The codebreaker was Elizebeth Smith Friedman (1892–1980), a self-taught American cryptanalyst who broke rum-runners' radio ciphers for the U.S. Coast Guard during Prohibition and later led the decryption of Nazi spy radio networks in South America. She did this work largely without public credit, and J. Edgar Hoover publicly took credit for some of her biggest cases.
Friedman's career shows how a codebreaker actually works. She did not guess passwords or read minds. She exploited the statistical fingerprints that every human communication leaves behind, then turned decrypted messages into evidence that courts and intelligence agencies could use. The sections below explain the mechanisms, the scale of the work, the myths, and the limits.
Mechanism: How Rum-Runner Ciphers Were Broken
Rum-runners did not use unbreakable encryption. They used commercial codebooks and simple ciphers, and that choice was their weakness. Smugglers needed speed and secrecy, so they picked systems that could be taught to ship crews and shore contacts quickly. Those systems left patterns.
The most common tool was the commercial codebook. A codebook maps whole words or phrases to groups of digits or letters, and both sender and receiver hold the same book. Coast Guard intercept operators recorded hundreds of coded messages, and Friedman matched the number groupings against known commercial codebooks. Once she identified the book, the messages became readable without knowing any key.
A second tool was the transposition cipher. Transposition rearranges the letters or symbols of a message without substituting them, like shuffling a deck rather than swapping the cards. Friedman tested column widths and reading orders until the underlying English emerged. Her statistical edge was letter frequency. In ordinary English, E appears roughly 12 to 13 percent of the time, and T, A, O, I, and N cluster close behind. A shuffled text keeps those frequencies, so a wrong column width produces gibberish statistics while the right one snaps into readable phrases.
The deepest layer was the polyalphabetic cipher. A simple substitution cipher uses one fixed mapping, so frequent letters stay frequent and are easy to spot. A polyalphabetic cipher cycles through several alphabets, flattening the frequency peaks and defeating that shortcut. Friedrich Kasiski's 1863 method could sometimes recover the key length from repeated letter groups, and Friedman used period-finding to split the message into shorter segments. Each segment then behaves like a simple substitution problem, which frequency analysis can solve. She pushed this work further than most of her contemporaries and helped formalize the statistical foundations that academic cryptanalysis later adopted.
Plain-language version: smugglers hid messages inside predictable habits of language, and Friedman attacked the habits rather than the secrecy. Her skill was patience plus arithmetic, not intuition.
The output mattered as much as the break. A decrypted message had to be transcribed, dated, tied to a vessel, and kept chain-of-custody clean so prosecutors could use it. Friedman's reports fed directly into Coast Guard seizure cases and criminal prosecutions.
Quantified Context: The Scale of the Work
Prohibition ran from January 1920 to December 1933, roughly 14 years. Enforcement required monitoring thousands of miles of coastline, and radio interception created a flood of raw material. During her Coast Guard years, Friedman and a small team solved an estimated 12,000 or more coded rum-runner messages, according to accounts of her career. That figure is widely repeated in biographies and institutional histories rather than in a single audited dataset, so treat it as an order of magnitude.
The operational stakes were concrete. In one dismantled smuggling case, Friedman reportedly produced a chart of 650 coded messages used to convict organizers, and her testimony helped secure a conviction in a major New York rum-running network in 1931. The Coast Guard credited her work with helping prosecute dozens of rum-running cases and with deterring smuggling through cracked communications.
Her later South American work came during World War II. Nazi intelligence ran clandestine radio networks in Brazil, Argentina, and Chile, transmitting weather data, ship movements, and instructions in encrypted form. By 1942, Friedman's unit had identified and broken a major network operating from Brazil, and the decrypted traffic led to the arrest and conviction of several agents. Historical accounts of the case describe a network of more than 50 agents, with about 40 arrested, organized under the cover name Bolívar. Those numbers also come from retrospective histories, not released raw archives.
The currency figure is smaller but telling. Friedman's early government salary was modest by modern standards. She earned on the order of USD 30 to USD 40 per week in the early 1920s, below what male cryptanalysts of comparable output received, a pay gap she came to understand as routine.
