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Digital Fortress

Digital Fortress is a techno-thriller novel by American author Dan Brown and published in 1996 by St. Martin's Press (ISBN 0-312-26312-1).

Plot summary

Susan Fletcher, a brilliant mathematician and head of the National Security Agency's (NSA's) cryptography division, finds herself faced with an unbreakable code named "Digital Fortress", that TRANSLTR cannot break. TRANSLTR is the NSA's 3 million processor supercomputer that can crack encrypted data using the brute force method in a short amount of time, with some codes broken in more than 3 hours, others within minutes. The code of "Digital Fortress" is written by Japanese cryptographer Ensei Tankado, a fired employee of the NSA, who is displeased with the agency's intrusion into people's privacy. Tankado posts a copy of Digital Fortress on his website, encrypted with itself. Tankado auctions the passkey to unlock the algorithm on his website, threatening that his accomplice "North Dakota" will release the algorithm for free if he dies. Tankado is found dead in Seville, Spain. Fletcher, along with her fiancé, David Becker, a skilled linguist with eidetic memory, must find a solution to stop the spread of the code.

Characters

Susan Fletcher - Head Cryptographer
Commander Trevor Strathmore - Head of the Cryptography Department ('Crypto') and Deputy Director of the NSA and the main antagonist of the novel.
David Becker - Susan Fletcher's fiance
Ensei Tankado - Ex-NSA employee
North Dakota (aka NDAKOTA) - A fictional accomplice of Ensei Tankado
Hulohot - An NSA assassin (wearing wire-rim glasses)
Midge Milken - Head of NSA internal security
Phil Chartrukian - System security technician
Jabba - Senior sys-sec officer who developed gauntlet, an anti-virus software.
Soshi Kuta - Head techi after Jabba.
Greg Hale - An employee of 'Crypto'
Leland Fontaine - Director of the NSA
Brinkerhoff - PA to Director of the NSA
Smith - Agents of NSA sent to Spain to watch Hulohot's back.

The Digital Fortress

Digital Fortress is actually a computer worm designed with the intent to open the entire NSA databank to the world. Tankado knew that the NSA would use TRANSLTR to try and crack the Digital Fortress algorithm so they could decode all messages encrypted with it. In doing so, Digital Fortress would then gain access to the databank and open all its firewalls so that "Any 3rd grader with a modem would get access."

Themes & Issues

This book deals with issues of civil rights and privacy from your government. There are many discussions in the book concerning whether or not having access to everyone's information in order to secure their safety is ethical.

Real life scenarios

The book is loosely based around recent history of cryptography. In 1976 the Data Encryption Standard (DES) was approved with a 56-bit key rather than the 64-bit key originally proposed. It was widely reckoned that the National Security Agency had pushed through this reduction in security on the assumption that it could crack codes before anyone else.

In fact the DES was first publicly broken in 1997, 96 days after the first of the DES Challenges. In 1998, the same year as Digital Fortress was published, the Electronic Frontier Foundation (featured in the book) built a piece of hardware costing less than $250,000 called the EFF DES cracker which broke it in 56 hours.

The brute force search used by TRANSLTR takes twice as long for each extra bit added to the key (if this is done sensibly), so the reaction of the industry has understandably been to lengthen the key. The Advanced Encryption Standard established in 2001 uses 128, 192 or 256 bits, which take at least 10²¹ times as long to solve by this technique.

Unbreakable codes are not new to the industry. The one-time pad, invented in 1917 and used for the cold-war era Moscow-Washington hotline, was proved to be unconditionally secure by Claude Shannon in 1949 when properly implemented. However it is inconvenient and expensive to use in practice and its use is generally limited to government and military agencies.

Public-key cryptography does not generally use fixed length keys and is not susceptible to the computer described in Digital Fortress although it is not unbreakable and may be broken in the future using quantum computing techniques.

Code solution

The code that appears in the end of the book

128-10-93-85-10-128-98-112-6-6-25-126-39-1-68-78

is decrypted by looking at the first letter of the chapter for each number. For example, chapter 128 starts 'When Susan awoke'. The resulting text is:

WECGEWHYAAIORTNU

Decryption is performed using a columnar transposition cipher, termed a "Caesar Square" cipher in the book (this is unrelated to the Caesar cipher). The letters are arranged into a square:

W E C G

E W H Y

A A I O

R T N U

and read from the top down.

