GetExternalPublicKey.swift

let ellipticCurveHeader = [UInt8]([0x30, 0x59, 0x30, 0x13, 0x06, 0x07, 0x2A, 0x86, 0x48,
                                      0xCE, 0x3D, 0x02, 0x01, 0x06, 0x08, 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x03,
                                      0x01, 0x07, 0x03, 0x42, 0x00])
let begin = "-----BEGIN PUBLIC KEY-----\n"
let end = "\n-----END PUBLIC KEY-----"

func retrivePublicKey() -> String? {
    var error: Unmanaged<CFError>?
    if let publicKey = getPublicKey(), 
       let keyRef = SecKeyCopyExternalRepresentation(publicKey, &error) {
          var keyData = Data()
          keyData.append(Data(ellipticCurveHeader))
          keyData.append(keyRef as Data)
          let b64Key = keyData.base64EncodedString(options: [.lineLength64Characters, 
                                                             .endLineWithCarriageReturn])
          return "\(begin)\(b64Key)\(end)"
    }
    return nil
}