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Use RSA encrypted AES-Key for encoding Text

First, please excuse my bad english, this is not my native language.
i am trying to encode and then decode some text of different lengths.
I tried to encode some text with RSA algorithm in java. In the progress, I found out that RSA-encoding is limited to a specific size of bytes, so this approach doesn't fit my specs as my text could be large.
As I want to stay with a public and a private key only, I came up with the idea to use a 'hard-coded' AES key which will be encoded with RSA public key and than use this to encode the data. (see code)
//harcoded secretkey base64 encoded
private static final String secretkey_base64 = "xtnN6Pove5AovbLXtGRJKw==";
//this is how RSA key are generated
public static KeyPair genKeys() throws Exception {
KeyPairGenerator kpg = KeyPairGenerator.getInstance( "RSA" );
kpg.initialize(2048);
KeyPair kp = kpg.generateKeyPair();
return kp;
}
//here text should be encoded
public static String encodeText(String plainText, PublicKey publicKey) throws Exception
{
Cipher encryptCipher = Cipher.getInstance("RSA");
encryptCipher.init(Cipher.ENCRYPT_MODE, publicKey);
byte[] cipherText = encryptCipher.doFinal(CryptoUtil.secretkey_base64.getBytes(UTF_8));
String encodedSecKey = Base64.getEncoder().encodeToString(cipherText);
SecretKeySpec k = new SecretKeySpec(Base64.getDecoder().decode(encodedSecKey), "AES");
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.ENCRYPT_MODE, k);
byte[] byteCipherText = aesCipher.doFinal(plainText.getBytes());
return Base64.getEncoder().encodeToString(byteCipherText);
}
Obviously this doesn't work because the RSA encoded AES key is too long for AES encryption. Throws "Invalid AES key length: 256 bytes"
Does anyone have an idea how I could get the encryption running with this particular approach ?
Is this even possible, or do I have to use a different approach?
Thanks in advance.
//edit - Solved thanks to #ewramner
// here is the working solution:
public static String encodeText(String plainText, PublicKey publicKey) throws Exception
{
KeyGenerator generator = KeyGenerator.getInstance("AES");
generator.init(128); // The AES key size in number of bits
SecretKey secKey = generator.generateKey();
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.PUBLIC_KEY, publicKey);
byte[] encSecKey = cipher.doFinal(secKey.getEncoded());
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.ENCRYPT_MODE, secKey);
byte[] byteCipherText = aesCipher.doFinal(plainText.getBytes());
return Base64.getEncoder().encodeToString(encSecKey) + "#" + Base64.getEncoder().encodeToString(byteCipherText);
}
public static String decodeText(String cipherText, PrivateKey privateKey) throws Exception
{
String[] split = cipherText.split("#");
String encSecKey = split[0];
String encText = split[1];
byte[] bytesSecKey = Base64.getDecoder().decode(encSecKey);
byte[] bytesText = Base64.getDecoder().decode(encText);
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.PRIVATE_KEY, privateKey);
byte[] decSecKey = cipher.doFinal(bytesSecKey);
SecretKey secKey = new SecretKeySpec(decSecKey , 0, decSecKey .length, "AES");
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.DECRYPT_MODE, secKey);
return new String(aesCipher.doFinal(bytesText));
}

Why Java class Cipher produce the same result in encrypt and decrypt mode on Android platform?

I'm trying to use Java class Cipher to encrypt/decrypt some of my data. But I'm confused why the same result is produced in encrypt and decrypt mode. Below is my code:
KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
KeyPair keyPair = generator.genKeyPair();
PrivateKey privateKey = keyPair.getPrivate();
byte[] data = "12345".getBytes();
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.ENCRYPT_MODE, privateKey);
byte[] encrypted = cipher.doFinal(data);
Cipher cipher2 = Cipher.getInstance("RSA");
cipher2.init(Cipher.DECRYPT_MODE, privateKey);
byte[] decrypted = cipher2.doFinal(data);
Here the encrypted has exactly same content with decrypted. It really confused me why same result is produced regardless of what mode I'm using.
What I have tried?
If I use Cipher.getInstance("DES") to decrypt "12345".getBytes(), exception will be thrown.
Does it mean class Cipher can automatically detect whether encrypt or decrypt should be used(ignore mode parameter that I've specified when initialize the Cipher class) when using RSA?
Really appreciate for your help.
===Update===
My bad, looks Java has different behavior with Android. I should limit my question to Android platform only. If I run the above code as a normal Java application, javax.crypto.BadPaddingException: Decryption error will be thrown when decrypting "123456".
===Update===

