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AES-256-CBC encrypted with PHP and decrypt in Java

I am in a situation where a JSON is encrypted in PHP's openssl_encrypt and needs to be decrypted in JAVA.
$encrypted = "...ENCRYPTED DATA...";
$secretFile = "/path/to/secret/saved/in/text_file";
$secret = base64_decode(file_get_contents($secretFile));
var_dump(strlen($secret)); // prints : int(370)
$iv = substr($encrypted, 0, 16);
$data = substr($encrypted, 16);
$decrypted = openssl_decrypt($data, "aes-256-cbc", $secret, null, $iv);
This $decrypted has correct data which is now decrypted.
Now, the problem is when I try to do same things in Java it doesn't work :(
String path = "/path/to/secret/saved/in/text";
String payload = "...ENCRYPTED DATA...";
StringBuilder output = new StringBuilder();
String iv = payload.substring(0, 16);
byte[] secret = Base64.getDecoder().decode(Files.readAllBytes(Paths.get(path)));
String data = payload.substring(16);
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
SecretKeySpec secretKeySpec = new SecretKeySpec(secret, "AES");
IvParameterSpec ivParameterSpec = new IvParameterSpec(iv.getBytes(), 0, cipher.getBlockSize());
cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivParameterSpec); // This line throws exception :
cipher.doFinal(data.getBytes(StandardCharsets.UTF_8));
Here it is:
Exception in thread "main" java.security.InvalidKeyException: Invalid AES key length: 370 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:591)
at com.sun.crypto.provider.AESCipher.engineInit(AESCipher.java:346)
at javax.crypto.Cipher.init(Cipher.java:1394)
at javax.crypto.Cipher.init(Cipher.java:1327)
at com.sample.App.main(App.java:70)
I have already visited similar question like
AES-256 CBC encrypt in php and decrypt in Java or vice-versa
openssl_encrypt 256 CBC raw_data in java
Unable to exchange data encrypted with AES-256 between Java and PHP
and list continues.... but no luck there
btw, this is how encryption is done in PHP
$secretFile = "/path/to/secret/saved/in/text_file";
$secret = base64_decode(file_get_contents($secretFile));
$iv = bin2hex(openssl_random_pseudo_bytes(8));
$enc = openssl_encrypt($plainText, "aes-256-cbc", $secret, false, $iv);
return $iv.$enc;
and yes, I forgot to mention that my JRE is already at UnlimitedJCEPolicy and I can't change PHP code.
I am totally stuck at this point and can't move forward. Please help out.
EDIT#1
byte[] payload = ....;
byte[] iv = ....;
byte[] secret = ....; // Now 370 bits
byte[] data = Base64.getDecoder().decode(payload);
Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
SecretKeySpec secretKeySpec = new SecretKeySpec(Arrays.copyOfRange(secret, 0, 32), "AES");
IvParameterSpec ivParameterSpec = new IvParameterSpec(iv, 0, cipher.getBlockSize());
cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivParameterSpec);
byte[] output = cipher.doFinal(data);
System.out.println(new String(output).trim());
Above snippet seems to be working with openssl_encrypt
EDIT#2
I am not sure if this is correct, but following is what I have done and encryption-decryption on both side are working fine.
Encrypt in PHP, Decrypt in JAVA use AES/CBC/NoPadding
Encrypt in JAVA, Decrypt in PHP use AES/CBC/PKCS5Padding

I won't provide a complete solution, but there are a few differences you should take care of
Encoding:
String iv = payload.substring(0, 16);
String data = payload.substring(16);
are you sure the IV and data are the same in Java and PHP (The IV is string?)? If the data are encrypted, they should be treated as a byte array, not string. Just REALLY make sure they are THE SAME (print hex/base64 in php and java)
For the IV you at the end call iv.getBytes(), but the locale encoding may/will corrupt your values. The String should be use only when it's really string (text). Don't use string for binaries.
Simply treat data and iv as byte[]
Key generation according to the openssl
AES key must have length of 256 bit for aes-256-cbc used. The thing is - openssl by default doesn't use the provided secret as a key (I believe it can, but I don't know how it is to be specified in PHP).
see OpenSSL EVP_BytesToKey issue in Java
and here is the EVP_BytesToKey implementation: https://olabini.com/blog/tag/evp_bytestokey/
you should generate a 256 bit key usging the EVP_BytesToKey function (it's a key derivation function used by openssl).
Edit:
Maarten (in the comments) is right. The key parameter is the key. Seems the PHP function is accepting parameter of any length which is misleading. According to some articles (e.g. http://thefsb.tumblr.com/post/110749271235/using-opensslendecrypt-in-php-instead-of) the key is trucated or padded to necessary length (so seems 370 bit key is truncated to length of 256 bits).

According to your example, I wrote fully working code for PHP and Java:
AesCipher class: https://gist.github.com/demisang/716250080d77a7f65e66f4e813e5a636
Notes:
-By default algo is AES-128-CBC.
-By default init vector is 16 bytes.
-Encoded result = base64(initVector + aes crypt).
-Encoded/Decoded results present as itself object, it gets more helpful and get possibility to check error, get error message and get init vector value after encode/decode operations.
PHP:
$secretKey = '26kozQaKwRuNJ24t';
$text = 'Some text'
$encrypted = AesCipher::encrypt($secretKey, $text);
$decrypted = AesCipher::decrypt($secretKey, $encrypted);
$encrypted->hasError(); // TRUE if operation failed, FALSE otherwise
$encrypted->getData(); // Encoded/Decoded result
$encrypted->getInitVector(); // Get used (random if encode) init vector
// $decrypted->* has identical methods
JAVA:
String secretKey = "26kozQaKwRuNJ24t";
String text = "Some text";
AesCipher encrypted = AesCipher.encrypt(secretKey, text);
AesCipher decrypted = AesCipher.decrypt(secretKey, encrypted);
encrypted.hasError(); // TRUE if operation failed, FALSE otherwise
encrypted.getData(); // Encoded/Decoded result
encrypted.getInitVector(); // Get used (random if encode) init vector
// decrypted.* has identical methods

CryptoJS AES encryption and Java AES decryption

I'm only asking this because I have read many posts for 2 days now about crypto AES encryption, and just when I thought I was getting it, I realized I wasn't getting it at all.
This post is the closest one to my issue, I have exactly the same problem but it is unanswered:
CryptoJS AES encryption and JAVA AES decryption value mismatch
I have tried doing it in many ways but I haven't gotten it right.
First Off
I'm getting the already encrypted string (I only got the code to see how they were doing it), so modifying the encryption way is not an option. That's why all the similar questions aren't that useful to me.
Second
I do have access to the secret key and I can modify it (so adjusting length is an option if neccessary).
The encryption is done on CryptoJS and they send the encrypted string as a GET parameter.
GetParamsForAppUrl.prototype.generateUrlParams = function() {
const self = this;
return new Promise((resolve, reject) => {
const currentDateInMilliseconds = new Date().getTime();
const secret = tokenSecret.secret;
var encrypted = CryptoJS.AES.encrypt(self.authorization, secret);
encrypted = encrypted.toString();
self.urlParams = {
token: encrypted,
time: currentDateInMilliseconds
};
resolve();
});
};
I can easily decrypt this on javascript using CryptoJS with:
var decrypted = CryptoJS.AES.decrypt(encrypted_string, secret);
console.log(decrypted.toString(CryptoJS.enc.Utf8));
But I don't want to do this on Javascript, for security reasons, so I'm trying to decrypt this on Java:
String secret = "secret";
byte[] cipherText = encrypted_string.getBytes("UTF8");
SecretKey secKey = new SecretKeySpec(secret.getBytes(), "AES");
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.DECRYPT_MODE, secKey);
byte[] bytePlainText = aesCipher.doFinal(byteCipherText);
String myDecryptedText = = new String(bytePlainText);
Before I had any idea of what I was doing, I tried base64 decoding, adding some IV and a lot of stuff I read, of course none of it worked.
But after I started to understand, kinda, what I was doing, I wrote that simple script above, and got me the same error on the post: Invalid AES key length
I don't know where to go from here. After reading a lot about this, the solution seems to be hashing or padding, but I have no control on the encryption method, so I can't really hash the secret or pad it.
But as I said, I can change the secret key so it can match some specific length, and I have tried changing it, but as I'm shooting in the dark here, I don't really know if this is the solution.
So, my question basically is, If I got the encrypted string (in javascript like the first script) and the secret key, is there a way to decrypt it (in Java)? If so, how to do it?

