A piece of software called Haystack, which claimed to be an "anti-censorship" system to let people in Iran use the internet anonymously, has been withdrawn by its author after experts raised serious questions about its security.
After having obtained access by other means, the experts now say that instead of making users anonymous, it could reveal key information about them to the Iranian authorities.
In a post on his blog on Monday, Heap says that in the "vigorous debate" about Haystack's security "many of the points made were valid" and that users have been asked to stop using it.
Daniel Colascione, who worked with Heap and says he came up with the "Haystack" name, tweeted on Tuesday that the Censorship Research Center (CRC) that he co-created with Heap to host Haystack is now being wound down. But he also maintained that the software that has been criticised was not intended for widespread use, and was only a test version.
In March the US government granted Haystack an export licence, required for "sensitive" cryptographic software, following a fast-track approval process which does not seem to have included independent verification of its security.