.efr is Encryptafile Private Key File
Key Features: .EFR File Extension
What is an .EFR file?
Private Key used to authenticate an encrypted Encryptafile (.EFL) file; contains a hexadecimal representation of the private key used to created the digital signature for the file.
The length of a private key file must be greater than or equal to 16 bits, and less than or equal to 7128 bits.