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IBM Java 8 still gets IllegalBlockSizeException when trying to decrypt valid data if IllegalBlockSizeException is once thrown

Question & Answer


Question

Encrypted data is decrypted with javax.crypto.Cipher class in the application code.
If trying to decrypt truncated data in the application, IllegalBlockSizeException is thrown as expected. After IllegalBlockSizeException is thrown, valid data can be decryted with the same cipher object when IBM Java 7 is used. However, IllegalBlockSizeException is still thrown in the same situation when IBM Java 8 is used.

Cause

As specification for doFinal in Cipher, cipher object may need to be reset before it can be used again if any exception is thrown. This specification is applied to both IBM Java 7 and IBM Java 8.

Answer

The application code needs to call Cipher.init again to reset cipher object to re-use its cipher object when doFinal gets exception.

The specification for doFinal in Cipher of Java 7 and Java 8 both indicate: "if any exception is thrown, this cipher object may need to be reset before it can be used again.".
> Note: if any exception is thrown, this cipher object may need to be reset before it can be used again.
There are different implementations for IBM Java 7 and IBM Java 8. But, both IBM Java 7 and IBM Java 8 are following the same specification for doFinal in Cipher. Therefore, the application code should be calling Cipher.init to reset cipher object like below when doFinal gets any exception for any Java or JCE version.
try{
    cipher.doFinal(...);
} catch (Exception e) {
    cipher.init(Cipher.DECRYPT_MODE, ...);
}
 

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Document Information

Modified date:
31 August 2021

UID

ibm16485131

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