Historical clue formulations

• Impossible to read, perhaps — 2026-04-12
• Like some spy messages — 2023-09-17
• 12 9 11 5 20 8 9 19 3 12 21 5 — 2016-10-16
• How some diplomats communicate — 2015-03-26
• Like secret messages — 2006-09-04
• Needing crackers, say — 2006-09-03
• One way to communicate — 2000-10-14
• Needing to be cracked? — 2000-06-24
• Unreadable — 1998-07-19