Adding support for bincode Encode/Decode for a struct defined elsewhere

1

I have a struct that I want to use. I want to be able to Encode/Decode it using bincode. How can I add a custom function to do that?

I know for Serialize/Deserialize I can add an attribute #[serde(with = "my_serde")] and define a module with the function serialize and deserialize . I found a great source here Custom serialization · Serde .

However, for bincode Encode/Decode I can't find what to do.

Specifically ibe/src/kem/kiltz_vahlis_one.rs at main · encryption4all/ibe · GitHub

Any help?

2

bincode supports a subset of serde. Isn't the compatibility layer between serde and bincode sufficient for your type to implement Encode and Decode out of the box?

3

I'm sorry, I'm new to rust. So I'm still learning what the terms "subset" and "compatibility layer" are. My struct that I want to add support to is not owned by me and does not support serde. So I added an attribute to support it myself

#[serde(with = "ciphertext_serde")]
cipher_text: CipherText

mod ciphertext_serde { ... }

Now I can derive Serialize/Deserialize from my struct that holds it. However, I cannot simply write

#[serde(with = "ciphertext_serde")]
#[bincode(with_serde)]
cipher_text: CipherText

Because when adding derive Encode/Decode I get the error the trait bound 'CipherText: Serialize' is not satisfied.

I don't understand the error, so I thought it would be easiest for me to implement support myself, like I did with ciphertext_serde.

4

If I try to add something like
#[bincode(with = "ciphertext_bincode")]
I get the error (pointing to with) saying Unknown field attribute. What does that mean?

5

Please post a complete MRE and full error messages.

6

Cargo.toml:

[package]
name = "test"
version = "0.1.0"
edition = "2024"

[lib]
path = "src/lib.rs"

[dependencies]
serde = { version = "1.0.219", features = ["derive"] }
bincode = { version = "2.0.1", features = ["derive", "serde"] }
ibe = { version = "0.3.0", features = ["kv1"] }

lib.rs:

use bincode::{Decode, Encode};
use serde::{Deserialize, Serialize};
use ibe::kem::kiltz_vahlis_one::CipherText;

#[derive(Serialize, Deserialize, Debug, Encode, Decode)]
pub enum Enum {
	Case {
		#[serde(with = "ciphertext_serde")]
		#[bincode(with = "ciphertext_bincode")]
		cipher_text: CipherText},
}

mod ciphertext_serde {
	use ibe::Compress;
	use super::CipherText;
	use serde::{Serializer, Deserializer};
	use serde::de::Error as DeError;

	pub fn serialize<S>(ct: &CipherText, s: S) -> Result<S::Ok, S::Error>
	where
		S: Serializer,
	{
		let bytes = ct.to_bytes();
		s.serialize_bytes(&bytes)
	}

	pub fn deserialize<'de, D>(d: D) -> Result<CipherText, D::Error>
	where
		D: Deserializer<'de>,
	{
		let bytes: &[u8] = serde::Deserialize::deserialize(d)?;
		let arr: &[u8; CipherText::OUTPUT_SIZE] =
			bytes.try_into()
				.map_err(|_| D::Error::custom("invalid ciphertext length"))?;
		let ct_opt = CipherText::from_bytes(arr);
		ct_opt
			.into_option()
			.ok_or_else(|| D::Error::custom("ciphertext decompression failed"))
	}
}

build message:

error: Unknown field attribute
 --> test/src/lib.rs:9:13
  |
9 |         #[bincode(with = "ciphertext_bincode")]
  |                   ^^^^
7

Hmm, maybe if you wrap CipherText in a newtype?

#[derive(Deserialize, Serialize)]
struct MyCipherText(#[serde_with = "ciphertext_serde")] CipherText);

and then

#[derive(Decode, Encode)]
struct Foo {
    #[bincode(with_serde)]
    cipher_text: MyCipherText,
}
8

presumably I could. I will if I have to. However, what is the reason that both #[bincode(with = "ciphertext_bincode")] and #[bincode(with_serde)] don't work?

9

serde_with = "..." doesn't implement Serialize or Deserialize for the type of the field, but rather is an ad-hoc way to insert alternative (de)serialisation logic into serde's processing. This is incompatible with the code generated by the bincode::Encode/bincode::Decode macros, which relies on Serialize/Deserialize actually being implemented for the type.

10

thanks. I guess I'll have to use another type (or not support Encode, Decode)

11

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