public class GdriveUsingJWT {
    public static String Sinatute_blob;
    public static blob Signature_b;
    public static blob privateKey;
    public string iss;
    public string scope;
    public string aud;
    public Long exp;
    public Long iat;
    
    public static HttpResponse accessToken(){
        http h = new Http();
        Httprequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        req.setEndpoint('https://accounts.google.com/o/oauth2/token');
        req.setMethod('POST');
        req.setHeader('ContentType','application/x-www-form-urlencoded');
        
        String Header = '{"alg":"RS256","typ":"JWT"}';
        String Header_Encode = EncodingUtil.base64Encode(blob.valueof(Header));
        
        String claim_set = '{"iss":"driveint@gd-proj-  211813.iam.gserviceaccount.com"';
        claim_set += ',"scope":"https://www.googleapis.com/auth/analytics.readonly"';
        claim_set += ',"aud":"https://accounts.google.com/o/oauth2/token"';
        claim_set += ',"exp":"'+datetime.now().addHours(1).getTime()/1000;
        claim_set += '","iat":"'+datetime.now().getTime()/1000+'"}';
        system.debug('Response ='+claim_set);        
        String claim_set_Encode = EncodingUtil.base64Encode(blob.valueof(claim_set));
        String Singature_Encode = Header_Encode+'.'+claim_set_Encode;
        String key = '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';
        privateKey = EncodingUtil.base64Decode(key);
        system.debug('Singature_Input ='+privateKey);
        Singature_Encode = Singature_Encode.replaceAll('=','');
        String Singature_Encode_Url = EncodingUtil.urlEncode(Singature_Encode,'UTF-8');
        Signature_b =   blob.valueof(Singature_Encode_Url);
        Sinatute_blob = base64(Crypto.sign('RSA-SHA256', Signature_b , privateKey));
        system.debug('Singature_blobinatute_blob'+Sinatute_blob);
        String JWT = Singature_Encode+'.'+Sinatute_blob;
        system.debug('JWT@@'+JWT);
        JWT = JWT.replaceAll('=','');
        
        String grt = 'urn:ietf:params:oauth:grant-type:jwt-bearer';
        req.setBody('grant_type='+grt+'&assertion='+JWT);
        res = h.send(req);
        if (res.getStatusCode() == 200) {
            String Response = res.getBody();
            system.debug('@@@response'+ Response);
            Integer stuscode = res.getStatusCode();
            system.debug('@@@statuccode'+ stuscode);
            Map<String, Object> results = (Map<String, Object>) JSON.deserializeUntyped(res.getBody());
            system.debug('result='+ results );
            List<Object> values = (List<Object>) results.get('values');
            System.debug('@@@values are:'+ values);
            //system.debug('result='+ results );
            //Map<String, String> accessMap= new Map<String, String>();
         }
        return res;
    }
    private static String base64(Blob b) {
        String ret = EncodingUtil.base64Encode(b);
        system.debug('@@@##blob@@'+ b);
        system.debug('@@@rett'+ret);
        ret = ret.replaceAll('\\+', '-');
        ret = ret.replaceAll('/', '_');
        ret = ret.replaceAll('=', '');
        system.debug('@@@rett'+ret);
        return ret;
    }
}