diff --git a/idsig/src/lib.rs b/idsig/src/lib.rs
new file mode 100644
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--- /dev/null
+++ b/idsig/src/lib.rs
@@ -0,0 +1,221 @@
+/*
+ * Copyright (C) 2021 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 ring::digest::{self, Algorithm};
+use std::io::{Cursor, Read, Result, Seek, SeekFrom, Write};
+
+/// `HashTree` is a merkle tree (and its root hash) that is compatible with fs-verity.
+pub struct HashTree {
+    pub tree: Vec<u8>,
+    pub root_hash: Vec<u8>,
+}
+
+impl HashTree {
+    /// Creates merkle tree from `input`, using the given `salt` and hashing `algorithm`. `input`
+    /// is divided into `block_size` chunks.
+    pub fn from<R: Read>(
+        input: &mut R,
+        input_size: usize,
+        salt: &[u8],
+        block_size: usize,
+        algorithm: &'static Algorithm,
+    ) -> Result<Self> {
+        let salt = zero_pad_salt(&salt, algorithm);
+        let tree = generate_hash_tree(input, input_size, &salt, block_size, algorithm)?;
+
+        // Root hash is from the first block of the hash or the input data if there is no hash tree
+        // generate which can happen when input data is smaller than block size
+        let root_hash = if tree.is_empty() {
+            hash_one_level(input, input_size, &salt, block_size, algorithm)?
+        } else {
+            let mut ctx = digest::Context::new(algorithm);
+            ctx.update(&salt);
+            ctx.update(&tree[0..block_size]);
+            ctx.finish().as_ref().to_vec()
+        };
+        Ok(HashTree { tree, root_hash })
+    }
+}
+
+/// Calculate hash tree for the blocks in `input`.
+///
+/// This function implements: https://www.kernel.org/doc/html/latest/filesystems/fsverity.html#merkle-tree
+///
+/// The file contents is divided into blocks, where the block size is configurable but is usually
+/// 4096 bytes. The end of the last block is zero-padded if needed. Each block is then hashed,
+/// producing the first level of hashes. Then, the hashes in this first level are grouped into
+/// blocksize-byte blocks (zero-padding the ends as needed) and these blocks are hashed,
+/// producing the second level of hashes. This proceeds up the tree until only a single block
+/// remains.
+fn generate_hash_tree<R: Read>(
+    input: &mut R,
+    input_size: usize,
+    salt: &[u8],
+    block_size: usize,
+    algorithm: &'static Algorithm,
+) -> Result<Vec<u8>> {
+    let digest_size = algorithm.output_len;
+    let (hash_level_offsets, tree_size) =
+        calc_hash_level_offsets(input_size, block_size, digest_size);
+
+    let mut hash_tree = Cursor::new(vec![0; tree_size]);
+    let mut input_size = input_size;
+    for (level, offset) in hash_level_offsets.iter().enumerate() {
+        let hashes = if level == 0 {
+            hash_one_level(input, input_size, salt, block_size, algorithm)?
+        } else {
+            // For the intermediate levels, input is the output from the previous level
+            hash_tree.seek(SeekFrom::Start(hash_level_offsets[level - 1] as u64)).unwrap();
+            hash_one_level(&mut hash_tree, input_size, salt, block_size, algorithm)?
+        };
+        hash_tree.seek(SeekFrom::Start(*offset as u64)).unwrap();
+        hash_tree.write_all(hashes.as_ref()).unwrap();
+        // Output from this level becomes input for the next level
+        input_size = hashes.len();
+    }
+    Ok(hash_tree.into_inner())
+}
+
+/// Calculate hashes for the blocks in `input`. The end of the last block is zero-padded if needed.
+/// Each block is then hashed, producing a stream of hashes for a level.
+fn hash_one_level<R: Read>(
+    input: &mut R,
+    input_size: usize,
+    salt: &[u8],
+    block_size: usize,
+    algorithm: &'static Algorithm,
+) -> Result<Vec<u8>> {
+    // Input is zero padded when it's not multiple of blocks. Note that `take()` is also needed to
+    // not read more than `input_size` from the `input` reader. This is required because `input`
+    // can be from the in-memory hashtree. We need to read only the part of hashtree that is for
+    // the current level.
+    let pad_size = round_to_multiple(input_size, block_size) - input_size;
+    let mut input = input.take(input_size as u64).chain(Cursor::new(vec![0; pad_size]));
+
+    // Read one block from input, write the hash of it to the output. Repeat that for all input
+    // blocks.
