Delphi DLL Requires Chilkat v11.0.0+

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

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);