Cryptography in the modern age

Code breakers help win wars, and encryption goes public

(CNN) -- Ancient civilizations were the first to practice cryptography, but modern efforts date back to World War I when the United States dedicated military units to capturing and deciphering communications intelligence and the British decoded a key telegram.

The famous Zimmermann telegram from Germany to Mexico was intercepted by the British, who decoded the message in which Germany promised Mexico a chunk of the United States if it entered World War I. The British leaked the message to the United States, which helped end U.S. neutrality.

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But it was during World War II that the United States and Great Britain achieved a great deal of success in cryptography.

German researchers created the cipher machine known as Enigma, which the German military used during World War II. However, a British team of code breakers, including the brilliant mathematician Alan Turing, managed to crack the Enigma code against staggering odds.

The British efforts, code-named Ultra, were based at Bletchley Park, 65 miles north of London. The researchers there also built Colossus, one of the world's first computers, to crack the Lorenz cipher, another encryption scheme used by the Nazis.

The 'Magic' effort to crack the Japanese code

While the British focused on Germany, U.S. intelligence worked on Japanese communications in the Pacific. The U.S. naval intelligence effort, code-named "Magic," cracked the Japanese diplomatic cipher that was produced by an encryption machine nicknamed "Purple." U.S. naval intelligence also cracked other Japanese codes, including the naval and merchant ship codes.

The ability to read the Japanese code helped the Navy in several important battles, most importantly at the Battle of Midway in 1942. The Japanese had six aircraft carriers to the three that the United States operated in the Pacific. Japanese naval commander Admiral Isoroku Yamamoto devised a plan to attack Midway Island in hopes of luring the entire U.S. fleet into battle and use the carrier advantage to crush the U.S. forces.

Yamamoto placed his main carrier force, consisting of four carriers, into a single formation, making all four vulnerable to attack if one was discovered. When naval code breakers pieced enough Japanese messages together, they discovered the plans and objectives of the Japanese fleet.

In the ensuing battle on June 4, 1942, U.S. pilots sank all four Japanese carriers from the main strike force while only one U.S. carrier was lost. The battle helped turn the tide of the Pacific war against the Japanese.

An intercepted Japanese message also led to Yamamoto's death. U.S. code breakers discovered plans for Yamamoto to inspect the naval base at Bougainville in the Solomon Islands. On April 18, 1943, Yamamoto's plane was ambushed as it approached Bougainville and was shot down by U.S. fighter planes.

U.S. code breakers also helped in the invasions of the islands of Guadalcanal and Tarawa. Decoded Japanese messages led to the U.S. Marines invading Guadalcanal before the Japanese could fortify the beaches. During the battle for Tarawa, naval intelligence discovered key details of the Japanese order of battle and logistical status.

Navajo code-talkers baffled the Japanese

While the Japanese were able to decipher the codes used by the U.S. Army and Army Air Corps, they never cracked the code of the U.S. Marines, which used Native Americans speaking in Navajo to communicate between units.

The idea for using Navajos belonged to Philip Johnston, who grew up on a Navajo reservation as the son of a missionary. He also was one of the few non-Navajos fluent in that language. A World War I veteran, Johnston knew that the military used Native American languages -- notably Choctaw -- in the earlier war to encode messages.

Johnston believed Navajo was suited for encoding messages because it was an unwritten language with no alphabet or symbols and was spoken only by Navajos in the American Southwest. Once the military agreed to the idea, 200 Navajos were recruited, and the first 29 recruits created the Navajo code, including a written dictionary and the creation of words for military terms.

Roughly 400 Navajos became what were called "code talkers," serving in various Marine units in the Pacific. Their primary job was to transmit battlefield communications via telephones and radios. The Navajo language stumped the Japanese, and they never cracked the code.

Venona captures Soviet communications

Also during the war, the U.S. Army's Signal Intelligence Service, a forerunner of the National Security Agency, began the top-secret Venona program, which captured and exploited encrypted Soviet diplomatic communications.

According to a declassified NSA history of the project, a Signal Corps reserve officer, Lt. Richard Hallock, discovered weaknesses in the cryptographic system used to encrypt Soviet trade communications. The discovery helped break four other Soviet cryptographic systems used by Soviet diplomats, the KGB and the Soviet army and naval intelligence services.

Eventually, more than 2,200 Soviet messages were translated in the Venona program, but not all the messages collected were successfully decrypted. However, enough information was garnered to help expose activities of some high-profile spy cases, including Kim Philby, the high-ranking British intelligence officer who defected to Moscow, and Ethel and Julius Rosenberg, U.S. citizens who were convicted of selling atomic secrets to the Soviets and executed in 1953.

The trial and execution of the Rosenbergs was protested worldwide. When the NSA released its Venona archive, the children of the Rosenbergs said it contained nothing that proved the guilt of their parents.

The Venona program ended in 1980. In 1995, the NSA ended its 50-year silence on the project and publicly released some of the declassified messages.

The development of public-key encryption

Cryptography and encryption were primarily the domain of intelligence agencies until the rise of the information age and the Internet. A huge step forward in encryption was the simultaneous development of public-key encryption in the 1970s.

A person who encrypts a document uses what's called a "key" to encode the message. Until public-key encryption, the same key had to be used to decrypt, or "unlock" the document. If a person sent an encoded document to someone else, the key also had to be sent to that person via a secure method, like a face-to-face meeting.

Public-key encryption eliminated the problem of securely transmitting the key by using two keys, a public and private one. The sender of a message encrypts the document using the recipient's public key and the recipient of the message decrypts the message using the private key, which the recipient keeps secret. The public and private keys are the inverse of each, so a person's private key -- which can be kept secret -- can unlock a message that was encrypted by the public key.

The idea of public-key encryption was first developed in 1976 by Martin Hellman, a Stanford University professor, and two graduate students who worked with him at the time, Ralph Merkle and Whitfield Diffie.

A similar idea was developed by three Massachusetts Institute of Technology professors, Ron Rivest, Adi Shamir and Len Adleman. Their approach was labeled RSA after their initials, and the three went on to form a business that is now RSA Data Security. RSA is one of primary encryption ideas used on the Internet.

However, in 1997, the British government released a document showing that three employees of its eavesdropping organization known as the Government Communications Headquarters discovered a similar approach earlier in the 1970s. British officials said the discovery by researchers James Ellis, Clifford Cocks and Malcolm Williamson was kept secret for national security reasons.

In 1991, programmer Phil Zimmermann released the first version of his encryption program, Pretty Good Privacy (PGP), as a free program. Zimmerman released the program after the FBI pushed for access to encrypted communications. The program has now become a worldwide standard.

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