As I read through the Harry Potter series, I am bothered by the apparent dearth of secure ways to transmit information. I would like to review various methods of magic communication, look at the drawbacks, and suggest methods that the magic-using world could employ to achieve higher security. I don’t expect to be exhaustive in my suggestions, but to provide some ideas and hopefully spark a discussion.
In Mundane society, we have developed various means of encryption and obfuscation to ensure that information can be passed between users without external interference (either obtaining the information, or changing it before it reaches its destination). In the magic-using world, however, it appears that this has not been widely achieved.
Magic-users communicate in various ways: direct person-to-person contact; letters sent by owl; Floo network head-only travel; Patronus charm; two-way mirror; Protean charm; a Secret Keeper. Each of these has various advantages and disadvantages, which I will discuss before providing some suggestions for additional security.
There are several instances in the Harry Potter series of direct contact being overheard, such as the Weasley twins’ Extendable Ears, and Rita Skeeter transforming into a bug so she can listen unnoticed. The Imperturbable Charm was used with great success by Mrs. Weasley to prevent the use of the Extendable Ears, as it creates an invisible barrier around its target. I’m not sure that the full extent of this barrier is detailed in the books, but we’re shown that it prevents eavesdropping as well as physical access to the protected space. In my opinion, this is a very useful charm and I’m not sure why it wasn’t used more extensively to protect the secrecy of direct communication.
The question of information security in the magic world first came to mind specifically because magic-users apparently have few methods available for use with the letters they send by owl. The owl can be intercepted, the letter read, and then sent on its way; both the owl and the letter itself are vulnerable. The parchment can be sealed, but it cannot be prevented from being opened and re-sealed by a powerful Witch or Wizard. They message can be vague or obscure, but this reduces the usefulness of the information being transmitted.
Sealing the letter provides several opportunities for security. One could place a charm on the seal which causes the contents of the letter to change or disappear if it’s opened by an unauthorized party; alternatively (depending on the type of information being shared), the seal could show no sign to the unauthorized opener that they were recognized as such, but could indicate to the true recipient that the information has been accessed.
Within the letter itself, the contents could be encrypted in such a way that it can only be decrypted either by any person with the appropriate key or by a specific recipient; we already know that magic can distinguish between people, so it should be able to ensure that only a given person can access specific information, though this may also be a point of vulnerability (if someone else were transfigured to appear to be the authorized recipient, for example).
The owl itself could be secured by an Invisibility charm, by increasing or altering its natural defenses (give it sharper claws and beak, stronger wings so it can fly faster and higher), or perhaps by disguising it to appear as though it’s not carrying a letter, or is a different type of animal (though owls aren’t the only bird that can carry a letter). The letter could also be disguised as something else, such as a newspaper or parcel.
According to the Harry Potter Wiki, the Ministry of Magic did start planning a Secret Messaging Service involving owls, but it’s not clear exactly how that was expected to work; I would have been very interested to see where that went.
Floo Network Head-Only Travel
I am regrettably unclear on the specifics of how the Floo network functions, so I don’t have much to suggest for this method of communication, That said, there must be spells that could be used to prevent third-parties from accessing the fireplace in use, or from hearing the conversation that they are not invited to participate in (perhaps the Imperturbable charm?).
This was my favourite method of communication, as it’s specifically created to be secure and the Patronus cannot be compromised. As explained in an old FAQ entry on J.K. Rowling’s website, the Patronus is secure because:
It is an anti-Dark Arts device, which makes it highly resilient to interference from Dark wizards; it is not hindered by physical barriers; each Patronus is unique and distinctive, so that there is never any doubt which Order member has sent it; nobody else can conjure another person's Patronus, so there is no danger of false messages being passed between Order members; nothing conspicuous needs to be carried by the Order member to create a Patronus.
Although the Patronus can be used to send a communication, it’s not clear if it can return to the sender with a reply, so it may not be a two-way method.
To my knowledge, the people using the two-way mirror cannot be seen or heard elsewhere, so this appears to also be a secure method of communication! It does have drawbacks, in that the people using the mirror need to have it in hand, and need to be in a location where they can speak freely without fear of being overheard, but this may be another use for the Imperturbable charm.
Although this isn’t truly a two-way communication method, I found it interesting and wanted to commend Hermione for finding a way to convey information from one source to many recipients, and obscure the communication by making it appear to be something else (in this case, the serial number on a coin updated to show the date and time of secret meetings).
Finally, the Secret Keeper. As explained in Prisoner of Azkaban, the Fidelius Charm is:
an immensely complex spell involving the magical concealment of a secret inside a single, living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is henceforth impossible to find -- unless, of course, the Secret-Keeper chooses to divulge it.
As with the Protean charm, this is not a two-way communication method, but it’s a good way to keep specific information hidden (as long as the Secret-Keeper can be trusted…)
As we have seen, magic-users have found several methods of conveying information with varying degrees of secrecy. Due to the high stakes of their fight against Voldemort, I would have expected more security-related magic to be employed, especially surrounding sending letters by owl. In the end, I can only guess that Rowling’s personal interests and goals for the book series lay elsewhere, resulting in the lack of focus on this question in the Harry Potter canon.