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How to use Key Interface for secret key in java?

I have a code from creating base 64 hashes
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import org.apache.commons.codec.binary.Base64;
public class ApiSecurityExample {
public static void main(String[] args) {
try {
String secret = "secret";
String message = "Message";
Mac sha256_HMAC = Mac.getInstance("HmacSHA256");
SecretKeySpec secret_key = new SecretKeySpec(secret.getBytes(), "HmacSHA256");
sha256_HMAC.init(secret_key);
String hash = Base64.encodeBase64String(sha256_HMAC.doFinal(message.getBytes()));
System.out.println(hash);
}
catch (Exception e){
System.out.println("Error");
}
}
}
there is secret_key in sha256_HMAC.init(secret_key);
when I read, it tells use Key key an Interface.
how to use it?

The example is doing it wrong, as strings should not be used to store keys.
A secret key should consist of bytes that are unpredictable to an adversary. The most logical method to generate those is to use a random number generator, but you can also generate them from key establishment (Diffie-Hellman), using a key derivation function upon another key, ratchets and many other ways.
A somewhat dangerous method is to generate them from a password. For that you normally use a Password Based Key Derivation Function or PBKDF. Java has direct support for PBKDF2 which can be used for this.
So you could create a HMAC key in the following way:
Mac mac = Mac.getInstance("HMACSHA256");
SecureRandom rng = new SecureRandom();
// key size can be anything but should default to the hash / MAC output size for HMAC
byte[] hmacKeyData = new byte[mac.getMacLength()];
rng.nextBytes(hmacKeyData);
SecretKey hmacKey = new SecretKeySpec(hmacKeyData, "HMACSHA256");
Arrays.fill(hmacKeyData, (byte) 0x00);
However, the following code is shorter, probably more descriptive. It also allows hardware devices to be used later on to implement the Mac, although that might be a bit out of your territory.
KeyGenerator kg = KeyGenerator.getInstance("HMACSHA256");
SecretKey hmacKey = kg.generateKey();
Finally, if you still want to use a password, then use PKBDF2 and don't forget to store the salt:
// you don't want to use a string, as you cannot delete strings in Java
char[] password = {'p', 'a', 's', 's' };
SecureRandom rng = new SecureRandom();
byte[] salt = new byte[128 / Byte.SIZE];
rng.nextBytes(salt);
int iterations = 1_000_000;
Mac mac = Mac.getInstance("HMACSHA256");
PBEKeySpec spec = new PBEKeySpec(password, salt, iterations, mac.getMacLength() * Byte.SIZE);
SecretKeyFactory pbkdf = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
byte[] hmacKeyData = pbkdf.generateSecret(spec).getEncoded();
SecretKey hmacKey = new SecretKeySpec(hmacKeyData, "HMACSHA256");
// clean up secret material
Arrays.fill(password, (char) 0x0000);
spec.clearPassword();
Arrays.fill(hmacKeyData, (byte) 0x00);
As an attacker may have forever to try passwords if he has a MAC to compare the result with, it would be a very good idea to choose a very complex password though; this is why password based encryption generally is not a good idea.
Key is a generic parent interface used for both SecretKey, PublicKey and PrivateKey. It is used in many classes that represent crypto algorithms as they may be used with any kind of key. For instance Cipher can be used for RSA but also for AES. So the implementation just checks at runtime if the correct key is given.
For Mac it might as well have been SecretKey as a Mac is really always a symmetrical algorithm (an asymmetric form of a Mac is called a Signature after all). Just a HMAC key would not be enough though, as there are also Mac algorithms based on block ciphers such as AES (thus requiring a SecretKey with algorithm "AES").
For convenience, SecretKeySpec also implements SecretKey; that way you don't need the SecretKeyFactory to create a SecretKey. The Java designers kind of forgot about hardware support that does require such as factory, but here we are.

