feat: Added KoalaBear field support to enable Rust hash-sig compatibility

README.md

@@ -4,28 +4,159 @@ A Zig implementation of the Poseidon2 cryptographic hash function.
 
 ## Supported Configurations
 
-Currently, this implementation provides:
+This implementation provides:
+
+### Finite Fields
+
+- **BabyBear field** (p = 2³¹ - 2²⁷ + 1 = 0x78000001)
+  - Width: 16 elements
+  - S-Box degree: 7
+  - Internal rounds: 13
+  - External rounds: 8
+  - Use case: Ethereum Lean chain
+
+- **KoalaBear field** (p = 2³¹ - 2²⁴ + 1 = 0x7f000001)  
+  - Width: 16 elements
+  - S-Box degree: 3
+  - Internal rounds: 20
+  - External rounds: 8
+  - Use case: plonky3, Rust hash-sig compatibility
+
+### Features
 
-- BabyBear finite field with a width of 16 elements
 - Generic Montgomery form implementation for finite fields of 31 bits or less
-- Compression mode, since it's the recommended mode for Merkle Trees compared to the sponge construction.
+- Compression mode (recommended for Merkle Trees)
+- Both naive and optimized (Montgomery) implementations for verification
+- Comprehensive test suite ensuring consistency between implementations
 
-The generic implementation makes it straightforward to add support for additional 31-bit fields.
+## Installation
 
-## Project Motivation
+Add `zig-poseidon` as a dependency in your `build.zig.zon`:
+
+```zig
+.dependencies = .{
+    .poseidon = .{
+        .url = "https://github.com/jsign/zig-poseidon/archive/<commit-hash>.tar.gz",
+        .hash = "<hash>",
+    },
+},
+```
+
+## Usage
+
+### Using BabyBear16
+
+```zig
+const std = @import("std");
+const babybear16 = @import("babybear16");
+
+pub fn main() !void {
+    const Field = babybear16.Poseidon2BabyBear.Field;
+
+    // Prepare input state (16 field elements)
+    var input_state: [16]u32 = .{0} ** 16;
+    input_state[0] = 42;
+
+    // Convert to Montgomery form
+    var mont_state: [16]Field.MontFieldElem = undefined;
+    for (0..16) |i| {
+        Field.toMontgomery(&mont_state[i], input_state[i]);
+    }
+
+    // Apply permutation
+    babybear16.Poseidon2BabyBear.permutation(&mont_state);
+
+    // Convert back to normal form
+    var output_state: [16]u32 = undefined;
+    for (0..16) |i| {
+        output_state[i] = Field.toNormal(mont_state[i]);
+    }
+
+    std.debug.print("Output: {any}\n", .{output_state});
+}
+```
+
+### Using KoalaBear16 (Rust hash-sig compatible)
+
+```zig
+const std = @import("std");
+const koalabear16 = @import("koalabear16");
 
-This repository was created primarily to support the upcoming Ethereum Beam chain. The implementation will be updated to match the required configuration once the specifications are finalized.
+pub fn main() !void {
+    const Field = koalabear16.Poseidon2KoalaBear.Field;
+
+    // Prepare input state (16 field elements)
+    var input_state: [16]u32 = .{0} ** 16;
+    input_state[0] = 42;
+
+    // Convert to Montgomery form
+    var mont_state: [16]Field.MontFieldElem = undefined;
+    for (0..16) |i| {
+        Field.toMontgomery(&mont_state[i], input_state[i]);
+    }
+
+    // Apply permutation
+    koalabear16.Poseidon2KoalaBear.permutation(&mont_state);
+
+    // Convert back to normal form
+    var output_state: [16]u32 = undefined;
+    for (0..16) |i| {
+        output_state[i] = Field.toNormal(mont_state[i]);
+    }
+
+    std.debug.print("Output: {any}\n", .{output_state});
+}
+```
 
-With time this repository can keep expaning on features:
+## Building and Testing
 
-- Add support for more finite fields.
-- Add support for the sponge construction.
-- Add benchmarks and optimizations.
+```bash
+# Build the library
+zig build
+
+# Run all tests (includes BabyBear and KoalaBear)
+zig build test
+```
+
+## Field Comparison
+
+| Feature | BabyBear | KoalaBear |
+|---------|----------|-----------|
+| **Prime** | 2³¹ - 2²⁷ + 1 | 2³¹ - 2²⁴ + 1 |
+| **Hex Value** | 0x78000001 | 0x7f000001 |
+| **Width** | 16 | 16 |
+| **S-Box Degree** | 7 | 3 |
+| **Internal Rounds** | 13 | 20 |
+| **External Rounds** | 8 | 8 |
+| **Compatible With** | Ethereum Lean | plonky3, Rust hash-sig |
+
+**Important:** Different fields produce completely different hash outputs! Choose the field that matches your target system.
+
+## Project Motivation
+
+This repository was created primarily to support the upcoming Ethereum Lean chain. The KoalaBear field was added to enable compatibility with Rust's hash-sig implementation and plonky3.
 
