Black fax

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The term black fax refers to a prank fax transmission, consisting of one or more pages entirely filled with a uniform black tone and often made into a loop the sending machine will transmit endlessly. The sender's intention is typically to consume as much of the recipient's fax ink, toner or thermal paper as possible, thus denying its owner its use (this is similar to computer-based denial of service attacks). A bonus for the malicious sender is that, in cases where the receiving fax machine prints the fax using water-based ink onto conventional paper, the saturated paper may disintegrate inside the fax machine's mechanism, thus entirely blocking it. Black faxes can be particularly effective as the CCITT fax algorithm compresses the solid black image very well, so a very short fax call can produce many pages.

Black faxes have been used to harass large institutions or government departments, to retaliate against the senders of junk faxes, or merely as simple pranks.

The basic principle of a black fax can be extended to form a fax attack. In this case, a single sheet is fed halfway through the sender's fax machine and taped end to end, forming an endless loop that cycles through the machine. Not only can solid black be used, but also images which will repeat endlessly on the receiver's machine until their toner runs out.

The introduction of computer-based facsimile systems (combined with integrated document imaging solutions) at major corporations now means that black faxes are unlikely to cause problems for larger corporations. Alternatively, the ability for computer modems to send faxes offers new avenues for abuse. A program could be used to generate hundreds of pages of perfectly-compressed, pure black and send them very quickly to the target fax machine.

Black faxes are similar (in both intention and implementation) to lace cards.