Public Key Encryption

Basic Information

Public-key cryptography, also known as asymmetric cryptography, is a form of cryptography in which the key used to encrypt a message differs from the key used to decrypt it. In public key cryptography, a user has a pair of cryptographic keys—a public key and a private key. The private key is kept secret, while the public key may be widely distributed. Incoming messages would have been encrypted with the recipient's public key and can only be decrypted with his corresponding private key. The keys are related mathematically, but the private key cannot be practically derived from the public key.

Sources

Wikipedia Link

See Also

• Cryptography
• One Time Pad - when "cannot be practically derived" isn't good enough
• Computational Complexity - the reason why public key cryptography works

Game and Story Uses

Fantasy

• In a high-magic setting, a similar principle on a magical basis could easily be used by magicians and wizards to keep their writings and communications secure.
  • One could even use spirits or magically self-encrypting messages to secure messages.
  • Like the "Teleport codes" of D&D4, a setting might include public communication centers where people deposit their communication codes and public keys so that random travelling adventures can still be reached securely while in the dungeon.
• Demons are generally summoned by various rituals, often found in dusty tomes and such. In a way, these could be considered "public keys" to secure any attempt to reach the demon or keep his enemies from intercepting and possibly redirecting invocations.
  • The same goes for prayers, really.
  • This would be especially appropriate in a "War in Heaven/Hell" scenario where there is active antagonism between the various supernatural parties.
• One thing to note is that most Public Key schemes can be broken trivially by a Time Master. The private key can be derived from the public key, it just takes millions or billions of years; and a Time Master can compress that into a split second.

Science Fiction

• Asymmetric Encryption is often only as secure as the password that protects the users secret key (Which might be stolen by carious means.), which makes it attackable by various social and technological means.
  • This might mean that the easiest way to get at a the Cleartext of an encrypted message is to get the user to tell you his password,either by force or regular social engineering. Characters might have to abduct somebody with the appropriate password to "break" the encryption.
    • This is where "deniable encryption" and similar things are useful.
    • Note that even this can be more complicated than it sounds. The user might know that their password is "passw0rd", but that won't get you their private key of 221B8E040DDB7177DD28B9BD3B109F1DA8D5D54E27E1085C8ADB20EEDF2BDD9A without access to the machine storing it, or their session key from last week at all. It may well be easier to simply get the Cleartext out of your user directly.
• Encryption is secure because the range of possible key values is too big to calculate in a reasonable time frame with current computers. Advances in technology might lead to ways to reduce this time frame to something usable, thus leading to adventures centered on a new McGuffin that might make any current encryption scheme easily breakable.
  • Alternatively, claiming to have developed such a McGuffin, even if not true, could be a powerful possibility for fraud or intimidation by the characters.
  • For those interested in this, a large enough quantum computer or a proof that P=NP¹ would let you break most or all public key schemes.
  • Likewise, current-day encryption might not stand up to time travel, if a time traveller can simply grab a computer from the future fast enough to break the encryption that exists now.
• Asymmetric Encryption also includes ways to sign messages, this making clear that they are really coming from a specific key the target knows. This could be an easy way to make sure that the sender is really who he claims to be, even when he sends through various anonymous ways.
  • So an AI or other "mysterious contact" that uses various unconnected means to contact the PCs could sign his messages to proof that they are really coming from the same … being.
  • Alternatively, breaking a key or otherwise getting control of it is a great way to assume a persons identity at least as long as that person has no other way to retract that key or contact the target. (Though disconnection of communication services, abduction or death.)
• Related to public key cryptography is the one-way hash algorithm, which allows someone to prove that two messages are identical without knowing what those messages are. This is how things like online passwords are protected.

Footnotes

1. The math behind this is complicated, but it essentially amounts to "can a computer pretend to have more cores than it actually has without losing too much speed" and has applications in protein folding, pathfinding, and a lot of other areas