A cipher's fatal weakness is often the human who trusts it, not the math

Singh opens with the story of Mary Queen of Scots, who used a substitution cipher to communicate secretly with conspirators plotting to assassinate Queen Elizabeth I. The cipher itself was reasonably sophisticated for its time, but England's chief codebreaker, Thomas Phelippes, successfully decrypted the messages using frequency analysis, providing the evidence that led to Mary's execution in 1587.

Singh's point is that Mary's downfall came not from an inherently flawed cipher but from an unwarranted confidence that the cipher's mere existence guaranteed safety, leading her and her co-conspirators to write incriminating details plainly once encrypted, assuming interception was impossible. The mathematics of the cipher mattered less than the human overconfidence surrounding its use.

This pattern — technically sound security undermined by human misuse or overconfidence — recurs throughout the book's later chapters and remains a central lesson in modern information security: systems fail as often through human behavior around them as through flaws in the underlying method itself.

Takeaway: no cipher protects you from your own careless trust in it.