Digital Fortress is the first novel by Dan Brown.Susan Fletcher, star of the NSA's cryptography division, is called in by NSA Deputy Director Trevor Strathmore after TRANSLTR, the NSA's code-breaking computer, encounters a code that it can't break. This code, called Digital Fortress, was created by Ensei Tankado, a disgruntled NSA operative. When Tankado is found dead in Seville, Spain, Strathmore dispatches David Becker, Fletcher's fiancée, to investigate in the hope of finding a clue to breaking the code, while Fletcher and Strathmore investigate Tankado's mysterious partner "North Dakota", who unknown to them is in talks with a Japanese corporation to release the Digital Fortress code publicly. But as in any Dan Brown novel, all is not as it seems...As with any Dan Brown book, Digital Fortress is infamous for some lapses of research, particularly some glaring flaws in its portrayal of cryptography and its portrayal of Seville as a poorly equipped city, with a medical service almost as bad as some in third-world countries. Despite this, it remains a readable book if you keep the MST3K Mantra in mind (though opinions differ on this point).Compare The Da Vinci Code, Angels and Demons, Deception Point and The Lost Symbol. Contrast with Cryptonomicon.
Anti-Villain: Ensei Tankado. Sure he conspired to infect the NSA's computer with a virus that would remove all its protection from hackers, but he never intended to let it actually happen, just force them to admit they really did have a big shiny computer and he wasn't spreading anti-US propaganda about 60 years too late.
Attempted Rape: Subverted twice. Susan thinks Hale's about to rape her, and David gets maced when a girl thinks he wants to hire her as a prostitute, then Strathmore actually does try to rape Susan.
Batman Gambit / Kansas City Shuffle: Tankado's "unbreakable code" is a virus that he tricked the NSA into downloading into their system so he can hold the NSA database to ransom and force the NSA to admit the existence of TRANSLTR.
Darwinist Desire: Hale has a few thoughts along these lines (feeling that Susan's genes would be wasted on her David while if he had children with her they would be "perfect"). Amongst other signs he's a jerkass.
Embarrassing Nickname: Hale is called "Halite" by his colleagues (he doesn't realize this is embarrassing though).
Hollywood Encryption: The NSA has a computer which is powerful enough to brute force (i.e. keep trying different passkeys until it gets the right answer) encryption. The plot is based around a new encryption algorithm which is resistant to brute force methods. This is a clear case of research failure, since a brute force search for a solution would try every possible key until the right one was found.
There does, however, exist an encryption method, the one-time pad, that is immune to brute force, and unbreakable if carried out correctly (doing so, however, is often logistically prohibitive). It's immune to brute force because with different keys you can get every possible message that has the same length as the one being sent, with no indication whatsoever of which possible decryption is the right one.
Actually most modern digital encryption is resistant to brute forcing, in that it's theoretically possible to do it, but would likely take an impossibly long time, possibly billions of years, assuming you dedicate all of the worlds computing power to brute forcing that one key.
My Country, Right or Wrong: Tankado went through a historical version in the backstory; as a child he was upset that the US used nukes on his country (and that he suffered deformities which caused his mother to die in childbirth and his father to abandon him), later in life he decided Japan was as at fault as the USA in WWII, dropped his grudge and even began working for the US government.
The protagonists fit this trope as well, being NSA employees who are perfectly fine with all manner of immoral or even outright illegal things as long as it's in the national interest (they occasionally monologue about it).
My God, What Have I Done?: The final chapter has the corrupt CEO discover Tankado, the man he'd practically ordered the death of for profit, was the deformed son he walked out on.
Nice Job Breaking It Strathmore: While he may not be that heroic, Strathmore having Tankado murdered ruins the latter's plan...by turning his casualty free plan for vindication into a full on attempt to destroy the NSA's giant shiny computer and let any interested hackers march straight into their data system.
Playful Hacker: Greg Hale was somewhere in between this and The Cracker (on the one hand, he causes damage and has a political agenda, on the other hand said damage was discovering an illegal backdoor the NSA planted in some encryption software).
Pretentious Latin Motto: Tankado's personal motto is "Qui Custodiet Ipsos Custodes?", most commonly translated as "Who Watches the Watchmen?" and "Who Will Guard the Guards?". The phrase handily sums up his motivations in the book: he doesn't believe that any government agency can be trusted enough to spy on its citizens in the name of national security.
Psycho for Hire: As is traditional for a Dan Brown book, we have an assassin with a Red Right Hand in Hulahot, the assassin hired by Strathmore to kill Tankado and David Becker and recover the Digital Fortress key.
Recruiting the Criminal: How Hale was originally hired. While what he did— find a back door they left in an encryption algorithm— wasn't exactly illegal, it certainly did negatively impact his future employers. Possibly Tankado as well, considering he was quite anti-American before he got over the Hiroshima and Nagasaki bombings (although he didn't get around to committing any crimes because of it).
Red Shirt: The computer tech who turns comes in by chance and gets suspicious. Also the assassin following David kills everyone he speaks to when he's left.
Red Herring: The password to shut down Digital Fortress is not written on Tankado's ring; it's just Tankado's personal motto, the Latin phrase "Qui Custodiet Ipsos Custodes?"
The Reveal: There is no Digital Fortress encryption, and North Dakota doesn't exist. The "code" that the NSA let into TRNSLTR is actually a virus designed to cripple their computers; Tankado created the myth of Digital Fortress to convince them to try to break it.
Significant Anagram: "NDAKOTA" is an anagram of "Tankado", which is a clue to the fact that there is no North Dakota.
Spanner in the Works: Strathmore ruins Tankado's plan when he has him killed, almost dooming the NSA database to destruction because Tankado can't give them the code to deactivate his virus. Fortunately the NSA figure out the code themselves before it's too late.
Spexico: Dan Brown's depiction of Spain as a Third World hellhole with rampant crime, corruption and poverty. He apparently confused some of the worst stereotypes about Mexico with Spain, which is a prosperous Western European country with a GDP about the same as the United States'.
The Password Is Always Swordfish: Tankado's password is the single digit '3'. He even leaves clues just to make absolutely sure that everyone can guess it. Possibly justified in that Tankado may have wanted them to guess it. Which begs the question of why he even bothered with a password in the first place.
Since he was trying to ransom the password off and he expected to be alive to give it to them and clearly didn't expect them to guess it (it's made quite clear that the fact he's dying is a massive Oh Crap moment for him), it's fair to assume that we're supposed to believe this is an absolutely, positively fiendishly clever password with the clue being there to taunt them.
Brown Fails Physics Forever, since the "difference" in the clue is not 3. Also, it's axiomatic that cryptosystem security is exponentially proportional to key length. This key is approximately 8 bits long. Good job, guy.
Who Watches the Watchmen?: The entire conflict is down to Tankado feeling that TRANSLTR gives the NSA too much control and not enough oversight (since they decided to just use it to decrypt everything they could find rather than go through some sort of requisition process). He's quite fond of the orinal Latin phrase as well and it's what's engraved on his ring, rather than the passcode as everyone assumed. Hale also seems to think about this a lot.
Artistic License - Physics: It has been amply established that uranium-235 and plutonium-239 were the two fissionable isotopes used, respectively, in the bombs which nuked Hiroshima and Nagasaki. However, Dan Brown is apparently smarter than all of us, and invoked a mythical 'widely historically inaccurate claim' to justify why Tankado was hand-signalling the number "3" - the difference in mass between uranium-235 and uranium-238.
Bonus fail: Brown could have legitimately used the prime number 2, the difference in atomic number between uranium and plutonium, and escaped failing physics forever.