[PATCH v3 1/2] rust: crypto: add AES-128, AES-CMAC, SHA-256, and HMAC bindings

[PATCH v3 1/2] rust: crypto: add AES-128, AES-CMAC, SHA-256, and HMAC bindings

[Mike Lothian]

Expose the synchronous lib/crypto AES-128, AES-CMAC, SHA-256, and
HMAC-SHA256 primitives through safe Rust APIs.

Aes128 prepares the key schedule once and reuses it for block encryption
and CMAC. The one-shot hash and HMAC helpers operate on slices, and
every API uses fixed-size outputs. C shims cover interfaces that bindgen
cannot represent and clear temporary key material before returning.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
---
 rust/bindings/bindings_helper.h |   3 +
 rust/helpers/crypto.c           |  37 ++++++++++
 rust/helpers/helpers.c          |   1 +
 rust/kernel/crypto.rs           | 115 ++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   1 +
 5 files changed, 157 insertions(+)
 create mode 100644 rust/helpers/crypto.c
 create mode 100644 rust/kernel/crypto.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 2d079f278a04..8d7489b8cce8 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -28,6 +28,9 @@
  */
 #include <linux/hrtimer_types.h>
 
+#include <crypto/aes.h>
+#include <crypto/sha2.h>
+
 #include <linux/acpi.h>
 #include <linux/gpu_buddy.h>
 #include <drm/drm_device.h>
diff --git a/rust/helpers/crypto.c b/rust/helpers/crypto.c
new file mode 100644
index 000000000000..a18780231ce0
--- /dev/null
+++ b/rust/helpers/crypto.c
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <crypto/aes.h>
+#include <crypto/aes-cbc-macs.h>
+#include <linux/string.h>
+
+/*
+ * aes_encrypt() takes a transparent union (aes_encrypt_arg) that bindgen cannot
+ * express, so the single-block encrypt step is wrapped here. The key schedule
+ * is prepared once (aes_prepareenckey() is a plain extern bound directly) and
+ * the resulting struct aes_enckey is reused across blocks by the caller, so the
+ * key is not re-expanded per block. SHA-256 and HMAC-SHA256 are plain extern
+ * functions and are bound directly.
+ */
+__rust_helper void
+rust_helper_aes_enckey_encrypt_block(const struct aes_enckey *key, u8 *out,
+				     const u8 *in)
+{
+	aes_encrypt(key, out, in);
+}
+
+/*
+ * AES-CMAC one-shot over the in-tree library (crypto/aes-cbc-macs.h): prepares
+ * the 128-bit key, MACs @data and writes the 16-byte tag to @out. A helper
+ * because both aes_cmac_preparekey()'s struct and the aes_cmac() one-shot are
+ * not expressible from Rust directly. The key length is fixed at 128 bits, so
+ * aes_cmac_preparekey() cannot fail; the prepared key is wiped before return.
+ */
+__rust_helper void
+rust_helper_aes_cmac(const u8 *key, const u8 *data, size_t data_len, u8 *out)
+{
+	struct aes_cmac_key cmac_key;
+
+	aes_cmac_preparekey(&cmac_key, key, AES_KEYSIZE_128);
+	aes_cmac(&cmac_key, data, data_len, out);
+	memzero_explicit(&cmac_key, sizeof(cmac_key));
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 0d85b5e68ec2..cb7c668bffa2 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -56,6 +56,7 @@
 #include "cpufreq.c"
 #include "cpumask.c"
 #include "cred.c"
+#include "crypto.c"
 #include "device.c"
 #include "dma.c"
 #include "dma-resv.c"
diff --git a/rust/kernel/crypto.rs b/rust/kernel/crypto.rs
new file mode 100644
index 000000000000..5f7c301b2bb8
--- /dev/null
+++ b/rust/kernel/crypto.rs
@@ -0,0 +1,115 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Safe wrappers over the kernel's synchronous library crypto.
+//!
+//! Exposes the one-shot `lib/crypto` primitives — AES-128 (an [`Aes128`] key
+//! prepared once for single-block encryption, the building block for modes the
+//! library does not yet provide such as AES-CTR), the in-tree AES-CMAC
+//! ([`aes_cmac`]), SHA-256 and HMAC-SHA256 — for use from Rust. They run
+//! synchronously in the calling context with no allocation; the hashes and the
+//! MAC are infallible.
+//!
+//! C headers: [`include/crypto/aes.h`](srctree/include/crypto/aes.h),
+//! [`include/crypto/aes-cbc-macs.h`](srctree/include/crypto/aes-cbc-macs.h),
+//! [`include/crypto/sha2.h`](srctree/include/crypto/sha2.h).
+
+use crate::{bindings, error::to_result, prelude::*};
+
+/// Size of a SHA-256 / HMAC-SHA256 digest, in bytes.
+pub const SHA256_DIGEST_SIZE: usize = 32;
+/// AES-128 block and key size, in bytes.
+pub const AES128_BLOCK_SIZE: usize = 16;
+
+/// Returns the SHA-256 digest of `data`.
+pub fn sha256(data: &[u8]) -> [u8; SHA256_DIGEST_SIZE] {
+    let mut out = [0u8; SHA256_DIGEST_SIZE];
+    // SAFETY: `data` is valid for `data.len()` reads and `out` is a valid
+    // `SHA256_DIGEST_SIZE`-byte output buffer, as `sha256()` requires.
+    unsafe { bindings::sha256(data.as_ptr(), data.len(), out.as_mut_ptr()) };
+    out
+}
+
+/// Returns `HMAC-SHA256(key, data)`.
+pub fn hmac_sha256(key: &[u8], data: &[u8]) -> [u8; SHA256_DIGEST_SIZE] {
+    let mut out = [0u8; SHA256_DIGEST_SIZE];
+    // SAFETY: `key` and `data` are valid for their respective lengths and `out`
+    // is a valid `SHA256_DIGEST_SIZE`-byte output buffer, as required.
+    unsafe {
+        bindings::hmac_sha256_usingrawkey(
+            key.as_ptr(),
+            key.len(),
+            data.as_ptr(),
+            data.len(),
+            out.as_mut_ptr(),
+        )
+    };
+    out
+}
+
+/// Returns `AES-CMAC-128(key, data)` (RFC 4493), computed by the in-tree
+/// AES-CMAC library ([`include/crypto/aes-cbc-macs.h`]). The 128-bit key is
+/// prepared and wiped internally; the call is infallible.
+///
+/// [`include/crypto/aes-cbc-macs.h`]: srctree/include/crypto/aes-cbc-macs.h
+pub fn aes_cmac(key: &[u8; AES128_BLOCK_SIZE], data: &[u8]) -> [u8; AES128_BLOCK_SIZE] {
+    let mut out = [0u8; AES128_BLOCK_SIZE];
+    // SAFETY: `key` is a valid 16-byte key, `data` is valid for `data.len()`
+    // reads, and `out` is a valid `AES128_BLOCK_SIZE`-byte output buffer, as the
+    // helper requires.
+    unsafe { bindings::aes_cmac(key.as_ptr(), data.as_ptr(), data.len(), out.as_mut_ptr()) };
+    out
+}
+
+/// An AES-128 key, expanded once for single-block encryption.
+///
+/// The key schedule is computed in [`Aes128::new`] and reused across every
+/// [`encrypt_block`](Aes128::encrypt_block) call, so encrypting a stream of
+/// blocks (e.g. an AES-CTR keystream) does not re-expand the key per block. This
+/// is a low-level building block: prefer a full mode of operation where the
+/// library provides one (see [`aes_cmac`]); the bare block cipher is here only
+/// for modes `lib/crypto` does not yet expose, such as AES-CTR.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::crypto::Aes128;
+/// let cipher = Aes128::new(&[0u8; 16])?;
+/// let _ct = cipher.encrypt_block(&[0u8; 16]);
+/// # Ok::<(), Error>(())
+/// ```
+pub struct Aes128(bindings::aes_enckey);
+
+impl Aes128 {
+    /// Expands an AES-128 key from 16 raw key bytes.
+    pub fn new(key: &[u8; AES128_BLOCK_SIZE]) -> Result<Self> {
+        // SAFETY: `aes_enckey` is a plain-old-data key schedule (integer arrays
+        // in a union of integer arrays); an all-zero bit pattern is a valid,
+        // inert initial value, fully overwritten by `aes_prepareenckey()` below.
+        let mut enckey: bindings::aes_enckey = unsafe { core::mem::zeroed() };
+        // SAFETY: `enckey` is a valid, owned `aes_enckey`; `key` is a valid
+        // 16-byte buffer; `AES128_BLOCK_SIZE` (16) is a supported key length.
+        let ret =
+            unsafe { bindings::aes_prepareenckey(&mut enckey, key.as_ptr(), AES128_BLOCK_SIZE) };
+        to_result(ret)?;
+        Ok(Self(enckey))
+    }
+
+    /// Encrypts one 16-byte block with the prepared key: returns
+    /// `AES-128-ECB(key, block)`.
+    pub fn encrypt_block(&self, block: &[u8; AES128_BLOCK_SIZE]) -> [u8; AES128_BLOCK_SIZE] {
+        let mut out = [0u8; AES128_BLOCK_SIZE];
+        // SAFETY: `self.0` is a prepared encryption key; `block` and `out` are
+        // valid 16-byte buffers, as the helper requires.
+        unsafe { bindings::aes_enckey_encrypt_block(&self.0, out.as_mut_ptr(), block.as_ptr()) };
+        out
+    }
+}
+
+impl Drop for Aes128 {
+    fn drop(&mut self) {
+        // SAFETY: `self.0` is a valid, owned `aes_enckey`; overwriting it with
+        // an all-zero `aes_enckey` clears the expanded key schedule.
+        // `write_volatile` keeps the store from being optimised away.
+        unsafe { core::ptr::write_volatile(&mut self.0, core::mem::zeroed()) };
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index d7ced2a4c11f..3c45e4730646 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -62,6 +62,7 @@
 pub mod cpufreq;
 pub mod cpumask;
 pub mod cred;
+pub mod crypto;
 pub mod debugfs;
 pub mod device;
 pub mod device_id;