First of all RSA is asymetric thus you use public key to encrypt and private key decrypt.
Second, those do not produce the same output.
KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
KeyPair keyPair = generator.genKeyPair();
PrivateKey privateKey = keyPair.getPrivate();
byte[] data = "12345".getBytes();
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.ENCRYPT_MODE, keyPair.getPublic());
byte[] encrypted = cipher.doFinal(data);
System.out.println(new String(encrypted));
Cipher cipher2 = Cipher.getInstance("RSA");
cipher2.init(Cipher.DECRYPT_MODE, privateKey);
byte[] decrypted = cipher2.doFinal(encrypted);
System.out.println(new String(decrypted));
with the output of
D=1��� 12345
Third, there is no autodetection, but cipher can detect that decrypt material is corrupted or invalid - wrong pading in your case causing the error.

Twofish encryption: "java.security.NosuchAlgorithnException:Twofish KeyGenerator not available" error

This is the code to generate key for twofish algo and to encrypt using it.
SecureRandom sr = new SecureRandom(key.getBytes());
KeyGenerator kg = KeyGenerator.getInstance("twofish");
kg.init(sr);
SecretKey sk = kg.generateKey();
// create an instance of cipher
Cipher cipher = Cipher.getInstance("twofish");
// initialize the cipher with the key
cipher.init(Cipher.ENCRYPT_MODE, sk);
byte[] encrypted = cipher.doFinal(toEncrypt.getBytes());

java - get key from byte array

I have a java program that encrypts file content with a random-generated key.
That key is encrpyted with RSA and saved into a text file.
Now, I have a java program that given the file and the keystore where the RSA key is stored, needs to first decrypt the encryped key and then with the key to decrypt the file.
Here's what I have so far:
// Fetch the other public key and decrypt the file encryption key
java.security.cert.Certificate cert2 = keystore.getCertificate("keyForSeckeyDecrypt");
Key secKeyPublicKey = cert2.getPublicKey();
Cipher cipher = Cipher.getInstance(secKeyPublicKey.getAlgorithm());
cipher.init(Cipher.DECRYPT_MODE, secKeyPublicKey);
keyFileFis = new FileInputStream(keyFile);
byte[] encryptedKey = new byte[128];
keyFileFis.read(encryptedKey);
byte[] realFileKey = cipher.doFinal(encryptedKey, 0, encryptedKey.length);
Key realKey = // THE PROBLEM!!!;
keyFileFis.close();
In short, I get the encrypted key from the key text file and decrypt it, now I have the decrypted key as a byte array, how would I make it a Key variable again?
I've generated the key this way:
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
KeyGenerator keyGen = KeyGenerator.getInstance("AES");
Key secKey = keyGen.generateKey();
cipher.init(Cipher.ENCRYPT_MODE, secKey);
And encrypted it this way:
KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(1024);
KeyPair kp = kpg.genKeyPair();
PrivateKey privateKey = kp.getPrivate();
Cipher keyCipher = Cipher.getInstance("RSA");
keyCipher.init(Cipher.ENCRYPT_MODE, privateKey);
byte[] encryptedKey = keyCipher.doFinal(secKey.getEncoded());
FileOutputStream keyStream = new FileOutputStream("key.txt");
keyStream.write(encryptedKey);
keyStream.close();

I haven't tried it but from clicking through the API SecretKeySpec could be what you are looking for.
SecretKeySpec(byte[] key, String algorithm)
It can be used to construct a SecretKey from a byte array, without having to go through a (provider-based) SecretKeyFactory.
This class is only useful for raw secret keys that can be represented as a byte array and have no key parameters associated with them, e.g., DES or Triple DES keys.

