libs/libvmbase/src/rand.rs

// Copyright 2023, The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Functions and drivers for obtaining true entropy.

use crate::arch::rand::{platform_entropy, PlatformError, MAX_BYTES_PER_CALL};
use core::fmt;

pub(crate) type Entropy = [u8; MAX_BYTES_PER_CALL];

/// Error type for rand operations.
pub enum Error {
    /// No source of entropy found.
    NoEntropySource,

    /// Platform specific error
    Platform(PlatformError),
}

impl From<PlatformError> for Error {
    fn from(e: PlatformError) -> Self {
        Error::Platform(e)
    }
}

/// Result type for rand operations.
pub type Result<T> = core::result::Result<T, Error>;

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Self::NoEntropySource => write!(f, "No source of entropy available"),
            Self::Platform(e) => write!(f, "Platform error: {e}"),
        }
    }
}

impl fmt::Debug for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{self}")
    }
}

/// Fills a slice of bytes with true entropy.
pub fn fill_with_entropy(s: &mut [u8]) -> Result<()> {
    for chunk in s.chunks_mut(MAX_BYTES_PER_CALL) {
        let entropy = platform_entropy(chunk.len())?;
        chunk.clone_from_slice(&entropy[..chunk.len()]);
    }

    Ok(())
}

/// Generate an array of fixed-size initialized with true-random bytes.
pub fn random_array<const N: usize>() -> Result<[u8; N]> {
    let mut arr = [0; N];
    fill_with_entropy(&mut arr)?;
    Ok(arr)
}