Re: [PATCH v3 1/2] rust: crypto: add AES-128, AES-CMAC, SHA-256, and HMAC bindings

[Eric Biggers]

On Wed, Aug 26, 2026 at 05:29:48PM +0100, Mike Lothian wrote:
> +//! Safe wrappers over the kernel's synchronous library crypto.
> +//!
> +//! Exposes the one-shot `lib/crypto` primitives — AES-128 (an [`Aes128`] key
> +//! prepared once for single-block encryption, the building block for modes the
> +//! library does not yet provide such as AES-CTR)

There's an aes_ctr() function now.

With that, is bare AES still needed?

- Eric

[PATCH v3 2/2] rust: crypto: add synchronous RSA akcipher support

[Mike Lothian]

Add an RAII Rust wrapper for RSA public keys backed by the kernel
crypto_akcipher implementation. Encode modulus and exponent components
in the PKCS#1 DER form accepted by the existing RSA transform and
expose synchronous public-key encryption.

Provide RSAES-OAEP with SHA-256 for consumers which need that standard
encoding, while keeping the OAEP seed explicit for use with the kernel
CSPRNG and deterministic known-answer tests. Wipe encoded messages,
expanded AES keys, and Secret byte strings with memzero_explicit().

Gate the AES, SHA-256, and akcipher bindings with dedicated Rust
Kconfig symbols. Cover the DER encoder and OAEP path with a published
Wycheproof vector.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
---
 crypto/Kconfig                  |  10 ++
 lib/crypto/Kconfig              |  19 ++
 rust/bindings/bindings_helper.h |   1 +
 rust/helpers/crypto.c           |  27 +++
 rust/kernel/Kconfig.test        |  14 ++
 rust/kernel/crypto.rs           |  72 +++++++-
 rust/kernel/crypto/akcipher.rs  | 298 ++++++++++++++++++++++++++++++++
 7 files changed, 435 insertions(+), 6 deletions(-)
 create mode 100644 rust/kernel/crypto/akcipher.rs

diff --git a/crypto/Kconfig b/crypto/Kconfig
index f1e372195273..9b7cfdbef7c4 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -116,6 +116,16 @@ config CRYPTO_AKCIPHER
 	select CRYPTO_AKCIPHER2
 	select CRYPTO_ALGAPI
 
