Here's one way to encrypt a string with openssl on the command line (must enter password twice):
echo -n "aaaabbbbccccdddd" | openssl enc -e -aes-256-cbc -a -salt
enter aes-256-cbc encryption password:
Verifying - enter aes-256-cbc encryption password:
Here's what the output looks like:
U2FsdGVkX1/6LATntslD80T2HEIn3A0BqxarNfwbg31D2kI00dYbmBo8Mqt42PIm
Edit: To my knowledge, you can't control the number of bytes out. You can b64 or hex encode it, but that's about it. Also, if you want to save that string to a file rather than stdout, use the -out option.
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