Vitamin C Protected

This crate is part of the Vitamin C framework to make cryptography code healthy.

Safe wrappers for sensitive data

Protected is a set of types that remove some of the sharp edges of working with sensitive data in Rust. Its interface is conceptually similar to Option or Result.

Sensitive data footguns

Rust is a safe language, but it's still possible to make mistakes when working with sensitive data. These can include (but are not limited to):

• Not zeroizing sensitive data when it's no longer needed
• Accidentally leaking sensitive data in logs or error messages
• Performing comparison operations on sensitive data in a way that leaks timing information
• Serializing sensitive data in a way that leaks information

Protected and the other types in this crate aim to make it easier to avoid these mistakes.

Usage

The Protected type is the most basic building block in this crate. You can use it to wrap any type that you want to protect so long as it implements the Zeroize trait.

use vitaminc_protected::Protected;

let x = Protected::new([0u8; 32]);

Protected will call zeroize on the inner value when it goes out of scope. Because that wipe is a real Drop, Protected is never Copy, even when the inner type is: unlike a move, a Copy value has no single owner whose drop can wipe it. Duplicating the wrapper is an explicit clone(), and each clone is wiped when it drops. See the Protected type docs for the full rationale. risky_unwrap is the escape hatch: it hands back the plain inner value along with the obligation to wipe it. Combinators such as map show the plain value only to their closure and return the result re-wrapped. It also provides an "opaque" implementation of the Debug trait so you can debug protected values without accidentally leaking their innards.

use vitaminc_protected::{Controlled, Protected};
let x = Protected::new([0u8; 32]);
assert!(format!("{x:?}").contains("Protected<[u8; 32]>"));

The inner value is not accessible directly, but you can use the risky_unwrap method as an escape hatch to get it back. risky_unwrap is defined in the [Controlled] trait so you'll need to bring that in scope.

use vitaminc_protected::{Controlled, Protected};

let x = Protected::new([0u8; 32]);
assert_eq!(x.risky_unwrap(), [0; 32]);

Protected does not implement Deref so you cannot access the data directly. This is to prevent accidental leakage of the inner value. It also means comparisons (like PartialEq) are not implemented for Protected.

If you want to safely compare values, you can use [Equatable].

Equatable

The Equatable type is a wrapper around Protected that implements constant-time comparison. It implements PartialEq for any inner type that implements [ConstantTimeEq].

use vitaminc_protected::{Equatable, Protected};

let x: Equatable<Protected<u32>> = Equatable::new(100);
let y: Equatable<Protected<u32>> = Equatable::new(100);
assert_eq!(x, y);

Exportable

The Exportable type is a wrapper around Protected that supports safe serialization via the SafeSerialize and SafeDeserialize traits.

Usage

The Usage type is a wrapper around Protected that allows you to specify a scope for the data.

This adapter is WIP.

Working with wrapped values

None of the adapters implement Deref so you can't access the inner value directly. This is to prevent accidental leakage of the inner value by being explicit about when and how you want to work with the inner value.

You can map over the inner value to transform it, so long as the adapter is the same type. For example, you can map a Protected<T> to a Protected<U>.

use vitaminc_protected::{Controlled, Protected};

// Calculate the sum of values in the array with the result as a `Protected`
let x: Protected<[u8; 4]> = Protected::new([1, 2, 3, 4]);
let result: Protected<u8> = x.map(|arr| arr.as_slice().iter().sum());
assert_eq!(result.risky_unwrap(), 10);

If you have a pair of Protected values, you can zip them together with a function that combines them.

use vitaminc_protected::{Controlled, Protected};

let x: Protected<u8> = Protected::new(1);
let y: Protected<u8> = Protected::new(2);
let z: Protected<u8> = x.zip(y, |a, b| a + b);

If the inner type is an Option you can call transpose to swap the Protected and the Option.

use vitaminc_protected::{Controlled, Protected};

let x = Protected::new(Some([0u8; 32]));
let y = x.transpose();
assert!(y.is_some());

A Protected of Protected can be "flattened" into a single Protected.

# use vitaminc_protected::{Controlled, Protected};

let x = Protected::new(Protected::new([0u8; 32]));
let y = x.flatten();
assert_eq!(y.risky_unwrap(), [0u8; 32]);

Use [flatten_array] to convert a [Protected<T>; N] into a Protected<[T; N]>.

Protected digests

ProtectedDigest requires a fixed-output implementation that zeroizes its internal state on drop. Enable the digest crate's zeroization feature, such as sha2 = { version = "0.11", features = ["zeroize"] }.

Unkeyed digests use new; keyed fixed-output functions implementing KeyInit use new_with_key so the key remains in a Controlled container at the API boundary.

Secret inputs use update and protected outputs use finalize_into. Public protocol framing and intentionally exposed outputs cross separate, explicitly named channels:

use sha2::Sha256;
use vitaminc_protected::{Controlled, Protected, ProtectedDigest};

let secret = Protected::new(*b"secret");
let mut digest = ProtectedDigest::<Sha256>::new();
digest.update_public(b"example/domain/v1");
digest.update(&secret);

let mut output = Protected::new([0u8; 32]);
digest.finalize_into(&mut output);
assert_ne!(output.risky_ref(), &[0u8; 32]);

Also in this crate

Beyond the adapters above, the crate exports TimingSafeEq and Choice (timing-safe comparison), OpaqueDebug and Redacted (leak-resistant Debug), ProtectedDigest, Zeroed, and AsProtectedRef — see the docs.rs API reference for details.

Non-empty contexts

An AEAD associated-data value or PRF context can legitimately be empty, but a caller that uses one value to domain-separate fields needs it not to be. NonEmpty<T> carries that invariant in the type, checked once at construction: nonempty!("users/email") is checked at compile time (an empty literal does not compile), and NonEmpty::new(value) checks a dynamic value structurally — "", None, Some("") and ("", "") are all rejected, without parsing any encoding. An API that requires the invariant takes NonEmpty<T> directly; a bare &str argument cannot be value-checked at compile time, so there is deliberately no implicit conversion from a string or byte slice. Integers are never empty and convert with From, and a proven value extends with .with(tail) without a second check.

use vitaminc_protected::{nonempty, EmptyError, NonEmpty};

// Compile-time checked: nonempty!("") does not compile.
assert_eq!(nonempty!("users/email").get(), &"users/email");

// Runtime checked, once, for dynamic values.
assert!(NonEmpty::new(String::from("users/email")).is_ok());
assert_eq!(NonEmpty::new(("", None::<&str>)).unwrap_err(), EmptyError);

Generators

Protected supports generating new values from functions that return the inner value.

# use vitaminc_protected::{Controlled, Protected};
fn array_gen<const N: usize>() -> [u8; N] {
    core::array::from_fn(|i| (i + 1) as u8)
}

let input: Protected<[u8; 8]> = Protected::generate(array_gen);

You can also generate values from functions that return a Result with the inner value.

# use vitaminc_protected::{Controlled, Protected};
use std::string::FromUtf8Error;

let input: Result<Protected<String>, FromUtf8Error> = Protected::generate_ok(|| {
  String::from_utf8(vec![1, 2, 3, 4, 5, 6, 7, 8])
});

CipherStash

Vitamin C is brought to you by the team at CipherStash.

License: MIT