So why even bother with the first parameter then? That looks like a badly designed method signature.
On Monday, October 9, 2023 at 8:12:33 AM UTC-6 Axel Wagner wrote: > I get the impression that the thing you are missing is that appending to a > slice does not modify it. That is, `append(s, x...)` modifies neither the > length, nor the content of `s`, you have to type `x = append(s, x...)`. > `Seal` (and `Open`) work exactly the same. They append to the `out` > parameter - and to enable you to get at the slice, they return it. > The docs could be slightly more clear, by explicitly stating that they > return the appended-to slice.0 > > On Mon, Oct 9, 2023 at 3:46 PM Dean Schulze <dean.w....@gmail.com> wrote: > >> If the docs are correct, how do you append to nil? That's what the docs >> say. Take a look at them. >> >> Your code example shows the first parameter to Seal() can be nil. So >> what does that parameter do? How do you append to it? >> >> On Sunday, October 8, 2023 at 11:19:13 PM UTC-6 Axel Wagner wrote: >> >>> For what it's worth, here is an example that demonstrates a typical >>> encryption/decryption roundtrip, perhaps more clearly: >>> https://go.dev/play/p/ZZry8IgTJQ_- >>> The `out` parameter can be used to make this more efficient by using >>> pre-allocated buffers (depending on use case) and there are cases where you >>> don't have to send the nonce, because you can derive them from common data, >>> which is why both of these parameters are there. But in the most common >>> usage, you'd do what this example does. >>> >>> The example code from the docs tries to be a little bit more efficient >>> and packs the `Box` struct into a single byte, perhaps at the cost of >>> understandability. >>> >>> On Mon, Oct 9, 2023 at 7:06 AM Axel Wagner <axel.wa...@googlemail.com> >>> wrote: >>> >>>> oh I forgot to emphasize: I don't believe the output is *really* >>>> `<encryptedMessage><hash>`. That is, I don't believe you can really treat >>>> the first N bytes as the encrypted text and decrypt it (say, if you didn't >>>> care about the authentication). It's just that you ultimately need to add >>>> 16 bytes of extra information to carry that authentication tag, which is >>>> why the box needs to be 16 bytes longer than the message. In reality, the >>>> two are probably cleverly mixed - I'm not a cryptographer. >>>> I just wanted to demonstrate where all the information ultimately goes. >>>> >>>> On Mon, Oct 9, 2023 at 7:03 AM Axel Wagner <axel.wa...@googlemail.com> >>>> wrote: >>>> >>>>> I don't really understand your issue. You call >>>>> encrypted := secretbox.Seal(nonce[:], []byte(s), &nonce, &secretKey) >>>>> That means you pass `nonce[:]` as the `out` argument, `s` as the >>>>> `message` argument, and the nonce and key and assign the result to >>>>> `encrypted`. >>>>> According to the docs of `secretbox`, `Seal` will `append` the >>>>> encrypted message to `nonce[:]` and that encrypted message will be 16 >>>>> bytes >>>>> longer than the message, which is 11 bytes long. Appending 37 (16+11) >>>>> bytes >>>>> to a 24 byte nonce gives you 51 bytes, which is what you observe as the >>>>> length of `encrypted`. >>>>> The length of `nonce` doesn't change (it's an array, after all) - but >>>>> passing `append` to a slice does not change the length of the slice, it >>>>> just returns a new slice, so that seems expected. >>>>> >>>>> So, from what I can tell, the code does exactly what the docs say it >>>>> should do. >>>>> >>>>> > In their example code the out parameter is nil. So what does it do? >>>>> >>>>> Appending to `nil` allocates a new slice. The point of still accepting >>>>> an `out` parameter is that you can potentially prevent an allocation by >>>>> passing in a slice with 0 length and extra capacity (which can then be >>>>> allocated on the stack, or which is from a `sync.Pool`). If you don't >>>>> need >>>>> that, passing in `nil` seems fine. >>>>> >>>>> > The second argument is encrypted[len(nonce):] which includes the >>>>> Overhead at the start of the []byte. Apparently that Overhead is >>>>> important. >>>>> >>>>> Yes, the Overhead is important. It is used to authenticate the >>>>> message. You can imagine the process of `Seal` as "encrypt the message >>>>> and >>>>> attach a hash". The hash is the Overhead. The process also needs a random >>>>> `nonce`, that both the sender and the receiver need to know. That's why >>>>> the >>>>> example code sends it along with the message (it doesn't have to be >>>>> secret). So that `Seal` call does, effectively (again, for illustrative >>>>> purposes): >>>>> encrypted := append(append(nonce, <encryptedMessage>), <hash>) >>>>> As `nonce` is an array, this allocates a new backing array for the >>>>> returned slice, which ends up filled with >>>>> <nonce><encryptedMessage><hash> >>>>> >>>>> The `Open` call then retrieves the `nonce` from the first 24 bytes (by >>>>> copying it into `decryptNonce`) and passes the `<encryptedMessage><hash>` >>>>> slice as the `box` argument. Which decrypts the message, authenticates >>>>> the >>>>> hash and appends the decrypted message to `out` (which is `nil` in the >>>>> example code). >>>>> >>>>> So, the docs are correct. And it seems to me, the code works as >>>>> expected. I'm not sure where the misunderstanding is. >>>>> >>>> -- >> > You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/3d7d75e5-d79a-4f81-978d-cb6feb7b12efn%40googlegroups.com >> >> <https://groups.google.com/d/msgid/golang-nuts/3d7d75e5-d79a-4f81-978d-cb6feb7b12efn%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/0fc63e62-0911-4ad1-b209-570c063a61c3n%40googlegroups.com.