+config RUST_CRYPTO_AKCIPHER
+	bool
+	depends on RUST
+	select CRYPTO_AKCIPHER
+	help
+	  Enable safe Rust abstractions for the public-key cipher API. The
+	  crypto API core is built into the kernel because Rust abstractions
+	  are part of the built-in kernel crate; individual algorithms may
+	  remain modules.
+
 config CRYPTO_KPP2
 	tristate
 	select CRYPTO_ALGAPI2
diff --git a/lib/crypto/Kconfig b/lib/crypto/Kconfig
index 591c1c2a7fb3..9d47fce62f54 100644
--- a/lib/crypto/Kconfig
+++ b/lib/crypto/Kconfig
@@ -36,6 +36,16 @@ config CRYPTO_LIB_AES_CBC_MACS
 	  this if your module uses any of the functions from
 	  <crypto/aes-cbc-macs.h>.
 
+config RUST_CRYPTO_LIB_AES
+	bool
+	depends on RUST
+	select CRYPTO_LIB_AES
+	select CRYPTO_LIB_AES_CBC_MACS
+	help
+	  Enable the Rust bindings for the synchronous AES library functions.
+	  The selected C libraries are built into the kernel because Rust
+	  abstractions are part of the built-in kernel crate.
+
 config CRYPTO_LIB_AESGCM
 	tristate
 	select CRYPTO_LIB_AES
@@ -216,6 +226,15 @@ config CRYPTO_LIB_SHA256
 	  Select this if your module uses any of these functions from
 	  <crypto/sha2.h>.
 
+config RUST_CRYPTO_LIB_SHA256
+	bool
+	depends on RUST
+	select CRYPTO_LIB_SHA256
+	help
+	  Enable the Rust bindings for the synchronous SHA-256 and HMAC-SHA256
+	  library functions. The selected C library is built into the kernel
+	  because Rust abstractions are part of the built-in kernel crate.
+
 config CRYPTO_LIB_SHA256_ARCH
 	bool
 	depends on CRYPTO_LIB_SHA256 && !UML
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 8d7489b8cce8..bb46317898c1 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -28,6 +28,7 @@
  */
 #include <linux/hrtimer_types.h>
 
+#include <crypto/akcipher.h>
 #include <crypto/aes.h>
 #include <crypto/sha2.h>
 
diff --git a/rust/helpers/crypto.c b/rust/helpers/crypto.c
index a18780231ce0..f0cdf7fa8a76 100644
--- a/rust/helpers/crypto.c
+++ b/rust/helpers/crypto.c
@@ -1,9 +1,30 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#include <crypto/akcipher.h>
 #include <crypto/aes.h>
 #include <crypto/aes-cbc-macs.h>
 #include <linux/string.h>
 
+__rust_helper void rust_helper_memzero_explicit(void *s, size_t count)
+{
+	memzero_explicit(s, count);
+}
+
+#ifdef CONFIG_RUST_CRYPTO_AKCIPHER
+__rust_helper void rust_helper_crypto_free_akcipher(struct crypto_akcipher *tfm)
+{
+	crypto_free_akcipher(tfm);
+}
+
+__rust_helper int
+rust_helper_crypto_akcipher_set_pub_key(struct crypto_akcipher *tfm,
+					const void *key, unsigned int key_len)
+{
+	return crypto_akcipher_set_pub_key(tfm, key, key_len);
+}
+#endif
+
+#ifdef CONFIG_RUST_CRYPTO_LIB_AES
 /*
  * aes_encrypt() takes a transparent union (aes_encrypt_arg) that bindgen cannot
  * express, so the single-block encrypt step is wrapped here. The key schedule
@@ -19,6 +40,11 @@ rust_helper_aes_enckey_encrypt_block(const struct aes_enckey *key, u8 *out,
 	aes_encrypt(key, out, in);
 }
 
+__rust_helper void rust_helper_aes_enckey_zero(struct aes_enckey *key)
+{
+	memzero_explicit(key, sizeof(*key));
+}
+
 /*
  * AES-CMAC one-shot over the in-tree library (crypto/aes-cbc-macs.h): prepares
  * the 128-bit key, MACs @data and writes the 16-byte tag to @out. A helper
@@ -35,3 +61,4 @@ rust_helper_aes_cmac(const u8 *key, const u8 *data, size_t data_len, u8 *out)
 	aes_cmac(&cmac_key, data, data_len, out);
 	memzero_explicit(&cmac_key, sizeof(cmac_key));
 }
+#endif
diff --git a/rust/kernel/Kconfig.test b/rust/kernel/Kconfig.test
index e6a5c7a795f0..32e1a0c17d08 100644
--- a/rust/kernel/Kconfig.test
+++ b/rust/kernel/Kconfig.test
@@ -83,4 +83,18 @@ config RUST_BITFIELD_KUNIT_TEST
 
 	  If unsure, say N.
 
+config RUST_CRYPTO_KUNIT_TEST
+	bool "KUnit tests for Rust crypto APIs" if !KUNIT_ALL_TESTS
+	default KUNIT_ALL_TESTS
+	select CRYPTO_RSA
+	select RUST_CRYPTO_AKCIPHER
+	select RUST_CRYPTO_LIB_SHA256
+	help
+	  This option enables KUnit tests for the safe Rust crypto APIs,
+	  including public-key encoding and encryption. The tests include
+	  published known-answer vectors and are intended for development and
+	  testing rather than regular kernel use cases.
+
+	  If unsure, say N.
+
 endif
diff --git a/rust/kernel/crypto.rs b/rust/kernel/crypto.rs
index 5f7c301b2bb8..0cf38541c8d1 100644
--- a/rust/kernel/crypto.rs
+++ b/rust/kernel/crypto.rs
@@ -9,18 +9,75 @@
 //! synchronously in the calling context with no allocation; the hashes and the
 //! MAC are infallible.
 //!
-//! C headers: [`include/crypto/aes.h`](srctree/include/crypto/aes.h),
+//! Public-key ciphers are available through [`akcipher`] when
+//! `CONFIG_RUST_CRYPTO_AKCIPHER` is enabled.
+//!
+//! C headers: [`include/crypto/akcipher.h`](srctree/include/crypto/akcipher.h),
+//! [`include/crypto/aes.h`](srctree/include/crypto/aes.h),
 //! [`include/crypto/aes-cbc-macs.h`](srctree/include/crypto/aes-cbc-macs.h),
 //! [`include/crypto/sha2.h`](srctree/include/crypto/sha2.h).
 
