Cloak and dagger
Concerned with, or characteristic of espionage or intrigue, especially in the context of drama.
'Cloak and dagger' has the same feel as the expression 'smoke and mirrors' in that they both conjure up images of subterfuge and deceit. A form of drama that was popular in France and Spain in the 18th century, which included protagonists who typically wore cloaks and carried daggers was called 'de cape et d'épée' and, in Spanish, 'de capa y espada', which both translate literally as 'of cloak and sword'. The cloak was wrapped around one arm as a form of shield and the dagger, or sword, was used for fighting. That appears not to be the source of the English term 'cloak and dagger', which was in use in English prior to the 18th century.
An alternative use of the cloak was to conceal the identity, in the manner of a treacherous assassin. That explanation seems to chime best with the earliest example of a figurative allusion to 'cloak and dagger' that I have found in print. That is found in a letter printed in The Derby Mercury, July 1769, under the title of "A Speech of a Nobel Earl to a Great Personage":
"...and those that endeavour to dissolve it [the Union of Great Britain], carry a dagger under the cloak of patriotism, to stab their country in the heart."
The 'Noble Earl' is anonymous but, given the date, we can assume that the 'Great Personage' was George III.
Other early examples in print also use the imagery of a dagger concealed under a cloak, like this from The Morning Post, September 1836:
"...carrying a dagger against the Church, under the capacious cloak of economy."
Cloaks and daggers had been referred to in print prior to the 1840s but, if anyone can claim to have brought the expression 'cloak and dagger' to the English language, it was Charles Dickens. In Barnaby Rudge, 1841, he made a sardonic reference to the type of melodramas that employed the cloak and dagger as stage devices:
...his servant brought in a very small scrap of dirty paper, tightly sealed in two places, on the inside whereof was inscribed in pretty large text these words: 'A friend. Desiring of a conference. Immediate. Private. Burn it when you've read it.'
'Where in the name of the Gunpowder Plot did you pick up this?' said his master.
It was given him by a person then waiting at the door, the man replied.
'With a cloak and dagger?' said Mr Chester.