If I get it right, this should work..
Key privateKey = keyStore.getKey("youralias", "password".toCharArray());
PublicKey publicKey = keyStore.getCertificate("youralias").getPublicKey();
KeyGenerator keyGen = KeyGenerator.getInstance("AES");
Key secKey = keyGen.generateKey();
Cipher keyCipher = Cipher.getInstance("RSA");
keyCipher.init(Cipher.ENCRYPT_MODE, privateKey);
byte[] encryptedKey = keyCipher.doFinal(secKey.getEncoded());
// Write & Read to/from file!
Cipher decryptCipher = Cipher.getInstance("RSA");
decryptCipher.init(Cipher.DECRYPT_MODE, publicKey);
byte[] decryptedKey = decryptCipher.doFinal(encryptedKey);
boolean equals = Arrays.equals(secKey.getEncoded(), new SecretKeySpec(decryptedKey, "AES").getEncoded());
System.out.println(equals?"Successfull!":"Failed!");

Using RSA encryption in Java without BouncyCastle

In my program, I'm trying to encrypt some plaintext with RSA using the following code:
static String RSAEncrypt(String pubkey, String plain){
return encrypt(pubkey,plain,"RSA");
}
static String encrypt(String stringKey, String plain, String algo){
String enc="failed";
try{
byte[] byteKey = new BASE64Decoder().decodeBuffer(stringKey);
Key key = new SecretKeySpec(byteKey,algo);
byte[] data = plain.getBytes();
Cipher c = Cipher.getInstance(algo);
c.init(Cipher.ENCRYPT_MODE, key);
byte[] encVal = c.doFinal(data);
enc = new BASE64Encoder().encode(encVal);
}catch(Exception e){e.printStackTrace();}
return enc;
}
However, when it runs, it shows the following error:
java.security.InvalidKeyException: No installed provider supports this key: javax.crypto.spec.SecretKeySpec
at javax.crypto.Cipher.chooseProvider(Cipher.java:877)
at javax.crypto.Cipher.init(Cipher.java:1212)
at javax.crypto.Cipher.init(Cipher.java:1152)
at Crypto.encrypt(Crypto.java:37)
at Crypto.RSAEncrypt(Crypto.java:62)
I have tried changing it to RSA/None/PKCS1Padding and RSA/ECB/PKCS1Padding to no avail.. I know that installing BouncyCastle may help but I'd like to avoid it (I'd like to avoid more dependencies and I've been having some issues installing it anyway). Thanks in advance for any ideas.

As was said in the comments, SecretKeySpec is for symmetric algorithms only. You mentioned that you got your byte[] containing the key by calling getEncoded.
There are two possibilities and two resulting formats:
Encoding of an RSA PrivateKey
Calling PrivateKey#getEncoded on an instance of an RSA private key will result in a PKCS#8 encoding for private keys, and it can be restored with the help of PKCS8EncodedKeySpec.
Encoding of an RSA PublicKey
PublicKey#getEncoded on an RSA public key results in the generic X.509 public key encoding, and can be restored with X509EncodedKeySpec.
Example Usage
byte[] data = "test".getBytes("UTF8");
KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(512);
KeyPair keyPair = kpg.genKeyPair();
byte[] pk = keyPair.getPublic().getEncoded();
X509EncodedKeySpec spec = new X509EncodedKeySpec(pk);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PublicKey pubKey = keyFactory.generatePublic(spec);
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pubKey);
byte[] encrypted = cipher.doFinal(data);
byte[] priv = keyPair.getPrivate().getEncoded();
PKCS8EncodedKeySpec spec2 = new PKCS8EncodedKeySpec(priv);
PrivateKey privKey = keyFactory.generatePrivate(spec2);
cipher.init(Cipher.DECRYPT_MODE, privKey);
byte[] plain = cipher.doFinal(encrypted);
System.out.println(new String(plain, "UTF8")); //=> "test"