The Kangaroo cipher, sometimes referred to as a jumping key cipher, is a classical polyalphabetic cipher whose exact origins are uncertain, but which appeared in cryptographic literature and practice in the late 19th century to early 20th century. Unlike fixed-key polyalphabetic systems such as the Vigenère cipher, the defining feature of the Kangaroo cipher is that the key does not advance in a simple, linear fashion. Instead, it “jumps” through the key according to a predefined rule, much like a kangaroo hopping unevenly forward. Because of this irregular progression, repeated plaintext patterns are less likely to align with repeated key positions, increasing resistance to straightforward frequency analysis.
The cipher is generally attributed to unknown cryptographers working in the era when manual encryption was evolving beyond simple substitution but before fully mechanized systems became widespread. Rather than being tied to a single named inventor, the Kangaroo cipher represents a family of techniques built around the same idea: irregular key movement. The plaintext is typically encrypted using a tableau or numeric mapping similar to Vigenère, where letters are converted to numbers such as A=0 through Z=25. A keyword is chosen, but instead of advancing one key letter per plaintext letter, the cipher follows a jumping pattern defined by a secondary rule, such as a numeric sequence or a repeating skip pattern.
To illustrate, consider encrypting the word “HELLO” using the key KEY. First, the plaintext letters are converted to numbers: H=7, E=4, L=11, L=11, O=14. The key letters are likewise converted: K=10, E=4, Y=24. In a simple Kangaroo-style scheme, the cipher might advance the key by jumps of 1, then 2, then 1, repeating this pattern. The first plaintext letter uses K, the second uses Y (skipping E), the third uses E, the fourth uses K again, and so on. Each plaintext number is combined with its corresponding key number modulo 26, producing ciphertext letters that vary more unpredictably than in standard polyalphabetic systems.
Decryption of the Kangaroo cipher requires the recipient to know both the keyword and the jumping rule. By reproducing the same key hops in the same order, the ciphertext can be reversed back into the original plaintext. This makes the cipher symmetric and fully reversible, but also places heavy importance on precise agreement between sender and receiver. Any mistake in the jump sequence immediately corrupts the output.
Historically, the Kangaroo cipher is significant as an example of cryptographers experimenting with irregularity to defeat analysis. By breaking the predictable rhythm of key usage, it complicates attacks that rely on repeated patterns and key length detection. While it is not secure by modern standards and has been superseded by mathematically rigorous algorithms, the cipher illustrates an important conceptual step in cryptographic thinking. It shows how altering key progression alone can meaningfully increase complexity, and it remains a useful teaching example when exploring the evolution of polyalphabetic encryption and the constant struggle between pattern and randomness in cipher design.