Se (e un grande se!) scegli i caratteri uniformemente a caso, da un insieme di 95, ciascuna scelta indipendentemente l'una dall'altra, quindi ciascuna delle 14 scelte casuali ha circa 6,6 bit di entropia. (Con matematica molto rozza che vale la pena memorizzare: una scelta casuale uniforme su 10 possibilità è di circa 3,3 bit, su 100 possibilità è di circa 6,6 bit.)
- 6,6 bit / carattere * 14 caratteri = 92,4 bit.
- 6,6 bit / carattere * 20 caratteri = 132 bit.
Sembra che la raccomandazione di TrueCrypt sia basata sulla scelta di utilizzare un algoritmo di crittografia a 128 bit come i caratteri ASCII stampabili AES-128-20 è la lunghezza più breve la cui massima sicurezza può essere all'altezza del livello di sicurezza a 128 bit dell'applicazione .
Direi che dovresti considerare di usare la loro raccomandazione di 20 caratteri, ma potresti anche seriamente prendere in considerazione l'idea di usare 14. Il Domande frequenti sulla passphrase di Diceware ha una voce in cui vengono discusse le lunghezze della passphrase consigliate per la crittografia, con un consiglio che, sebbene non sia per niente definitivo, offre una prospettiva importante:
A five-word Diceware passphrase has an entropy of at least 64.6 bits; six words have 77.5 bits, seven words 90.4 bits, eight words 103 bits, nine words 116 bits, ten words 129 bits. (Four words only provide 51.6 bits, about the same as an 8 character password made up of random ASCII characters. Both are breakable in less than a day with two dozen graphics processors.) Inserting one extra letter at random adds about 10 bits of entropy. Here is a rough idea of how much protection various lengths provide, based on updated estimates by A.K. Lenstra (See www.kelength.com). Needless to say, projections for the far future have the most uncertainty.
- Five words are breakable with a thousand or so PCs equipped with high-end graphics processors. (Criminal gangs with botnets of infected PCs can marshal such resources.)
- Six words may be breakable by an organization with a very large budget, such as a large country's security agency.
- Seven words and longer are unbreakable with any known technology, but may be within the range of large organizations by around 2030.
- Eight words should be completely secure through 2050.
Pick your passphrase size based on the level of security you want.
Inoltre, questo poco più in basso è sicuramente divertente e ottimo spunto di riflessione:
Another way to think about passphrase length is to consider what security precautions you take to physically protect your computer and data. Here is a list of possible passphrase lengths and commensurate security precautions. The list of precautions is not intended to be complete. I am not trying to discourage anyone from using longer passphrases if they feel up to it, but the added strength without comparable physical security for your computer is of limited value.
[...]
7 words
- Your computer is protected from unauthorized access at all times when not in your personal possession by being locked in a room or cabinet in a building where access is controlled 24 hours a day or that is protected by a high quality alarm service.
- Routine cleaning and building maintenance people do not have physical access to your computer when you are not present.
- You regularly use an up-to-date anti-virus program purchased off the floor at a computer store.
- You have verified the signatures on your copy of PGP or GPG, etc.
- You never run unverified downloaded software, e-mail attachments or unsolicited disks received through the mail on your computer.
[...]
8 words
You take all the steps listed under 7 words above, and:
- Your computer is kept in a safe or vault at all times when it is not in sight of you or someone you trust.
- Your computer was purchased off the floor at a randomly selected computer store.
- All the software used on your computer was distributed with a strong, independently verified electronic signature that you checked, or was purchased off the floor in a randomly selected computer store
- Your computer has never been repaired or upgraded by anyone you do not trust completely.
- All disks and tapes used with your computer are either kept in a safe or physically destroyed.
- You take precautions against audio and video surveillance when entering passphrases.
- You change your PGP encryption key regularly (at least once a year).
- You have taken precautions against TEMPEST attacks. See the chapter "Commonsense and Cryptography," in Internet Secrets, from IDG Books Worldwide, for a discussion of what this involves.
Penso che questi siano forse un po 'paranoici per la maggior parte delle persone (è la NSA o l'FSB che vogliono prenderti?), ma trasmettono un messaggio importante: rendere la tua password super-strong ha rendimenti decrescenti. Se sei vulnerabile a attacchi molto più semplici, sei così tanto bravo ad avere la passphrase più strong del mondo. Ecco come si giustifica la più comoda password di 14 caratteri su quella di 20 caratteri.