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... thank you for trying to explain. I am still too thick to get it on first read. I have saved this in Evernote and will keep coming back to it after researchi

... thank you for trying to explain. I am still too thick to get it on first read. I have saved this in Evernote and will keep coming back to it after researching terms etc., until I understand in full.

schoen 11y ago
I'm sure you'll want to keep reading about this, but let me just set out: Diffie-Hellman involves four numbers, which are called g, p, a, and b (and which are used to calculate other numbers). g and p are public, and are often used "for the long term" (including sharing g and p values between different Internet sites and services). The researchers found that the problem is that a lot of sites use the same p values, and the ones they use are too small, so that there is a single calculation that a government can do which allows them to break DH that uses those particular p values. Although this calculation is phenomenally expensive, it seems likely that NSA did it several years ago. a and b are private and must remain secret. Commonly a different a and b are chosen each time Diffie-Hellman is used (like for each new connection or session with an Internet service). Diffie-Hellman provides a property called forward secrecy because it allows two ends of a connection to derive a session key (which gets calculated using information derived from g, p, a, and b, and gets then used as a key for some other cryptosystem, normally some form of AES today, but in theory it could be anything) but then later "forget" what the session key was and how it was derived. Unlike other ways of doing key exchange, when the parties choose to "forget" their session key and associated information this way, they no longer have a way to recover or reconstruct it! This is useful because, for example, if someone hacks the computer of someone who was using protocols with ephemeral DH, the computer likely won't contain information that could be used to reconstruct the session key and decrypt old communications. (That's assuming that the software scrubs key material from memory when it's no longer needed... which might not always be true.)

schoen 11y ago

I'm sure you'll want to keep reading about this, but let me just set out: Diffie-Hellman involves four numbers, which are called g, p, a, and b (and which are used to calculate other numbers). g and p are public, and are often used "for the long term" (including sharing g and p values between different Internet sites and services). The researchers found that the problem is that a lot of sites use the same p values, and the ones they use are too small, so that there is a single calculation that a government can do which allows them to break DH that uses those particular p values. Although this calculation is phenomenally expensive, it seems likely that NSA did it several years ago. a and b are private and must remain secret. Commonly a different a and b are chosen each time Diffie-Hellman is used (like for each new connection or session with an Internet service). Diffie-Hellman provides a property called forward secrecy because it allows two ends of a connection to derive a session key (which gets calculated using information derived from g, p, a, and b, and gets then used as a key for some other cryptosystem, normally some form of AES today, but in theory it could be anything) but then later "forget" what the session key was and how it was derived. Unlike other ways of doing key exchange, when the parties choose to "forget" their session key and associated information this way, they no longer have a way to recover or reconstruct it! This is useful because, for example, if someone hacks the computer of someone who was using protocols with ephemeral DH, the computer likely won't contain information that could be used to reconstruct the session key and decrypt old communications. (That's assuming that the software scrubs key material from memory when it's no longer needed... which might not always be true.)