The ability to hold up a smartphone camera to a face in the street or a room or a crowd and have the device recognise that face and give the biographical details associated with it, is the K A of the C.
In June, Facebook, owner of the largest repository of peoples' photos on the planet, bought face.com, an Israeli firm with face recognition software.
The great killer app looked to be on the way. In fact a limited version was in use on Facebook.
But now Facebook has been ordered by the Irish Data Protection Commissioner, Billy Hawkes, to turn off its face recognition feature.
However Facebook intends the disabling of the face recognition tool to be only temporary.
"Our intention is to reinstate the tag-suggest feature, but consistent with new guidelines,'" says Facebook's EMEA director of policy Richard Allan.
Somehow, one doesn't think that Facebook is too scared of the Irish Data Protection Commission, especially when Facebook's European HQ is in Ireland to take advantage of Ireland's 12.5% corporation tax rate.
Would Ireland stand up for the human race's right to privacy if it came to losing the tax take on Facebook?
No one knows. But if Irish sinews need stiffening, other countries are there to help.
The EC's Article 29 Data Protection Working Party has ruled that: "Once consent is withdrawn, processing for the purposes of facial recognition should stop immediately."