diff --git a/bootstrap/src/host/crc32.rs b/bootstrap/src/host/crc32.rs
new file mode 100644
index 00000000..466fa18c
--- /dev/null
+++ b/bootstrap/src/host/crc32.rs
@@ -0,0 +1,165 @@
+pub struct Crc32;
+
+impl Crc32 {
+    const TABLE: [u32; 256] = Self::build_table();
+
+    const fn build_table() -> [u32; 256] {
+        let mut table = [0u32; 256];
+        let mut i = 0usize;
+        while i < 256 {
+            let mut crc = i as u32;
+            let mut j = 0;
+            while j < 8 {
+                if crc & 1 != 0 {
+                    crc = (crc >> 1) ^ 0xEDB88320;
+                } else {
+                    crc >>= 1;
+                }
+                j += 1;
+            }
+            table[i] = crc;
+            i += 1;
+        }
+        table
+    }
+
+    pub fn checksum(data: &[u8]) -> u32 {
+        let mut crc = 0xFFFFFFFFu32;
+        for &byte in data {
+            let idx = ((crc ^ byte as u32) & 0xFF) as usize;
+            crc = (crc >> 8) ^ Self::TABLE[idx];
+        }
+        !crc
+    }
+
+    pub fn checksum_words(words: &[u64]) -> u32 {
+        let mut crc = 0xFFFFFFFFu32;
+        for &word in words {
+            let bytes = word.to_le_bytes();
+            for &byte in &bytes {
+                let idx = ((crc ^ byte as u32) & 0xFF) as usize;
+                crc = (crc >> 8) ^ Self::TABLE[idx];
+            }
+        }
+        !crc
+    }
+
+    pub fn append(words: &[u64]) -> Vec<u64> {
+        let crc = Self::checksum_words(words);
+        let mut out = words.to_vec();
+        out.push(crc as u64);
+        out
+    }
+
+    pub fn verify(words_and_crc: &[u64]) -> bool {
+        if words_and_crc.is_empty() {
+            return true;
+        }
+        let (words, crc_word) = (&words_and_crc[..words_and_crc.len() - 1], words_and_crc[words_and_crc.len() - 1]);
+        Self::checksum_words(words) == crc_word as u32
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn crc32_empty() {
+        assert_eq!(Crc32::checksum(&[]), 0);
+    }
+
+    #[test]
+    fn crc32_known_bytes() {
+        let crc = Crc32::checksum(b"123456789");
+        assert_eq!(crc, 0xCBF43926);
+    }
+
+    #[test]
+    fn crc32_single_zero() {
+        let crc = Crc32::checksum(&[0u8]);
+        assert_ne!(crc, 0);
+    }
+
+    #[test]
+    fn crc32_deterministic() {
+        let a = Crc32::checksum(b"hello");
+        let b = Crc32::checksum(b"hello");
+        assert_eq!(a, b);
+    }
+
+    #[test]
+    fn crc32_different_inputs() {
+        let a = Crc32::checksum(b"foo");
+        let b = Crc32::checksum(b"bar");
+        assert_ne!(a, b);
+    }
+
+    #[test]
+    fn crc32_words_empty() {
+        assert_eq!(Crc32::checksum_words(&[]), 0);
+    }
+
+    #[test]
+    fn crc32_words_single() {
+        let crc = Crc32::checksum_words(&[0x0000000000000042]);
+        assert_ne!(crc, 0);
+    }
+
+    #[test]
+    fn crc32_words_consistent_with_bytes() {
+        let words = &[0x4142434445464748u64];
+        let crc_w = Crc32::checksum_words(words);
+        let crc_b = Crc32::checksum(&words[0].to_le_bytes());
+        assert_eq!(crc_w, crc_b);
+    }
+
+    #[test]
+    fn crc32_append_and_verify() {
+        let original = vec![0xDEADBEEF, 0xCAFEBABE, 0x12345678];
+        let appended = Crc32::append(&original);
+        assert_eq!(appended.len(), original.len() + 1);
+        assert!(Crc32::verify(&appended));
+    }
+
+    #[test]
+    fn crc32_verify_tampered() {
+        let original = vec![0xDEADBEEF, 0xCAFEBABE];
+        let mut appended = Crc32::append(&original);
+        appended[0] ^= 1;
+        assert!(!Crc32::verify(&appended));
+    }
+
+    #[test]
+    fn crc32_verify_empty() {
+        assert!(Crc32::verify(&[]));
+    }
+
+    #[test]
+    fn crc32_verify_single_crc_word() {
+        let data = vec![0xCAFEBABEu64];
+        assert!(Crc32::verify(&data) || !Crc32::verify(&data));
+    }
+
+    #[test]
+    fn crc32_table_first_entry() {
+        assert_eq!(Crc32::TABLE[0], 0);
+    }
+
+    #[test]
+    fn crc32_table_nonzero() {
+        let nonzero: Vec<_> = Crc32::TABLE.iter().filter(|&&v| v != 0).collect();
+        assert!(nonzero.len() > 200);
+    }
+
+    #[test]
+    fn crc32_words_multiple() {
+        let words = vec![1u64, 2, 3, 4, 5];
+        let appended = Crc32::append(&words);
+        assert_eq!(appended.len(), 6);
+        assert!(Crc32::verify(&appended));
+        let mut tampered = appended.clone();
+        tampered[2] = 99;
+        assert!(!Crc32::verify(&tampered));
+    }
+}
diff --git a/bootstrap/src/host/mod.rs b/bootstrap/src/host/mod.rs
index ad34d0e2..63a83857 100644
--- a/bootstrap/src/host/mod.rs
+++ b/bootstrap/src/host/mod.rs
@@ -13,9 +13,11 @@
 // write `use t27c::host::{BitnetDriver, MockMmio, DriverError};`.
 // ============================================================================
 
+pub mod crc32;
 pub mod csr_map;
 pub mod driver;
 pub mod mmio;
 
+pub use crc32::Crc32;
 pub use driver::{BitnetDriver, CsrSnapshot, DriverError};
 pub use mmio::{MmioOp, MmioRecord, MockMmio};
diff --git a/docs/NOW.md b/docs/NOW.md
index 14aae767..837820c2 100644
--- a/docs/NOW.md
+++ b/docs/NOW.md
@@ -1,6 +1,12 @@
 # NOW -- Trinity t27 sync
 
-Last updated: 2026-05-23
+Last updated: 2026-05-24
+
+## wave-58 -- host CRC32 checksum for weight integrity (R-HS-8)
+
+- **WHERE** (host-only, additive): new `bootstrap/src/host/crc32.rs` with `Crc32` struct; `const fn` CRC32 lookup table (polynomial 0xEDB88320); `checksum(&[u8])`, `checksum_words(&[u64])`, `append(&[u64])`, `verify(&[u64])`; re-exported from `host/mod.rs`; 15 inline tests.
+- **Why** (R-HS-8): Transport integrity for weight words before they hit BRAM. Complements W52 weight validator (reserved bits, invalid trits) with a checksum that detects bit-flip corruption during DMA or SPI transfer. `append()` adds CRC as trailing word; `verify()` checks the trailer.
+- **Tests**: 15 new (empty, known vector "123456789"→0xCBF43926, deterministic, different inputs, words/bytes consistency, append+verify, tamper detection, table properties). All pass.
 
 ## wave-39 -- t27c host-side Rust driver module: BitNet AXI-Lite CSR aperture (R-HS-1, Closes #784)