WEAREWATCHINGYOU

Add spaces and you get the plaintext,

"We are watching you"

a reference to the NSA's monitoring systems.

Technical Errors

Although the book's website cites reviews lauding Digital Fortress for being extremely realistic, the book contains a number of technical errors and misunderstandings in computers, math and technology.

In describing how David Becker helped the NSA translate Chinese characters early in the book, there is no point in emphasizing Mandarin Chinese characters, as all Chinese characters are the same, it is only the spoken dialects (Mandarin, Cantonese, Shanghainese, Hakka, etc.) that differ. The linguistic knowledge of the author is brought into question when he writes: "He’d [Becker] mastered six Asian dialects as well as Spanish, French and Italian". While Mandarin and Cantonese are dialects of Chinese, Japanese is a different language. The author is either unaware of this, or is using the word "dialect" in the derogatory sense of "unimportant foreign language" while he refers to the modern descendents of Latin (Spanish, French and Italian) by name.
Also while describing how David Becker translates Chinese characters, Brown implies that the characters might be Japanese Kanji (incorrectly referred to as Kanji language)) instead. However, (modern) Japanese is not written solely with Kanji; it is always accompanied by syllabic kana. Furthermore, it is implied that Becker is shown only single characters, but Kanji is highly dependent on context for their reading/meaning. For example, by itself 天 might be read as 'ama' = 'sky' but the combination 天才 is always read 'tensai' = 'genius'. Unless he was shown more substantial parts of the text, it would be impossible to make sense of it. And if he were, then it should have been immediately obvious which language he was dealing with.
There is an error in the description of the usage of a public key (chapter 4):

Public-key encryption was a concept as simple as it was brilliant. [...] The only way to unscramble the message was to enter the sender’s “pass-key”[...].

In asymmetric cryptography, it is in fact the recipient’s private key that is used to decrypt the message, and not the sender’s one.

