From 3d0fd474fb82572aff89a42101483d6bce293bab Mon Sep 17 00:00:00 2001
From: Tony Arcieri <bascule@gmail.com>
Date: Sat, 31 Jan 2026 17:56:58 -0700
Subject: [PATCH] frodo-kem: preliminary `no_std` support

The existing uses of `std` can probably be worked around, but for now
this allows the crate to compile in `no_std` contexts
---
 frodo-kem/Cargo.toml           |  4 +++-
 frodo-kem/src/hazmat/models.rs |  1 +
 frodo-kem/src/hazmat/traits.rs |  4 ++++
 frodo-kem/src/lib.rs           | 19 +++++++++++++++----
 4 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/frodo-kem/Cargo.toml b/frodo-kem/Cargo.toml
index 1ea8aa0..5384fca 100644
--- a/frodo-kem/Cargo.toml
+++ b/frodo-kem/Cargo.toml
@@ -19,6 +19,7 @@ default = [
     "frodo",
     "efrodo",
     "serde",
+    "std"
 ]
 hazmat = []
 efrodo = [
@@ -52,7 +53,8 @@ frodo1344shake = []
 openssl = ["openssl-aes", "openssl-shake"]
 openssl-aes = ["dep:openssl-sys"]
 openssl-shake = ["dep:openssl-sys"]
-serde = ["dep:hex", "dep:serde"]
+serde = ["std", "dep:hex", "dep:serde"]
+std = []
 
 [dependencies]
 aes = { version = "0.9.0-rc.2", optional = true }
diff --git a/frodo-kem/src/hazmat/models.rs b/frodo-kem/src/hazmat/models.rs
index 0208a44..1e6e86c 100644
--- a/frodo-kem/src/hazmat/models.rs
+++ b/frodo-kem/src/hazmat/models.rs
@@ -2,6 +2,7 @@
 
 use super::{Expanded, Kem, Params, Sample};
 use crate::{Error, FrodoResult};
+use alloc::{boxed::Box, vec::Vec};
 use core::marker::PhantomData;
 use zeroize::{Zeroize, ZeroizeOnDrop};
 
diff --git a/frodo-kem/src/hazmat/traits.rs b/frodo-kem/src/hazmat/traits.rs
index 342d905..cc271b1 100644
--- a/frodo-kem/src/hazmat/traits.rs
+++ b/frodo-kem/src/hazmat/traits.rs
@@ -4,6 +4,7 @@ use crate::hazmat::{
     Ciphertext, CiphertextRef, DecryptionKey, DecryptionKeyRef, EncryptionKey, EncryptionKeyRef,
     SharedSecret,
 };
+use alloc::{string::String, vec::Vec};
 use rand_core::CryptoRng;
 use sha3::digest::{ExtendableOutput, ExtendableOutputReset, Update};
 use subtle::{Choice, ConditionallySelectable};
@@ -27,6 +28,7 @@ pub trait Sample: Default {
 /// Expand the seed to produce the matrix A
 pub trait Expanded: Default {
     /// The method used to expand the seed
+    #[cfg_attr(not(feature = "std"), allow(dead_code))]
     const METHOD: &'static str;
     /// Expand the seed to produce the matrix A
     /// Generate matrix A (N x N) column-wise
@@ -106,6 +108,7 @@ pub trait Params: Sized + Default {
 /// The base FrodoKEM methods
 pub trait Kem: Params + Expanded + Sample {
     /// The name of the frodoKEM algorithm
+    #[cfg_attr(not(feature = "std"), allow(dead_code))]
     const NAME: &'static str;
 
     /// Generate a keypair
@@ -426,6 +429,7 @@ pub trait Kem: Params + Expanded + Sample {
     }
 
     /// Get the algorithm name
+    #[cfg_attr(not(feature = "std"), allow(dead_code))]
     fn algorithm(&self) -> String {
         format!("{}-{}-{}", Self::NAME, Self::N, Self::METHOD)
     }
diff --git a/frodo-kem/src/lib.rs b/frodo-kem/src/lib.rs
index 292bca4..6e100c3 100644
--- a/frodo-kem/src/lib.rs
+++ b/frodo-kem/src/lib.rs
@@ -1,4 +1,4 @@
-// #![no_std] TODO
+#![no_std]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![doc = include_str!("../README.md")]
 #![doc(
@@ -96,21 +96,29 @@
 )))]
 compile_error!("no algorithm feature enabled");
 
-mod error;
-pub use error::*;
+#[macro_use]
+extern crate alloc;
+#[cfg(feature = "std")]
+extern crate std;
 
 #[cfg(feature = "hazmat")]
 pub mod hazmat;
 #[cfg(not(feature = "hazmat"))]
 mod hazmat;
 
-use hazmat::*;
+mod error;
+pub use error::*;
 
+use alloc::vec::Vec;
 use core::marker::PhantomData;
+use hazmat::*;
 use rand_core::CryptoRng;
 use subtle::{Choice, ConstantTimeEq};
 use zeroize::{Zeroize, ZeroizeOnDrop};
 
+#[cfg(feature = "serde")]
+use alloc::string::{String, ToString};
+
 macro_rules! serde_impl {
     ($name:ident, $from_method:ident) => {
         #[cfg(feature = "serde")]
@@ -508,6 +516,7 @@ impl ConstantTimeEq for Algorithm {
             (Self::FrodoKem976Shake, Self::FrodoKem976Shake) => Choice::from(1),
             #[cfg(feature = "frodo1344shake")]
             (Self::FrodoKem1344Shake, Self::FrodoKem1344Shake) => Choice::from(1),
+            #[allow(unreachable_patterns)]
             _ => Choice::from(0),
         }
     }
@@ -519,6 +528,7 @@ impl Default for Algorithm {
     }
 }
 
+#[cfg(feature = "std")]
 impl core::fmt::Display for Algorithm {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         static ALGORITHMS: std::sync::LazyLock<std::collections::HashMap<Algorithm, String>> =
@@ -592,6 +602,7 @@ impl core::fmt::Display for Algorithm {
     }
 }
 
+#[cfg(feature = "std")]
 impl core::str::FromStr for Algorithm {
     type Err = Error;