Disclaimer: Do not use encryption unless you understand encryption concepts including chaining mode, key derivation functions, IV and block size. And don't roll your own security scheme but stick to an established one. Just throwing in encryption algorithms doesn't mean an application has become any more secure.
CryptoJS implements the same key derivation function as OpenSSL and the same format to put the IV into the encrypted data. So all Java code that deals with OpenSSL encoded data applies.
Given the following Javascript code:
var text = "The quick brown fox jumps over the lazy dog. 👻 👻";
var secret = "René Über";
var encrypted = CryptoJS.AES.encrypt(text, secret);
encrypted = encrypted.toString();
console.log("Cipher text: " + encrypted);
We get the cipher text:
U2FsdGVkX1+tsmZvCEFa/iGeSA0K7gvgs9KXeZKwbCDNCs2zPo+BXjvKYLrJutMK+hxTwl/hyaQLOaD7LLIRo2I5fyeRMPnroo6k8N9uwKk=
On the Java side, we have
String secret = "René Über";
String cipherText = "U2FsdGVkX1+tsmZvCEFa/iGeSA0K7gvgs9KXeZKwbCDNCs2zPo+BXjvKYLrJutMK+hxTwl/hyaQLOaD7LLIRo2I5fyeRMPnroo6k8N9uwKk=";
byte[] cipherData = Base64.getDecoder().decode(cipherText);
byte[] saltData = Arrays.copyOfRange(cipherData, 8, 16);
MessageDigest md5 = MessageDigest.getInstance("MD5");
final byte[][] keyAndIV = GenerateKeyAndIV(32, 16, 1, saltData, secret.getBytes(StandardCharsets.UTF_8), md5);
SecretKeySpec key = new SecretKeySpec(keyAndIV[0], "AES");
IvParameterSpec iv = new IvParameterSpec(keyAndIV[1]);
byte[] encrypted = Arrays.copyOfRange(cipherData, 16, cipherData.length);
Cipher aesCBC = Cipher.getInstance("AES/CBC/PKCS5Padding");
aesCBC.init(Cipher.DECRYPT_MODE, key, iv);
byte[] decryptedData = aesCBC.doFinal(encrypted);
String decryptedText = new String(decryptedData, StandardCharsets.UTF_8);
System.out.println(decryptedText);
The result is:
The quick brown fox jumps over the lazy dog. 👻 👻
That's the text we started with. And emojis, accents and umlauts work as well.
GenerateKeyAndIV is a helper function that reimplements OpenSSL's key derivation function EVP_BytesToKey (see https://github.com/openssl/openssl/blob/master/crypto/evp/evp_key.c).
/**
* Generates a key and an initialization vector (IV) with the given salt and password.
* <p>
* This method is equivalent to OpenSSL's EVP_BytesToKey function
* (see https://github.com/openssl/openssl/blob/master/crypto/evp/evp_key.c).
* By default, OpenSSL uses a single iteration, MD5 as the algorithm and UTF-8 encoded password data.
* </p>
* #param keyLength the length of the generated key (in bytes)
* #param ivLength the length of the generated IV (in bytes)
* #param iterations the number of digestion rounds
* #param salt the salt data (8 bytes of data or <code>null</code>)
* #param password the password data (optional)
* #param md the message digest algorithm to use
* #return an two-element array with the generated key and IV
*/
public static byte[][] GenerateKeyAndIV(int keyLength, int ivLength, int iterations, byte[] salt, byte[] password, MessageDigest md) {
int digestLength = md.getDigestLength();
int requiredLength = (keyLength + ivLength + digestLength - 1) / digestLength * digestLength;
byte[] generatedData = new byte[requiredLength];
int generatedLength = 0;
try {
md.reset();
// Repeat process until sufficient data has been generated
while (generatedLength < keyLength + ivLength) {
// Digest data (last digest if available, password data, salt if available)
if (generatedLength > 0)
md.update(generatedData, generatedLength - digestLength, digestLength);
md.update(password);
if (salt != null)
md.update(salt, 0, 8);
md.digest(generatedData, generatedLength, digestLength);
// additional rounds
for (int i = 1; i < iterations; i++) {
md.update(generatedData, generatedLength, digestLength);
md.digest(generatedData, generatedLength, digestLength);
}
generatedLength += digestLength;
}
// Copy key and IV into separate byte arrays
byte[][] result = new byte[2][];
result[0] = Arrays.copyOfRange(generatedData, 0, keyLength);
if (ivLength > 0)
result[1] = Arrays.copyOfRange(generatedData, keyLength, keyLength + ivLength);
return result;
} catch (DigestException e) {
throw new RuntimeException(e);
} finally {
// Clean out temporary data
Arrays.fill(generatedData, (byte)0);
}
}
Note that you have to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy. Otherwise, AES with key size of 256 won't work and throw an exception:
java.security.InvalidKeyException: Illegal key size
Update
I have replaced Ola Bini's Java code of EVP_BytesToKey, which I used in the first version of my answer, with a more idiomatic and easier to understand Java code (see above).
Also see How to decrypt file in Java encrypted with openssl command using AES?.

