(C) Verify Signature of Alexa Custom Skill Request

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

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#include <C_CkHttp.h>
#include <C_CkStringBuilder.h>
#include <C_CkPem.h>
#include <C_CkCert.h>
#include <C_CkPublicKey.h>
#include <C_CkRsa.h>

void ChilkatSample(void)
    {
    const char *signature;
    const char *certChainUrl;
    const char *jsonBody;
    HCkHttp http;
    HCkStringBuilder sbPem;
    BOOL success;
    HCkPem pem;
    HCkCert cert;
    HCkPublicKey pubKey;
    HCkRsa rsa;
    BOOL bVerified;

    // 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) {
        printf("%s\n",CkHttp_lastErrorText(http));
        CkHttp_Dispose(http);
        CkStringBuilder_Dispose(sbPem);
        return;
    }

    pem = CkPem_Create();
    success = CkPem_LoadPem(pem,CkStringBuilder_getAsString(sbPem),"passwordNotUsed");
    if (success == FALSE) {
        printf("%s\n",CkPem_lastErrorText(pem));
        CkHttp_Dispose(http);
        CkStringBuilder_Dispose(sbPem);
        CkPem_Dispose(pem);
        return;
    }

    // The 1st certificate should be the signing certificate.
    cert = CkPem_GetCert(pem,0);
    if (CkPem_getLastMethodSuccess(pem) == FALSE) {
        printf("%s\n",CkPem_lastErrorText(pem));
        CkHttp_Dispose(http);
        CkStringBuilder_Dispose(sbPem);
        CkPem_Dispose(pem);
        return;
    }

    // Get the public key from the cert.
    pubKey = CkCert_ExportPublicKey(cert);
    if (CkCert_getLastMethodSuccess(cert) == FALSE) {
        printf("%s\n",CkCert_lastErrorText(cert));
        CkCert_Dispose(cert);
        CkHttp_Dispose(http);
        CkStringBuilder_Dispose(sbPem);
        CkPem_Dispose(pem);
        return;
    }

    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_ImportPublicKeyObj(rsa,pubKey);
    if (success == FALSE) {
        printf("%s\n",CkCert_lastErrorText(cert));
        CkPublicKey_Dispose(pubKey);
        CkHttp_Dispose(http);
        CkStringBuilder_Dispose(sbPem);
        CkPem_Dispose(pem);
        CkRsa_Dispose(rsa);
        return;
    }

    CkPublicKey_Dispose(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...)
    CkRsa_putEncodingMode(rsa,"base64");
    bVerified = CkRsa_VerifyStringENC(rsa,jsonBody,"sha1",signature);
    if (bVerified == TRUE) {
        printf("The signature is verified against the JSON body of the request. Yay!\n");
    }
    else {
        printf("Sorry, not verified.  Crud!\n");
    }



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

    }