A book written in 1977 has recently been declassified by the United States National Security Agency (NSA), and it reveals a three-step process for breaking "impossible" encryptions.
The book was written by cryptologist Lambros Callimahos, and it explains how to break all types of codes that are commonly used by the military. For the aforementioned reason, the NSA decided to claim that the book's release could endanger national security, as it could reveal the NSA's "code-breaking prowess". The NSA later declassified the book in December 2020.
One of the chapters of the book is titled "Principles of Cryptodiagnosis", and within that chapter, Callimahos reveals a systematic three-step approach to deciphering an encrypted message that used an unknown method. For example, the NSA may encounter thousands of encrypted messages, but if the cryptographers don't know the method for how the messages were encrypted, then they won't be able to break the encryption.
Phys.org reports, "As Callimahos details in his chapter, the code-breaker must begin with all the necessary data. This includes the ciphertext (the enciphered text hiding the real message), any known underlying plaintext (text from before the encryption was applied), as well as important contextual information."
As for puzzles, "For puzzles, part of the plaintext may be given to help the solver. With confidential military messages, the solver may suspect certain words have been encoded into the ciphertext, based on past knowledge. For example, there may be key terms such as "message begins," "message ends" or "secret," or specific names, places or addresses." per Phys.org.
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