My Company is attempting to use Amazon S3 for storage of various image files that need to be kept secure.  To do this we have setup Amazon's s3 service with an encrypted key security system.  This system fails about 50% of the time for some unknown reason that we think is to do with the clocks at Amazon and Sales-force being slightly off.  

We would like to know why our system is failing sometimes (producing broken image links) and if their is a simple fix to allow the passthrough to work.

Code:

    public static String GenerateAWSLink(Boolean isFront, String barcode){
    	String bucketName = '';
    	String fileName = '';
    	if(isFront){
    		bucketName = 'front_' + AWSAmazonS3.key.toLowerCase();
    		fileName = barcode.toUpperCase() + '_front.jpg';
    	}
    	else{
    		bucketName = 'back_' + AWSAmazonS3.key.toLowerCase();
    		fileName = barcode.toUpperCase() + '_back.jpg';
    	}

        fileName = EncodingUtil.urlEncode(fileName, 'UTF-8');
        
        Datetime now = DateTime.now();
        //Not working due to being 1 second off of AWS or something along those lines.
        
        String stringToSign = 'GET\n\n\n' + now.addSeconds(240).getTime()/1000 + '\n/' + bucketName + '/' + fileName;
        System.debug(stringToSign);
        String signed = GetAWSSignature(stringToSign);
        System.debug(signed);
        String codedsigned = EncodingUtil.urlEncode(signed, 'UTF-8');
        System.debug(codedsigned);
        String url = 'https://s3.amazonaws.com/' + bucketName + '/' + fileName + '?AWSAccessKeyId=' + AWSCredentials.key + '&Expires=' + now.addSeconds(240).getTime()/1000 + '&Signature=' + signed;
        System.debug(url);
        return url;
    }
    
    private static String GetAWSSignature(String toSign){
        String macUrl;
        String signingKey = EncodingUtil.base64Encode(Blob.valueOf(AWSCredentials.secret));
        Blob mac = Crypto.generateMac('HMacSHA1', blob.valueof(toSign), blob.valueof(AWSCredentials.secret)); 
        macUrl = EncodingUtil.base64Encode(mac);                
        return macUrl;
    }
}

Thanks for your help!