Java AES encryption issue

Every time the encryption values changed by using AES, let anyone investigate the below code and let me know the issue
code:
private static final String secretKeys = "58BA833E57A51CBF9BF8BAB696BF9"
public static String encrypt() throws Exception {
byte[] salt = new byte[16];
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA256");
PBEKeySpec pbeKeySpec = new PBEKeySpec(secretKeys.getChars(),salt,1000, 256);
Key secretKey = factory.generateSecret(pbeKeySpec);
byte[] key = new byte[32];
byte[] iv = new byte[16];
SecretKeySpec secret = new SecretKeySpec(key, "AES");
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, secret);
byte[] result = cipher.doFinal("welcome".getBytes("UTF-8"));
String s = Base64.getEncoder().encodeToString(result);
return s
}
Output
first time I got the below-encrypted string
CZRIP35M4CnJtuDQ6YpmaQ==
The second time I got the below-encrypted string
/fylTjohAZDsnCaHhiZo3A==
I have three questions:
why the encrypted string not a constant?
how can I set the Blocksize? ( AES.BlockSize = 128;)
How can I set the padding mode? (AES.Padding = PaddingMode.PKCS7;)

For the first question, #Freiheit already answered this.
Long story short, based on the iv (initilization vector) which acts as a salt and will be different for each encryption.
Having that said, encrypting the same plain text will result in different encrypted text, but the decryption (if necessary) will result back into the same plain text.
IV is helpful to make the encryption predictable.
Having stored the same password for 2 different users in a database will have different values, but will be the same password.
With the current cipher configured, you already have 128 block size. You can read more about the different cypher transformation here. You can also find more information of the block sizes for different algorithms here
You just need to change the Cipher.getInstance() to AES/CBC/PKCS7Padding

1) the encrypted text is always different because the Cipher initialization is providing it's own IV since you are not providing one. You need to provide the IV you've "computed" in order to have a consistent output. Remember you never want to use an IV more than once for whatever this code is ultimately intended to do.
2) The keysize can be 128, 192 or 256 but the blocksize is always 128.
3) Java only provides PKCS5, but there is no difference in the implementation for AES. see what-is-the-difference-between-pkcs5-padding-and-pkcs7-padding
As was already pointed out there are several problems with the code provided such as the first lines not actually doing anything and the key and iv both being uninitialized. I would additionally suggest you use SecureRandom to initialize your key and iv. If you plan on using only a single AES key, this can be computed once and placed in the code or configuration file instead of running PBKDF2 every time.

Only adding to the answer provided by #micker, you need to invoke another version of Cipher.init(); one that takes the IV into account:
...
byte[] iv = new byte[16];
IvParameterSpec ivSpec = new IvParameterSpec(iv); // <= Wrap your IV bytes here.
SecretKeySpec secret = new SecretKeySpec(key, "AES");
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, secret, ivSpec); // <= Add IV here.
...
That being said, the implementation suffers from a slew of other issues (key being all zeroes, IV being all zeroes, first 4 line don't do anything for you (as #JBNizet pointed out)). I hope you are only using it to study how Java's encryption mechanics works.

Reusing PBKDF2 salt for AES/GCM as IV: dangerous?