-use crate::{bindings, error::to_result, prelude::*};
+use core::ops::{Deref, DerefMut};
+
+use crate::bindings;
+#[cfg(any(
+    CONFIG_RUST_CRYPTO_AKCIPHER,
+    CONFIG_RUST_CRYPTO_LIB_AES,
+    CONFIG_RUST_CRYPTO_LIB_SHA256
+))]
+use crate::{error::to_result, prelude::*};
+
+#[cfg(CONFIG_RUST_CRYPTO_AKCIPHER)]
+pub mod akcipher;
 
 /// Size of a SHA-256 / HMAC-SHA256 digest, in bytes.
 pub const SHA256_DIGEST_SIZE: usize = 32;
 /// AES-128 block and key size, in bytes.
 pub const AES128_BLOCK_SIZE: usize = 16;
 
+/// A fixed-size byte string which is wiped when dropped.
+///
+/// This is intended for cryptographic keys and other sensitive intermediate
+/// values. Borrowing the contained bytes can still create copies which this
+/// type cannot track; callers should avoid copying them unnecessarily.
+pub struct Secret<const N: usize>([u8; N]);
+
+impl<const N: usize> Secret<N> {
+    /// Wraps bytes which should be wiped when their owner is dropped.
+    pub const fn new(bytes: [u8; N]) -> Self {
+        Self(bytes)
+    }
+
+    /// Creates an all-zero byte string.
+    pub const fn zeroed() -> Self {
+        Self([0; N])
+    }
+}
+
+impl<const N: usize> Deref for Secret<N> {
+    type Target = [u8; N];
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<const N: usize> DerefMut for Secret<N> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<const N: usize> Drop for Secret<N> {
+    fn drop(&mut self) {
+        // SAFETY: `self.0` is valid for exactly `N` writable bytes. The helper
+        // uses `memzero_explicit()`, so the wipe is not optimised away.
+        unsafe { bindings::memzero_explicit(self.0.as_mut_ptr().cast(), N) };
+    }
+}
+
 /// Returns the SHA-256 digest of `data`.
+#[cfg(CONFIG_RUST_CRYPTO_LIB_SHA256)]
 pub fn sha256(data: &[u8]) -> [u8; SHA256_DIGEST_SIZE] {
     let mut out = [0u8; SHA256_DIGEST_SIZE];
     // SAFETY: `data` is valid for `data.len()` reads and `out` is a valid
@@ -30,6 +87,7 @@
 }
 
 /// Returns `HMAC-SHA256(key, data)`.
+#[cfg(CONFIG_RUST_CRYPTO_LIB_SHA256)]
 pub fn hmac_sha256(key: &[u8], data: &[u8]) -> [u8; SHA256_DIGEST_SIZE] {
     let mut out = [0u8; SHA256_DIGEST_SIZE];
     // SAFETY: `key` and `data` are valid for their respective lengths and `out`
@@ -51,6 +109,7 @@
 /// prepared and wiped internally; the call is infallible.
 ///
 /// [`include/crypto/aes-cbc-macs.h`]: srctree/include/crypto/aes-cbc-macs.h
+#[cfg(CONFIG_RUST_CRYPTO_LIB_AES)]
 pub fn aes_cmac(key: &[u8; AES128_BLOCK_SIZE], data: &[u8]) -> [u8; AES128_BLOCK_SIZE] {
     let mut out = [0u8; AES128_BLOCK_SIZE];
     // SAFETY: `key` is a valid 16-byte key, `data` is valid for `data.len()`
@@ -77,8 +136,10 @@
 /// let _ct = cipher.encrypt_block(&[0u8; 16]);
 /// # Ok::<(), Error>(())
 /// ```
+#[cfg(CONFIG_RUST_CRYPTO_LIB_AES)]
 pub struct Aes128(bindings::aes_enckey);
 
+#[cfg(CONFIG_RUST_CRYPTO_LIB_AES)]
 impl Aes128 {
     /// Expands an AES-128 key from 16 raw key bytes.
     pub fn new(key: &[u8; AES128_BLOCK_SIZE]) -> Result<Self> {
@@ -105,11 +166,10 @@ impl Aes128 {
     }
 }
 
