trusted time stamping
To: cypherpunks@toad.com -----BEGIN PGP SIGNED MESSAGE----- I thought my idea about having trusted entities digitally sign a document in order to establish its existence at a particular time was a new idea, but I just read about it in _Applied Cryptography_. Anyway, I wrote some C code to do automatic time stamping with PGP (source code is in the next e-mail). If you just want to try it, simply send an e-mail to weidai@eskimo.com with the subject "Time Stamp This Mail". The body of the mail will be signed with a PGP private key (public key is at the end of this message) and returned to the sender. E-mail with any other subject will be piped to my regular mailbox. What's the use of this? Well, here is an interesting application of time stamping that wasn't covered in _Applied Cryptography_. Let's say Alice would like to publish an article anonymously but retain the ability to claim authorship some time later. She can follow this protocol: 1. Alice signs her article with RSA 2. She encrypts her signed article with IDEA 3. She sends the encrypted article to several trusted time stamping servers 4. She places the signatures she gets back along with the encrypted article in a safe place 5. She waits a random length of time 6. She posts the plain article (without encryption or signature) anonymously 7. When Alice wants to claim authorship, she publishes the encrypted article, the IDEA key, and the signatures she got back from the time servers Now, people can be reasonably sure that Alice actually wrote the original article because the time server signatures prove that she signed the article before it was made public. One problem here is that at least one of the time servers she used must have remained secure until step 7. Comments? Wei Dai PGP Public Key avaliable -----BEGIN PGP SIGNATURE----- Version: 2.3a iQCVAgUBLlRtiTl0sXKgdnV5AQFr+gQAsymOrN/Zd3C94NebWZOVFcl2tCkd/cSW EehvHxJMD1qO5fmmsDelhA+YKqqjLz8Dyp94pIqZXtWSu+kx/p5OUjB173PdAyN0 TSNaVMyZX266B/JIRqHI6+/5F2EWysFTXXH23v0mEH/us82Dvdb8rcqyKwQvjGZf mOvhObHf8Fo= =w0Q+ -----END PGP SIGNATURE----- -----BEGIN PGP PUBLIC KEY BLOCK----- Version: 2.3a mQBNAi5UWcgAAAECALp+QU9dtN2N4BjVr8OSxRPXtZ6UX4bLGq8PxpXru6WpsBD/ SJUl6jK4YcnatNJmjkl9oEHC6fjTpwMbZVOWJE0ABRG0MFdlaSdzIFBHUCBUaW1l IFN0YW1wIFNlcnZpY2UgPHdlaWRhaUBlc2tpbW8uY29tPokAVQIFEC5UWpenAxtl U5YkTQEBzDQB/3+eNgnW22sRaZFpBY3Wfzj4uEVXXcYU4vrdS1fsSRixJSEKta/N uyvmkeiB4GyyahhtHTtybywrRzD1y9IlwMmJAJUCBRAuVFmZOXSxcqB2dXkBAYNZ A/4/KHOQ1gjPEkdLhdPJ/yaXyQilqWV+MWiHblrqcDOrsFu1dKizJrBdWa5+vuIX nCu5DSq9cd3/cGrMOYK3OJGQC8JkPc6LNw7siuRGuVn413JBlM3wnCEXnFsAUhpG hDLTPUC2JqmiCwQP6OpxwqlTxPmZk8wKE0Sh/iaGRwZnBg== =vpgO -----END PGP PUBLIC KEY BLOCK----- PGP Public Key available
Wei Dai wrote:
I thought my idea about having trusted entities digitally sign a document in order to establish its existence at a particular time was a new idea, but I just read about it in _Applied Cryptography_. Anyway, I wrote some C code to do automatic time stamping with PGP (source code is in the next e-mail).
Stu Haber (who reads this list, sometimes) and Scott Stornetta of Bellcore developed a system which solves the more important problem of the time stamper reliability, which I don't think W.D. has addressed. I've written up a couple of summaries, the last of which got a favorable reaction from Stu on. So I'll mail it later today, when I fire up my off-line archives and retrieve it. The hard part is time stamper reliability, i.e., how does the world (and the courts) know that the time stamper(s) did not simply reset his clock and thus fake the times? Haber and Stornetta came up with two clever ideas: 1. Publish a one-way hash of the text to be stamped in a very public place, e.g., one's latest bestselling novel or the "New York Times." This is similar to the crypto methods used by scientists through the ages to prove ownership. H & S call this a "widely witnessed event," the idea being that millions of copies of archived issued of the NYT (or the novel!) would have to be retrieved and reprinted in order to change at a later date the text. Economically impractical. 2. But it may also be economically impractical for the NYT to print page after page of such hashes...they may choose not to, understandably. So H & S developed a "tree"-like way to merge customer-provided hashes with many other hashes (and earlier hashes, to, thus adding to the difficulty of faking) and so to only have to publish a comparatively small number. These two clevernesses are the crux of time-stamping. They are trying to build a company to do this; perhaps Stu can update us on the status. --Tim May -- .......................................................................... Timothy C. May | Crypto Anarchy: encryption, digital money, tcmay@netcom.com | anonymous networks, digital pseudonyms, zero 408-688-5409 | knowledge, reputations, information markets, W.A.S.T.E.: Aptos, CA | black markets, collapse of governments. Higher Power: 2^859433 | Public Key: PGP and MailSafe available. "National borders are just speed bumps on the information superhighway."
They are trying to build a company to do this; perhaps Stu can update us on the status. I don't know if Stu's on the list right now or not, but I saw him Tuesday in Manhattan. They're in the middle of development, which includes much more than simply writing the crypto protocol that's at the core of any real business. Eric
participants (3)
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hughes＠ah.com -
tcmay＠netcom.com -
weidai＠eskimo.com