Cryptographic locks guarding the secret files of a Brazilian banker suspected of financial crimes have defeated law enforcement officials.

Brazilian police seized five hard drives when they raided the Rio apartment of banker Daniel Dantas as part of Operation Satyagraha in July 2008. But subsequent efforts to decrypt files held on the hardware using a variety of dictionary-based attacks failed even after the South Americans called in the assistance of the FBI.

The files were encrypted using Truecrypt and an unnamed algorithm, reportedly based on the 256-bit AES standard. In the UK, Dantas would be compelled to reveal his passphrase under threat of imprisonment, but no such law exists in Brazil.

The Brazilian National Institute of Criminology (INC) tried for five months to obtain access to the encrypted data without success before turning over the job to code-breakers at the FBI in early 2009. US computer specialists also drew a blank even after 12 months of efforts to crack the code, Brazil's Globo newspaper reports.

The case is an illustration of how care in choosing secure (hard-to-guess) passwords and applying encryption techniques to avoid leaving file fragments that could aid code breakers are more important in maintaining security than the algorithm a code maker chooses. In other cases, law enforcement officials have defeated suspects' use of encryption because of weak cryptographic trade craft or poor passwords, rather than inherent flaws in encryption packages. ®