(MFC) Verify Signature of Alexa Custom Skill Request

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

Chilkat C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

#include <CkHttp.h>
#include <CkStringBuilder.h>
#include <CkPem.h>
#include <CkCert.h>
#include <CkPublicKey.h>
#include <CkRsa.h>

void ChilkatSample(void)
    {
    CkString strOut;

    // 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:
    const char *signature = "dSUmPwxc9...aKAf8mpEXg==";
    const char *certChainUrl = "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem";
    const char *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 
    CkHttp http;
    CkStringBuilder sbPem;
    bool success = http.QuickGetSb(certChainUrl,sbPem);
    if (success == false) {
        strOut.append(http.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    CkPem pem;
    success = pem.LoadPem(sbPem.getAsString(),"passwordNotUsed");
    if (success == false) {
        strOut.append(pem.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // The 1st certificate should be the signing certificate.
    CkCert *cert = pem.GetCert(0);
    if (pem.get_LastMethodSuccess() == false) {
        strOut.append(pem.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    // Get the public key from the cert.
    CkPublicKey *pubKey = cert->ExportPublicKey();
    if (cert->get_LastMethodSuccess() == false) {
        strOut.append(cert->lastErrorText());
        strOut.append("\r\n");
        delete cert;
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    delete cert;

    // Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
    CkRsa rsa;
    success = rsa.ImportPublicKeyObj(*pubKey);
    if (success == false) {
        strOut.append(cert->lastErrorText());
        strOut.append("\r\n");
        delete pubKey;
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        return;
    }

    delete pubKey;

    // 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...)
    rsa.put_EncodingMode("base64");
    bool bVerified = rsa.VerifyStringENC(jsonBody,"sha1",signature);
    if (bVerified == true) {
        strOut.append("The signature is verified against the JSON body of the request. Yay!");
        strOut.append("\r\n");
    }
    else {
        strOut.append("Sorry, not verified.  Crud!");
        strOut.append("\r\n");
    }



    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }