How to encrypt with a string as input,but not a key object

Question

As far as I know,when encrypt or decrypt ,a key must first be got,and the key is first generate by a tool or from SecurityRandom,that means I can not generate the same key with the same input.Does there is a method which can generate a same with the same input string?
There is a need to transfer file between to site,and the customer wish to encrypt these files during transfering,and they want to store a string into each database at each site,when sending and receiving files,get the string from the database to encrypt or decrypt the file,they alse want to have a control of the string,they can change the string randomly if they wish as long as long keep each string is equal,how to realize this?

Answer 1

I think what you are looking for is a password based encryption (PBE). An example of using the same is mentioned in JCE reference guide. Below is the relevant code/docs from the rference guide.
Using Password-Based Encryption
In this example, we prompt the user for a password from which we derive an encryption key.
It would seem logical to collect and store the password in an object of type java.lang.String. However, here's the caveat: Objects of type String are immutable, i.e., there are no methods defined that allow you to change (overwrite) or zero out the contents of a String after usage. This feature makes String objects unsuitable for storing security sensitive information such as user passwords. You should always collect and store security sensitive information in a char array instead.
For that reason, the javax.crypto.spec.PBEKeySpec class takes (and returns) a password as a char array.
The following method is an example of how to collect a user password as a char array:
     * Reads user password from given input stream.
    public char[] readPasswd(InputStream in) throws IOException {
        char[] lineBuffer;
        char[] buf;
        int i;
        buf = lineBuffer = new char[128];
        int room = buf.length;
        int offset = 0;
        int c;
loop:   while (true) {
            switch (c = in.read()) {
              case -1:
              case '\n':
                break loop;
              case '\r':
                int c2 = in.read();
                if ((c2 != '\n') && (c2 != -1)) {
                    if (!(in instanceof PushbackInputStream)) {
                        in = new PushbackInputStream(in);
                    ((PushbackInputStream)in).unread(c2);
                } else
                    break loop;
              default:
                if (--room < 0) {
                    buf = new char[offset + 128];
                    room = buf.length - offset - 1;
                    System.arraycopy(lineBuffer, 0, buf, 0, offset);
                    Arrays.fill(lineBuffer, ' ');
                    lineBuffer = buf;
                buf[offset++] = (char) c;
                break;
        if (offset == 0) {
            return null;
        char[] ret = new char[offset];
        System.arraycopy(buf, 0, ret, 0, offset);
        Arrays.fill(buf, ' ');
        return ret;
    }In order to use Password-Based Encryption (PBE) as defined in PKCS #5, we have to specify a salt and an iteration count. The same salt and iteration count that are used for encryption must be used for decryption:
    PBEKeySpec pbeKeySpec;
    PBEParameterSpec pbeParamSpec;
    SecretKeyFactory keyFac;
    // Salt
    byte[] salt = {
        (byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,
        (byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99
    // Iteration count
    int count = 20;
    // Create PBE parameter set
    pbeParamSpec = new PBEParameterSpec(salt, count);
    // Prompt user for encryption password.
    // Collect user password as char array (using the
    // "readPasswd" method from above), and convert
    // it into a SecretKey object, using a PBE key
    // factory.
    System.out.print("Enter encryption password:  ");
    System.out.flush();
    pbeKeySpec = new PBEKeySpec(readPasswd(System.in));
    keyFac = SecretKeyFactory.getInstance("PBEWithMD5AndDES");
    SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);
    // Create PBE Cipher
    Cipher pbeCipher = Cipher.getInstance("PBEWithMD5AndDES");
    // Initialize PBE Cipher with key and parameters
    pbeCipher.init(Cipher.ENCRYPT_MODE, pbeKey, pbeParamSpec);
    // Our cleartext
    byte[] cleartext = "This is another example".getBytes();
    // Encrypt the cleartext
    byte[] ciphertext = pbeCipher.doFinal(cleartext);Hope this helps
Sai Pullabhotla