where access is controlled 24 hours a day or that\nis protected by a high quality alarm service.\n * Routine cleaning and building maintenance people do not have\nphysical access to your computer when you are not present.\n * You regularly use an up-to-date anti-virus program purchased off\nthe floor at a computer store.\n * You have verified the signatures on your copy of PGP or your\ninstalled Hushmail 2 client.\n * You never run unverified downloaded software, e-mail attachments\nor unsolicited disks received through the mail on your computer.\n\nNote: However I do encourage using six or more words on systems that\nuse the passphrase directly to form a transmission key. Such systems\ninclude Hushmail, disk encryption (e.g. Apple's FileVault),\nCiphersaber, and WiFi's WPA.\n\n7 words\n * You take all the steps listed under 6 words above, and:\n * Your computer is kept in a safe or vault at all times when it is\nnot in sight of you or someone you trust.\n * Your computer was purchased off the floor at a randomly selected\ncomputer store.\n * All the software used on your computer was distributed with a\nstrong, independently verified electronic signature that you checked,\nor was purchased off the floor in a randomly selected computer store\n * Your computer has never been repaired or upgraded by anyone you\ndo not trust completely.\n * All disks and tapes used with your computer are either kept in a\nsafe or physically destroyed.\n * You take precautions against audio and video surveillance when\nentering passphrases.\n * You change your PGP encryption key regularly (at least once a\nyear).\n * You have taken precautions against TEMPEST attacks. See the\nchapter "Commonsense and Cryptography," in Internet Secrets, from IDG\nBooks Worldwide, for a discussion of what this involves.\n\nFor people seeking long term data protection (greater than 10 years) I\nwould recommend adding one word to the above suggestions.