(PureBasic) Verify an RSA Signature Against the Signed String

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Demonstrates how to validate an RSA Signature against the string that was signed.

Chilkat PureBasic Module Download Chilkat PureBasic Module

IncludeFile "CkStringBuilder.pb"
IncludeFile "CkRsa.pb"
IncludeFile "CkPublicKey.pb"

Procedure ChilkatExample()

    ; Create the same string we previously signed in this example: 
    ; RSA Sign utf-8 Bytes of String to get Base64 RSA Signature

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

    crlfLineEnding.i = 1
    i.i
    For i = 0 To 10
        CkStringBuilder::ckAppendLine(sb,"This is a test.",crlfLineEnding)
    Next

    base64_rsa_sig.s = "gWgpEXQqvXN6wh0MuFXPiw2xCb8cnmdizBT1TD1Tpm2GlJ8gnD59DGSj35GGXk1tM+mCOvMa2uW/9gmz8p6A90JLbn918i/2wKGvcde4wXnzPO7JADSNn2UwYzf3rp4/q/JrHv1GZETRJhABSFqTxOgdmdbEJQE/zijOSctCtQAM2CTb6t2BO8uHKSSUAwPH2cbeWxqsaVRLT4ruk5nZOxhoLgLMSjupiSzbE8zSTkkFOEHD+Mbb2xwg85AI0E4DsUnp+hLwwtS0blesgSyvtBY8oJODJeFcJsR7JDOTPdzHQHgJRl/2A4ij5xT91TpXy7ok43jhT6O9j7Q3qrafIQ=="

    ; Get the public key to be used for signature verification.
    pubKey.i = CkPublicKey::ckCreate()
    If pubKey.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success.i = CkPublicKey::ckLoadFromFile(pubKey,"rsaKeys/chilkat-rsa-2048.pem")
    If success = 0
        Debug CkPublicKey::ckLastErrorText(pubKey)
        CkStringBuilder::ckDispose(sb)
        CkPublicKey::ckDispose(pubKey)
        ProcedureReturn
    EndIf

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

    CkRsa::ckImportPublicKeyObj(rsa,pubKey)

    ; Verify the string against the signature.
    ; We signed the utf-8 byte representation of the string, so we must indicate
    ; to also validate against the utf-8 byte representation.
    CkRsa::setCkCharset(rsa, "utf-8")
    CkRsa::setCkEncodingMode(rsa, "base64")
    signedString.s = CkStringBuilder::ckGetAsString(sb)
    success = CkRsa::ckVerifyStringENC(rsa,signedString,"sha256",base64_rsa_sig)

    If success = 0
        Debug CkRsa::ckLastErrorText(rsa)
        Debug "Signature invalid."
    Else
        Debug "Signature valid."
    EndIf



    CkStringBuilder::ckDispose(sb)
    CkPublicKey::ckDispose(pubKey)
    CkRsa::ckDispose(rsa)


    ProcedureReturn
EndProcedure