hard to crack
- toughDurable and strong
- impenetrableImpossible to pass through
- stubbornRefusing to change course
- puzzleChallenge testing ingenuity
- enigmaSomething hard to fathom
cipher · riddle · safe · vault · walnut · shell · code · mystery · unbreakable · resilient · fortified · locked · secret · encrypted · nut · dense · solid · resistant
difficulty of access or penetration
hard to crackwide open
Fully accessible, unrestricted
accessible · penetrable · transparent · open · vulnerable
emotional/psychological accessibility
hard to crackopen book
One easily understood
open · transparent · approachable · warm · forthcoming
intellectual challenge
hard to cracksimple
Free of complexity
straightforward · easy · elementary · transparent · obvious