When encrypting on one system and decrypting on another you are at the mercy of system defaults. If any system defaults do not match (and they often don't) then your decryption will fail.
Everything has to be byte for byte the same on both sides. Effectively that means specifying everything on both sides rather than relying on defaults. You can only use defaults if you are using the same system at both ends. Even then, it is better to specify exactly.
Key, IV, encryption mode, padding and string to bytes conversion all need to be the same at both ends. It is especially worth checking that the key bytes are the same. If you are using a Key Derivation Function (KDF) to generate your key, then all the parameters for that need to be the same, and hence specified exactly.
Your "Invalid AES key length" may well indicate a problem with generating your key. You use getBytes(). That is probably an error. You need to specify what sort of bytes you are getting: ANSI, UTF-8, EBCDIC, whatever. The default assumption for the string to byte conversion is the likely cause of this problem. Specify the conversion to be used explicitly at both ends. That way you can be sure that they match.
Crypto is designed to fail if the parameters do not match exactly for encryption and decryption. For example, even a one bit difference in the key will cause it to fail.

Encrypt in python and decrypt in Java with AES-CFB

I am aware of a question very similar to this (How do I encrypt in Python and decrypt in Java?) but I have a different problem.
My problem is, I am not able to decrypt in Java correctly. Despite using the correct key and IV, I still get garbage characters after decryption. I don't have any compile/run-time errors or exceptions in Java so I believe I am using the right parameters for decryption.
Python Encryption Code -
from Crypto.Cipher import AES
import base64
key = '0123456789012345'
iv = 'RandomInitVector'
raw = 'samplePlainText'
cipher = AES.new(key,AES.MODE_CFB,iv)
encrypted = base64.b64encode(iv + cipher.encrypt(raw))
Java Decryption Code -
private static String KEY = "0123456789012345";
public static String decrypt(String encrypted_encoded_string) throws NoSuchAlgorithmException, NoSuchPaddingException,
InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
String plain_text = "";
try{
byte[] encrypted_decoded_bytes = Base64.getDecoder().decode(encrypted_encoded_string);
String encrypted_decoded_string = new String(encrypted_decoded_bytes);
String iv_string = encrypted_decoded_string.substring(0,16); //IV is retrieved correctly.
IvParameterSpec iv = new IvParameterSpec(iv_string.getBytes());
SecretKeySpec skeySpec = new SecretKeySpec(KEY.getBytes("UTF-8"), "AES");
Cipher cipher = Cipher.getInstance("AES/CFB/NoPadding");
cipher.init(Cipher.DECRYPT_MODE, skeySpec, iv);
plain_text = new String(cipher.doFinal(encrypted_decoded_bytes));//Returns garbage characters
return plain_text;
} catch (Exception e) {
System.err.println("Caught Exception: " + e.getMessage());
}
return plain_text;
}
Is there anything obvious that I am missing?

