Pronunciation: [ɹˈʌnɪŋ kˈiː] (IPA) 
The term "running key" is commonly used in telegraphy and cryptography to refer to a type of cipher where a keyword is used to encrypt a message. The spelling of the word itself is quite straightforward, with the first syllable pronounced like "run." The second syllable is pronounced with a soft "i" sound as in "pin," and the final syllable rhymes with "key." The IPA transcription for "running key" would be /ˈrʌnɪŋ kiː/.
A running key is a cryptographic technique used to encrypt or decrypt messages by shifting the alphabet with each letter or a regular interval. It is a substitution cipher where each letter of the plaintext is replaced with another letter from the alphabet, determined by a predefined pattern or key.
In the running key technique, the key is a sequence of letters that is repeated over and over again. The key can be generated using different methods, such as using a keyword or phrase, a random number generator, or a predetermined pattern. This key is then used to determine the letter substitution for each corresponding letter of the plaintext or ciphertext.
To encrypt or decrypt a message using a running key, one must align the key and the plaintext or ciphertext, either by repeating the key if it is shorter than the message or by truncating the key if it is longer. By substituting each letter with the corresponding letter from the key, the message is encoded or decoded.
Running key encryption offers a higher level of security compared to simple substitution ciphers, as the use of a pattern key minimizes the likelihood of frequency analysis attacks. However, the security of the running key technique heavily relies on the secrecy of the key and the chosen pattern, as a knowledgeable attacker who discovers the pattern can easily crack the ciphertext.
The etymology of the word "running key" can be traced back to the printing industry.
In traditional typesetting, a "running head" was a small piece of text, typically the title or chapter name, that was printed at the top of each page to help readers navigate through a book. It "ran" across the top of the pages.
Similarly, a "running foot" was a small piece of text, often the page number, that was printed at the bottom of each page, also helping readers keep track of their progress through the book. It "ran" along the bottom of the pages.
The term "key" in this context refers to the guide or reference point that helps ensure the proper alignment of these running elements on each page. It serves as a visual cue for the printer or typesetter, allowing them to accurately position the running head or foot on every page of the book.