 ## Compatibility
 
-This implementation has been cross-validated against the [reference repository](https://github.com/HorizenLabs/poseidon2) cited in the Poseidon2 paper to ensure correctness.
+- **BabyBear**: Cross-validated against the [Poseidon2 reference repository](https://github.com/HorizenLabs/poseidon2)
+- **KoalaBear**: Compatible with [plonky3](https://github.com/Plonky3/Plonky3) and [Rust hash-sig](https://github.com/b-wagn/hash-sig)
+
+Both implementations include tests ensuring the naive and optimized (Montgomery) implementations produce identical outputs.
+
+## Future Enhancements
+
+- Add support for more finite fields
+- Add support for the sponge construction
+- Add benchmarks and performance optimizations
+- Add more S-Box degrees as needed
 
 ## License
 
 MIT
+
+## References
+
+- [Poseidon2 Paper](https://eprint.iacr.org/2023/323)
+- [HorizenLabs/poseidon2 Reference Implementation](https://github.com/HorizenLabs/poseidon2)
+- [Plonky3](https://github.com/Plonky3/Plonky3)
+- [Rust hash-sig](https://github.com/b-wagn/hash-sig)

build.zig

@@ -4,23 +4,32 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    _ = b.addModule("poseidon", .{
-        .root_source_file = .{
-            .cwd_relative = "src/poseidon2/poseidon2.zig",
-        },
+    // Generic Poseidon2 module
+    _ = b.addModule("poseidon2", .{
+        .root_source_file = b.path("src/poseidon2/poseidon2.zig"),
+    });
+
+    // BabyBear16 instance
+    _ = b.addModule("babybear16", .{
+        .root_source_file = b.path("src/instances/babybear16.zig"),
+    });
+
+    // KoalaBear16 instance (compatible with Rust hash-sig)
+    _ = b.addModule("koalabear16", .{
+        .root_source_file = b.path("src/instances/koalabear16.zig"),
     });
 
     const lib = b.addStaticLibrary(.{
         .name = "zig-poseidon",
-        .root_source_file = .{ .cwd_relative = "src/main.zig" },
+        .root_source_file = b.path("src/main.zig"),
         .target = target,
         .optimize = optimize,
     });
 
     b.installArtifact(lib);
 
     const main_tests = b.addTest(.{
-        .root_source_file = .{ .cwd_relative = "src/main.zig" },
+        .root_source_file = b.path("src/main.zig"),
         .target = target,
         .optimize = optimize,
     });

src/fields/koalabear/montgomery.zig

@@ -0,0 +1,3 @@
+// KoalaBear field: p = 2^31 - 2^24 + 1 = 127 * 2^24 + 1 = 2130706433 = 0x7f000001
+// This field is used in plonky3 and Rust hash-sig implementations
+pub const MontgomeryField = @import("../generic_montgomery.zig").MontgomeryField31(127 * (1 << 24) + 1);

src/fields/koalabear/naive.zig

@@ -0,0 +1,32 @@
+const std = @import("std");
+
+// KoalaBear field: p = 2^31 - 2^24 + 1 = 127 * 2^24 + 1 = 2130706433 = 0x7f000001
+const modulus = 127 * (1 << 24) + 1;
+pub const FieldElem = u32;
+pub const MontFieldElem = u32;
+
+pub fn toMontgomery(out1: *MontFieldElem, value: FieldElem) void {
+    out1.* = value;
+}
+
+pub fn toNormal(out1: MontFieldElem) FieldElem {
+    return out1;
+}
+
+pub fn square(out1: *MontFieldElem, value: MontFieldElem) void {
+    mul(out1, value, value);
+}
+
+pub fn add(out1: *MontFieldElem, elem1: MontFieldElem, elem2: MontFieldElem) void {
+    var tmp: u64 = elem1;
+    tmp += elem2;
+    tmp %= modulus;
+    out1.* = @intCast(tmp);
+}
+
+pub fn mul(out1: *MontFieldElem, elem1: MontFieldElem, elem2: MontFieldElem) void {
+    var tmp: u64 = elem1;
+    tmp *= elem2;
+    tmp %= modulus;
+    out1.* = @intCast(tmp);
+}