Bouncy castle 128bit AES/CBC/PKCS7 in java
Some observations:


It is best to always specify the encoding when converting from a string to a byte array:
input.getBytes()     -> input.getBytes("UTF-8")
keyString.getBytes() -> keyString.getBytes("UTF-8")
The IV should be an array of random bytes unique for each encryption, usually created from a CSPRNG. Then prefix the encrypted data with the IV for use during decryption, the IV does not need to be secret.
When using a password it is necessary to derive the key from it with a function such as PBKDF2.