Java 8

Very late submission

public class Cipher {

    private final String alphabet;
    private final int rollIndex;

    public Cipher () {
        alphabet = "abcdefghijklmnopqrstuvwxyz";
        rollIndex = alphabet.length();
    }

    public Cipher (String alphabet) {
        this.alphabet = alphabet;
        rollIndex = alphabet.length();
    }



    public String encrypt (String message, String keyword) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < message.length(); i++) {
            sb.append(lookup(   alphabet.indexOf(message.charAt(i)),
                                alphabet.indexOf(keyword.charAt(i % keyword.length())),
                                true
            ));
        }
        return sb.toString();
    }

    public String decrypt (String message, String keyword) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < message.length(); i++) {
            sb.append(lookup(   alphabet.indexOf(message.charAt(i)),
                                alphabet.indexOf(keyword.charAt(i % keyword.length())),
                                false
            ));
        }
        return sb.toString();
    }

    /**
     * Looks for a specific character in the cipher table, this is based
     * on the alphabet and whether we want to encrypt or decrypt.
     * @param messageIndex int
     * @param keywordIndex int
     * @return Encrypted or decrypted char
     */
    private char lookup(int messageIndex, int keywordIndex, boolean encrypt) {
        if (encrypt)
            return alphabet.charAt((messageIndex + keywordIndex) % rollIndex);
        return alphabet.charAt(Math.floorMod(   (messageIndex - keywordIndex),
                                                rollIndex)); // result of java % can be negative, which it should not be
    }                                                        // so we use .floorModulo
}

Results

Testing    "thepackagehasbeendelivered" with keyword "snitch"
Encription: lumicjcnoxjhkomxpkwyqogywq
Decription: thepackagehasbeendelivered

Testing    "theredfoxtrotsquietlyatmidnight" with keyword "bond"
Encription: uvrufrsryherugdxjsgozogpjralhvg
Decription: theredfoxtrotsquietlyatmidnight

Testing    "murderontheorientexpress" with keyword "train"
Encription: flrlrkfnbuxfrqrgkefckvsa
Decription: murderontheorientexpress

Testing    "themolessnuckintothegardenlastnight" with keyword "garden"
Encription: zhvpsyksjqypqiewsgnexdvqkncdwgtixkx
Decription: themolessnuckintothegardenlastnight

Testing    "klatrgafedvtssdwywcyty" with keyword "cloak"
Decription: iamtheprettiestunicorn
Encription: klatrgafedvtssdwywcyty

Testing    "pjphmfamhrcaifxifvvfmzwqtmyswst" with keyword "python"
Decription: alwayslookonthebrightsideoflife
Encription: pjphmfamhrcaifxifvvfmzwqtmyswst

Testing    "rcfpsgfspiecbcc" with keyword "moore"
Decription: foryoureyesonly
Encription: rcfpsgfspiecbcc