it works!

Cargo.lock

@@ -0,0 +1,80 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
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+ "subtle",
+]
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Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "poly1305"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+crypto-bigint = "0.6.1"

src/lib.rs

@@ -0,0 +1,79 @@
+use std::os::unix::raw::mode_t;
+use crypto_bigint::{BitOps, ConstZero, Encoding, Integer, Limb, MulMod, NonZero, Pow, Uint, U192};
+use crypto_bigint::subtle::ConstantTimeEq;
+
+pub fn oneoff_authenticate(message: &[u8], key: &[u8; 32]) -> [u8; 16] {
+    let r = u128::from_le_bytes(key[0..16].try_into().unwrap());
+    let s = u128::from_le_bytes(key[16..32].try_into().unwrap());
+
+    let poly = poly1305(message, clamp(r)).wrapping_add(s).to_le_bytes();
+
+    poly
+}
+
+/// the prime $2^130 - 5$
+const MODULO_PRIME: NonZero<Uint<3>> = NonZero::<U192>::new_unwrap(U192::ONE.shl(130).sub_mod(&U192::from_u8(5), &U192::MAX));
+
+fn clamp(secret: u128) -> u128 {
+    secret & 0x0ffffffc0ffffffc0ffffffc0fffffff
+}
+
+fn poly1305(data: &[u8], secret: u128) -> u128 {
+    let secret = U192::from_u128(secret);
+
+    let blocks = data.chunks(16);
+
+    let mut sum = U192::ZERO;
+    for block in blocks {
+        let c = block_to_coefficient(block);
+
+        sum = sum.add_mod(&c, &MODULO_PRIME).mul_mod(&secret, &MODULO_PRIME);
+    }
+
+    let mut out_bytes = [0u8; 16];
+    for i in 0..16 {
+        out_bytes[i] = sum.to_le_bytes()[i]
+    }
+
+    u128::from_le_bytes(out_bytes)
+}
+
+fn block_to_coefficient(block: &[u8]) -> U192 {
+    let mut bytes = [0u8; 192/8];
+
+    let mut i = 0;
+    for byte in block {
+        bytes[i] = *byte;
+        i += 1;
+    }
+
+    bytes[i] = 1;
+
+    U192::from_le_bytes(bytes)
+}
+
+
+#[cfg(test)]
+mod tests {
+    use crypto_bigint::U256;
+    use super::*;
+
+
+    const MESSAGE: &[u8] = b"Cryptographic Forum Research Group";
+    const KEY: [u8; 32] = U256::from_be_hex("1bf54941aff6bf4afdb20dfb8a800301a806d542fe52447f336d555778bed685").to_le_bytes();
+    const EXPECTED_TAG: [u8; 16] = 0xa927010caf8b2bc2c6365130c11d06a8_u128.to_le_bytes();
+    #[test]
+    fn it_works() {
+        let tag = oneoff_authenticate(MESSAGE, &KEY);
+
+        assert_eq!(tag, EXPECTED_TAG);
+    }
+
+    #[test]
+    fn single_block() {
+        let secret = clamp(0xa806_d542_fe52_447f_336d_5557_78be_d685_u128);
+        let block = poly1305(b"Cryptographic Fo", secret);
+
+        assert_eq!(block, 0xc88c77849d64ae9147ddeb88e69c83fc)
+    }
+}