Urkel Tree: Cryptographically Provable Key-Value Store

Urkel Tree is an optimized, cryptographically provable key-value store implemented as a base-2 merkelized trie, specifically designed for the Handshake protocol. It significantly outperforms alternatives like Ethereum's base-16 trie by operating as its own database, storing nodes in flat, append-only files for snapshotting and crash consistency. The current stable version is 1.0.3, with releases focusing on stability, minor bug fixes, and internal optimizations rather than rapid feature additions. Key differentiators include its architectural simplicity (only two node types), compact internal node size (76 bytes), and extremely small proof sizes (32 bytes per sibling node), which are crucial for maintaining proof sizes under 1KB even with millions of leaves. It offers fully transactional capabilities and inherently provides history independence and non-destruction properties. A critical usage requirement is that Urkel must be used with uniformly distributed keys, typically achieved through hashing. While compaction is available, it is currently inefficient and manual, with future C implementation planned for optimization.