I'm developing an encryption utility class to be reused for common operations.
A very common case is to encrypt a plaintext with a user-provided password.
In this case, I'm using PBKDF2 to derive a valid AES key, then use it in GCM mode to encrypt the plaintext.
Some code:
// IV_LEN = 96
// ITERATIONS = 1000 ~ 4000
// KEY_LEN = 128 ~ 256
// TAG_LEN = 128
public static String encrypt(byte[] plain, char[] password) throws GeneralSecurityException
{
SecureRandom rng = SecureRandom.getInstanceStrong();
byte[] iv = new byte[IV_LEN / 8];
rng.nextBytes(iv);
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA512");
SecretKey derivedKey = factory.generateSecret(new PBEKeySpec(password, iv, ITERATIONS, KEY_LEN));
SecretKey secretKey = new SecretKeySpec(derivedKey.getEncoded(), "AES");
Cipher c = Cipher.getInstance("AES/GCM/NoPadding");
c.init(Cipher.ENCRYPT_MODE, secretKey, new GCMParameterSpec(TAG_LEN, iv));
byte[] encrypted = c.doFinal(plain);
Encoder encoder = Base64.getUrlEncoder().withoutPadding();
return encoder.encodeToString(iv) + ":" + encoder.encodeToString(encrypted);
}
Currently, I'm using the PBKDF2 salt (96 bit - SecureRandom) also as the IV for AES/GCM encryption.
Both the salt and IV can be public, but they shouldn't be reused.
Is it to be understood that they shouldn't be reused within the same Feature/Service/Algorithm, or that they shouldn't be reused anywhere?
It's very easy to modify this method to generate different salt and IV, but is there a reason to do it?
Thanks

Note that you may not need to re-generate a random IV as long as you change the salt and therefore the resulting key. If you do not change the salt each time you (re-)encrypt then you do need a separate IV or you may leak information to an adversary.
You can keep the salt and IV the same. But it is generally easier to derive the IV together with the key from the password and salt. If you use PBKDF2 with a SHA-512 hash this is easy: just take 128, 192 or 256 bits of AES key from the resulting hash and then take another 128 subsequent bits as IV and use that.
If you need multiple keys or if you use a smaller hash then you may need to derive more keys from the result of PBKDF2. In that case it is best to mark the result of PBKDF2 as a master secret and perform N key derivations from it, one for each key and one for the IV. You could use e.g. HKDF for this, which Bouncy Castle has an implementation of (for which I provided the initial source code).

How to properly recreate SecretKey from string

I'm trying to make an encryption-decryption app. I've got two classes - one with functions to generate the key, encrypt and decrypt, second one for JavaFX GUI. In the GUI class I've got 4 textareas: 1st to write text to encrypt, 2nd for encrypted text, 3rd for the key (String encodedKey = Base64.getEncoder().encodeToString(klucz.getEncoded());) and 4th for decrypted text.
The problem is, I am not able to decrypt the text. I'm trying to recreate the SecretKey like this:
String encodedKey = textAreaKey.getText();
byte[] decodedKey = Base64.getDecoder().decode(encodedKey);
SecretKey klucz = new SecretKeySpec(decodedKey, "DESede");
When I encrypt the key looks like this: com.sun.crypto.provider.DESedeKey#4f964d80 and when I try to recreate it: javax.crypto.spec.SecretKeySpec#4f964d80 and I'm getting javax.crypto.IllegalBlockSizeException: Input length must be multiple of 8 when decrypting with padded cipher
Here is my 1st class:
public class Encryption {
public static SecretKey generateKey() throws NoSuchAlgorithmException {
Security.addProvider(new com.sun.crypto.provider.SunJCE());
KeyGenerator keygen = KeyGenerator.getInstance("DESede");
keygen.init(168);
SecretKey klucz = keygen.generateKey();
return klucz;
}
static byte[] encrypt(byte[] plainTextByte, SecretKey klucz)
throws Exception {
Cipher cipher = Cipher.getInstance("DESede/ECB/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, klucz);
byte[] encryptedBytes = cipher.doFinal(plainTextByte);
return encryptedBytes;
}
static byte[] decrypt(byte[] encryptedBytes, SecretKey klucz)
throws Exception {
Cipher cipher = Cipher.getInstance("DESede/ECB/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, klucz);
byte[] decryptedBytes = cipher.doFinal(encryptedBytes);
return decryptedBytes;
}
}
edit
btnEncrypt.setOnAction((ActionEvent event) -> {
try {
String plainText = textAreaToEncrypt.getText();
SecretKey klucz = Encryption.generateKey();
byte[] plainTextByte = plainText.getBytes();
byte[] encryptedBytes = Encryption.encrypt(plainTextByte, klucz);
String encryptedText = Base64.getEncoder().encodeToString(encryptedBytes);
textAreaEncryptedText.setText(encryptedText);
byte[] byteKey = klucz.getEncoded();
String stringKey = Base64.getEncoder().encodeToString(byteKey);
textAreaKey.setTextstringKey
} catch (Exception ex) {
ex.printStackTrace();
}
});
btnDecrypt.setOnAction((ActionEvent event) -> {
try {
String stringKey = textAreaKey.getText();
byte[] decodedKey = Base64.getDecoder().decode(encodedKey);
SecretKey klucz2 = new SecretKeySpec(decodedKey, "DESede");
String encryptedText = textAreaEncryptedText.getText();
byte[] encryptedBytes = Base64.getDecoder().decode(encryptedText.getBytes());
byte[] decryptedBytes = Encryption.decrypt(encryptedBytes, klucz2;
String decryptedText = Base64.getEncoder().encodeToString(decryptedBytes);
textAreaDecryptedText.setText(decryptedText);
} catch (Exception ex) {
ex.printStackTrace();
}
});

