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aes: disable soft backend if not needed #593

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3 changes: 2 additions & 1 deletion aes/Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,8 @@ level = "warn"
check-cfg = [
'cfg(aes_backend_soft, values("compact"))',
'cfg(aes_backend, values("soft"))',
'cfg(cpubits, values("16", "32", "64"))'
'cfg(cpubits, values("16", "32", "64"))',
'cfg(__enable_aes_soft_backend)'
]

[package.metadata.docs.rs]
Expand Down
30 changes: 30 additions & 0 deletions aes/build.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,30 @@
use std::collections::HashSet;

fn main() {
let needs_soft = needs_soft();
if needs_soft {
println!("cargo::rustc-cfg=__enable_aes_soft_backend");
}
}

/// Returns whether the soft backend is required.
///
/// The soft backend may not be required if the target's features are
/// sufficient for one of the accelerated backends and the user did not
/// explicitly opt into the soft backend.
fn needs_soft() -> bool {
if std::env::var("CARGO_CFG_AES_BACKEND").is_ok_and(|value| value == "soft") {
return true;
}

let target_arch = std::env::var("CARGO_CFG_TARGET_ARCH").expect("missing target_arch");
let target_features = std::env::var("CARGO_CFG_TARGET_FEATURE").unwrap_or_default();
let target_features = target_features.split(',').collect::<HashSet<_>>();
let miri = std::env::var("CARGO_CFG_MIRI").is_ok();

match &*target_arch {
"x86" | "x86_64" => !target_features.contains("aes"),
"aarch64" if !miri => !target_features.contains("aes"),
_ => true,
}
}
1 change: 1 addition & 0 deletions aes/src/backends.rs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
#[cfg(__enable_aes_soft_backend)]
pub(crate) mod soft;

cpubits::cfg_if! {
Expand Down
48 changes: 42 additions & 6 deletions aes/src/hazmat.rs
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Expand Up @@ -11,6 +11,7 @@
//! We do NOT recommend using it to implement any algorithm which has not
//! received extensive peer review by cryptographers.

#[cfg(__enable_aes_soft_backend)]
use crate::backends::soft::hazmat as soft;

pub use crate::Block;
Expand Down Expand Up @@ -42,6 +43,17 @@ macro_rules! if_intrinsics_available {
}};
}

/// Execute the provided body if the soft backend is available.
macro_rules! if_soft_backend_available {
($body:expr) => {{
#[cfg(__enable_aes_soft_backend)]
if true {
$body;
return;
}
}};
}

/// ⚠️ AES cipher (encrypt) round function.
///
/// This API performs the following steps as described in FIPS 197 Appendix C:
Expand All @@ -62,7 +74,11 @@ pub fn cipher_round(block: &mut Block, round_key: &Block) {
intrinsics::cipher_round(block, round_key)
}

soft::cipher_round(block, round_key);
if_soft_backend_available! {
soft::cipher_round(block, round_key)
}

unreachable!();
}

/// ⚠️ AES cipher (encrypt) round function: parallel version.
Expand All @@ -79,7 +95,11 @@ pub fn cipher_round_par(blocks: &mut Block8, round_keys: &Block8) {
intrinsics::cipher_round_par(blocks, round_keys)
}

soft::cipher_round_par(blocks, round_keys);
if_soft_backend_available! {
soft::cipher_round_par(blocks, round_keys)
}

unreachable!();
}

/// ⚠️ AES equivalent inverse cipher (decrypt) round function.
Expand All @@ -102,7 +122,11 @@ pub fn equiv_inv_cipher_round(block: &mut Block, round_key: &Block) {
intrinsics::equiv_inv_cipher_round(block, round_key)
}

soft::equiv_inv_cipher_round(block, round_key);
if_soft_backend_available! {
soft::equiv_inv_cipher_round(block, round_key)
}

unreachable!();
}

/// ⚠️ AES equivalent inverse cipher (decrypt) round function: parallel version.
Expand All @@ -119,7 +143,11 @@ pub fn equiv_inv_cipher_round_par(blocks: &mut Block8, round_keys: &Block8) {
intrinsics::equiv_inv_cipher_round_par(blocks, round_keys)
}

