National security letters permit the executive branch to seek records about people in terrorism and spy investigations without a judge's approval or grand jury subpoena.
"The Defense Department gets involved because we've got hundreds of bases inside the United States that are potential terrorist targets," Cheney said.
"The Department of Defense has legitimate authority in this area. This is an authority that goes back three or four decades. It was reaffirmed in the Patriot Act," he said. "It's perfectly legitimate activity. There's nothing wrong with it or illegal. It doesn't violate people's civil rights."
The Pentagon and the CIA, to a lesser extent, have used this little- known power, officials said. The FBI, the lead agency on domestic counterterrorism and espionage, has issued thousands of such letters since the attacks of Sept. 11, 2001.
The letters have generated criticism and court challenges from civil liberties advocates who claim they invade the privacy of Americans' lives, even though banks and other financial institutions typically turn over the financial records voluntarily.
The vast majority of national security letters are issued by the FBI, but in rare circumstances they have been used by the CIA before and after Sept. 11, according to a U.S. intelligence official. The CIA has used these noncompulsory letters in espionage investigations and other circumstances, the official said.
The New York Times, which reported Sunday on the expanded use of the technique by the Pentagon and CIA, said military intelligence officers have sent the letters in up to 500 investigations.
Cheney was interviewed on Fox News Sunday.