(DataFlex) Verify Signature of Alexa Custom Skill Request

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

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

Use ChilkatAx-win32.pkg

Procedure Test
    String sSignature
    String sCertChainUrl
    String sJsonBody
    Handle hoHttp
    Variant vSbPem
    Handle hoSbPem
    Boolean iSuccess
    Handle hoPem
    Variant vCert
    Handle hoCert
    Variant vPubKey
    Handle hoPubKey
    Handle hoRsa
    Boolean iBVerified
    String sTemp1
    Boolean bTemp1

    // 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:
    Move "dSUmPwxc9...aKAf8mpEXg==" To sSignature
    Move "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem" To sCertChainUrl
    Move '{"version":"1.0","session":{"new":true,"sessionId":"amzn1.echo-api.session.433 ... }}' To sJsonBody

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

    // First, download the PEM-encoded X.509 certificate chain that Alexa used to sign the message 
    Get Create (RefClass(cComChilkatHttp)) To hoHttp
    If (Not(IsComObjectCreated(hoHttp))) Begin
        Send CreateComObject of hoHttp
    End
    Get Create (RefClass(cComChilkatStringBuilder)) To hoSbPem
    If (Not(IsComObjectCreated(hoSbPem))) Begin
        Send CreateComObject of hoSbPem
    End
    Get pvComObject of hoSbPem to vSbPem
    Get ComQuickGetSb Of hoHttp sCertChainUrl vSbPem To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoHttp To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    Get Create (RefClass(cComChilkatPem)) To hoPem
    If (Not(IsComObjectCreated(hoPem))) Begin
        Send CreateComObject of hoPem
    End
    Get ComGetAsString Of hoSbPem To sTemp1
    Get ComLoadPem Of hoPem sTemp1 "passwordNotUsed" To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoPem To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    // The 1st certificate should be the signing certificate.
    Get ComGetCert Of hoPem 0 To vCert
    If (IsComObject(vCert)) Begin
        Get Create (RefClass(cComChilkatCert)) To hoCert
        Set pvComObject Of hoCert To vCert
    End
    Get ComLastMethodSuccess Of hoPem To bTemp1
    If (bTemp1 = False) Begin
        Get ComLastErrorText Of hoPem To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    // Get the public key from the cert.
    Get ComExportPublicKey Of hoCert To vPubKey
    If (IsComObject(vPubKey)) Begin
        Get Create (RefClass(cComChilkatPublicKey)) To hoPubKey
        Set pvComObject Of hoPubKey To vPubKey
    End
    Get ComLastMethodSuccess Of hoCert To bTemp1
    If (bTemp1 = False) Begin
        Get ComLastErrorText Of hoCert To sTemp1
        Showln sTemp1
        Send Destroy of hoCert
        Procedure_Return
    End

    Send Destroy of hoCert

    // Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
    Get Create (RefClass(cComChilkatRsa)) To hoRsa
    If (Not(IsComObjectCreated(hoRsa))) Begin
        Send CreateComObject of hoRsa
    End
    Get ComImportPublicKeyObj Of hoRsa vPubKey To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoCert To sTemp1
        Showln sTemp1
        Send Destroy of hoPubKey
        Procedure_Return
    End

    Send Destroy of hoPubKey

    // 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...)
    Set ComEncodingMode Of hoRsa To "base64"
    Get ComVerifyStringENC Of hoRsa sJsonBody "sha1" sSignature To iBVerified
    If (iBVerified = True) Begin
        Showln "The signature is verified against the JSON body of the request. Yay!"
    End
    Else Begin
        Showln "Sorry, not verified.  Crud!"
    End



End_Procedure