The Cipher Feedback (CFB) mode of operation is a family of modes. It is parametrized by the segment size (or register size). PyCrypto has a default segment size of 8 bit and Java (actually OpenJDK) has a default segment size the same as the block size (128 bit for AES).
If you want CFB-128 in pycrypto, you can use AES.new(key, AES.MODE_CFB, iv, segment_size=128). If you want CFB-8 in Java, you can use Cipher.getInstance("AES/CFB8/NoPadding");.
Now that we have that out the way, you have other problems:
Always specify the character set you're using, because it can change between different JVMs: new String(someBytes, "UTF-8") and someString.getBytes("UTF-8"). When you do, be consistent.
Never use a String to store binary data (new String(encrypted_decoded_bytes);). You can copy the bytes directly: IvParameterSpec iv = new IvParameterSpec(Arrays.copyOf(encrypted_decoded_bytes, 16)); and cipher.doFinal(Arrays.copyOfRange(encrypted_decoded_bytes, 16, encrypted_decoded_bytes.length)).
In Java, you're assuming that the IV is written in front of the ciphertext and then encoded together, but in Python, you're never doing anything with the IV. I guess you posted incomplete code.
It is crucial for CFB mode to use a different IV every time if the key stays the same. If you don't change the IV for every encryption, you will create a multi-time pad which enables an attacker to deduce the plaintext even without knowing the key.

AES-256: IV vector misunderstanding between Ruby and Java implementations

I have "inherited" a Ruby on Rails app, and I must translate this app from Ruby to Java, and the most important thing, I don't have contact with the creator.
My problem is with the IV vector in AES-256 authentication. Ruby app uses AESCrypt gem to encrypt and decrypt user's password. It works fine, and I have already some thousands of users in DB.
The problem is when I try to do the same in Java (I've already updated JCE to allow 256bit key lenght). The Key and the IV are writen as binary strings in ruby source code (see bellow), and when I try to use it in Java I get a exception which say that the IV lenght must be 16 bytes long (I know that it must be 16 bytes long, but the binary string in Ruby has 32 characters).
Ruby code (works fine):
require 'openssl'
require 'digest/md5'
require 'base64'
module AESCrypt
KEY = "AB1CD237690AF13B6721AD237A"
IV = "por874hyufijdue7w63ysxwet4320o90"
TYPE = "AES-256-CBC"
def AESCrypt.key(key)
key = Digest::MD5.hexdigest(key)
key.slice(0..32)
end
# Encrypts a block of data given an encryption key and an
# initialization vector (iv). Keys, iv's, and the data returned
# are all binary strings. Cipher_type should be "AES-256-CBC",
# "AES-256-ECB", or any of the cipher types supported by OpenSSL.
# Pass nil for the iv if the encryption type doesn't use iv's (like
# ECB).
#:return: => String
#:arg: data => String
#:arg: key => String
#:arg: iv => String
#:arg: cipher_type => String
def AESCrypt.encrypt(data)
return nil if data.nil?
return data if data.blank?
aes = OpenSSL::Cipher::Cipher.new(TYPE)
aes.encrypt
aes.key = AESCrypt.key(KEY)
aes.iv = IV if IV != nil
result = aes.update(data) + aes.final
Base64.encode64(result)
end
end
and this is my Java code (it should do the same, seems that works with a 16 chars/bytes IV):
public static void main(String[] args) throws UnsupportedEncodingException {
String KEY = "AB1CD237690AF13B6721AD237A";
String IV = "por874hyufijdue7w63ysxwet4320o90";
SecretKeySpec key = generateKey(KEY);
String message = "password";
final Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
IvParameterSpec ivSpec = new IvParameterSpec(IV.getBytes());
cipher.init(Cipher.ENCRYPT_MODE, key, ivSpec);
byte[] ciphedText = cipher.doFinal(message.getBytes());
String encoded = Base64.encodeBase64String(ciphedText);
System.out.println("ENCRYPTED text= " + encoded);
}
public static SecretKeySpec generateKey(final String password) throws NoSuchAlgorithmException, UnsupportedEncodingException {
final MessageDigest digest = MessageDigest.getInstance("MD5");
byte[] bytes = password.getBytes("UTF-8");
digest.update(bytes, 0, bytes.length);
byte[] key = digest.digest();
SecretKeySpec secretKeySpec = new SecretKeySpec(key, "AES");
return secretKeySpec;
}
And I'm getting this exception (obviously):
java.security.InvalidAlgorithmParameterException: Wrong IV length: must be 16 bytes long
at com.sun.crypto.provider.CipherCore.init(CipherCore.java:516)
at com.sun.crypto.provider.AESCipher.engineInit(AESCipher.java:339)
at javax.crypto.Cipher.implInit(Cipher.java:801)
at javax.crypto.Cipher.chooseProvider(Cipher.java:859)
at javax.crypto.Cipher.init(Cipher.java:1370)
at javax.crypto.Cipher.init(Cipher.java:1301)
at com.javi.test.security.Test.main(Test.java:129)
I guess my problem is the way I convert the IV java string in byte[]. I think that openSSL code in ruby is unpacking (or doing something internally) the 32 bytes of the IV to 16 bytes. I have tried a lot of things, but I'm going crazy.
Anyone had the same problem or figure out where could be my problem?
I have posted the encryption code but I hace the same issue with decryption.
Thanks in advance, I'll be very grateful with every answer. :)

