(PureBasic) Verify Signature of Alexa Custom Skill Request

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

Chilkat PureBasic Module Download Chilkat PureBasic Module

IncludeFile "CkPem.pb"
IncludeFile "CkHttp.pb"
IncludeFile "CkPublicKey.pb"
IncludeFile "CkRsa.pb"
IncludeFile "CkCert.pb"
IncludeFile "CkStringBuilder.pb"

Procedure ChilkatExample()

    ; 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.s = "dSUmPwxc9...aKAf8mpEXg=="
    certChainUrl.s = "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem"
    jsonBody.s = "{" + Chr(34) + "version" + Chr(34) + ":" + Chr(34) + "1.0" + Chr(34) + "," + Chr(34) + "session" + Chr(34) + ":{" + Chr(34) + "new" + Chr(34) + ":true," + Chr(34) + "sessionId" + Chr(34) + ":" + Chr(34) + "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.i = CkHttp::ckCreate()
    If http.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    sbPem.i = CkStringBuilder::ckCreate()
    If sbPem.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success.i = CkHttp::ckQuickGetSb(http,certChainUrl,sbPem)
    If success = 0
        Debug CkHttp::ckLastErrorText(http)
        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbPem)
        ProcedureReturn
    EndIf

    pem.i = CkPem::ckCreate()
    If pem.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkPem::ckLoadPem(pem,CkStringBuilder::ckGetAsString(sbPem),"passwordNotUsed")
    If success = 0
        Debug CkPem::ckLastErrorText(pem)
        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbPem)
        CkPem::ckDispose(pem)
        ProcedureReturn
    EndIf

    ; The 1st certificate should be the signing certificate.
    cert.i = CkPem::ckGetCert(pem,0)
    If CkPem::ckLastMethodSuccess(pem) = 0
        Debug CkPem::ckLastErrorText(pem)
        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbPem)
        CkPem::ckDispose(pem)
        ProcedureReturn
    EndIf

    ; Get the public key from the cert.
    pubKey.i = CkCert::ckExportPublicKey(cert)
    If CkCert::ckLastMethodSuccess(cert) = 0
        Debug CkCert::ckLastErrorText(cert)
        CkCert::ckDispose(cert)

        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbPem)
        CkPem::ckDispose(pem)
        ProcedureReturn
    EndIf

    CkCert::ckDispose(cert)

    ; Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
    rsa.i = CkRsa::ckCreate()
    If rsa.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkRsa::ckImportPublicKeyObj(rsa,pubKey)
    If success = 0
        Debug CkCert::ckLastErrorText(cert)
        CkPublicKey::ckDispose(pubKey)

        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbPem)
        CkPem::ckDispose(pem)
        CkRsa::ckDispose(rsa)
        ProcedureReturn
    EndIf

    CkPublicKey::ckDispose(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::setCkEncodingMode(rsa, "base64")
    bVerified.i = CkRsa::ckVerifyStringENC(rsa,jsonBody,"sha1",signature)
    If bVerified = 1
        Debug "The signature is verified against the JSON body of the request. Yay!"
    Else
        Debug "Sorry, not verified.  Crud!"
    EndIf



    CkHttp::ckDispose(http)
    CkStringBuilder::ckDispose(sbPem)
    CkPem::ckDispose(pem)
    CkRsa::ckDispose(rsa)


    ProcedureReturn
EndProcedure