Delphi DLL

Verify Signature of Alexa Custom Skill Request

See more HTTP Misc Examples

This example verifies the signature of an Alexa Custom Skill Request. Delphi DLL

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, PublicKey, Pem, StringBuilder, Rsa, Cert, Http;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Boolean;
signature: PWideChar;
certChainUrl: PWideChar;
jsonBody: PWideChar;
http: HCkHttp;
sbPem: HCkStringBuilder;
pem: HCkPem;
cert: HCkCert;
pubKey: HCkPublicKey;
rsa: HCkRsa;
bVerified: Boolean;

begin
success := False;

// This example assumes you have a web service that will receive requests from Alexa.
// A sample request sent by Alexa will look like the following:

// Connection: Keep-Alive
// Content-Length: 2583
// Content-Type: application/json; charset=utf-8
// Accept: application/json
// Accept-Charset: utf-8
// Host: your.web.server.com
// User-Agent: Apache-HttpClient/4.5.x (Java/1.8.0_172)
// Signature: dSUmPwxc9...aKAf8mpEXg==
// SignatureCertChainUrl: https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem
// 
// {"version":"1.0","session":{"new":true,"sessionId":"amzn1.echo-api.session.433 ... }}

// First, assume we've written code to get the 3 pieces of data we need:
signature := 'dSUmPwxc9...aKAf8mpEXg==';
certChainUrl := 'https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem';
jsonBody := '{"version":"1.0","session":{"new":true,"sessionId":"amzn1.echo-api.session.433 ... }}';

// To validate the signature, we do the following:

// First, download the PEM-encoded X.509 certificate chain that Alexa used to sign the message 
http := CkHttp_Create();
sbPem := CkStringBuilder_Create();
success := CkHttp_QuickGetSb(http,certChainUrl,sbPem);
if (success = False) then
  begin
    Memo1.Lines.Add(CkHttp__lastErrorText(http));
    Exit;
  end;

pem := CkPem_Create();
success := CkPem_LoadPem(pem,CkStringBuilder__getAsString(sbPem),'passwordNotUsed');
if (success = False) then
  begin
    Memo1.Lines.Add(CkPem__lastErrorText(pem));
    Exit;
  end;

// The 1st certificate should be the signing certificate.
cert := CkPem_GetCert(pem,0);
if (CkPem_getLastMethodSuccess(pem) = False) then
  begin
    Memo1.Lines.Add(CkPem__lastErrorText(pem));
    Exit;
  end;

// Get the public key from the cert.
pubKey := CkPublicKey_Create();
CkCert_GetPublicKey(cert,pubKey);

CkCert_Dispose(cert);

// Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
rsa := CkRsa_Create();
success := CkRsa_UsePublicKey(rsa,pubKey);
if (success = False) then
  begin
    Memo1.Lines.Add(CkCert__lastErrorText(cert));
    Exit;
  end;

// RSA "decrypt" the signature.
// (Amazon's documentation is confusing, because we're simply verifiying the signature against the SHA-1 hash
// of the request body.  This happens in a single call to VerifyStringENC...)
CkRsa_putEncodingMode(rsa,'base64');
bVerified := CkRsa_VerifyStringENC(rsa,jsonBody,'sha1',signature);
if (bVerified = True) then
  begin
    Memo1.Lines.Add('The signature is verified against the JSON body of the request. Yay!');
  end
else
  begin
    Memo1.Lines.Add('Sorry, not verified.  Crud!');
  end;

CkHttp_Dispose(http);
CkStringBuilder_Dispose(sbPem);
CkPem_Dispose(pem);
CkPublicKey_Dispose(pubKey);
CkRsa_Dispose(rsa);

end;