What is X15?

X15 is a chained hashing algorithm that extends the X13 design by adding two more cryptographic hash functions, bringing the total to fifteen sequential operations. The chain includes all thirteen functions from X13 — BLAKE, Blue Midnight Wish, Grøstl, JH, Keccak, Skein, Luffa, CubeHash, SHAvite-3, SIMD, ECHO, Hamsi, and Fugue — followed by Shabal and Whirlpool. Each function in the chain processes the output of the previous one, with the input data cascading through all fifteen hash functions to produce the final 256-bit hash value. This sequential chaining approach follows the same design philosophy established by X11.

With fifteen chained hash functions, X15 provides an even deeper layer of defense-in-depth security than its predecessors X11 and X13. The increased number of cryptographic functions means that an attacker would need to find vulnerabilities in a larger number of independently designed algorithms to compromise the overall hash output. The algorithm maintains reasonable computational efficiency despite the additional functions, as each individual hash operation is relatively fast. The additional complexity also increased the initial barrier to ASIC development, though this advantage proved temporary as the mining hardware industry continued to advance. X15 strikes a balance between enhanced security through algorithm diversity and practical usability for mining operations.

X15 emerged in 2014 during the proliferation of X-series algorithms that followed X11's successful introduction with Dash. It was adopted by several cryptocurrency projects looking to offer a differentiated mining experience while benefiting from the proven multi-hash security model. Notable cryptocurrencies that have used X15 include Kobocoin and HTMLCoin (in its earlier iterations). While X15 never reached the prominence of X11 or even X13, it played a role in the evolutionary progression of chained hashing algorithms and demonstrated the scalability of the X-series approach. The algorithm eventually led to X17, which added two more functions and represented the practical upper limit of the X-series concept before more fundamentally different approaches to ASIC resistance emerged.