In Chapter 1 of The Code Book, author Simon Singh states, "The cipher of Mary Queen of Scots clearly demonstrates that a weak encryption can be worse than no encryption at all." What this essentially means is that overconfidence with a cipher, especially a relatively weak one, can be dangerous in that it creates an illusion of privacy that may lead to careless communication. This was problematic for Mary and continues to be problematic today.
The encryption method used by Mary and Babington was called nomenclator, in which both letters and common words are replaced with corresponding symbols in the ciphertext. In their minds, that system was more than effective, but they were unaware of the advancements in cryptanalysis that were being made at the time which allowed Walsingham and Phelippes to decipher it. As a result, Mary and Babington had the false impression that they could say anything to each other without their messages being understood if intercepted. This ended up proving worse for them than if they had no encryption method at all. Had that been the case, they would have consciously made efforts to be vague and discreet when discussing sensitive information because there would be an obvious threat of self-incrimination. However, their blind confidence in the encryption masked that threat and led them to speak directly and openly about their plans to assassinate Queen Elizabeth. When it turned out that Walsingham was able to decipher their messages, they were caught completely off guard.
The issue of reliance on weak encryption methods is arguably even more prevalent today in the digital age. The internet allows more information than ever before to be accessible to more people than ever before, so weak encryption can pose extreme privacy and security risks. That is why it is important to be careful what information you put online, even if it is protected by a password. There is always a possibility that hackers can gain access to your personal info. For that reason, it is important to utilize the best encryption methods, and even then, to avoid putting out sensitive information when possible.