One of your problems is here:
String encryptedText = new String(encryptedBytes, "UTF8");
Generally, many byte sequences in cipher text are not valid UTF-8–encoded characters. When you try to create a String, this malformed sequences will be replaced with the "replacement character", and then information from the the cipher text is irretrievably lost. When you convert the String back to bytes and try to decrypt it, the corrupt cipher text raises an error.
If you need to represent the cipher text as a character string, use base-64 encoding, just as you do for the key.
The other principal problem is that you are aren't specifying the full transformation. You should specify the "mode" and "padding" of the cipher explicitly, like "DESede/ECB/PKCS5Padding".
The correct mode will depend on your assignment. ECB is generally not secure, but more secure modes add a bit of complexity that may be outside the scope of your assignment. Study your instructions and clarify the requirements with your teacher if necessary.

There are two main issues:
You should not use user entered password as a key (there are difference between them). The key must have specific size depending on the cipher (16 or 24 bytes for 3des)
Direct 3DES (DESede) is a block cipher encrypting 8 bytes at once. To encrypt multiple blocks, there are some methods defined how to do that properly. It is calls Block cipher mode.
For proper encryption you need to take care of a few more things
Creating a key from the password
Let's assume you want to use DESede (3des). The key must have fixed size - 16 or 24 bytes. To properly generate a key from password you should use PBKDF. Some people are sensitive to "must use", however neglecting this step really compromises the encryption security mainly using user-entered passwords.
For 3DES you can use :
int keySize = 16*8;
int iterations = 800000;
char[] password = "password".toCharArray();
SecureRandom random = new SecureRandom();
byte[] salt = random.generateSeed(8);
SecretKeyFactory secKeyFactory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA512");
KeySpec spec = new PBEKeySpec(password, salt, iterations, keySize);
SecretKey pbeSecretKey = secKeyFactory.generateSecret(spec);
SecretKey desSecret = new SecretKeySpec(pbeSecretKey.getEncoded(), "DESede");
// iv needs to have block size
// we will use the salt for simplification
IvParameterSpec ivParam = new IvParameterSpec(salt);
Cipher cipher = Cipher.getInstance("DESEde/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, desSecret, ivParam);
System.out.println("salt: "+Base64.getEncoder().encodeToString(salt));
System.out.println(cipher.getIV().length+" iv: "+Base64.getEncoder().encodeToString(cipher.getIV()));
byte[] ciphertext = cipher.doFinal("plaintext input".getBytes());
System.out.println("encrypted: "+Base64.getEncoder().encodeToString(ciphertext));
if you can ensure that your password has good entropy (is long and random enough) you may be good with a simple hash
MessageDigest dgst = MessageDigest.getInstance("sha-1");
byte[] hash = dgst.digest("some long, complex and random password".getBytes());
byte[] keyBytes = new byte[keySize/8];
System.arraycopy(hash, 0, keyBytes, 0, keySize/8);
SecretKey desSecret = new SecretKeySpec(keyBytes, "DESede");
The salt serves to randomize the output and should be used.
The output of the encryption should be salt | cipthertext | tag (not necessarily in this order, but you will need all of these for proper encryption).
To decrypt the output, you will need to split the output to salt, ciphertext and the tag.
I see zero vectors ( static salt or iv ) very often in examples from StackOverflow, but in many cases it may lead to broken ciphers revelaling key or plaintext.
The initialization vector iv is needed for block chain modes (encrypting longer input than a single block), we could use the salt from the key as well
when having the same size ( 8 bytes in our case). For really secure solution the password salt should be longer.
The tag is an authentication tag, to ensure that nobody has manipulated with the ciphertext. You could use HMAC of the plaintext or ciphertext. It is important you should use different key for HMAC than for encryption. However - I believe in your case your homework will be ok even without the hmac tag

