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7-Zip is a free and open source encryption program.
Specs
Pros
Pro Password protected
The encrypted file cannot be viewed without the same password inputted by the sender in the encryption process and the recipient in the decryption process. Although this password is another bit of sensitive information to transfer this may be a good solution if the sender and recipient can meet once for the password and then use password protected encrypted .7z files for future information transfers.
Cons
Con Encrypted .zips are suspicious
It is rather suspicious for an encrypted zip file to be found in your email. If it is seriously important, people can find a way to coerce you to give over the password. A steganography technique is important if secrecy is of utmost important.
Con Both the sender and recipient need an encryption program
Not only does the sender need an encryption program to encrypt the file, but the recipient needs an encryption program to open it.
Windows provides a basic level of encryption natively without AES, but most users craving security will want to use a more advanced utility with AES such as 7-Zip. If the recipient is not good with computers this may be difficult.
Con The standard encyption method ZipCrypto is unsecure
Be sure to select AES encryption instead of the standard and much less secure ZipCrypto encryption. And use .7z files with Filename encryption, to prevent anyone from even viewing the contents and filenames, instead of .zip files, which can only encrypt Data.