(Go) Duplicate openssl rsautl -verify -inkey pubKey.pem -pubin -in rsautl.sig -out originalFile.dat

Demonstrates how to duplicate this OpenSSL command:

openssl rsautl -verify -inkey pubKey.pem -pubin -in rsautl.sig -out originalFile.dat

The openssl rsautl command signs the data directly, and therefore the maximum amount of data that can signed is very small. (To sign any amount of data, use openssl dgst -sign.) The maximum amount of data that can be signed (with rsautl) is the key size minus the overhead for the padding. The overhead size depends on the padding. With OAEP padding, the overhead is 42 bytes. With the default PKCSv1.5 padding, the overhead is 11 bytes.

Given a 2048-bit RSA key (256 bytes) and using PKCSv1.5 padding, the max data size that can be signed is 256 - 11 = 245 bytes.

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    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    pubKey := chilkat.NewPublicKey()

    // Load the public key from an PEM file:
    success := pubKey.LoadFromFile("pubKey.pem")
    if success != true {
        fmt.Println(pubKey.LastErrorText())
        pubKey.DisposePublicKey()
        return
    }

    // Load the signature.
    bd := chilkat.NewBinData()
    success = bd.LoadFile("rsautl.sig")

    rsa := chilkat.NewRsa()

    // Import the public key into the RSA component:
    success = rsa.ImportPublicKeyObj(pubKey)
    if success != true {
        fmt.Println(rsa.LastErrorText())
        pubKey.DisposePublicKey()
        bd.DisposeBinData()
        rsa.DisposeRsa()
        return
    }

    // OpenSSL uses big-endian.
    rsa.SetLittleEndian(false)

    // If the signature is verified, then the signature contents of bd
    // are replaced with the extracted original data.
    success = rsa.OpenSslVerifyBd(bd)
    if success != true {
        fmt.Println(rsa.LastErrorText())
        fmt.Println("The signature was invalid.")
        pubKey.DisposePublicKey()
        bd.DisposeBinData()
        rsa.DisposeRsa()
        return
    }

    fmt.Println("The signature was verified.")

    // Save the original data that was extracted from the signature:
    success = bd.WriteFile("originalFile.dat")
    if success != true {
        fmt.Println("Failed to save extracted data.")
        pubKey.DisposePublicKey()
        bd.DisposeBinData()
        rsa.DisposeRsa()
        return
    }


    pubKey.DisposePublicKey()
    bd.DisposeBinData()
    rsa.DisposeRsa()