The Gold-Bug: Edgar Allan Poe's story that taught me about cryptography (2024)

During my middle school years, I attended an English-medium institution in India. It was there that I encountered a short story by Edgar Allan Poe titled "The Gold-Bug," a piece of fiction that served as my gateway into the world of cryptography—the scientific study of secret communication.

Today, cryptography is invisible but omnipresent. Every time you:

Swipe a credit card for a purchase
Access your online banking portal

...you are utilizing these principles. The infrastructure of modern existence relies on breakthroughs made after World War II, and Poe's narrative helped inspire the pioneers of those advancements.

The Narrative and the Author

In terms of character development and plot, The Gold-Bug is relatively sparse. The story follows a white man and his black manservant who, while searching for a rare beetle on a beach, stumble upon a piece of ancient parchment. This document contains a message written in an enigmatic code.

The bulk of the narrative is dedicated to the process of deciphering this code to locate a buried pirate's hoard. Despite the thin plot, Poe maintains a gripping atmosphere through:

Mystery
The Macabre (e.g., skulls nailed to trees, a high body count)
Gothic elements (e.g., disappearing ink)

At the time, I was unfamiliar with Poe. However, after moving to Boston—his birthplace—I encountered his statue on Boylston Street and began exploring the life of this tormented writer. Around 1840, Poe's obsession with logic and reasoning led him to the study of information encoding.

In that era, the act of writing secret messages was often viewed as a form of occult magic.

Poe disagreed, believing that anyone could encode or decode messages through the application of logical reasoning. He wrote "The Gold-Bug" as an introductory primer on the subject, eventually winning a $100 prize in a Philadelphia newspaper's writing competition. It stands as perhaps the earliest work of fiction to feature cryptography and is certainly the first to use the specific term.

Decoding the Puzzle

The cipher used in the story is a simple substitution cipher. In this system, every letter of the plaintext is replaced by a specific symbol or number. To decrypt it, one needs a key—a mapping table.

The Logic of Frequency Analysis

If the key is missing, the code can be broken using logical deduction. Assuming the text is English, one can use the known frequency of letters. For example, the probability of a letter being 'e' is the highest in English:

$P(\text{letter} = 'e') \approx 12.7\%$

The general frequency order is often cited as: e, t, a, o, i, n, s, h, r, d, l.

Symbol in Story	Frequency Rank	Likely English Letter
`8`	1st	e
`;`	2nd	t
`4`	3rd	a

The protagonist follows this exact workflow:

Count the occurrences of every symbol on the parchment.
Create a frequency table (e.g., 8 is most common, then ;, then 4).
Substitute symbols with the most likely letters.
Iteratively refine the substitutions until coherent English words emerge.

This analytical approach mirrors the mathematical foundations used in modern generative text models, such as ChatGPT. Notably, Claude Shannon, the father of information theory, mentioned in an interview that he had read The Gold-Bug as a child.

Addendum: The Typography Problem

When I first attempted to solve the cipher in middle school, I followed the protagonist's method but hit a snag. My result was ~~"fosty"~~ "forty".

It turns out that various published versions of the story contain typographical errors on the parchment. I have encountered at least three different versions with unique mistakes.

Example: The Gutenberg scan contains the error that leads to "fosty".

Poe had actually analyzed hundreds of reader-submitted messages in another magazine before writing this story. Fortunately, the Edgar Allan Poe Society of Baltimore has provided a meticulously proofread scan of the original Dollar Newspaper version, which is error-free.

The original cipher text appears as follows:

5 3 ‡ ‡ 3 0 5 ) ) 6 * ; 4 8 2 6 ) 4 ‡ .
) 4 ‡ ) ; 8 0 6 * ; 4 8 8 6 0 ) ) 8 5 ; 1 ‡ ( ; : ‡ * 8 8 3 ( 8 8 ) 5 * ; 4 6 ( ; 8 8 * 9 6 * ?
; 8 ) * ‡ ( ; 4 8 5 ) ; 5 * 2 : * ‡ ( ; 4 9 5 6 * 2 ( 5 * 4 ) 8 8 * ; 4 0 6 9 2 8 5 ) ; ) 6 8 ) 4 ‡ ‡ ; 1 ( ‡ 9 ; 4 8 0 8 1 ; 8 : 8 ‡ 1 ; 4 8 8 5 ; 4 ) 4 8 5 5 2 8 8 0 6 * 8 1 ( ‡ 9 ; 4 8 ; ( 8 8 ; 4 ( ‡ ?