Using public private key combination for symmetric encryption with Java 8

I'm trying to use an asymmetric private and public key combination to generate a symmetric key for encrypting and decrypting some text, but, I'm stuck unable to use the generated key as it is 128bytes in size and this is unacceptable for the AES encryption. I'd like to solve this problem using just the JRE (no external libraries). Do you have a solution?
I've included my example code below, there's a comment indicating the line I get the exception thrown.
(encryptCipher.init(Cipher.ENCRYPT_MODE, tomSecretKeySpec, iv);)
I read about KDF hashing, but Java doesn't seem to have an obvious way of invoking this on my 128byte key. Also, Im not sure this is the right answer since my understanding is that the longer the key, the more secure the encryption (for a given algorithm). Perhaps I need to switch from using AES/CBC/PKCS5Padding, but none of the other algorithms included with the JDK as standard seem to support the 128byte key either.
public void demoSymmetricEncryption() throws NoSuchAlgorithmException, InvalidKeyException, NoSuchPaddingException, InvalidAlgorithmParameterException, UnsupportedEncodingException, IllegalBlockSizeException, BadPaddingException {
String keyAlgorithm = "DiffieHellman";
String keyAgreementAlgorithm = "DiffieHellman";
String keySpecAlgorithm = "AES";
String cipherAlgorithm = "AES/CBC/PKCS5Padding";
KeyPairGenerator keyGenerator = KeyPairGenerator.getInstance(keyAlgorithm);
keyGenerator.initialize(1024, new SecureRandom());
KeyPair tomKeyPair = keyGenerator.generateKeyPair();
PrivateKey tomPrivateKey = tomKeyPair.getPrivate();
PublicKey tomPublicKey = tomKeyPair.getPublic();
KeyPair steveKeyPair = keyGenerator.generateKeyPair();
PrivateKey stevePrivateKey = steveKeyPair.getPrivate();
PublicKey stevePublicKey = steveKeyPair.getPublic();
int maxKeyLen = Cipher.getMaxAllowedKeyLength("AES");
System.out.println("Limited encryption policy files installed : " + (maxKeyLen == 128)); // returns false
KeyAgreement tomKeyAgreement = KeyAgreement.getInstance(keyAgreementAlgorithm);
keyGenerator.initialize(1024, new SecureRandom());
tomKeyAgreement.init(tomPrivateKey);
tomKeyAgreement.doPhase(stevePublicKey, true);
byte[] tomSecret = tomKeyAgreement.generateSecret();
SecretKeySpec tomSecretKeySpec = new SecretKeySpec(tomSecret, keySpecAlgorithm);
KeyAgreement steveKeyAgreement = KeyAgreement.getInstance(keyAgreementAlgorithm);
steveKeyAgreement.init(stevePrivateKey);
steveKeyAgreement.doPhase(tomPublicKey, true);
byte[] steveSecret = steveKeyAgreement.generateSecret();
SecretKeySpec steveSecretKeySpec = new SecretKeySpec(steveSecret, keySpecAlgorithm);
System.out.println("Secret Keys are identical : " + steveSecretKeySpec.equals(tomSecretKeySpec)); // returns true
String initVector = "RandomInitVector";
Cipher encryptCipher = Cipher.getInstance(cipherAlgorithm);
IvParameterSpec iv = new IvParameterSpec(initVector.getBytes("UTF-8"));
// fails because AES key is 128 bytes not 128 bits in length - think I need to use KDF hash to shrink it appropriately.
encryptCipher.init(Cipher.ENCRYPT_MODE, tomSecretKeySpec, iv);
// Attempt to use the cipher
byte[] encryptedData = encryptCipher.doFinal("Hello".getBytes());
Cipher decryptCipher = Cipher.getInstance(cipherAlgorithm);
iv = new IvParameterSpec(initVector.getBytes("UTF-8"));
decryptCipher.init(Cipher.DECRYPT_MODE, steveSecretKeySpec, iv);
byte[] decryptedData = decryptCipher.doFinal(encryptedData);
System.out.println("Decrypted Data : " + new String(decryptedData));
}
The output from the program is as follows:
Limited encryption policy files installed : false
Secret Keys are identical : true
Exception in thread "main" java.security.InvalidKeyException: Invalid AES key length: 128 bytes
at com.sun.crypto.provider.AESCrypt.init(AESCrypt.java:87)
at com.sun.crypto.provider.CipherBlockChaining.init(CipherBlockChaining.java:91)
at com.sun.crypto.provider.CipherCore.init(CipherCore.java:582)
at com.sun.crypto.provider.AESCipher.engineInit(AESCipher.java:339)
at javax.crypto.Cipher.implInit(Cipher.java:806)
at javax.crypto.Cipher.chooseProvider(Cipher.java:864)
at javax.crypto.Cipher.init(Cipher.java:1396)
at javax.crypto.Cipher.init(Cipher.java:1327)
at crypto.SymetricEncryptionTest.demoSymmetricEncryption(SymetricEncryptionTest.java:76)
at crypto.SymetricEncryptionTest.main(SymetricEncryptionTest.java:29)

