7-Zip is a free and open source encryption program.
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Pro AES-256 encryption
7-Zip can encrypt files with extremely secure AES-256 encryption. AES is used by governments and businesses and is believed to be impossible to crack with today's technology at least. AES-256 has 1.1x10^77 combinations.
Pro 7-Zip is free and open-source
One of the best programs to encrypt .zip files is a free and open source program called 7-Zip. Although there are many other options out there such as WinZip, many cost money.
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 .zip files for future information transfers.
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.
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.