diff --git a/authfs/src/crypto.rs b/authfs/src/crypto.rs
index a51acca..8b8905c 100644
--- a/authfs/src/crypto.rs
+++ b/authfs/src/crypto.rs
@@ -14,8 +14,6 @@
  * limitations under the License.
  */
 
-#![allow(dead_code)]
-
 use std::mem::MaybeUninit;
 
 use thiserror::Error;
diff --git a/authfs/src/fsverity.rs b/authfs/src/fsverity.rs
new file mode 100644
index 0000000..f32ccab
--- /dev/null
+++ b/authfs/src/fsverity.rs
@@ -0,0 +1,195 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+use std::io;
+use thiserror::Error;
+
+use crate::crypto::{CryptoError, Sha256Hasher};
+use crate::reader::ReadOnlyDataByChunk;
+
+const ZEROS: [u8; 4096] = [0u8; 4096];
+
+#[derive(Error, Debug)]
+pub enum FsverityError {
+    #[error("Cannot verify a block")]
+    CannotVerify,
+    #[error("I/O error")]
+    Io(#[from] io::Error),
+    #[error("Crypto")]
+    UnexpectedCryptoError(#[from] CryptoError),
+}
+
+type HashBuffer = [u8; Sha256Hasher::HASH_SIZE];
+
+fn divide_roundup(dividend: u64, divisor: u64) -> u64 {
+    (dividend + divisor - 1) / divisor
+}
+
+fn hash_with_padding(chunk: &[u8], pad_to: usize) -> Result<HashBuffer, CryptoError> {
+    let padding_size = pad_to - chunk.len();
+    Sha256Hasher::new()?.update(&chunk)?.update(&ZEROS[..padding_size])?.finalize()
+}
+
+#[allow(dead_code)]
+fn verity_check<T: ReadOnlyDataByChunk>(
+    chunk: &[u8],
+    chunk_index: u64,
+    file_size: u64,
+    merkle_tree: &T,
+) -> Result<HashBuffer, FsverityError> {
+    // The caller should not be able to produce a chunk at the first place if `file_size` is 0. The
+    // current implementation expects to crash when a `ReadOnlyDataByChunk` implementation reads
+    // beyone the file size, including empty file.
+    assert_ne!(file_size, 0);
+
+    let chunk_hash = hash_with_padding(&chunk, T::CHUNK_SIZE as usize)?;
+
+    fsverity_walk(chunk_index, file_size, merkle_tree)?.try_fold(
+        chunk_hash,
+        |actual_hash, result| {
+            let (merkle_chunk, hash_offset_in_chunk) = result?;
+            let expected_hash =
+                &merkle_chunk[hash_offset_in_chunk..hash_offset_in_chunk + Sha256Hasher::HASH_SIZE];
+            if actual_hash != expected_hash {
+                return Err(FsverityError::CannotVerify);
+            }
+            Ok(hash_with_padding(&merkle_chunk, T::CHUNK_SIZE as usize)?)
+        },
+    )
+}
+
+fn log128_ceil(num: u64) -> Option<u64> {
+    match num {
+        0 => None,
+        n => Some(divide_roundup(64 - (n - 1).leading_zeros() as u64, 7)),
+    }
+}
+
+/// Given a chunk index and the size of the file, returns an iterator that walks the Merkle tree
+/// from the leaf to the root. The iterator carries the slice of the chunk/node as well as the
+/// offset of the child node's hash. It is up to the iterator user to use the node and hash,
+/// e.g. for the actual verification.
+#[allow(clippy::needless_collect)]
+fn fsverity_walk<'a, T: ReadOnlyDataByChunk>(
+    chunk_index: u64,
+    file_size: u64,
+    merkle_tree: &'a T,
+) -> Result<impl Iterator<Item = Result<([u8; 4096], usize), FsverityError>> + 'a, FsverityError> {
+    let hashes_per_node = T::CHUNK_SIZE / Sha256Hasher::HASH_SIZE as u64;
+    let hash_pages = divide_roundup(file_size, hashes_per_node * T::CHUNK_SIZE);
+    debug_assert_eq!(hashes_per_node, 128u64);
+    let max_level = log128_ceil(hash_pages).expect("file should not be empty") as u32;
+    let root_to_leaf_steps = (0..=max_level)
+        .rev()
+        .map(|x| {
+            let leaves_per_hash = hashes_per_node.pow(x);
+            let leaves_size_per_hash = T::CHUNK_SIZE * leaves_per_hash;
+            let leaves_size_per_node = leaves_size_per_hash * hashes_per_node;
+            let nodes_at_level = divide_roundup(file_size, leaves_size_per_node);
+            let level_size = nodes_at_level * T::CHUNK_SIZE;