The error is: * Invalid AES key length: 128 bytes*
Valid AES key sizes are 128-bits, 192-bits and 256-bits or in bytes: 16-bytes, 24-bytes and 32-bytes.
Use an AES key size that is valid.
The general method of generation a symmetric key is just to get the bytes from a cryptographic PRNG. For Java see Class SecureRandom.
For key derivation use PBKDF2, see Class SecretKeyFactory and Java Cryptography Architecture Standard Algorithm Name Documentation "PBKDF2WithHmacSHA1" (Constructs secret keys using the Password-Based Key Derivation Function function).
For an example see OWASP Hashing Java but use "PBKDF2WithHmacSHA1" as the algorithm.

The reason the code wasn't working was that I was using incompatible algorithms. The corrections are as follows:
Replace lines:
String keyAlgorithm = "DiffieHellman";
String keyAgreementAlgorithm = "DiffieHellman";
with
String keyAlgorithm = "EC";
String keyAgreementAlgorithm = "ECDH";
int keySize = 128;
and replace lines
keyGenerator.initialize(1024, new SecureRandom());
with
keyGenerator.initialize(keySize, new SecureRandom());
Program now produces output:
Limited encryption policy files installed : false
Secret Keys are identical : true
Decrypted Data : Hello
Technically, you probably also want to Base64 encode the encrypted output and then decode it again prior to the decode as below:
String encryptedData = Base64.encode(encryptCipher.doFinal("Hello".getBytes()));
byte[] decryptedData = decryptCipher.doFinal(Base64.decode(encryptedData));