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*To*: [email protected]*Subject*: trusted time stamping*From*: [email protected] (Wei Dai)*Date*: Fri, 19 Aug 1994 01:42:57 -0700*Sender*: [email protected]

To: [email protected] -----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 [email protected] 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

**Follow-Ups**:**Re: trusted time stamping***From:*[email protected] (Timothy C. May)

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