Method for Non-Invasive Certified Mail
Original Publication Date: 2001-Apr-14
Included in the Prior Art Database: 2003-Jun-18
A method to achieve certfied mail in a fair as well as non-invasive manner is described. This is the first technique to achieve non-invasiveness. Certified mail is a well studied problem where one player (A) sends a message to another player (B) and expects a non-repudiable receipt in return. This is an instance of the general problem of "fair exchange". As usual, the requirement from A's point of view is that if she does not get a receipt, then B does not gain any information about the content of the message. Similarly B's requirement is that if he does not get the whole message, A must not get any advantage in obtaining a receipt that is non-repudiable by B. There have been various solutions to this problem with various levels of practicality. The so-called "optimistic fair exchange" solutions, which currently represent the most practical approaches to certified mail, make use of a trusted third party to implement certified mail. However, the third party is used only in exceptional circumstances. All the certified mail protocols of this type specify the format of the receipt. In this sense, these protocols are "invasive". It is likely that the format of receipts are decided due to other considerations (e.g., legal and business practice). Hence non-invasive protocols can be very attractive. In this article, a certified mail protocol is described which is non-invasive. The protocol is based on the known optimistic fair-exchange protocol (N. Asokan, M. Schunter, M. Waidner: "Optimistic Protocols for Fair Exchange"; Proc. 4th ACM Conf. on Computer and Communication Security, Zurich, April 1997, pp.6-17, US patent appl. 09/181442) and uses the same building block called verifiable encryption.