chore(csprng): use getrandom as random source for unix seeder

Cargo.toml

@@ -33,6 +33,7 @@ rand = "0.8"
 rayon = "1.11"
 serde = { version = "1.0", default-features = false }
 wasm-bindgen = "0.2.100"
+getrandom = "0.2.8"
 
 [profile.bench]
 lto = "fat"

tfhe-csprng/Cargo.toml

@@ -14,6 +14,7 @@ rust-version = "1.72"
 [dependencies]
 aes = "0.8.2"
 rayon = { workspace = true, optional = true }
+getrandom = { workspace = true }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 libc = "0.2.133"

tfhe-csprng/src/seeders/implem/unix.rs

@@ -1,12 +1,11 @@
 use crate::seeders::{Seed, Seeder};
-use std::fs::File;
-use std::io::Read;
 
-/// A seeder which uses the `/dev/random` source on unix-like systems.
+/// A seeder which uses the system entropy source on unix-like systems.
+///
+/// If available, this will use `getrandom` or `getentropy` system call. Otherwise it will draw from
+/// `/dev/urandom` after successfully polling `/dev/random`.
 pub struct UnixSeeder {
-    counter: u128,
     secret: u128,
-    file: File,
 }
 
 impl UnixSeeder {
@@ -15,35 +14,34 @@ impl UnixSeeder {
     /// Important:
     /// ----------
     ///
-    /// This secret is used to ensure the quality of the seed in scenarios where `/dev/random` may
-    /// be compromised.
+    /// This secret is used to ensure the quality of the seed in scenarios where the system random
+    /// source may be compromised.
     ///
     /// The attack hypotheses are as follow:
-    /// - `/dev/random` output can be predicted by a process running on the machine by just
+    /// - The kernel random output can be predicted by a process running on the machine by just
     ///   observing various states of the machine
     /// - The attacker cannot read data from the process where `tfhe-csprng` is running
     ///
     /// Using a secret in `tfhe-csprng` allows to generate values that the attacker cannot
-    /// predict, making this seeder secure on systems were `/dev/random` outputs can be
+    /// predict, making this seeder secure on systems were the kernel random outputs can be
     /// predicted.
     pub fn new(secret: u128) -> UnixSeeder {
-        let file = std::fs::File::open("/dev/random").expect("Failed to open /dev/random .");
-        let counter = std::time::UNIX_EPOCH
-            .elapsed()
-            .expect("Failed to initialize unix seeder.")
-            .as_nanos();
-        UnixSeeder {
-            secret,
-            counter,
-            file,
-        }
+        UnixSeeder { secret }
     }
 }
 
 impl Seeder for UnixSeeder {
+    /// Draws entropy from a system source to seed a CSPRNG.
+    ///
+    /// It may be blocking at system startup if the kernel entropy pool has not been initialized,
+    /// but should not be blocking after.
+    ///
+    /// # Panics
+    /// This may panic if the `getrandom` system call is not available and no file descriptor is
+    /// available on the system.
     fn seed(&mut self) -> Seed {
-        let output = self.secret ^ self.counter ^ dev_random(&mut self.file);
-        self.counter = self.counter.wrapping_add(1);
+        let output = self.secret ^ get_system_entropy();
+
         Seed(output)
     }
 
@@ -52,11 +50,14 @@ impl Seeder for UnixSeeder {
     }
 }
 
-fn dev_random(random: &mut File) -> u128 {
+fn get_system_entropy() -> u128 {
     let mut buf = [0u8; 16];
-    random
-        .read_exact(&mut buf[..])
-        .expect("Failed to read from /dev/random .");
+    // This will use the getrandom syscall if possible (from linux 3.17). This syscall is not
+    // vulnerable to fd exhaustion since it directly pulls from kernel entropy sources.
+    //
+    // This syscall will use the urandom entropy source but block at startup until it is correctly
+    // seeded. See <https://www.2uo.de/myths-about-urandom/> for a rational around random/urandom.
+    getrandom::getrandom(&mut buf).expect("Failed to read entropy from system");
     u128::from_ne_bytes(buf)
 }
 

tfhe-fft/Cargo.toml

@@ -37,7 +37,7 @@ serde_json = "1.0.96"
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 wasm-bindgen-test = "0.3"
 wasm-bindgen = { workspace = true }
-getrandom = { version = "0.2", features = ["js"] }
+getrandom = { workspace = true, features = ["js"] }
 
 [target.'cfg(all(not(target_os = "windows"), not(target_arch = "wasm32")))'.dev-dependencies]
 rug = "1.19.1"

tfhe/Cargo.toml

@@ -86,7 +86,7 @@ wasm-bindgen-rayon = { version = "1.3.0", optional = true }
 js-sys = { version = "0.3", optional = true }
 console_error_panic_hook = { version = "0.1.7", optional = true }
 serde-wasm-bindgen = { version = "0.6.0", optional = true }
-getrandom = { version = "0.2.8", optional = true }
+getrandom = { workspace = true, optional = true }
 bytemuck = { workspace = true }
 
 tfhe-hpu-backend = { version = "0.2", path = "../backends/tfhe-hpu-backend", optional = true }