+            let offset_in_level = (chunk_index / leaves_per_hash) * Sha256Hasher::HASH_SIZE as u64;
+            (level_size, offset_in_level)
+        })
+        .scan(0, |level_offset, (level_size, offset_in_level)| {
+            let this_level_offset = *level_offset;
+            *level_offset += level_size;
+            let global_hash_offset = this_level_offset + offset_in_level;
+            Some(global_hash_offset)
+        })
+        .map(|global_hash_offset| {
+            let chunk_index = global_hash_offset / T::CHUNK_SIZE;
+            let hash_offset_in_chunk = (global_hash_offset % T::CHUNK_SIZE) as usize;
+            (chunk_index, hash_offset_in_chunk)
+        })
+        .collect::<Vec<_>>();
+
+    Ok(root_to_leaf_steps.into_iter().rev().map(move |(chunk_index, hash_offset_in_chunk)| {
+        let mut merkle_chunk = [0u8; 4096];
+        let _ = merkle_tree.read_chunk(chunk_index, &mut merkle_chunk)?;
+        Ok((merkle_chunk, hash_offset_in_chunk))
+    }))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::reader::ReadOnlyDataByChunk;
+    use anyhow::Result;
+
+    fn total_chunk_number(file_size: u64) -> u64 {
+        (file_size + 4095) / 4096
+    }
+
+    #[test]
+    fn fsverity_verify_full_read_4k() -> Result<()> {
+        let file = &include_bytes!("../testdata/input.4k")[..];
+        let merkle_tree = &include_bytes!("../testdata/input.4k.merkle_dump")[..];
+
+        let mut buf = [0u8; 4096];
+
+        for i in 0..total_chunk_number(file.len() as u64) {
+            let size = file.read_chunk(i, &mut buf[..])?;
+            assert!(verity_check(&buf[..size], i, file.len() as u64, &merkle_tree).is_ok());
+        }
+        Ok(())
+    }
+
+    #[test]
+    fn fsverity_verify_full_read_4k1() -> Result<()> {
+        let file = &include_bytes!("../testdata/input.4k1")[..];
+        let merkle_tree = &include_bytes!("../testdata/input.4k1.merkle_dump")[..];
+
+        let mut buf = [0u8; 4096];
+        for i in 0..total_chunk_number(file.len() as u64) {
+            let size = file.read_chunk(i, &mut buf[..])?;
+            assert!(verity_check(&buf[..size], i, file.len() as u64, &merkle_tree).is_ok());
+        }
+        Ok(())
+    }
+
+    #[test]
+    fn fsverity_verify_full_read_4m() -> Result<()> {
+        let file = &include_bytes!("../testdata/input.4m")[..];
+        let merkle_tree = &include_bytes!("../testdata/input.4m.merkle_dump")[..];
+
+        let mut buf = [0u8; 4096];
+        for i in 0..total_chunk_number(file.len() as u64) {
+            let size = file.read_chunk(i, &mut buf[..])?;
+            assert!(verity_check(&buf[..size], i, file.len() as u64, &merkle_tree).is_ok());
+        }
+        Ok(())
+    }
+
+    #[test]
+    fn fsverity_verify_bad_merkle_tree() -> Result<()> {
+        let file = &include_bytes!("../testdata/input.4m")[..];
+        // First leaf node is corrupted.
+        let merkle_tree = &include_bytes!("../testdata/input.4m.merkle_dump.bad")[..];
+
+        // A lowest broken node (a 4K chunk that contains 128 sha256 hashes) will fail the read
+        // failure of the underlying chunks, but not before or after.
+        let mut buf = [0u8; 4096];
+        let num_hashes = 4096 / 32;
+        let last_index = num_hashes;
+        for i in 0..last_index {
+            let size = file.read_chunk(i, &mut buf[..])?;
+            assert!(verity_check(&buf[..size], i, file.len() as u64, &merkle_tree).is_err());
+        }
+        let size = file.read_chunk(last_index, &mut buf[..])?;
+        assert!(verity_check(&buf[..size], last_index, file.len() as u64, &merkle_tree).is_ok());
+        Ok(())
+    }
+}
diff --git a/authfs/src/lib.rs b/authfs/src/lib.rs
index c128f62..26de157 100644
--- a/authfs/src/lib.rs
+++ b/authfs/src/lib.rs
@@ -23,4 +23,5 @@
 //! The implementation is not finished.
 
 mod crypto;
+mod fsverity;
 mod reader;