Класс для отправки запросов к Мегаплан API.

type
  TMegaplanRequest = class
  private
    FAccessID: string; //ключ AccessId, полученный при авторизации
    FSecretKey: string; //ключ SecretKey, полученный при авторизации
    FData    : TStringStream;//даные для отправки с помощью POST
    procedure SetData(Str: string);
    function GetData:string;
    function CalcSignarure(const Method,URL: string; Date: TDateTime): string;
    function GetURI(const URL:string):string;
  public
    constructor Create(AccessID,SecretKey: string);
    destructor Destroy;
    function Get(const URL: string):string;
    function Post(const URL:string;Params:TStringList):string;
    property Data: string read GetData write SetData;
end;

constructor TMegaplanRequest.Create(AccessID,SecretKey: string);
begin
  inherited Create;
  FAccessID:=AccessID;
  FSecretKey:=SecretKey;
  FData:=TStringStream.Create;
end;

function TMegaplanRequest.GetData: string;

begin
  Result:=FData.DataString
end;

procedure TMegaplanRequest.SetData(Str: string);
begin
  FData.Clear;
  FData.WriteString(Str);
end;

function TMegaplanRequest.GetURI(const URL: string): string;
begin
  if pos('http://',URL)>0 then
    Result:=Copy(URL,8,length(URL)-7)
  else
    if pos('https://',URL)>0 then
      Result:=Copy(URL,9,length(URL)-8)
    else
      Result:=URL;
end;

function TMegaplanRequest.CalcSignarure(const Method, URL: string;
  Date: TDateTime): string;
var BaseString: TStringStream;
    HMACsig,RFCDate: String;
begin
BaseString:=TStringStream.Create;
try
  BaseString.WriteString(Method+LF);//метод
  if FData.Size=0 then
    BaseString.WriteString(LF)//ContentMD5 отсутствует
  else
    BaseString.WriteString(StrToHex(MD5(FData.DataString))+LF);
  if UpperCase(Method)='POST' then
    BaseString.WriteString('application/x-www-form-urlencoded'+LF)
  else
    BaseString.WriteString(LF);//Content-Type отсутствует
  RFCDate:=Rfc822DateTime(Date);//формируем дату в формате RFC-822
  BaseString.WriteString(RFCDate+LF);
  BaseString.WriteString(URL);//HOST+URI
  HMACsig:=HMAC_SHA1(BaseString.DataString,FSecretKey);//зашифровали по HMAC-SHA1
  Result:=EncodeBase64((StrToHex(HMACsig)));//перевели в HEX и шифроали по Base64
finally
  BaseString.Free;
end;
end;

function TMegaplanRequest.Get(const URL: string): string;
var URI:string;
    Date: TDateTime;
    DataStream: TstringStream;
begin
  Date:=Now;
  DataStream:=TStringStream.Create;
try
  with THTTPSend.Create do
    begin
      Headers.Add('Date: '+Rfc822DateTime(Date));
      Headers.Add('X-Authorization: '+FAccessID+':'+CalcSignarure('GET',GetURI(URL),Date));
      Headers.Add('Accept: application/json');
      if HTTPMethod('GET',URL) then
        DataStream.LoadFromStream(Document)
      else
        Exception.Create(Format(cErrString,[ResultCode,ResultString]));
    end;
  Result:=DataStream.DataString;
finally
  DataStream.Free;
end;
end;

function TMegaplanRequest.Post(const URL: string): string;
var URI:string;
    Date: TDateTime;
    DataStream: TstringStream;
begin
  Date:=Now;
  DataStream:=TStringStream.Create;
try
  with THTTPSend.Create do
    begin
      Headers.Add('Date: '+Rfc822DateTime(Date));
      Headers.Add('X-Authorization: '+FAccessID+':'+CalcSignarure('POST',GetURI(URL),Date));
      Headers.Add('Accept: application/json');
      FData.Position:=0;
      Document.LoadFromStream(FData);
      if HTTPMethod('POST',URL) then
        DataStream.LoadFromStream(Document)
      else
        Exception.Create(Format(cErrString,[ResultCode,ResultString]));
    end;
  Result:=DataStream.DataString;
finally
  DataStream.Free;
end;
end;

var
 MegaAPI: TMegaplanRequest;
begin
  MegaAPI:=TMegaplanRequest.Create('AccessID','SecretKey');
  Memo1.Lines.Add(MegaAPI.Get('http://webdelphi.megaplan.ru/BumsTaskApiV01/Task/list.api'));
  ...
end;