From e56a806dbba5dc2495c85cf700e79aa9cd909393 Mon Sep 17 00:00:00 2001
From: jason-nitro <jason@nitro.technology>
Date: Wed, 30 Jul 2025 20:07:19 -0500
Subject: [PATCH 1/2] Added v0.3.3 backend for zkvm

---
 src/backends.rs      |  8 +++++++-
 src/backends/zkvm.rs | 49 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 src/backends/zkvm.rs

diff --git a/src/backends.rs b/src/backends.rs
index d7b73cec0..fc6a65427 100644
--- a/src/backends.rs
+++ b/src/backends.rs
@@ -7,8 +7,14 @@
 //! The function MUST NOT ever write uninitialized bytes into `dest`,
 //! regardless of what value it returns.
 
+mod zkvm;
+
 cfg_if! {
-    if #[cfg(getrandom_backend = "custom")] {
+    if #[cfg(target_arch = "riscv32")] {
+        // Auto-detect SP1 zkVM target
+        pub use zkvm::*;
+    }
+    else if #[cfg(getrandom_backend = "custom")] {
         mod custom;
         pub use custom::*;
     } else if #[cfg(getrandom_backend = "linux_getrandom")] {
diff --git a/src/backends/zkvm.rs b/src/backends/zkvm.rs
new file mode 100644
index 000000000..e2219670a
--- /dev/null
+++ b/src/backends/zkvm.rs
@@ -0,0 +1,49 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! zkVM implementation using custom backend
+use crate::Error;
+use core::mem::MaybeUninit;
+
+// Main function for getrandom 0.3.3 API - note the MaybeUninit
+pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    unsafe {
+        __getrandom_v03_custom(dest.as_mut_ptr() as *mut u8, dest.len())
+    }
+}
+
+pub fn inner_u32() -> Result<u32, Error> {
+    let mut buf = [MaybeUninit::<u8>::uninit(); 4];
+    fill_inner(&mut buf)?;
+    // Safe because fill_inner initialized the bytes
+    let buf: [u8; 4] = unsafe { core::mem::transmute(buf) };
+    Ok(u32::from_ne_bytes(buf))
+}
+
+pub fn inner_u64() -> Result<u64, Error> {
+    let mut buf = [MaybeUninit::<u8>::uninit(); 8];
+    fill_inner(&mut buf)?;
+    // Safe because fill_inner initialized the bytes
+    let buf: [u8; 8] = unsafe { core::mem::transmute(buf) };
+    Ok(u64::from_ne_bytes(buf))
+}
+
+// Custom backend function - to be overridden by user implementations
+#[no_mangle]
+pub unsafe extern "Rust" fn __getrandom_v03_custom(
+    dest: *mut u8,
+    len: usize,
+) -> Result<(), Error> {
+    // Should be overridden by the correct sp1 implementation. Previous version was:
+    // unsafe { sp1_zkvm::syscalls::sys_rand(s.as_mut_ptr(), s.len()) };
+
+    for i in 0..len {
+        *dest.add(i) = (i as u8).wrapping_mul(17).wrapping_add(42);
+    }
+    Ok(())
+}

From 291d8218807fdf6dc71928c72925558a8bcc646a Mon Sep 17 00:00:00 2001
From: jason-nitro <jason@nitro.technology>
Date: Thu, 7 Aug 2025 13:51:32 -0500
Subject: [PATCH 2/2] Removed redundant getrandom core code, already in SP1

---
 src/backends/zkvm.rs | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/src/backends/zkvm.rs b/src/backends/zkvm.rs
index e2219670a..538cf7473 100644
--- a/src/backends/zkvm.rs
+++ b/src/backends/zkvm.rs
@@ -11,12 +11,18 @@ use crate::Error;
 use core::mem::MaybeUninit;
 
 // Main function for getrandom 0.3.3 API - note the MaybeUninit
+#[allow(dead_code)]
 pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    extern "Rust" { // Call SP1's version
+        fn __getrandom_v03_custom(dest: *mut u8, len: usize) -> Result<(), Error>;
+    }
+
     unsafe {
         __getrandom_v03_custom(dest.as_mut_ptr() as *mut u8, dest.len())
     }
 }
 
+#[allow(dead_code)]
 pub fn inner_u32() -> Result<u32, Error> {
     let mut buf = [MaybeUninit::<u8>::uninit(); 4];
     fill_inner(&mut buf)?;
@@ -25,6 +31,7 @@ pub fn inner_u32() -> Result<u32, Error> {
     Ok(u32::from_ne_bytes(buf))
 }
 
+#[allow(dead_code)]
 pub fn inner_u64() -> Result<u64, Error> {
     let mut buf = [MaybeUninit::<u8>::uninit(); 8];
     fill_inner(&mut buf)?;
@@ -32,18 +39,3 @@ pub fn inner_u64() -> Result<u64, Error> {
     let buf: [u8; 8] = unsafe { core::mem::transmute(buf) };
     Ok(u64::from_ne_bytes(buf))
 }
-
-// Custom backend function - to be overridden by user implementations
-#[no_mangle]
-pub unsafe extern "Rust" fn __getrandom_v03_custom(
-    dest: *mut u8,
-    len: usize,
-) -> Result<(), Error> {
-    // Should be overridden by the correct sp1 implementation. Previous version was:
-    // unsafe { sp1_zkvm::syscalls::sys_rand(s.as_mut_ptr(), s.len()) };
-
-    for i in 0..len {
-        *dest.add(i) = (i as u8).wrapping_mul(17).wrapping_add(42);
-    }
-    Ok(())
-}