The terms character and bit are used interchangeably throughout the novel, the term bit being erroneously used to describe a character. In reality, a character is generally equivalent to 8 bits - possibly more, depending on the encoding used (ASCII, Unicode etc).
Ten-thousand, million, and even ten-billion bit keys are referred to in the text (Chapter 5) as being unrealistic, but possible, scenarios. Simple calculations can be made to determine that, given the number of keys each processor is able to audit per hour (Chapter 5), even if TRANSLTR had as many processors as there are atoms in the universe (about 10⁸⁰) it would take TRANSLTR more than 1 million years to crack a 320 bit key. Adding a single bit, (a 321 bit key) would double the time taken to use a brute force attack (more than 2 million years). This demonstrates the absurdity of the claim that TRANSLTR could break a ten-billion bit key in 15 hours (Chapter 5).
The main character is purported to have created a program called a tracer. The tracer is sent from the NSA to an email address, and upon arrival at the end host's terminal, the tracer sends the host's IP address back to the NSA. The problem is that this program is said to be undetectable (it even self destructs itself). But in order for the tracer program to function, it would have to execute on the host's machine. The host would at the very least need to open the attachment on the email, thus contradicting the supposed stealthiness of the tracer program. In addition, the tracer could only execute on architectures with which it was binary compatible. A similar (and simpler) implementation of this idea is known as a web beacon, widely used to surreptitiously determine if an email message has been read by adding a link to a graphic on a web site, along with an inline unique parameter to identify the recipient. When the recipient reads the email, the mail program fetches the graphic from the server, sending the unique parameter in the process. The server records the unique parameter so that whoever analyzes the information knows the particular message has been read, and by whom.
The book emphasizes that the Ph.D cryptographer characters believe in the fundamental impossibility of an unbreakable encryption scheme. See one-time pad.
The main character's boss has developed a scheme to replace digital fortress with a modified, NSA-breakable, algorithm. However, the original digital fortress software has been published online for some time, allegedly with a large number of downloads. The public would easily be able to tell that the new version of digital fortress has been modified from the original and isolate the changes.
The boss also relies on making the pass-key known to the public, in order to have a backdoor for information coded with the modified Digital Fortress, but the pass-key would also allow the public to code information with the original, non-modified, already downloaded and supposedly unbreakable Digital Fortress.
Many errors are made when describing the time taken to break keys. Most of these errors stem from a misunderstanding of the relationship between the length of the key and the number of possible keys. In general, this relationship is exponential (that is, as the key length grows linearly the number of possible keys grows exponentially). In the book it is stated that TRANSLTR is able to audit 100 billion keys in one hour (Chapter 5). Beginning with this assumption, then TRANSLTR should be able to break a 32-bit key in about 2.4 minutes. However, in reality, since this relationship is exponential, it would then take 4.8 minutes to break a 33-bit key, about 10 minutes to break a 34-bit key, and 21,058 years to break a 64-bit key. This is inconsistent with the estimation in the book that a 64-bit key would be broken in about 10 minutes.
The characters attempt to decrypt the digital fortress software, but acknowledge that it is encoded with an unknown algorithm. It would not be possible to use brute force to decrypt a message without first knowing the algorithm that was used. Without knowing the algorithm, a brute force attack might generate a sufficient number of keys, but would have no method of applying the keys for decryption, because the algorithm is unknown. Similarly, even if the receiver of an encrypted message has the correct key, if the receiver does not know how to apply the key, the key is useless.
The book also contains several typos, perhaps the most glaring of which is a reference to 'VSLI' chips in chapter 18. Brown was most likely referring to VLSI chips.
There are many errors in the use of the Japanese language throughout the book, the most glaring of which being the unrealistic name "Ensei Tankado." The seven lucky gods are referred to as "Shichigosan" when in fact they are "Shichifukujin". Shichigosan is a rite of passage for 7- 5- and 3-year old children in Japan.
The description of the construction of TRANSLTR makes reference to three million chips being hand-soldered into place. There is no good reason for hand soldering to feature in the construction of any modern computer, especially not a massively parallel government-funded supercomputer. Even with the (very generous) assumption that a technician could solder a chip into place with negligible chance of error in one hour, it would take a team of 100 technicians working eight hour days and seven day weeks over ten years just to complete this soldering process - not to mention that TRANSLTR is said to have been constructed using only 500,000 man-hours of work.
Both TRANSLTR and the NSA database are compromised by the virus. There is no good reason for a password cracking machine to execute the data it is provided with and it is equally implausible that a database would attempt to execute the data stored on it.
Brown mentions a Hungarian mathematician, Josef Harne, who in 1987 proposed an encryption algorithm that, in addition to encrypting, shifted decrypted cleartext over a time variant. However, neither Harne nor the concept of rotating-cleartext ever existed.
Mr. Brown makes a significant error when describing a climactic chase scene up the Giralda Tower of the Cathedral of Seville. Though he describes Becker as dashing up the stairs of the Giralda, one of the well-known features of this tower is that it has no stairs leading to the top, but a series of ramps.
Events occurring at Fort Meade in the middle of the night are concurrent with those in Seville at 02:00.
An error appears in the 2004 Corgi paperback edition, on page 143 the assassin Hulohot transmits the message "SUBJECT: P. CLOUCHARDE - TERMINATED", but on page 425 the list of messages reads "SUBJECT: PIERRE CLOUCHARDE - TERMINATED". The same error appears on pages 113 and 357, respectively, in the St. Martin's Paperbacks 2004 edition.
There's no reason why NSA shouldn't back-up its database.
The book states that it is a "misconception" that the atomic bomb dropped on Nagasaki was a Plutonium bomb. Instead, the book claims, the Nagasaki bomb used Uranium 238, "a neutron enriched isotope" of Uranium 235 that was used at Hiroshima. In fact, (1) Uranium 235 is the isotope that exists in very small quantities in nature. (2) "Enrichment" consists not of adding neutrons to Uranium 235 but rather of extracting the small percentage of Uranium 235, either through effusion or the gas centrifuge method. (3) Uranium 238 (i.e. the majority of the Uranium found in nature) is not capable of nuclear fission because the extra three neutrons add to the strong force that binds the nucleus together, without adding to the electrostatic repulsion of like charges that tends to force it apart. This makes the nucleus of the Uranium 238 atom more stable than that of uranium 235. (4) The material used in the atomic bomb at Nagasaki WAS plutonium!

Notes

One briefly described character is mentioned as an alumnus of Amherst College, which Brown graduated in 1986.
The assassin, Hulohot, is described as using a revolver with the finest silencer available. Except for the unusual Nagant 1895, revolvers don't benefit from a silencer because noise comes through the gap between the cylinder and the barrel of the gun. Even with the silencer on the Nagant, the gun makes a loud clicking noise, impractical for use in public areas (as described in the book).