+#[cfg(CONFIG_RUST_CRYPTO_LIB_AES)]
 impl Drop for Aes128 {
     fn drop(&mut self) {
-        // SAFETY: `self.0` is a valid, owned `aes_enckey`; overwriting it with
-        // an all-zero `aes_enckey` clears the expanded key schedule.
-        // `write_volatile` keeps the store from being optimised away.
-        unsafe { core::ptr::write_volatile(&mut self.0, core::mem::zeroed()) };
+        // SAFETY: `self.0` is a valid, owned `aes_enckey`.
+        unsafe { bindings::aes_enckey_zero(&mut self.0) };
     }
 }
diff --git a/rust/kernel/crypto/akcipher.rs b/rust/kernel/crypto/akcipher.rs
new file mode 100644
index 000000000000..c5fc2ca0cfbe
--- /dev/null
+++ b/rust/kernel/crypto/akcipher.rs
@@ -0,0 +1,298 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Safe wrappers for the kernel public-key cipher API.
+//!
+//! C header: [`include/crypto/akcipher.h`](srctree/include/crypto/akcipher.h)
+
+use core::ptr::NonNull;
+
+use crate::{
+    alloc::{Flags, KVec},
+    bindings, c_str,
+    crypto::{sha256, SHA256_DIGEST_SIZE},
+    error::{from_err_ptr, to_result},
+    prelude::*,
+};
+
+/// A configured RSA public key.
+///
+/// The key is backed by the existing kernel `"rsa"` akcipher implementation.
+/// Creating it converts unsigned big-endian modulus and exponent components
+/// into the PKCS#1 DER form consumed by the crypto API.
+pub struct RsaPublicKey {
+    tfm: NonNull<bindings::crypto_akcipher>,
+    size: usize,
+}
+
+impl RsaPublicKey {
+    /// Create an RSA public key from unsigned big-endian components.
+    pub fn new(modulus: &[u8], exponent: &[u8], flags: Flags) -> Result<Self> {
+        let modulus = trim_unsigned(modulus).ok_or(EINVAL)?;
+        let exponent = trim_unsigned(exponent).ok_or(EINVAL)?;
+        let der = encode_rsa_public_key(modulus, exponent, flags)?;
+
+        // SAFETY: The name is NUL-terminated and remains live for the call.
+        let tfm = from_err_ptr(unsafe {
+            bindings::crypto_alloc_akcipher(c_str!("rsa").as_char_ptr(), 0, 0)
+        })?;
+        let tfm = NonNull::new(tfm).ok_or(ENOMEM)?;
+
+        // SAFETY: `tfm` is a live akcipher transform and `der` contains
+        // `der.len()` initialized bytes.
+        let result = to_result(unsafe {
+            bindings::crypto_akcipher_set_pub_key(
+                tfm.as_ptr(),
+                der.as_ptr().cast(),
+                der.len().try_into()?,
+            )
+        });
+        if let Err(err) = result {
+            // SAFETY: `tfm` was returned by `crypto_alloc_akcipher()` and has
+            // not been freed.
+            unsafe { bindings::crypto_free_akcipher(tfm.as_ptr()) };
+            return Err(err);
+        }
+
+        Ok(Self {
+            tfm,
+            size: modulus.len(),
+        })
+    }
+
+    /// Return the RSA modulus size in bytes.
+    pub fn size(&self) -> usize {
+        self.size
+    }
+
+    /// Apply the raw RSA public-key operation to one already-encoded message.
+    ///
+    /// Both buffers must have the modulus size. The output is fixed-width,
+    /// unsigned, and big-endian. Prefer a padded scheme such as
+    /// [`oaep_sha256_encrypt`](Self::oaep_sha256_encrypt).
+    pub fn encrypt(&mut self, encoded: &[u8], out: &mut [u8]) -> Result {
+        if encoded.len() != self.size || out.len() != self.size {
+            return Err(EINVAL);
+        }
+
+        out.fill(0);
+        // SAFETY: `self.tfm` remains live and exclusively borrowed for the
+        // synchronous operation; both buffers are valid for their lengths.
+        to_result(unsafe {
+            bindings::crypto_akcipher_sync_encrypt(
+                self.tfm.as_ptr(),
+                encoded.as_ptr().cast(),
+                encoded.len().try_into()?,
+                out.as_mut_ptr().cast(),
+                out.len().try_into()?,
+            )
+        })
+    }
+
+    /// Encrypt a message using RSAES-OAEP with SHA-256 and an empty label.
+    ///
+    /// `seed` is a caller-provided random OAEP seed. It is explicit so callers
+    /// can use the kernel CSPRNG while tests can use published deterministic
+    /// vectors.
+    #[cfg(CONFIG_RUST_CRYPTO_LIB_SHA256)]
+    pub fn oaep_sha256_encrypt(
+        &mut self,
+        message: &[u8],
+        seed: &[u8; SHA256_DIGEST_SIZE],
+        out: &mut [u8],
+        flags: Flags,
+    ) -> Result {
+        let overhead = 2 * SHA256_DIGEST_SIZE + 2;
+        if out.len() != self.size || self.size < overhead || message.len() > self.size - overhead {
+            return Err(EINVAL);
+        }
+
+        let mut encoded = KVec::from_elem(0u8, self.size, flags)?;
+        let result = (|| {
+            encoded[1..1 + SHA256_DIGEST_SIZE].copy_from_slice(seed);
+            let db = &mut encoded[1 + SHA256_DIGEST_SIZE..];
+            db[..SHA256_DIGEST_SIZE].copy_from_slice(&sha256(&[]));
+            let separator = db.len() - message.len() - 1;
+            db[separator] = 1;
+            db[separator + 1..].copy_from_slice(message);
+
+            mgf1_sha256_xor(seed, db, flags)?;
+            let (seed_block, masked_db) = encoded[1..].split_at_mut(SHA256_DIGEST_SIZE);
+            mgf1_sha256_xor(masked_db, seed_block, flags)?;
+
+            self.encrypt(&encoded, out)
+        })();
+        encoded.fill(0);
+        result
+    }
+}
+
+impl Drop for RsaPublicKey {
+    fn drop(&mut self) {
+        // SAFETY: `self.tfm` was returned by `crypto_alloc_akcipher()` and is
+        // owned by this object.
+        unsafe { bindings::crypto_free_akcipher(self.tfm.as_ptr()) };
+    }
+}
+
+#[cfg(CONFIG_RUST_CRYPTO_LIB_SHA256)]
+fn mgf1_sha256_xor(seed: &[u8], output: &mut [u8], flags: Flags) -> Result {
+    let mut input = KVec::with_capacity(seed.len().checked_add(4).ok_or(EOVERFLOW)?, flags)?;
+    let result = (|| {
+        let mut counter = 0u32;
+        for chunk in output.chunks_mut(SHA256_DIGEST_SIZE) {
+            input.clear();
+            input.extend_from_slice(seed, flags)?;
+            input.extend_from_slice(&counter.to_be_bytes(), flags)?;
+            let digest = sha256(&input);
+            for (byte, mask) in chunk.iter_mut().zip(digest) {
+                *byte ^= mask;
+            }
+            counter = counter.checked_add(1).ok_or(EOVERFLOW)?;
+        }
+        Ok(())
+    })();
+    input.fill(0);
+    result
+}
+
+fn trim_unsigned(value: &[u8]) -> Option<&[u8]> {
+    let value = value
+        .iter()
+        .position(|byte| *byte != 0)
+        .map(|i| &value[i..])?;
+    Some(value)
+}
+
+fn der_length_size(length: usize) -> Result<usize> {
