The bigger they are, the harder they fall. In chapter one of The Code Bookby Simon Singh, Singh states that “…a weak encryption can be worse than no encryption at all” (Singh, 41). When it comes to cryptography, this could not be more true.
A successfully encrypted message should only be decipherable to the intended recipient, otherwise it fails to accomplish its purpose. As a result, those responsible for encrypting the message must be certain that, without the proper key, their message is indecipherable. This, however, is a dangerous assumption. False confidence can lull cryptographers and their intended recipients into a false sense of security, thereby causing them to let their guard down. For example, in the instance of the Babington plot, both Mary Queen of Scots and Anthony Babington assumed that their cipher was unbreakable and spoke quite openly about their plans in their correspondences. As a result, when Thomas Phelippes managed to crack their cipher, he effectively signed their death warrants. Had Mary Queen of Scots and Babington been less assured of the strength of their code, they would never have written their plans out as obviously as they did.
Additionally, there is much that depends on the abilities of the cryptanalysts of the times. For example, the Spanish cryptographers that Singh refers to on pages 28 and 29 of his book believed their code to be indecipherable. When they discovered that their codes were, in fact, quite obvious to a French cryptographer, Philibert Babou, they could not accept it. They had been so confident in their ciphers that they went so far as to suggest that Babou was in league with the devil. Such overconfidence is a constant danger to cryptographers.
Confidence is one of the most basic conundrums of cryptography. On the one hand, if cryptographers are overly confident in their ciphers they risk exposure should their ciphers be broken. On the other, if a cryptographer is not confident enough in their cipher, then there would be no sensible reason risk using it for secret correspondence. The answer must be somewhere in the middle. Cryptographers must have enough faith in their own work to use their ciphers, and yet they must be wary enough to watch what they say.
Singh, Simon. The Code Book: The Science of Secrecy From Ancient
Egypt to Quantum Cryptography. Anchor Books, 2000.