Why does encrypted information present challenges to forensic investigators?

What will be an ideal response?

Encrypted information can present challenges to forensic investigators. A fairly common encryption product accepts the encryption key when the user logs on and decrypts information on the fly. When the system goes into screen saver mode or is powered down, the encryption key is destroyed and must be reentered. Unfortunately, data collected by a forensic investigator is encrypted and will not be readable without the key.