How can I encrypt / decrypt AES-256 CBC with OpenSSL?

Prepare input text:

echo "We're blown. Run" >input.txt

Encrypt:

openssl enc -aes-256-cbc -nosalt -e \
        -in input.txt -out input.txt.enc \
        -K '2222233333232323' -iv '5a04ec902686fb05a6b7a338b6e07760'

Decrypt to stdout original text:

openssl enc -aes-256-cbc -nosalt -d \
        -in input.txt.enc \
        -K '2222233333232323' -iv '5a04ec902686fb05a6b7a338b6e07760'

Note 1: for -K and -iv you must pass a string comprised only of hex digits. You can get this string from a binary file like this:

hexdump -e '16/1 "%02x"' FILE_WITH_KEY

Note 2: Here I used AES-256 algo that get key of 256-bit length. But in -K there is only 8 bytes/16 hex/64 bits. In this case openssl pads private key with zeros, so in example above used the following key: '2222233333232323000000000000000000000000000000000000000000000000'. This is a significant weakening, please use more strong keys in real life. The same story refer to -iv, but it's length depends on chosen algorithm's mode and block length, see related question.