A hallmark of "Mission: Impossible" was the message that would self-destruct after a spy played it.

Now a startup communications company promises that same level of secrecy with a Web-based messaging system designed to leave no traces.

The VaporStream system from Void Communications LLC is envisioned as a complement to e-mail and instant messaging, both of which leave abundant records.

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Let's say Alice wants to discuss something privately with Bob. Alice calls up a VaporStream Web page, which is encrypted by the same method that secures Internet commerce and banking. Then she selects Bob on her list of VaporStream chat partners.

That brings up a new window, where she can type a message. Neither her name nor Bob's appears anywhere. The individual messages cannot be copied or pasted into other programs.

When she sends the message, it no longer is visible on her computer. It goes to a server maintained by VaporStream, where it sits in a sort of holding pattern in a temporary segment of the server's memory.

When Bob checks his VaporStream Web page, he can see that he has a message from Alice and clicks to read it. When it is delivered, it leaves the VaporStream server for good.

When Bob responds, Alice's original message disappears from his computer. On and on it goes, in a conversation in which both parties have to remember their previous lines, making VaporStream more like a time-shifted phone conversation than an e-mail thread.

"Neither the sender nor the recipient has a full copy," said Amit Shah, the co-founder and chief technologist.

VaporStream is scheduled to be unveiled at the influential DEMOfall tech show in San Diego on Tuesday and become generally available in October.

Shah and co-founder Joseph Collins Jr. hope VaporStream's design and low cost — $40 per user annually — will attract companies that are swamped with the challenge of archiving business-critical e-mails and throwing away those of a personal or inconsequential nature.

A company could tell its employees to do all of their informal communications in VaporStream, for example. Besides PCs, VaporStream will be available for mobile gadgets such as BlackBerrys.

That's not to say that this is a natural for the business world.

Financial services firms, for example, are likely to reject VaporStream because of regulatory requirements governing the retention of their electronic communications. Other companies simply might not trust their workers enough to give them a record-less method of communication.

"I don't typically have customers come to me and say, 'I'm looking for a messaging system where I can hide all traces of what I'm saying,'" said Matt Brown, a senior analyst at Forrester Research.

Of VaporStream's overall prospects, he said, "I'm highly skeptical."

Companies also can set up "blacklists" and "whitelists" for their employees that dictate who can and cannot send VaporStreams to each other.

However, Nancy Flynn, founder of the ePolicy Institute, which trains companies on proper use of e-mail, said she suspects some businesses will welcome VaporStream because it could help them better articulate rules about when employees should and should not use e-mail.

Many e-mails have to be kept for audits, regulatory purposes or lawsuits, but personal messages that invariably get swept into that mix are often embarrassing, not to mention costly to store, she noted.

VaporStream isn't entirely dependent on businesses. Anyone can sign up for $40 a year.

But secrecy-seeking criminals, take note: While the system records no conversation logs, Collins said VaporStream will comply with wiretapping laws. That means the authorities would not be able to review past chats, but they could get warrants giving them the right to put an ear to future ones.