Click the Apex link in the Apex encryption challenge tab of the Secret Storage app. Make sure you switch to Salesforce Classic. The Secret Storage app isn't available in Lightning Experience.
Click the Apex Controller link at the bottom of the page to open the EncryptChallenge Apex class.
Modify the save() function to implement CRUD/FLS check on the Name field of the Encrypt_Decrypt__c object.
Modify the Save function to encrypt the lovers' messages. Use cryptoKey as the crypto key to encrypt data and implement AES256 as the encryption cipher in Crypto.encryptWithManagedIV() function.
Modify the DecryptData function to decrypt the lovers' messages. Modify the DecryptData function and use AES256 with cryptoKey as the crypto key to decrypt data. Decrypt the data using AES256 as the decryption cipher in Crypto.decryptWithManagedIV() function.

This Code works for me.

public class EncryptChallenge {
     public String paramValue{get;set;}
     public String pitemId{get;set;}    
     public String pitemName{get;set;}
     public Id recordId{get;set;}
     
     String testing;
     public List<EnCrypt_DecryptC__c> deals;
     public EnCrypt_DecryptC__c encrypt{get;set;}
     public List<EnCrypt_DecryptC__c> Messages {get;set;}
 
     Blob cryptoKey = Blob.valueOf([SELECT bkey__c FROM secretkey__mdt].bkey__c);
     
     
     public EncryptChallenge(ApexPages.StandardController controller) {
         if (Schema.sObjectType.EnCrypt_DecryptC__c.fields.Name.isAccessible()){
        getMessages();
          
         
    
        recordId = Apexpages.CurrentPage().getParameters().get('id');
        if(recordId !=null){
            encrypt = [SELECT Name From EnCrypt_DecryptC__c WHERE id=:recordId];
        }
        else{
            encrypt = new EnCrypt_DecryptC__c();
        }
         }
    }
     
     public PageReference DeleteMe(){
         if (!EnCrypt_DecryptC__c.sObjectType.getDescribe().isDeletable()){

        ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL, 'Insufficient access'));

        return null;
      }

         
        if(pitemId != ''){
            deals  = [select Id, Name from EnCrypt_DecryptC__c where id =:pitemId];
        }
   
     if(deals.size() > 0 ){ delete deals; }
       getMessages();
       return null; 
     }
    
   
       public PageReference DecryptMe(){
       try{     
            if(pitemName != NULL && pitemName != ''){
              Blob data = EncodingUtil.base64Decode(pitemName);
              Blob decryptedData = Crypto.decryptWithManagedIV('AES256', cryptoKey, data);
              String dryptData = decryptedData.toString();
              paramValue = dryptData;
           }
           else {
               System.debug('PitemName was NULL ');
           }
           paramValue = pitemName;
          }
          catch (Exception e){
                ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.FATAL,'Handling an error!');
                    ApexPages.addMessage(myMsg);
                }
       return null;    
    }
    
    
    public List<EnCrypt_DecryptC__c> getMessages() {
         if (!Schema.sObjectType.EnCrypt_DecryptC__c.fields.Name.isAccessible()){

                    return NULL;

        }
        Messages =  [select Id, Name from EnCrypt_DecryptC__c ORDER BY LastModifiedDate DESC NULLS FIRST];
        return Messages;
    }
    
    
    
    
     public PageReference Save(){
       if(!Schema.sObjectType.EnCrypt_DecryptC__c.fields.Name.isCreateable()){
           return NULL;
       }
       try{
           if(pitemName != NULL && pitemName != '')
           {
           if(encrypt.Name.length() < 80) 
            {
            Blob data = Blob.valueOf(encrypt.Name);
            Blob encryptedData = Crypto.encryptWithManagedIV('AES256', cryptoKey, data );
            String b64Data = EncodingUtil.base64Encode(encryptedData);
            encrypt.name = b64Data;
            insert encrypt;
            encrypt.id = null;
            getMessages();
            }
           }
       }
       catch (Exception e){
           ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.FATAL, 'String empty or handling an error!');
           ApexPages.addMessage(myMsg);
       }
       return null;
       }

     
}