soft::equiv_inv_cipher_round_par(blocks, round_keys);
if_soft_backend_available! {
soft::equiv_inv_cipher_round_par(blocks, round_keys)
}

unreachable!();
}

/// ⚠️ AES mix columns function.
Expand All @@ -133,7 +161,11 @@ pub fn mix_columns(block: &mut Block) {
intrinsics::mix_columns(block)
}

soft::mix_columns(block);
if_soft_backend_available! {
soft::mix_columns(block)
}

unreachable!();
}

/// ⚠️ AES inverse mix columns function.
Expand All @@ -149,5 +181,9 @@ pub fn inv_mix_columns(block: &mut Block) {
intrinsics::inv_mix_columns(block)
}

soft::inv_mix_columns(block);
if_soft_backend_available! {
soft::inv_mix_columns(block)
}

unreachable!();
}
53 changes: 40 additions & 13 deletions aes/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -247,9 +247,15 @@ macro_rules! impl_key_init {
}
}

let soft = backends::soft::$soft_name::new(key);
let inner = Inner { soft };
Self { inner, token }
cfg_if! {
if #[cfg(__enable_aes_soft_backend)] {
let soft = backends::soft::$soft_name::new(key);
let inner = Inner { soft };
return Self { inner, token };
} else {
unreachable!();
}
}
}
}
};
Expand Down Expand Up @@ -297,9 +303,15 @@ macro_rules! impl_encrypt {
}
}

// SAFETY: we access correct union variant
let backend = unsafe { &self.inner.soft };
f.call(backend);
cfg_if! {
if #[cfg(__enable_aes_soft_backend)] {
// SAFETY: we access correct union variant
let backend = unsafe { &self.inner.soft };
f.call(backend);
} else {
unreachable!();
}
}
}
}
};
Expand Down Expand Up @@ -347,9 +359,15 @@ macro_rules! impl_decrypt {
}
}

// SAFETY: we access correct union variant
let backend = unsafe { &self.inner.soft };
f.call(backend);
cfg_if! {
if #[cfg(__enable_aes_soft_backend)] {
// SAFETY: we access correct union variant
let backend = unsafe { &self.inner.soft };
f.call(backend);
} else {
unreachable!();
}
}
}
}
};
Expand Down Expand Up @@ -387,10 +405,16 @@ macro_rules! impl_from_enc {
}
}

// SAFETY: we access correct union variant
let soft = unsafe { enc.inner.soft };
let inner = Inner { soft };
Self { inner, token }
cfg_if! {
if #[cfg(__enable_aes_soft_backend)] {
// SAFETY: we access correct union variant
let soft = unsafe { enc.inner.soft };
let inner = Inner { soft };
Self { inner, token }
} else {
unreachable!();
}
}
}
}

Expand Down Expand Up @@ -457,6 +481,7 @@ macro_rules! define_aes_impl {
pub(super) aes: backends::x86_aes::$name,
#[cfg(all(target_arch = "aarch64", not(miri), not(aes_backend = "soft")))]
pub(super) aes: backends::aarch64_aes::$name,
#[cfg(__enable_aes_soft_backend)]
pub(super) soft: backends::soft::$name,
}

Expand All @@ -469,6 +494,7 @@ macro_rules! define_aes_impl {
pub(super) aes: backends::x86_aes::$name_enc,
#[cfg(all(target_arch = "aarch64", not(miri), not(aes_backend = "soft")))]
pub(super) aes: backends::aarch64_aes::$name_enc,
#[cfg(__enable_aes_soft_backend)]
pub(super) soft: backends::soft::$name,
}

Expand All @@ -481,6 +507,7 @@ macro_rules! define_aes_impl {
pub(super) aes: backends::x86_aes::$name_dec,
#[cfg(all(target_arch = "aarch64", not(miri), not(aes_backend = "soft")))]
pub(super) aes: backends::aarch64_aes::$name_dec,
#[cfg(__enable_aes_soft_backend)]
pub(super) soft: backends::soft::$name,
}
}
Expand Down