+    if length < 128 {
+        return Ok(1);
+    }
+
+    let bytes = (usize::BITS - length.leading_zeros()).div_ceil(8) as usize;
+    if bytes > 126 {
+        return Err(EOVERFLOW);
+    }
+    Ok(1 + bytes)
+}
+
+fn push_der_length(out: &mut KVec<u8>, length: usize, flags: Flags) -> Result {
+    if length < 128 {
+        out.push(length as u8, flags)?;
+        return Ok(());
+    }
+
+    let bytes = der_length_size(length)? - 1;
+    out.push(0x80 | bytes as u8, flags)?;
+    for shift in (0..bytes).rev() {
+        out.push((length >> (shift * 8)) as u8, flags)?;
+    }
+    Ok(())
+}
+
+fn der_integer_size(value: &[u8]) -> Result<usize> {
+    let leading_zero = usize::from(value[0] & 0x80 != 0);
+    1usize
+        .checked_add(der_length_size(value.len() + leading_zero)?)
+        .and_then(|size| size.checked_add(value.len() + leading_zero))
+        .ok_or(EOVERFLOW)
+}
+
+fn push_der_integer(out: &mut KVec<u8>, value: &[u8], flags: Flags) -> Result {
+    let leading_zero = value[0] & 0x80 != 0;
+    out.push(0x02, flags)?;
+    push_der_length(out, value.len() + usize::from(leading_zero), flags)?;
+    if leading_zero {
+        out.push(0, flags)?;
+    }
+    out.extend_from_slice(value, flags)?;
+    Ok(())
+}
+
+fn encode_rsa_public_key(modulus: &[u8], exponent: &[u8], flags: Flags) -> Result<KVec<u8>> {
+    let content_len = der_integer_size(modulus)?
+        .checked_add(der_integer_size(exponent)?)
+        .ok_or(EOVERFLOW)?;
+    let total_len = 1usize
+        .checked_add(der_length_size(content_len)?)
+        .and_then(|size| size.checked_add(content_len))
+        .ok_or(EOVERFLOW)?;
+    let mut der = KVec::with_capacity(total_len, flags)?;
+    der.push(0x30, flags)?;
+    push_der_length(&mut der, content_len, flags)?;
+    push_der_integer(&mut der, modulus, flags)?;
+    push_der_integer(&mut der, exponent, flags)?;
+    Ok(der)
+}
+
+#[cfg(CONFIG_RUST_CRYPTO_KUNIT_TEST)]
+#[crate::macros::kunit_tests(rust_kernel_crypto_akcipher)]
+mod tests {
+    use super::*;
+    use crate::alloc::flags::GFP_KERNEL;
+
+    // Wycheproof rsa_oaep_2048_sha256_mgf1sha256_test.json, test case 3.
+    const OAEP_MODULUS: [u8; 256] = [
+        0xa2, 0xb4, 0x51, 0xa0, 0x7d, 0x0a, 0xa5, 0xf9, 0x6e, 0x45, 0x56, 0x71, 0x51, 0x35, 0x50,
+        0x51, 0x4a, 0x8a, 0x5b, 0x46, 0x2e, 0xbe, 0xf7, 0x17, 0x09, 0x4f, 0xa1, 0xfe, 0xe8, 0x22,
+        0x24, 0xe6, 0x37, 0xf9, 0x74, 0x6d, 0x3f, 0x7c, 0xaf, 0xd3, 0x18, 0x78, 0xd8, 0x03, 0x25,
+        0xb6, 0xef, 0x5a, 0x17, 0x00, 0xf6, 0x59, 0x03, 0xb4, 0x69, 0x42, 0x9e, 0x89, 0xd6, 0xea,
+        0xc8, 0x84, 0x50, 0x97, 0xb5, 0xab, 0x39, 0x31, 0x89, 0xdb, 0x92, 0x51, 0x2e, 0xd8, 0xa7,
+        0x71, 0x1a, 0x12, 0x53, 0xfa, 0xcd, 0x20, 0xf7, 0x9c, 0x15, 0xe8, 0x24, 0x7f, 0x3d, 0x3e,
+        0x42, 0xe4, 0x6e, 0x48, 0xc9, 0x8e, 0x25, 0x4a, 0x2f, 0xe9, 0x76, 0x53, 0x13, 0xa0, 0x3e,
+        0xff, 0x8f, 0x17, 0xe1, 0xa0, 0x29, 0x39, 0x7a, 0x1f, 0xa2, 0x6a, 0x8d, 0xce, 0x26, 0xf4,
+        0x90, 0xed, 0x81, 0x29, 0x96, 0x15, 0xd9, 0x81, 0x4c, 0x22, 0xda, 0x61, 0x04, 0x28, 0xe0,
+        0x9c, 0x7d, 0x96, 0x58, 0x59, 0x42, 0x66, 0xf5, 0xc0, 0x21, 0xd0, 0xfc, 0xec, 0xa0, 0x8d,
+        0x94, 0x5a, 0x12, 0xbe, 0x82, 0xde, 0x4d, 0x1e, 0xce, 0x6b, 0x4c, 0x03, 0x14, 0x5b, 0x5d,
+        0x34, 0x95, 0xd4, 0xed, 0x54, 0x11, 0xeb, 0x87, 0x8d, 0xaf, 0x05, 0xfd, 0x7a, 0xfc, 0x3e,
+        0x09, 0xad, 0xa0, 0xf1, 0x12, 0x64, 0x22, 0xf5, 0x90, 0x97, 0x5a, 0x19, 0x69, 0x81, 0x6f,
+        0x48, 0x69, 0x8b, 0xcb, 0xba, 0x1b, 0x4d, 0x9c, 0xae, 0x79, 0xd4, 0x60, 0xd8, 0xf9, 0xf8,
+        0x5e, 0x79, 0x75, 0x00, 0x5d, 0x9b, 0xc2, 0x2c, 0x4e, 0x5a, 0xc0, 0xf7, 0xc1, 0xa4, 0x5d,
+        0x12, 0x56, 0x9a, 0x62, 0x80, 0x7d, 0x3b, 0x9a, 0x02, 0xe5, 0xa5, 0x30, 0xe7, 0x73, 0x06,
+        0x6f, 0x45, 0x3d, 0x1f, 0x5b, 0x4c, 0x2e, 0x9c, 0xf7, 0x82, 0x02, 0x83, 0xf7, 0x42, 0xb9,
+        0xd5,
+    ];
+    const OAEP_SEED: [u8; 32] = [
+        0x70, 0x97, 0x14, 0xb0, 0x48, 0xc3, 0x69, 0x73, 0x22, 0x69, 0xa3, 0xd8, 0xf9, 0x23, 0x02,
+        0x50, 0x87, 0x70, 0xa4, 0x43, 0x68, 0x01, 0x4b, 0x3a, 0x5c, 0xb1, 0x85, 0xc0, 0xc9, 0x1d,
+        0x97, 0x2c,
+    ];
+    const OAEP_CIPHERTEXT: [u8; 256] = [
+        0x5e, 0xab, 0x3f, 0x07, 0x41, 0xe6, 0x39, 0x86, 0xed, 0x64, 0x7d, 0x53, 0xe1, 0xcd, 0x71,
+        0xdf, 0x04, 0x19, 0x86, 0x90, 0x08, 0x03, 0xd0, 0xf9, 0x9c, 0x68, 0x35, 0x5d, 0x24, 0x9a,
+        0x15, 0xa4, 0x7d, 0xc5, 0xb4, 0xf7, 0x0a, 0x19, 0x14, 0x77, 0x65, 0x42, 0x99, 0xe5, 0xa2,
+        0x73, 0x1f, 0x3b, 0x4e, 0xec, 0x76, 0xde, 0xa1, 0x82, 0x62, 0xfc, 0x69, 0x6a, 0xc7, 0x94,
+        0xe5, 0xf6, 0x6c, 0xbf, 0xcd, 0xda, 0xc4, 0x47, 0x2c, 0x57, 0x8e, 0x24, 0x6c, 0x26, 0x70,
+        0x75, 0x98, 0x05, 0x55, 0x84, 0x54, 0x0b, 0x83, 0x98, 0x36, 0xb1, 0x40, 0x4c, 0x56, 0x11,
+        0xae, 0x55, 0x8a, 0x98, 0x4c, 0xee, 0x8f, 0xd0, 0x36, 0xce, 0xa9, 0x24, 0xe0, 0xbe, 0x24,
+        0x74, 0xa9, 0x40, 0xf6, 0x1e, 0x0a, 0xcc, 0x14, 0xfc, 0xae, 0x95, 0xeb, 0xdc, 0x59, 0x94,
+        0x2a, 0x9c, 0xe9, 0xaf, 0x9a, 0x9c, 0x81, 0x99, 0x9f, 0x7f, 0x68, 0x15, 0xf0, 0x57, 0xff,
+        0xdc, 0x25, 0x33, 0xcb, 0x15, 0xd6, 0x39, 0x1d, 0x1e, 0x2d, 0x95, 0xf1, 0x6f, 0x9c, 0x04,
+        0x20, 0x9c, 0x88, 0x9a, 0x4c, 0x35, 0x9c, 0x7d, 0x29, 0x26, 0xd2, 0x8a, 0x66, 0xe2, 0xb0,
+        0x30, 0xa4, 0x16, 0xb9, 0x28, 0xd2, 0x82, 0x56, 0x27, 0x99, 0x8e, 0x51, 0x91, 0xfb, 0x49,
+        0x83, 0xa6, 0xe6, 0x50, 0x24, 0x26, 0x2d, 0x94, 0xfc, 0x09, 0x18, 0x7a, 0x2d, 0x78, 0x16,
+        0x21, 0x22, 0x43, 0x32, 0x51, 0xd1, 0xbf, 0xcc, 0x8e, 0x50, 0x7d, 0x06, 0xeb, 0xa2, 0xd2,
+        0x29, 0xc1, 0x00, 0x31, 0x26, 0x1d, 0xa3, 0x2a, 0xb8, 0xcc, 0xd1, 0x5f, 0x1c, 0x5f, 0x9f,
+        0xbf, 0x07, 0xed, 0x15, 0x84, 0x83, 0xd7, 0x36, 0xa1, 0x10, 0xaf, 0x4b, 0x44, 0xd6, 0xa4,
+        0xda, 0x60, 0xd6, 0xcb, 0x51, 0x9b, 0x44, 0x54, 0x21, 0x3c, 0xf9, 0xf0, 0xdc, 0x56, 0x0f,
+        0x2b,
+    ];
+
+    #[test]
+    fn rsa_der_encoding() -> Result {
+        let der = encode_rsa_public_key(&[0x80, 0x01], &[0x01, 0x00, 0x01], GFP_KERNEL)?;
+        assert_eq!(
+            der.as_slice(),
+            &[0x30, 0x0a, 0x02, 0x03, 0x00, 0x80, 0x01, 0x02, 0x03, 0x01, 0x00, 0x01]
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn rsa_oaep_sha256_wycheproof() -> Result {
+        let mut key = RsaPublicKey::new(&OAEP_MODULUS, &[0x01, 0x00, 0x01], GFP_KERNEL)?;
+        let mut out = [0u8; 256];
+        key.oaep_sha256_encrypt(b"Test", &OAEP_SEED, &mut out, GFP_KERNEL)?;
+        assert_eq!(out, OAEP_CIPHERTEXT);
+        Ok(())
+    }
+}

