Let your plans be dark and impenetrable as night, and when you move, fall like a thunderbolt.

Yes. This is Lionel Giles's own English for The Art of War, chapter VII, section 19, in his numbering of the chapter. The Chinese is 难知如阴，动如雷震. His sentence is the one already in circulation: let your plans be dark and impenetrable as night, and when you move, fall like a thunderbolt. The two images are a plan that cannot be read, and a move that arrives all at once. The match is word for word. It is not a paraphrase, and it is not a modern compression.

The line sits in the chapter on manoeuvre, beside the comparisons with wind, forest, fire and mountain. It is one of the passages that really is in the book, in the 1910 words, which is rarer than quotation sites suggest. Cite it as Sun Tzu, VII.19, translated by Lionel Giles. The section number follows his numbering. Do not file this thunderbolt with later sentences that use his name and were never in the thirteen chapters. This one you can check.