The key to BLACK was zero

The key to BLACK was zero is the expression for the cipher’s decisive paradox: what appeared to be an extraordinarily difficult lock had no real lock at all. The answer turns a search for a hidden key into recognition of its absence. It also links BLACK’s strange design to the adversary’s fear of Rong Jinzhen, and to the limits of a method that assumes every cipher must yield to effort, patience, and deduction.

A cipher approached through keys and doors

The account sets the discovery against the ordinary work of cryptography. Intelligence agents first gather relevant information; cryptographers then form hypotheses and test them, in effect making keys for a limitless number of doors. The amount of material available and the solver’s sensitivity to the cipher shape the search. Though the method is described as simple and even stupid, it is also safe, secure, effective, and particularly useful against high-level ciphers. Its strength lies in systematic elimination rather than a sudden leap to an answer.

Rong Jinzhen did not want to proceed in this traditional way. He entered the history of cryptography, drawing on the work of earlier generations even though the field is portrayed as a lonely forest in which practitioners cannot readily follow an open path. That historical search might narrow an immense field of possibilities: if there were ten thousand byroads, it could perhaps eliminate half, or fewer. But narrowing the search was not the same as finding the key. In the later metaphor, Rong Jinzhen had located BLACK’s distant house and its one genuine door among countless false ones, yet the door remained unopened.

The long failure and the missing key

For years Rong Jinzhen waited for a weakness in BLACK. The account likens ciphers to an enormous, seamless net that becomes vulnerable through the slips made when it is used. A leak can split the net like a rivulet of blood, giving a sharp mind an opening into the cipher’s labyrinth. Rong Jinzhen watched for such gaps with patience through countless days and nights, but found no errors. This failure was presented as irregular and strange precisely because the established method depended on finding such an opening.

The contrast with PURPLE sharpened the puzzle. Breaking PURPLE had made the adversary more cautious: its users took care not to let anything slip. Unit 701 consequently felt invulnerable, but Rong Jinzhen’s inability to find a flaw in BLACK suggested a further possibility. Liseiwicz knew Rong Jinzhen’s talent and could have warned BLACK’s creators, helping them devise countermeasures. The two men had once been like father and son, but their political positions and beliefs had created a spiritual gulf. When Unit 701 learned that Liseiwicz was Weinacht, its members wanted to warn Rong Jinzhen about the deception. His angry response—“Tell him to go to hell, this devil in the temple of science!”—shows the personal charge behind the technical impasse. It did not, however, reveal BLACK’s key.

The answer: nothing

The answer came in the account of an older man’s interview. In the middle of the night, the speaker used the Forty-Thousand-Calculation Calculator gifted to Rong Jinzhen by Headquarters to test an assumption. He describes finally deciphering BLACK, then rushing out of the underground mountain-cave offices and falling to the ground, wailing and worshipping heaven and earth. It was still dark, just before dawn. The dramatic emotional release marks the reversal: what had resisted years of patient searching could be opened by recognizing that the expected mechanism did not exist.

The revelation is given in emphatic steps. The cryptographic key to BLACK was in a leather attaché case; BLACK had no real lock; its key was the number zero. The explanation immediately strips the number of any suggestion of a concealed, elaborate device: it was nothing, absolute nothing. The wording holds two ideas together. Zero is named as the key, but the cipher’s trick is that there is no operative key to discover. The solution is not another intricate instrument made to fit an intricate lock; it is the realization that the apparent lock is a false façade.

The speaker’s image of BLACK as a house concealed in the sky clarifies why the difference matters. A solver must find the house, distinguish one genuine door from countless identical false doors, and then locate the key that opens it. Rong Jinzhen had already found the house and the real door, but not the key. The zero solution recasts his failure: his painstaking work had taken him remarkably close, while the last step depended on questioning the assumption that the door was truly locked. The account stresses that even after the façade yielded, the result seemed unreal to the person who had found it.

Why BLACK was made, and what the solution leaves behind

The interview’s retrospective explanation connects BLACK’s design to the breaking of PURPLE. The adversary had been dejected and felt its work had reached a dead end. A single confrontation with Rong Jinzhen had shown that he was a formidable talent; continuing to challenge him directly seemed likely to bring about the adversary’s destruction. BLACK was therefore created as a freakish, malicious response: a cipher with no key, devised in the hope of frustrating the codebreaker who had defeated PURPLE.

Yet the response did not erase Rong Jinzhen’s agency. The older man says that Rong Jinzhen had a final countermove ready: through his notebook, he had already passed on to his colleagues the secret of BLACK’s strange birth. This is distinct from the older man’s later account of discovering the numerical answer. BLACK’s origin had been recorded and transmitted by Rong Jinzhen, while the interview discloses how its apparent lock could be opened. The expression therefore names a technical solution and marks a reversal in the contest: the enemy’s attempt to turn Rong Jinzhen’s exceptional skill against him could not prevent him from leaving knowledge behind. In the interview’s telling, zero is at once the cipher’s key and the exposure of a trap built on the expectation that every lock must have one.