+    let mut hashes = Cursor::new(Vec::new());
+    let mut buf = vec![0; block_size];
+    let mut num_blocks = (input_size + block_size - 1) / block_size;
+    while num_blocks > 0 {
+        input.read_exact(&mut buf)?;
+        let mut ctx = digest::Context::new(algorithm);
+        ctx.update(salt);
+        ctx.update(&buf);
+        let hash = ctx.finish();
+        hashes.write_all(hash.as_ref())?;
+        num_blocks -= 1;
+    }
+    Ok(hashes.into_inner())
+}
+
+/// Calculate the size of hashes for each level, and also returns the total size of the hash tree.
+/// This function is needed because hash tree is stored upside down; hashes for level N is stored
+/// "after" hashes for level N + 1.
+fn calc_hash_level_offsets(
+    input_size: usize,
+    block_size: usize,
+    digest_size: usize,
+) -> (Vec<usize>, usize) {
+    // The input is split into multiple blocks and each block is hashed, which becomes the input
+    // for the next level. Size of a single hash is `digest_size`.
+    let mut level_sizes = Vec::new();
+    loop {
+        // Input for this level is from either the last level (if exists), or the input parameter.
+        let input_size = *level_sizes.last().unwrap_or(&input_size);
+        if input_size <= block_size {
+            break;
+        }
+        let num_blocks = (input_size + block_size - 1) / block_size;
+        let hashes_size = round_to_multiple(num_blocks * digest_size, block_size);
+        level_sizes.push(hashes_size);
+    }
+    if level_sizes.is_empty() {
+        return ([].to_vec(), 0);
+    }
+
+    // The hash tree is stored upside down. The top level is at offset 0. The second level comes
+    // next, and so on. Level 0 is located at the end.
+    //
+    // Given level_sizes [10, 3, 1], the offsets for each label are ...
+    //
+    // Level 2 is at offset 0
+    // Level 1 is at offset 1 (because Level 2 is of size 1)
+    // Level 0 is at offset 4 (because Level 1 is of size 3)
+    //
+    // This is done by accumulating the sizes in reverse order (i.e. from the highest level to the
+    // level 1 (not level 0)
+    let mut offsets = level_sizes.iter().rev().take(level_sizes.len() - 1).fold(
+        vec![0; 1], // offset for the top level
+        |mut offsets, size| {
+            offsets.push(offsets.last().unwrap() + size);
+            offsets
+        },
+    );
+    offsets.reverse(); // reverse the offsets again so that index N is for level N
+    let tree_size = level_sizes.iter().sum();
+    (offsets, tree_size)
+}
+
+/// Round `n` up to the nearest multiple of `unit`
+fn round_to_multiple(n: usize, unit: usize) -> usize {
+    (n + unit - 1) & !(unit - 1)
+}
+
+/// Pad zero to salt if necessary.
+///
+/// According to https://www.kernel.org/doc/html/latest/filesystems/fsverity.html:
+///
+/// If a salt was specified, then it’s zero-padded to the closest multiple of the input size of the
+/// hash algorithm’s compression function, e.g. 64 bytes for SHA-256 or 128 bytes for SHA-512. The
+/// padded salt is prepended to every data or Merkle tree block that is hashed.
+fn zero_pad_salt(salt: &[u8], algorithm: &Algorithm) -> Vec<u8> {
+    if salt.is_empty() {
+        salt.to_vec()
+    } else {
+        let padded_len = round_to_multiple(salt.len(), algorithm.block_len);
+        let mut salt = salt.to_vec();
+        salt.resize(padded_len, 0);
+        salt
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+    use ring::digest;
+    use std::fs::{self, File};
+
+    #[test]
+    fn compare_with_golden_output() -> Result<()> {
+        // The golden outputs are generated by using the `fsverity` utility.
+        let sizes = ["512", "4K", "1M", "10000000"];
+        for size in sizes.iter() {
+            let input_name = format!("testdata/input.{}", size);
+            let mut input = File::open(&input_name)?;
+            let golden_hash_tree = fs::read(format!("testdata/input.{}.hash", size))?;
+            let golden_descriptor = fs::read(format!("testdata/input.{}.descriptor", size))?;
+            let golden_root_hash = &golden_descriptor[16..16 + 32];
+
+            let size = std::fs::metadata(&input_name)?.len() as usize;
+            let salt = vec![1, 2, 3, 4, 5, 6];
+            let ht = HashTree::from(&mut input, size, &salt, 4096, &digest::SHA256)?;
+
+            assert_eq!(golden_hash_tree.as_slice(), ht.tree.as_slice());
+            assert_eq!(golden_root_hash, ht.root_hash.as_slice());
+        }
+        Ok(())
+    }
+}