The Law of Mutual Exclusion Between Cryptographic Creation and Decryption

The Law of Mutual Exclusion Between Cryptographic Creation and Decryption is an unwritten, nearly ironclad principle of cryptography: an individual may create ciphers or crack them, but should not undertake both. In Rong Jinzhen’s case, the law is more than professional caution. His ability to make and destroy ciphers is described as a force that can enthrall his own mind, turning exceptional talent into a risk of fragmentation and madness. Yet Unit 701 entrusts him with BLACK anyway, making his assignment a test of whether even a mind already claimed by one cipher can cross into another domain and return.

The Principle and Its Rationale

The rule is tied to cryptography’s demand for distinct, non-repeating systems. If similar ciphers appeared, the account warns, they would become worthless. Against that background, creation and decryption are treated as opposing callings rather than complementary skills: an individual who makes ciphers should not also be responsible for breaking them. The law is unwritten, but its force is presented as practical and psychological, not merely bureaucratic.

The danger is especially acute for Rong Jinzhen because he possesses both capacities. His absorption in his own mind is figured as a kind of loss: to be enraptured by the ability to create and destroy ciphers is, paradoxically, to risk discarding or losing that mind. The rule therefore describes a boundary meant to protect the person as much as the integrity of cryptographic work. For Rong Jinzhen, the prescribed condition for taking on another cipher is the ability to re-forge the fragmentation that the work has made inevitable.

Methods and the Forbidden Past

The law also sets Rong Jinzhen apart from the ordinary procedure for decryption. The conventional method begins with intelligence agents gathering relevant information; the cryptographer develops hypotheses from that material and tests them through a process likened to making keys for an enormous number of doors. The work is painstaking, but the account calls it safe, secure, and effective, particularly against high-level ciphers. Its slow elimination of possibilities makes available information and the analyst’s sensitivity to the cipher part of the method itself.

Rong Jinzhen does not want to proceed in this conventional way. Instead, he studies cryptographic history, entering what the narrative calls forbidden territory and trying to stand on the work of earlier generations. That choice reverses cryptography’s usual isolation: its history is portrayed as a silent, lonely forest in which practitioners cannot freely ask directions or simply follow an open path. Historical knowledge might narrow the search—perhaps by eliminating some of many possible side roads—but cannot guarantee success. The method offers a possible advantage while exposing Rong Jinzhen to precisely the transgression the rule warns against.

The Assignment to Break BLACK

The contrast between PURPLE and BLACK makes the law immediate. After PURPLE was cracked, the adversary became more guarded, taking care to avoid the mistakes that might reveal a cipher’s weaknesses. Rong Jinzhen found no such errors in BLACK. Unit 701 considered whether Liseiwicz, who understood Rong Jinzhen’s decryption ability, could have warned BLACK’s creators and helped them develop countermeasures. Their former closeness, described as resembling a father-and-son relationship, had given way to a spiritual gulf shaped by their political positions and beliefs.

Despite the rule, Unit 701 places the burden of BLACK on Rong Jinzhen. The organization does not treat his case as ordinary: it trusts his talent to rebuild his mind, even when it cannot trust the rule or its own ability to judge what is possible for him. This is not the self-directed incursion into cryptographic history that marked his work on PURPLE. The account emphasizes that the second entry into the forbidden zone is imposed by others and by the force of Rong Jinzhen’s reputation. That reputation, once a source of confidence, becomes a pressure drawing him toward danger.

The Zero Key and the Law’s Limit

BLACK is eventually deciphered through a discovery that overturns the expectation of a conventional lock. Rong Jinzhen recognizes that its key is in a leather attaché case: the key is zero, and the apparent lock has no real lock behind it. The door seems secured, but it opens with a push. The solution is so strange that even after seeing what lies beyond it, the narrator struggles to believe the result.

The discovery does not erase the law’s warning about what the work can do to Rong Jinzhen, nor does it establish that the principle is universally false. Rather, it places the rule beside a case in which the expected barriers mislead: BLACK’s apparent complexity conceals the absence of a conventional key. The story’s account of cryptography thus holds two dangers together. A cipher may be made difficult to read by the caution of its creators, while the cryptographer may be endangered by the very capacity that allows him to cross its boundary. The zero key resolves BLACK; it does not resolve the cost of asking Rong Jinzhen to create and break ciphers alike.