How to use OpenSSL to encrypt/decrypt files?

Solution 1:

Security Warning: AES-256-CBC does not provide authenticated encryption and is vulnerable to padding oracle attacks. You should use something like age instead.

Encrypt:

openssl aes-256-cbc -a -salt -pbkdf2 -in secrets.txt -out secrets.txt.enc

Decrypt:

openssl aes-256-cbc -d -a -pbkdf2 -in secrets.txt.enc -out secrets.txt.new

More details on the various flags

Solution 2:

Short Answer:

You likely want to use gpg instead of openssl so see "Additional Notes" at the end of this answer. But to answer the question using openssl:

To Encrypt:

openssl enc -aes-256-cbc -in un_encrypted.data -out encrypted.data

To Decrypt:

openssl enc -d -aes-256-cbc -in encrypted.data -out un_encrypted.data

Note: You will be prompted for a password when encrypting or decrypt.

Long Answer:

Your best source of information for openssl enc would probably be: https://www.openssl.org/docs/man1.1.1/man1/enc.html

Command line: openssl enc takes the following form:

openssl enc -ciphername [-in filename] [-out filename] [-pass arg]
[-e] [-d] [-a/-base64] [-A] [-k password] [-kfile filename] 
[-K key] [-iv IV] [-S salt] [-salt] [-nosalt] [-z] [-md] [-p] [-P] 
[-bufsize number] [-nopad] [-debug] [-none] [-engine id]

Explanation of most useful parameters with regards to your question:

-e
    Encrypt the input data: this is the default.

-d    
    Decrypt the input data.

-k <password>
    Only use this if you want to pass the password as an argument. 
    Usually you can leave this out and you will be prompted for a 
    password. The password is used to derive the actual key which 
    is used to encrypt your data. Using this parameter is typically
    not considered secure because your password appears in 
    plain-text on the command line and will likely be recorded in 
    bash history.

-kfile <filename>
    Read the password from the first line of <filename> instead of
    from the command line as above.

-a
    base64 process the data. This means that if encryption is taking 
    place the data is base64 encoded after encryption. If decryption 
    is set then the input data is base64 decoded before being 
    decrypted.
    You likely DON'T need to use this. This will likely increase the
    file size for non-text data. Only use this if you need to send 
    data in the form of text format via email etc.

-salt
    To use a salt (randomly generated) when encrypting. You always
    want to use a salt while encrypting. This parameter is actually
    redundant because a salt is used whether you use this or not 
    which is why it was not used in the "Short Answer" above!

-K key    
    The actual key to use: this must be represented as a string
    comprised only of hex digits. If only the key is specified, the
    IV must additionally be specified using the -iv option. When 
    both a key and a password are specified, the key given with the
    -K option will be used and the IV generated from the password 
    will be taken. It probably does not make much sense to specify 
    both key and password.

-iv IV
    The actual IV to use: this must be represented as a string 
    comprised only of hex digits. When only the key is specified 
    using the -K option, the IV must explicitly be defined. When a
    password is being specified using one of the other options, the 
    IV is generated from this password.

-md digest
    Use the specified digest to create the key from the passphrase.
    The default algorithm as of this writing is sha-256. But this 
    has changed over time. It was md5 in the past. So you might want
    to specify this parameter every time to alleviate problems when
    moving your encrypted data from one system to another or when
    updating openssl to a newer version.

Additional Notes:

Though you have specifically asked about OpenSSL you might want to consider using GPG instead for the purpose of encryption based on this article OpenSSL vs GPG for encrypting off-site backups?

To use GPG to do the same you would use the following commands:

To Encrypt:

gpg --output encrypted.data --symmetric --cipher-algo AES256 un_encrypted.data

To Decrypt:

gpg --output un_encrypted.data --decrypt encrypted.data

Note: You will be prompted for a password when encrypting or decrypt.

Solution 3:

Encrypt:

openssl enc -in infile.txt -out encrypted.dat -e -aes256 -k symmetrickey

Decrypt:

openssl enc -in encrypted.dat -out outfile.txt -d -aes256 -k symmetrickey

For details, see the openssl(1) docs.