At the time, diplomatic dispatches were far from secure. Anyone could open and read them (as Scotland's James IV did in 1496, discovering that Spain's envoys had been instructed to string him along in his delusions of marrying a Spanish infanta).
"The only protection was cipher. This, however, was a science in its infancy. Only the Spanish rulers made a stab at it for diplomatic letters, and it was not a success.
"The simplest form of their cipher was to replace certain words with Roman numerals, but the code kept changing and expanding, until MCCCCLXXXVIII meant 'sea,' DCCCXXXIX meant 'the' and DCCCCLXVIIII meant 'in'. Few envoys got the hang of it. ('Impossible,' came back the cries from Madrid. 'Nonsense.' 'Cannot be understood.' 'Order the ambassador to send another dispatch.')
"One royal letter sarcastically thanks de Puebla for not only encoding his letter, but for thoughtfully including the code-book."