(Go) ECDSA Sign Data and Get Raw R and S Values

Demonstrates getting the raw R and S value of an ECDSA signature.

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

    // To create an ECDSA signature, the data first needs to be hashed.  Then the hash
    // is signed.

    crypt := chilkat.NewCrypt2()
    crypt.SetHashAlgorithm("SHA256")
    crypt.SetCharset("utf-8")
    crypt.SetEncodingMode("base64")

    // Hash a string.
    hash1 := crypt.HashStringENC("The quick brown fox jumps over the lazy dog")
    fmt.Println("hash1 = ", *hash1)

    // -----------------------------------------------------------
    // An ECDSA private key is used for signing.  The public key is for signature verification.
    // Load our ECC private key.
    // Our private key file contains this:

    // 	// -----BEGIN PRIVATE KEY-----
    // 	MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg3J8q/24D1sEKGdP9
    // 	72MGYElLGpw/a56Y3t6pfON3uhShRANCAATlSmoizyhAwoYZAOuFBATl07/1RR54
    // 	a1Dzfm16grxJe666AGKR+bSs24hk7TEpaeCTvT8YOOM3l+xKFg7zq6Q9
    // 	-----END PRIVATE KEY-----

    privKey := chilkat.NewPrivateKey()
    success := privKey.LoadPemFile("qa_data/ecc/secp256r1-key-pkcs8.pem")
    if success != true {
        fmt.Println(privKey.LastErrorText())
        crypt.DisposeCrypt2()
        privKey.DisposePrivateKey()
        return
    }

    // Sign the hash..
    prng := chilkat.NewPrng()
    ecdsa := chilkat.NewEcc()
    ecdsaSigBase64 := ecdsa.SignHashENC(*hash1,"base64",privKey,prng)
    if ecdsa.LastMethodSuccess() != true {
        fmt.Println(ecdsa.LastErrorText())
        crypt.DisposeCrypt2()
        privKey.DisposePrivateKey()
        prng.DisposePrng()
        ecdsa.DisposeEcc()
        return
    }

    // The ECDSA signature is ASN.1 that contains a sequence of 2 large integers (r and s)
    // For example:
    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9
    //   INTEGER (255 bit) 481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5

    fmt.Println("Base64 ECDSA signature = ", *ecdsaSigBase64)

    // If the raw R and S values are needed, here's how to get them:
    asn := chilkat.NewAsn()
    success = asn.LoadEncoded(*ecdsaSigBase64,"base64")
    if success == false {
        fmt.Println(asn.LastErrorText())
        crypt.DisposeCrypt2()
        privKey.DisposePrivateKey()
        prng.DisposePrng()
        ecdsa.DisposeEcc()
        asn.DisposeAsn()
        return
    }

    // The R and X will be in hexidecimal in the XML.
    xml := chilkat.NewXml()
    xml.LoadXml(*asn.AsnToXml())
    fmt.Println(*xml.GetXml())

    // The XML looks like this:
    // <sequence>
    // <int>792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9</int>
    // <int>481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5</int>
    // </sequence>

    // Copy raw R and S hex values into a Chilkat BinData object.
    bd := chilkat.NewBinData()
    r := xml.GetChildContent("int[0]")
    s := xml.GetChildContent("int[1]")
    bd.AppendEncoded(*r,"hex")
    bd.AppendEncoded(*s,"hex")

    fmt.Println("Number of bytes in bd: ", bd.NumBytes())

    crypt.DisposeCrypt2()
    privKey.DisposePrivateKey()
    prng.DisposePrng()
    ecdsa.DisposeEcc()
    asn.DisposeAsn()
    xml.DisposeXml()
    bd.DisposeBinData()