using randomly generated IV for encryption

[jimgardener]

hi
in a text book by David Hook, I came across creation of random IV for
encryption.It goes like this
<code snippet>
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS7Padding", "BC");
int blksz=cipher.getBlockSize();
byte[] ivBytes=new byte[blksz];
SecureRandom random=new SecureRandom();
random.nextBytes(ivBytes);
IvParameterSpec ivSpec=new IvParameterSpec(ivBytes);
//encryption pass
cipher.init(Cipher.ENCRYPT_MODE,key,ivSpec);

</code snippet>

then the example in the book takes the iv and encrypts it into
ciphertext and then works on the message to be encoded

<code snippet>
byte[] cipherText=new byte[cipher.getOutputSize(ivBytes.length
+input.length)];
int ctLength=cipher.update(ivBytes,0,ivBytes.length,cipherText,0);
ctLength+=cipher.update(input,0,input.length,cipherText,ctLength);
ctLength+=cipher.doFinal(cipherText,ctLength);
</code snippet>

In decryption pass,the ciphertext is decrypted and then the IV is
removed from the byte array to recover the plaintext bytes.

Is this the proper way to do this?Or is there a better alternative?
thanks
jim

 

[Arne Vajhøj]

hi
in a text book by David Hook, I came across creation of random IV for
encryption.It goes like this
<code snippet>
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS7Padding", "BC");
int blksz=cipher.getBlockSize();
byte[] ivBytes=new byte[blksz];
SecureRandom random=new SecureRandom();
random.nextBytes(ivBytes);
IvParameterSpec ivSpec=new IvParameterSpec(ivBytes);
//encryption pass
cipher.init(Cipher.ENCRYPT_MODE,key,ivSpec);

</code snippet>

then the example in the book takes the iv and encrypts it into
ciphertext and then works on the message to be encoded

<code snippet>
byte[] cipherText=new byte[cipher.getOutputSize(ivBytes.length
+input.length)];
int ctLength=cipher.update(ivBytes,0,ivBytes.length,cipherText,0);
ctLength+=cipher.update(input,0,input.length,cipherText,ctLength);
ctLength+=cipher.doFinal(cipherText,ctLength);
</code snippet>

In decryption pass,the ciphertext is decrypted and then the IV is
removed from the byte array to recover the plaintext bytes.

Is this the proper way to do this?

No.

The other end needs to know iv to decrypt.

So there are absolutely no point in having iv in the message.

Arne