Timeline: From Riverbank to the Spy Trials
| Period | Setting | Work and outcome |
|---|---|---|
| 1916–1917 | Riverbank Laboratories, Illinois | Cryptanalysis for the wealthy patron George Fabyan; met William Friedman |
| 1918–1920 | U.S. government wartime services | Cipher work during and after World War I |
| 1921–1925 | U.S. Coast Guard | Broke rum-runner radio codes; produced courtroom charts and testified |
| 1926–1929 | Department of Justice | Cryptanalysis in the Prohibition enforcement era |
| 1933 | USS Akron investigation | Broke messages tied to the airship disaster inquiry |
| 1940–1942 | U.S. Navy and intelligence services | Led decryption of Nazi radio networks in South America |
| 1944–1945 | Cryptographic security work | Postwar transition into security testing and analysis |
| 1945–1970s | Retirement era | Historical writing; died in 1980 |
The timeline shows the shape of a career: a wartime start, a Prohibition peak, a law-enforcement phase, and a global-conflict finale. The through-line is her consistent method, refined across every assignment.
Edge Cases and Professional Caveat
The history has real complications. Friedman's South American work was entangled with politics, and Hoover's FBI took public credit for intelligence her team produced. She was also constrained by the era. Women codebreakers were paid less, promoted less, and erased from press coverage, which distorted the historical record for decades. Any list of "who broke what" drawn from press clippings of the 1930s and 1940s will undercount her.
A counterintuitive detail matters for anyone who thinks about modern security. Many rum-runner ciphers were solved not by mathematical genius but by bad tradecraft. Human operators reused codebooks for years, kept the same call-sign schedules, and transmitted predictable message formats. Breaking the system was less about cracking an algorithm and more about profiting from operational laziness.
The professional caveat: decryption is evidence, not proof. Even a correct plaintext requires authentication, provenance, and corroboration before it can support a prosecution. Friedman's career unfolded inside that discipline, and her charts and affidavits were built to survive cross-examination.
Misconceptions About Prohibition Codebreaking
Several myths persist and deserve correction.
Myth: Rum-runners used unbreakable machine ciphers. Reality: most used commercial codebooks or simple ciphers, and machines like the Enigma came later and elsewhere. Myth: One brilliant codebreaker defeated the Navy's cryptanalysis. Reality: Friedman worked on small teams, and her results depended on intercept operators, tabulators, and clerks. Myth: The decrypted messages alone won prosecutions. Reality: prosecutors needed vessel records, witness testimony, and chain of custody, and weak links caused cases to fail. Myth: The South American breaks were a single dramatic raid. Reality: they were months of surveillance and accumulated traffic. Myth: Her contribution was rediscovered recently. Reality: her role was known in the profession; public awareness grew later through biographies and museum exhibits.
Red-flag warnings for readers evaluating claims about her legacy:
Suspiciously precise case counts with no archive citation. Attributions that credit Hoover or the FBI exclusively. Modern encodings, such as the Enigma machine, assigned to her rum-running years. Descriptions of her as formally trained. She was not, and her early statistical work was self-taught.
Plain-language version: the legend is real, but the legend inflates one genius and shrinks the team, the era, and the paperwork.
Practical Actions: How to Study or Replicate the Method
Anyone researching this history, or teaching the mechanics behind it, can follow concrete steps. Find primary sources first. The National Security Agency's Center for Cryptologic History and the Smithsonian hold material on Friedman, and the Coast Guard historian's office has records on Prohibition enforcement. Read the technical foundations before the biography. William Friedman's The Index of Coincidence and Its Applications in Cryptanalysis and the original Kasiski and Friedman papers on periodicity teach the math she used. Practice on real ciphertext. Transcribe a short transposition or codebook message and attempt frequency analysis. A 200-letter sample is a practical minimum for reliable letter-frequency conclusions. Track the evidence chain in any case study. Note how a decrypted message moved from intercept to prosecutor and where the chain broke. Flag the gaps. Record what the archive does not state, especially any figure described as "approximately."
Red-flag warning for researchers: a biography that cites no archives and gives precise conviction counts is probably repeating secondary sources with errors.
Limitations and What the Record Cannot Show
The record has hard limits. Friedman's South American files remain partly classified, and declassified summaries are not raw traffic. Many Prohibition files were destroyed or scattered among agencies, so her exact case totals cannot be verified. The 12,000-message figure is plausible and widely repeated, but I could not confirm it against primary archives and would not present it as audited.
The technical limits matter too. Frequency analysis varies by language and message length. Short messages, mixed-language text, or codebooks with irregular mappings can defeat it, and polyalphabetic ciphers with short keys were sometimes solved only by luck. The method is statistical, not certain.
A final caution for citation. The story is on firmer ground when it points to the Coast Guard, federal archives, the NSA's cryptologic history program, and museum exhibits rather than to press clippings of the 1930s. Many of those clippings credited the wrong people by design.