In the controller of an externally exposed VF page, I am accepting a bank notification after the payment completion.

The parameter being sent are encrypted using AES with Cipher Block Chaining, using PCKS-5 Padding. The decryption algorithm should be initialised with a 16 byte, zero-filled initialization vector, and should use your encryption key.  

Before the decryption the parameters look like:

EncryptedParameters=QzFtdn0%2B66KJV5L8ihbr6ofdmrkEQwqMXI3ayF7UpVlRheR7r5fA6
IqBszeKFoGSyR7c7J4YsXgaOergu5SWD%2FvL%2FzPSrZER9BS7mZGckriBrhYt%2FKMAbTSS8F

XR72gWJZsul9aGyGbFripp7XxE9NQHVMWCko0NlpWe7oZ0RBIgNpIZ3JojAfX7b1j%2F5ACJ79S
VeOIK80layBwCmIPOpB%2B%2BNI6krE0wekvkkLKF7CXilj5qITvmv%2FpMqwVDchv%2FUNMfCi
4uUA4igHGhaZDQcV8U%2BcYRO8dv%2FnqVbAjkNwBqxqN3UPNFz0Tt76%2BP7H48PDpU23c61eM
7mx%2FZh%2Few5Pd0WkiCwZVkSZoov97BWdnMIw5tOAiqHvAR3%2BnfmGsx

Signature=huq1shmZ6k7L5BYxjGI2lJvQxffqa%2FogZR5oO8Ln2oc%3D

The signature is a base-64 encoded MD5 hash of the encrypted text, and can be used to verify that the text was transmitted correctly.

After decryption, the parameters will appear as follows:
bank_reference=1234&card_type=VI&payment_amount=100...

Following I specify the VF controller constructor:

    public sbBankNotification() {
        System.debug('>>>>> sbPDC->>>>> 100 >>>>>sbBankNotification contructor' );

        String myEncrypPar	= Apexpages.currentPage().getParameters().get( 'EncryptedParameters' ) ;
        String mySignature	= Apexpages.currentPage().getParameters().get( 'Signature' );

        if ( myEncrypPar != null ) this.encrypPar	= myEncrypPar ;
        if ( mySignature != null ) this.signature	= mySignature ;
	
        System.debug('>>>>> sbPDC->>>>> 103 >>>>>sbBankNotification:encrypPar('+this.encrypPar.length()+')=['+this.encrypPar+']' );
        System.debug('>>>>> sbPDC->>>>> 105 >>>>>sbBankNotification:Signature=['+this.signature+']' );
        
        try {
           String algorithmName = 'AES192' ;
           Blob privateKey = Blob.valueOf(ENCRYPTIONKEY) ;
           Blob initializationVector = Blob.valueOf('0000000000000000') ;
           Blob cipherText = Blob.valueOf(this.encrypPar) ;
           Blob params = Crypto.decrypt(algorithmName, privateKey, initializationVector, cipherText ) ;
           System.debug('>>>>> sbPDC->>>>> 105 >>>>>sbBankNotification:params=['+params.toString()+']' );

        } catch (Exception ex) {
           System.debug('>>>>> sbPN->>>>> 107 >>>>>sbBankNotification:Exception['+ ex +'] ');
        }
    }

 I keep on getting the debug information followed by exception message:

USER_DEBUG|[18]|DEBUG|>>>>> sbPDC->>>>> 103 >>>>>sbBankNotification:encrypPar(408)=[xxUSjEJ2Hp2pycHLe13Xy9N0CGhnqixzD8ZpNZyyBx0oFemyzq8QGnTMoqDwmreT0OzYPVOkQ2iFsUT2gxHxansC3Bp2G31G0V1IxnjZckKzWNCf6o5n6OrTsCeQbgr0YAFzSowY6MJV2yY2RB//xFxlRJ0ShWfn4EAsodWsP6L25PuzIM0XxdpEyWuqgBl1DWOcqKQurnye2cdQJiAXvpP+lpogCikXf0KbZ9WnDTTV4ABdefU5wlCobUomz7x5SldmFhyHLs1hUXxFhF0inM+Bkii5zBPVWNf2OlEfs8uG94kczxDHmw3T7qDtlayW0mOtDw5GstMtl1K4KM/VksGzNbfL1wGM0ONTDmH0liXTaxlSj+SBmO4ouYq30bpnCYoVtx5VUnP1jvCjbicoeg==]
USER_DEBUG|[19]|DEBUG|>>>>> sbPDC->>>>> 105 >>>>>sbBankNotification:Signature=[XhIsA4bPLbCMVHhdYN+5ieZHKWkF3JDt4uso+A5v8Og=]
SYSTEM_METHOD_ENTRY|[26]|system.Crypto.decrypt(String, Blob, Blob, Blob)
SYSTEM_METHOD_EXIT|[26]|system.Crypto.decrypt(String, Blob, Blob, Blob)
USER_DEBUG|[30]|DEBUG|>>>>> sbPN->>>>> 107 >>>>>sbBankNotification:Exception[System.SecurityException: Input length must be multiple of 16 when decrypting with padded cipher] 

The length of the encrypted parameters I get from the bank is 408 chars, which are using pcks-5 padding.

I have tried shortening the last lengh of the string up to 400 chars (to be mulple of 16) but the I got another exception error notifying that the string does not finalize with the correct character.

Please help