Re: [PATCH v3 2/2] rust: crypto: add synchronous RSA akcipher support

[Eric Biggers]

On Wed, Aug 26, 2026 at 05:29:49PM +0100, Mike Lothian wrote:
> +config RUST_CRYPTO_LIB_AES
> +	bool
> +	depends on RUST
> +	select CRYPTO_LIB_AES
> +	select CRYPTO_LIB_AES_CBC_MACS
> +	help
> +	  Enable the Rust bindings for the synchronous AES library functions.
> +	  The selected C libraries are built into the kernel because Rust
> +	  abstractions are part of the built-in kernel crate.

This is being added in the wrong patch.

>  config CRYPTO_LIB_AESGCM
>  	tristate
>  	select CRYPTO_LIB_AES
> @@ -216,6 +226,15 @@ config CRYPTO_LIB_SHA256
>  	  Select this if your module uses any of these functions from
>  	  <crypto/sha2.h>.
>  
> +config RUST_CRYPTO_LIB_SHA256
> +	bool
> +	depends on RUST
> +	select CRYPTO_LIB_SHA256
> +	help
> +	  Enable the Rust bindings for the synchronous SHA-256 and HMAC-SHA256
> +	  library functions. The selected C library is built into the kernel
> +	  because Rust abstractions are part of the built-in kernel crate.

