PureBasic

Create a DKIM-Signature for a MIME Message

Demonstrates the Chilkat Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to a complete MIME message held in a BinData, modifying it in place. The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount properties configure the generated signature.

Note: The file paths are relative to the application's current working directory. Absolute paths may also be used. Supply the paths appropriate to your own environment.

Background: The signature is computed over a hash of the body plus a chosen set of headers, so the message must be completely built — every header, encoding, and boundary — before signing; any later change invalidates it. The d= (domain) and s= (selector) tags tell a verifier where to find the public key: at selector._domainkey.domain in DNS. relaxed/relaxed canonicalization tolerates the minor whitespace changes mail systems make in transit, which is why it is the common choice.

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IncludeFile "CkBinData.pb"
IncludeFile "CkDkim.pb"
IncludeFile "CkPrivateKey.pb"

Procedure ChilkatExample()

    success.i = 0

    ;  Demonstrates the Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to
    ;  a complete MIME message.  The only argument is a BinData holding the MIME, which is modified in
    ;  place.
    ;  
    ;  The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount
    ;  properties configure the signature that is generated.

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

    ;  Configure the DKIM signature.
    CkDkim::setCkDkimDomain(dkim, "example.com")
    CkDkim::setCkDkimSelector(dkim, "myselector")

    ;  Signing algorithm written to the a= tag.
    CkDkim::setCkDkimAlg(dkim, "rsa-sha256")

    ;  Canonicalization for headers and body, written to the c= tag.
    CkDkim::setCkDkimCanon(dkim, "relaxed/relaxed")

    ;  Colon-separated list of header fields to sign, written to the h= tag.
    CkDkim::setCkDkimHeaders(dkim, "From:To:Subject:Date:Message-ID")

    ;  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
    CkDkim::setCkDkimBodyLengthCount(dkim, 0)

    ;  Load the RSA private key and give it to the Dkim object.
    privKey.i = CkPrivateKey::ckCreate()
    If privKey.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkPrivateKey::ckLoadPemFile(privKey,"qa_data/dkim_private.pem")
    If success = 0
        Debug CkPrivateKey::ckLastErrorText(privKey)
        CkDkim::ckDispose(dkim)
        CkPrivateKey::ckDispose(privKey)
        ProcedureReturn
    EndIf

    success = CkDkim::ckSetDkimPrivateKey(dkim,privKey)
    If success = 0
        Debug CkDkim::ckLastErrorText(dkim)
        CkDkim::ckDispose(dkim)
        CkPrivateKey::ckDispose(privKey)
        ProcedureReturn
    EndIf

    ;  Load the complete MIME message to be signed.  It must already contain all headers, transfer
    ;  encodings, MIME boundaries, and body bytes.
    mimeData.i = CkBinData::ckCreate()
    If mimeData.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkBinData::ckLoadFile(mimeData,"qa_data/message.eml")
    If success = 0
        Debug CkBinData::ckLastErrorText(mimeData)
        CkDkim::ckDispose(dkim)
        CkPrivateKey::ckDispose(privKey)
        CkBinData::ckDispose(mimeData)
        ProcedureReturn
    EndIf

    ;  Sign.  The DKIM-Signature header is prepended to the MIME in place.
    success = CkDkim::ckDkimSign(dkim,mimeData)
    If success = 0
        Debug CkDkim::ckLastErrorText(dkim)
        CkDkim::ckDispose(dkim)
        CkPrivateKey::ckDispose(privKey)
        CkBinData::ckDispose(mimeData)
        ProcedureReturn
    EndIf

    ;  Save the signed message.
    success = CkBinData::ckWriteFile(mimeData,"qa_output/signed.eml")
    If success = 0
        Debug CkBinData::ckLastErrorText(mimeData)
        CkDkim::ckDispose(dkim)
        CkPrivateKey::ckDispose(privKey)
        CkBinData::ckDispose(mimeData)
        ProcedureReturn
    EndIf

    Debug "Message signed."


    CkDkim::ckDispose(dkim)
    CkPrivateKey::ckDispose(privKey)
    CkBinData::ckDispose(mimeData)


    ProcedureReturn
EndProcedure