gistfile1.txt

private func loadCFunction<Function>(named name: String, ofType _: Function.Type = Function.self) -> Function {
    let sym = dlsym(UnsafeMutablePointer<Void>(bitPattern: -2), name) // RTLD_DEFAULT
    return unsafeBitCast(sym, Function.self)
}

private let CC_SHA1_DIGEST_LENGTH = 20
private let CC_SHA1: @convention(c) (UnsafePointer<Void>, Int32, UnsafeMutablePointer<UInt8>) -> UnsafeMutablePointer<UInt8> = loadCFunction(named: "CC_SHA1")

extension String {

    public func SHA1Sum() -> [UInt8] {
        return withCString { ptr -> [UInt8] in
            var array = [UInt8](count: CC_SHA1_DIGEST_LENGTH, repeatedValue: 0)
            _ = CC_SHA1(ptr, numericCast(strlen(ptr)), &array)
            return array
        }
    }
    
}

"Hello, world!".SHA1Sum()