Likewise.

As I've been commenting on other of these bindings patches, it also
doesn't really make sense to have the kconfig symbol be in lib/ but then
have the actual code be in rust/.  They should be in the same place.

> +__rust_helper void rust_helper_memzero_explicit(void *s, size_t count)
> +{
> +	memzero_explicit(s, count);
> +}

Isn't there a standard Rust solution for this?

> +#ifdef CONFIG_RUST_CRYPTO_AKCIPHER
> +__rust_helper void rust_helper_crypto_free_akcipher(struct crypto_akcipher *tfm)
> +{
> +	crypto_free_akcipher(tfm);
> +}

If you need RSA, then please just create an API for RSA specifically.
The crypto_akcipher abstraction has never worked well, due to
differences between the algorithms and various other reasons.

> +__rust_helper void rust_helper_aes_enckey_zero(struct aes_enckey *key)
> +{
> +	memzero_explicit(key, sizeof(*key));
> +}

Similarly, isn't there a standard Rust solution to zeroize memory?

- Eric

Re: [PATCH v3 2/2] rust: crypto: add synchronous RSA akcipher support

[Miguel Ojeda]

On Thu, Aug 27, 2026 at 5:05 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> As I've been commenting on other of these bindings patches, it also
> doesn't really make sense to have the kconfig symbol be in lib/ but then
> have the actual code be in rust/.  They should be in the same place.

Did you see my reply at

  https://lore.kernel.org/rust-for-linux/CANiq72=uUR4Vo9W55Kq6tQjc+6Q4_+wiWhCo-VLdkvg6PJeoAw@mail.gmail.com/

?

But if you feel strongly about it, I guess it can be temporarily
placed elsewhere, i.e. Kconfig symbols are not tied to the path anyway
so they should be easy to move later on.

> > +__rust_helper void rust_helper_memzero_explicit(void *s, size_t count)
> > +{
> > +     memzero_explicit(s, count);
> > +}
>
> Isn't there a standard Rust solution for this?

If you mean a function in the standard library to zero memory without
being optimized out: no, there isn't.

For context: I added `memset_explicit` to the ISO C standard and
informally asked upstream Rust about adding an equivalent function
many years ago, but at least back then they didn't want to add it (or
at least in a way similar to how it is specified in C, which doesn't
really give guarantees, letting compiler writers do their best
effort).

I guess I can ask again since now we would finally have an actual user
in Linux... :)

I hope that helps!

Cheers,
Miguel

Re: [PATCH v3 2/2] rust: crypto: add synchronous RSA akcipher support

[Eric Biggers]

On Thu, Aug 27, 2026 at 04:46:55PM +0200, Miguel Ojeda wrote:
> On Thu, Aug 27, 2026 at 5:05 AM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > As I've been commenting on other of these bindings patches, it also
> > doesn't really make sense to have the kconfig symbol be in lib/ but then
> > have the actual code be in rust/.  They should be in the same place.
> 
> Did you see my reply at
> 
>   https://lore.kernel.org/rust-for-linux/CANiq72=uUR4Vo9W55Kq6tQjc+6Q4_+wiWhCo-VLdkvg6PJeoAw@mail.gmail.com/
> 
> ?
> 
> But if you feel strongly about it, I guess it can be temporarily
> placed elsewhere, i.e. Kconfig symbols are not tied to the path anyway
> so they should be easy to move later on.

Yes, it doesn't explain why they need to be inconsistent though.

And if the code is "conceptually part of the subsystem", why is the
MAINTAINERS entry for "CRYPTO LIBRARY" not being updated to include it?

FWIW: I do think the Rust bindings need to live alongside the code
itself and be maintained by the same people.

I just don't think it makes sense to mostly *not* be doing it that way,
but then also having the kconfig options randomly be different.  We
should at least be consistent.

> If you mean a function in the standard library to zero memory without
> being optimized out: no, there isn't.
> 
> For context: I added `memset_explicit` to the ISO C standard and
> informally asked upstream Rust about adding an equivalent function
> many years ago, but at least back then they didn't want to add it (or
> at least in a way similar to how it is specified in C, which doesn't
> really give guarantees, letting compiler writers do their best
> effort).
> 
> I guess I can ask again since now we would finally have an actual user
> in Linux... :)

Userspace crypto libraries need this too, so it's kind of surprising it
would just be coming up now.

- Eric

Re: [PATCH v3 2/2] rust: crypto: add synchronous RSA akcipher support

[Miguel Ojeda]

On Thu, Aug 27, 2026 at 8:29 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> Yes, it doesn't explain why they need to be inconsistent though.

It is simply because it is the end state that we want, i.e. both
Kconfig and source code to be in the usual folder (so less things to
move later), plus one normally wants the Kconfig options to show in
the right place in the menus.

> And if the code is "conceptually part of the subsystem", why is the
> MAINTAINERS entry for "CRYPTO LIBRARY" not being updated to include it?

I don't know, but it should be, indeed. That is what we have been
asking subsystems to do since the beginning (modulo exceptional
cases). Some take direct ownership, some request to have a maintainer
dedicated to that in the entry, some add a new sub-subsystem, etc.

I hope that clarifies.

Cheers,
Miguel

Re: [PATCH v3 2/2] rust: crypto: add synchronous RSA akcipher support

[Miguel Ojeda]

On Wed, Aug 26, 2026 at 6:30 PM Mike Lothian <mike@fireburn.co.uk> wrote:
>
> +__rust_helper void rust_helper_aes_enckey_zero(struct aes_enckey *key)
> +{
> +       memzero_explicit(key, sizeof(*key));
> +}

Hmm... why does this one need to be a Rust helper? i.e. why doesn't it
call the other helper from Rust?

(Same for the other calls I notice here in the context of the patch to
`memzero_explicit`.)

i.e. helpers are meant to be as minimal as possible -- just forwarders
to the same functions unless there is a good reason not to.

And then you can have a safe function abstracting the call and use
that from the different places you may need in Rust (e.g. from
`Secret` and `Aes128`).

Thanks!

Cheers,
Miguel