Tuesday, July 5, 2011

Len Sassaman on Bitcoin

Len Sassaman was a brilliant cryptographer and a true champion for privacy rights and privacy enhancing technologies. He passed away on July 3rd, 2011 at the age of 31. He will be sorely missed.

I had several opportunities to discuss bitcoin and cryptocurrencies generally with Len, with the most recent exchange on June 25th below. In posting this, it is my hope that anonymity considerations with bitcoin are well understood by the individual user and appropriate for the type of bitcoin transaction that is undertaken, as all security is relative. I had asked him "if his concerns about bitcoin traceability were more of a non-cryptographic nature? i.e., real-world identity?" Len's reply, in his own words:
"Well, it's hard to decouple cryptography from identity in a crypto-based currency, but, yes, my concerns are real-world identity issues. Bitcoin is less anonymous than physical cash. Compare to Digicash, which was 'more' anonymous. It's useful to speak about these things in more precise terms; using the Pfitzmann terminology, Bitcoin lacks unlinkability as a property. 'Throw Tor or Mixmaster at it' isn't a very satisfying answer, at least not to someone with an understanding of how those systems fail. My fear is people will associate 'bitcoin' and 'anonymous', get seriously burnt by the fact that it's 'not' anonymous, but rather a persistent ledger of all transactions ever, and then dismiss future e-cash that actually 'does' provide anonymity and unlinkability, etc., because they've 'heard that before.'

It may be possible to use bitcoin as a building-block in constructing an anonymous payment system; I'm skeptical, but what 'is' clear is the people currently advertising such systems haven't ever worked on traffic-analysis defeating protocols before. Any state-level adversary can link bitcoin back to the user's real-world identity — or at least, real-world computer. You're probably marginally better off with pre-paid visa cards exchanged through some kind of swap system like the cypherpunks used to do for Safeway cards, though I don't know if that exists. This is all somewhat of a distraction, though, since it's unclear to me that Bitcoin will survive the speculators; that's not what it was designed for, and it's showing. Further, the competence differential between the designers of bitcoin itself & the exchanges is staggering. Sorry for that flood of messages. I've added you to Skype; when's a good time to chat?"
For more Len, here's his presentation, "Anonymity for 2015: Why not just use Tor?" and another, "Towards a formal theory of computer insecurity: a language-theoretic approach".

7 comments:

  1. John,

    Can you clarify? what Len meant (or was referring to) when he wrote:

    "This is all somewhat of a distraction, though, since it's unclear to me that Bitcoin will survive the speculators; that's not what it was designed for, and it's showing. Further, the competence differential between the designers of bitcoin itself & the exchanges is staggering."

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  2. I cannot be certain but I believe that Len was referring to the secure systems immaturity of the leading web-based bitcoin exchanges as they appear to be patched together quickly. This makes sense for gaining market share rapidly but it does not make sense for stable resiliency.

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  3. Jon,

    Thanks! Thank you for providing this valuable resource!

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  4. Typo: "likability" should be "linkability"

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  5. @Anonymous,

    Thank you, it is corrected. I had originally left it the way Len had spelled it without correction.

    For more on the Pfitzmann terminology, see "A terminology for talking about privacy by data minimization: Anonymity, Unlinkability, Undetectability, Unobservability, Pseudonymity, and Identity Management" http://dud.inf.tu-dresden.de/literatur/Anon_Terminology_v0.32.pdf

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  6. Great insights from a very competent tech guy. He said: "it's unclear to me that Bitcoin will survive the speculators; that's not what it was designed for, and it's showing." He seems to be referring to bitcoin's price increases recently--up to 30 down to 12--as if to suggest that this volatility will hurt bitcoin's adoption by merchants. But what about recent events would be fatal to bitcoin?

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  7. From The Register (UK), Cryptographer Len Sassaman, RIP

    http://www.theregister.co.uk/2011/07/06/len_sassaman/

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