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matthew_d_green's profile
Matthew Green
Matthew Green
Matthew Green
@matthew_d_green

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Matthew Green

@matthew_d_green

I teach cryptography at Johns Hopkins.

Baltimore, MD
blog.cryptographyengineering.com
Joined January 2010

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    Matthew Green‏ @matthew_d_green 16 Apr 2018
    • Report Tweet

    Guide to iOS estimated passcode cracking times (assumes random decimal passcode + an exploit that breaks SEP throttling): 4 digits: ~13min worst (~6.5avg) 6 digits: ~22.2hrs worst (~11.1avg) 8 digits: ~92.5days worst (~46avg) 10 digits: ~9259days worst (~4629avg)

    7:17 AM - 16 Apr 2018
    • 1,290 Retweets
    • 2,008 Likes
    • Alex О'Кейси Mackenzie Price Łukasz Woźnicki W. Tad Morgan Rena 🦊 Manou🤘🏻 Joshua Olvera int0x13h
    55 replies 1,290 retweets 2,008 likes
      1. New conversation
      2. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        A lot of people responding to this saying “why decimal, why not use an alphanumeric passcode?” Sure, go for it. But keep in mind that unless you choose your password very well you might not be *that* much better off.

        5 replies 10 retweets 82 likes
        Show this thread
      3. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        In iOS when you choose a passcode, the system defaults to six digits. But under “passcode options” you get a choice of a custom numeric passcode or a custom alphanumeric passcode. (Let’s not talk about the 4-digit option. 💩)pic.twitter.com/3LouPSYYbz

        4 replies 26 retweets 87 likes
        Show this thread
      4. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        If you choose “numeric” then each time you log in you’ll see a big, friendly number keypad. If you choose alphanumeric you’ll get a typical iOS keyboard with tiny keys, and a Shift/123 key for caps/symbols/numbers.pic.twitter.com/7OmOHv6ddG

        3 replies 17 retweets 82 likes
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      5. Matthew Green‏ @matthew_d_green 16 Apr 2018
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        So the question now is: for a given level of security, which of these interfaces is easier for *you* to operate quickly. Which type of passcode is easier for you memorize?

        6 replies 6 retweets 75 likes
        Show this thread
      6. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        Alphanumeric has two hypothetical advantages. If you pick the password *truly at random* you can get more entropy (strength) from each alpha button press, so your password could be shorter. But those fewer keystrokes come at the cost of squinting at small keys.

        1 reply 6 retweets 54 likes
        Show this thread
      7. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        But of course we all know that none of you are picking your alphanumeric passcode at random. You’re all using Kitty123.

        12 replies 14 retweets 152 likes
        Show this thread
      8. Matthew Green‏ @matthew_d_green 16 Apr 2018
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        (Crap now I have to change my passcode.)

        7 replies 9 retweets 213 likes
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      9. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        In all seriousness, if you’re using the alphanumeric option to pick a *non-random* passcode (as most people do) then it’s much harder to tell how much password strength you’re getting. Some, a lot. Some less than they think.

        6 replies 7 retweets 60 likes
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      10. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        In practice (given the apparent limitations of current iOS attacks) it’s probably fine to use an alphanumeric passcode. It may be very secure. The real question is whether it’s worth the hassle of using the non-numeric keypad.

        8 replies 6 retweets 44 likes
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      11. Matthew Green‏ @matthew_d_green 16 Apr 2018
        • Report Tweet

        So in summary: random (long) numeric passcodes may be a more ergonomic choice for a given security level, especially if you force yourself to memorize the thing over a week or two (even if that involves carrying a post-it during that time.) But do whatever works for you.

        17 replies 13 retweets 85 likes
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      12. End of conversation

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