Gentlemen do not read each other's mail."- Henry L. Stimson, Secretary of State (1929-33)

Cracking the communications code of an adversary's secret governmental transmissions is the crown jewel of the intelligence business.

Almost nothing can compare in the cloak and dagger world to being able to read another country's innermost secrets - completely unbeknownst to them. After all, these communications may include diplomatic negotiating strategies, military plans, or even the names of undercover spies.

Cracked codes provide a veritable mother lode of information for the intelligence analyst (who is required to predict the future) and the policymaker (who is required to plan the future.)

Which explains why the saga of former Iraqi Governing Council member Ahmed Chalabi is so unfortunate. Though this story is still unfolding, it appears that Chalabi may have squealed to Iran's Baghdad spy chief that the U.S. was reading the encrypted transmissions of the Iranian intelligence service, the nefarious Ministry of Intelligence and Security.

If true, the revelation of this information would gravely undermine one of the U.S.'s most vital sources of information about Iran - one of the world's most hostile regimes.

Code breaking is one of the most secretive and sophisticated elements of the intelligence business. Countries spend millions of dollars developing impenetrable codes to keep their communications secure.

At the same time, their adversaries spend countless hours trying to break the ciphers, attempting to pull prized nuggets of guarded information from the streams of seemingly nonsensical ones and zeros.

During the Cold War, American intelligence's code-breaking organization, the National Security Agency, was the largest agency in the U.S. spy biz with over 50,000 employees. NSA, known for years jokingly as "No Such Agency," still employs as many as 30,000 employees and is twice the size of the CIA.

As a result of this disclosure, the Iranians will undoubtedly change their codes, leaving us unable to read their mail - at least for a period. This is no small matter. The change in codes could include losing windows of insight into:

-The MOIS' counter-Coalition and intelligence activities in Iraq. Tehran is trying to infiltrate the Iraqi Shi'a community and the new Iraqi government.

Iranian intelligence was bankrolling rebel Shi'a cleric Moqtada al Sadr and his insurgency to the tune of $80 million a year and are surely recruiting other snitches and agents of influence. Establishing a pro-Iran government in Baghdad is high on Tehran's to-do list.

-Tehran's terrorist activities, including support for Hamas, Hezbollah, Palestinian Islamic Jihad and Al Qaeda. Usama bin Laden's son, Saad bin Laden, mastermind Saif al Adel and other Al Qaeda operatives are currently being harbored in Iran. And Hezbollah may be operating in Iraq amongst the Shi'a population under the direction of the MOIS.

-The Iranian clandestine nuclear (weapons) program. The U.N.'s International Atomic Energy Agency just issued a report saying that Iran is not coming clean on its "peaceful" nuclear program. Iran also has not provided access to military sites suspected of housing weapons programs related to nukes.

-Iran's proliferation activities with Syria and North Korea, including ballistic missiles and nuclear weapons dealings. Iran has missiles that can range the entire Middle East and is working on a missile with intercontinental range - meaning the U.S. - supported by North Korea.

Gathering intelligence on Iran is a tremendous challenge for the American intelligence community under the best of circumstances. One of the avenues of doing so has just closed, with potentially dire circumstances.

Secretary Stimson, incidentally, ultimately changed his principled, peacetime position on code breaking when he served as FDR's and Harry Truman's Secretary of War (1940-45). He became a voracious consumer of cracked codes, coming to understand the value of reading other people's mail - especially in times of war.

Stimson would surely appreciate - in today's equally dangerous world - the need to read other people's mail.

Peter Brookes, a veteran of the CIA and naval intelligence, is a senior fellow at  The Heritage Foundation.