How to define &mut [u8] in Rust - rust

Question

I want to call openssl::sign::Signer::sign, which has the signature:
pub fn sign(&self, buf: &mut [u8]) -> Result<usize, ErrorStack>
I'm trying to hand over buf. How to I make it &mut [u8]?
cargo.toml
[dependencies]
openssl = { version = "0.10", features = ["vendored"] }
src/main.rs
use openssl::{base64, sign::Signer};
fn main() {
let mut buffer = [];
let str = base64::encode_block(signer.sign(&mut buffer));
}
But get an error:
openssl::base64::encode_block(signer.sign(&mut buffer));
^^^^^^^^^^^^^^^^^^^ expected `&[u8]`, found enum `std::result::Result`

Answer 1

I'm not familiar with this crate, but base64::encode_block expects &[u8], while the return value of Signer::sign is a Result that returns the number of bytes written as usize.
I'm not sure if the number of bytes written is what you want to encode?
If you want to encode buffer to base64, you could do:
use openssl;
fn main() {
let mut buf = [104, 101, 108, 108, 111];
let encoded = openssl::base64::encode_block(&buf);
println!("{}", encoded);
}
That should give you:
aGVsbG8=
Which is hello as base64.
Or, as an example, if you want to sign the given buf and encode that as base64:
use openssl::{base64, hash::MessageDigest, pkey::PKey, rsa::Rsa, sign::Signer};
fn main() {
let keypair = Rsa::generate(2048).unwrap();
let keypair = PKey::from_rsa(keypair).unwrap();
let signer = Signer::new(MessageDigest::sha256(), &keypair).unwrap();
let mut buf = [104, 101, 108, 108, 111].repeat(52);
let written = signer.sign(&mut buf).unwrap();
println!("bytes written: {}", written);
let encoded = base64::encode_block(&buf);
println!("base64: {}", encoded);
}
Which returns:
bytes written: 256
base64: wB4lBbyzpfRPInxhwm0XVKI3dQYqcUZWNdKyb4RTGDUmwq1DDDbQjMRmFBobRse3pNOxoMy+QQNSXsgI46b75hPfkar9TUowrIGk4Y+ZWWX/cwNJCJazC1dfanP4uft0fzpYJKMSfJTAxswccu1g4yT+u0V5yq+eHbeGDJ+bF2MMhCPds7wGjJguxO0e4wx+HQdVGbU9jrHQ38oIYTChG92iKLRpciiyB8vrbNEBcNNi4hlw6U0sUdz6scpXceREdPFVA6wr0otY3wSZLfcIeKELoBQkR2KPNTTCROreVJ49tXwiQdGe7Ky0NDeNba2H5tKu3uLAtAiG/hVoKEAJG2VsbG8=