(Swift 2) ECDSA Sign and Verify

Demonstrates how to create an ECDSA signature on the SHA256 hash of some data, and then verify.

Chilkat Downloads for the Swift Programming Language MAC OS X (Cocoa) Objective-C/Swift Libs iOS Objective-C/Swift Libs

func chilkatTest() {
    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // First load an ECDSA private key to be used for signing.
    let privKey = CkoPrivateKey()
    var success: Bool = privKey.LoadEncryptedPemFile("qa_data/ecc/secp256r1-key-pkcs8-secret.pem", password: "secret")
    if success == false {
        print("\(privKey.LastErrorText)")
        return
    }

    // Sign the SHA256 hash of some data.
    let bd = CkoBinData()
    success = bd.LoadFile("qa_data/hamlet.xml")
    if success == false {
        print("Failed to load file to be hashed.")
        return
    }

    let crypt = CkoCrypt2()
    crypt.HashAlgorithm = "sha256"
    crypt.EncodingMode = "base64"
    var hashStr: String? = crypt.HashBdENC(bd)

    let ecdsa = CkoEcc()
    let prng = CkoPrng()
    // Returns ASN.1 signature as a base64 string.
    var sig: String? = ecdsa.SignHashENC(hashStr, encoding: "base64", privkey: privKey, prng: prng)
    print("sig = \(sig!)")

    // The signature is in ASN.1 format (which may be described as the "encoded DSS signature").
    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
    //   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...

    // If you wish, you can get the r and s components of the signature like this:
    let asn = CkoAsn()
    asn.LoadEncoded(sig, encoding: "base64")
    let xml = CkoXml()
    xml.LoadXml(asn.AsnToXml())

    print("\(xml.GetXml())")

    // We now have this:
    // <?xml version="1.0" encoding="utf-8"?>
    // <sequence>
    //     <int>6650D422D86BA4A228B5617604E59052591B9B2C32EF324C44D09EF67E5F0060</int>
    //     <int>0CFD9F6AC85042FC70F672C141BA6B2A4CAFBB906C3D907BCCC1BED62B28326F</int>
    // </sequence>

    // Get the "r" and "s" as hex strings
    var r: String? = xml.GetChildContentByIndex(0)
    var s: String? = xml.GetChildContentByIndex(1)

    print("r = \(r!)")
    print("s = \(s!)")

    // --------------------------------------------------------------------
    // Now verify against the hash of the original data.

    // Get the corresponding public key.
    let pubKey = CkoPublicKey()
    success = pubKey.LoadFromFile("qa_data/ecc/secp256r1-pub.pem")
    if success == false {
        print("\(pubKey.LastErrorText)")
        return
    }

    // We already have the SHA256 hash of the original data (hashStr) so no need to re-do it..
    let ecc2 = CkoEcc()
    var result: Int = ecc2.VerifyHashENC(hashStr, encodedSig: sig, encoding: "base64", pubkey: pubKey).intValue
    if result != 1 {
        print("\(ecc2.LastErrorText)")
        return
    }

    print("Verified!")

    // Note: If we have only r,s and wish to reconstruct the ASN.1 signature, we do it like this:
    let xml2 = CkoXml()
    xml2.Tag = "sequence"
    xml2.NewChild2("int", content: r)
    xml2.NewChild2("int", content: s)

    let asn2 = CkoAsn()
    asn2.LoadAsnXml(xml2.GetXml())
    var encodedSig: String? = asn2.GetEncodedDer("base64")
    print("encoded DSS signature: \(encodedSig!)")

    // You can go to https://lapo.it/asn1js/  and copy/paste the base64 encodedSig into the online tool, then press the "decode" button.
    // You will see the ASN.1 such as this:

    // SEQUENCE (2 elem)
    //   INTEGER (255 bit) 4849395540832462044300553275435608522154141569743642905628579547100940...
    //   INTEGER (255 bit) 3680701124244788134409868118208591399799457104230118295614152238560005...

}