First, your IV is not actually iv, IV should be HEX encoded, but you have ASCII string "por874hyufijdue7w63ysxwet4320o90", may be it is some how encoded?
Second, IV.getBytes() will transofr IV's each character to hex encoding like p = 0x70, o = 0x6F, r = 0x72, etc...
It is not a useful answer, but may be hint.
Actually IV must be the same length as block cipher single block length. You have 32 bytes long IV itself, if you make IV.getBytes() IV length should match the cipher block length

How can I decrypt in objective-c from a file encrypted by Java Cipher

I have an encrypt method in Java.
public static String encrypt(String orignal){
SecretKeySpec key = new SecretKeySpec(keyString.getBytes(), "AES");
IvParameterSpec initalVector = new IvParameterSpec(initialVectorParam.getBytes());
try{
Cipher cipher = Cipher.getInstance("AES/CFB8/NoPadding");
/////////////// encrypt /////////////////
cipher.init(Cipher.ENCRYPT_MODE, key, initalVector);
Log.d("AES", "oriByte: "+ orignal.getBytes());
int length = orignal.length();
for(int i=0; i<length; i++){
}
byte[] test = cipher.doFinal(orignal.getBytes());
Log.d("AES", "encByte: "+ test);
return bytes2Hex(test);
}catch (Exception e) {
Log.d("AES", "Encrypt Exception:"+orignal);
return "";
}
}
For compatibility with PHP, I use "AES/CFB8/NoPadding" options.
In PHP: $sCipher = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $sKey, $sStr, MCRYPT_MODE_CFB, $sIV);
And I have a Objective-c Cipher code from here.
https://gist.github.com/838614
I found that there is no IvParameterSpec in Objective-c Cipher like java.
Besides, the getBytes method returns a different value with java.
(I think this is because java uses different encoding way.)
So, how can I apply IvParameterSpec in Objective-c.
And is there any way to get 'getBytes' value like java in Objective-c?

For the initialization vector, see line 24 in your pastie:
NULL /* initialization vector (optional) */,
That's where you would pass your IV.
But without knowing the string encoding the Java code used to create the bytes used as the IV, you won't be able to seed the encryption properly to decrypt the data, even if you know what the string displays to the screen as. Put another way, just because the IV looks like "abc123" doesn't mean the bytes Java is writing to the IV buffer are going to be the same bytes you'll get if you strncpy() from a C character literal buffer. You have to agree on the encoding as part of the protocol for handling the data.
You will also need to agree on a key size. Your Java code does not specify how many bits are in the AES key.
Once you've got that worked out, you'll want to use a call like:
const void *key = /* KEY BYTES */;
const void *iv = /* IV BYTES */;
const void *text = /* CIPHER TEXT */;
size_t textlen = /*...*/;
size_t outlen = 0;
(void)CCCrypt(kCCDecrypt, kCCAlgorithmAES128, 0/*use CBC mode*/,
key, kCCKeySizeAES128, iv,
text, textlen,
&text, textlen, &outlen);
The plaintext will be written over the ciphertext, assuming all goes well. The amount o data written to text during decryption will be stored in outlen. Error checking is your responsibility; the header is well-commented.
Once you have the data, you'll want to slurp it into an NSString with the correct encoding (+[NSString initWithData:encoding:] would work), and then you have a string you can work with from Obj-C like any other string.