diff --git a/metropolis/pkg/verity/encoder.go b/metropolis/pkg/verity/encoder.go
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+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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.
+
+// This package implements the minimum of functionality needed to generate and
+// map dm-verity images. It's provided in order to avoid a perceived higher
+// long term cost of packaging, linking against and maintaining the original C
+// veritysetup tool.
+//
+// dm-verity is a Linux device mapper target that allows integrity verification of
+// a read-only block device. The block device whose integrity should be checked
+// (the 'data device') must be first processed by a tool like veritysetup to
+// generate a hash device and root hash.
+// The original data device, hash device and root hash are then set up as a device
+// mapper target, and any read performed from the data device through the verity
+// target will be verified for integrity by Linux using the hash device and root
+// hash.
+//
+// Internally, the hash device is a Merkle tree of all the bytes in the data
+// device, layed out as layers of 'hash blocks'. Starting with data bytes, layers
+// are built recursively, with each layer's output hash blocks becoming the next
+// layer's data input, ending with the single root hash.
+//
+// For more information about the internals, see the Linux and cryptsetup
+// upstream code:
+//
+// https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
+package verity
+
+import (
+	"bytes"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/binary"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+// superblock represents data layout inside of a dm-verity hash block
+// device superblock. It follows a preexisting verity implementation:
+//
+// https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity#verity-superblock-format
+type superblock struct {
+	// signature is the magic signature of a verity hash device superblock,
+	// "verity\0\0".
+	signature [8]byte
+	// version specifies a superblock format. This structure describes version
+	// '1'.
+	version uint32
+	// hashType defaults to '1' outside Chrome OS, according to scarce dm-verity
+	// documentation.
+	hashType uint32
+	// uuid contains a UUID of the hash device.
+	uuid [16]byte
+	// algorithm stores an ASCII-encoded name of the hash function used.
+	algorithm [32]byte
+
+	// dataBlockSize specifies a size of a single data device block, in bytes.
+	dataBlockSize uint32
+	// hashBlockSize specifies a size of a single hash device block, in bytes.
+	hashBlockSize uint32
+	// dataBlocks contains a count of blocks available on the data device.
+	dataBlocks uint64
+
+	// saltSize encodes the size of hash block salt, up to the maximum of 256 bytes.
+	saltSize uint16
+
+	// padding
+	_ [6]byte
+	// exactly saltSize bytes of salt are prepended to data blocks before hashing.
+	saltBuffer [256]byte
+	// padding
+	_ [168]byte
+}
+
+// newSuperblock builds a dm-verity hash device superblock based on
+// hardcoded defaults. dataBlocks is the only field left for later
+// initialization.
+// It returns either a partially initialized superblock, or an error.
+func newSuperblock() (*superblock, error) {
+	// This implementation only handles SHA256-based verity hash images
+	// with a specific 4096-byte block size.
+	// Block sizes can be updated by adjusting the struct literal below.
+	// A change of a hashing algorithm would require a refactor of
+	// saltedDigest, and references to sha256.Size.
+	//
+	// Fill in the defaults (compare with superblock definition).
+	sb := superblock{
+		signature:     [8]byte{'v', 'e', 'r', 'i', 't', 'y', 0, 0},
+		version:       1,
+		hashType:      1,
+		algorithm:     [32]byte{'s', 'h', 'a', '2', '5', '6'},
+		saltSize:      64,
+		dataBlockSize: 4096,
+		hashBlockSize: 4096,
+	}
+
+	// Fill in the superblock UUID and cryptographic salt.
+	if _, err := rand.Read(sb.uuid[:]); err != nil {
+		return nil, fmt.Errorf("when generating UUID: %w", err)
+	}
+	if _, err := rand.Read(sb.saltBuffer[:]); err != nil {
+		return nil, fmt.Errorf("when generating salt: %w", err)
+	}
+
+	return &sb, nil
+}
+
+// salt returns a slice of sb.saltBuffer actually occupied by
+// salt bytes, of sb.saltSize length.
+func (sb *superblock) salt() []byte {
+	return sb.saltBuffer[:int(sb.saltSize)]
+}
+
+// algorithmName returns a name of the algorithm used to hash data block
+// digests.
+func (sb *superblock) algorithmName() string {
+	size := bytes.IndexByte(sb.algorithm[:], 0x00)
+	return string(sb.algorithm[:size])
+}
+
+// saltedDigest computes and returns a SHA256 sum of a block prepended
+// with a Superblock-defined salt.
+func (sb *superblock) saltedDigest(data []byte) (digest [sha256.Size]byte) {
+	h := sha256.New()
+	h.Write(sb.salt())
+	h.Write(data)
+	copy(digest[:], h.Sum(nil))
+	return
+}
+
+// dataBlocksPerHashBlock returns the count of hash operation outputs that
+// fit in a hash device block. This is also the amount of data device
+// blocks it takes to populate a hash device block.
+func (sb *superblock) dataBlocksPerHashBlock() uint64 {
+	return uint64(sb.hashBlockSize) / sha256.Size
+}
+
+// computeHashBlock reads at most sb.dataBlocksPerHashBlock blocks from
+// the given reader object, returning a padded hash block of length
+// defined by sb.hashBlockSize, the count of digests output, and an
+// error, if encountered.
+// In case a non-nil block is returned, it's guaranteed to contain at
+// least one hash. An io.EOF signals that there is no more to be read.
+func (sb *superblock) computeHashBlock(r io.Reader) ([]byte, uint64, error) {
+	// dcnt stores the total count of data blocks processed, which is the
+	// as the count of digests output.
+	var dcnt uint64
+	// Preallocate a whole hash block.
+	hblk := bytes.NewBuffer(make([]byte, 0, sb.hashBlockSize))
+
+	// For every data block, compute a hash and place it in hblk. Continue
+	// till EOF.
+	for b := uint64(0); b < sb.dataBlocksPerHashBlock(); b++ {
+		dbuf := make([]byte, sb.dataBlockSize)
+		// Attempt to read enough data blocks to make a complete hash block.
+		n, err := io.ReadFull(r, dbuf)
+		// If any data was read, make a hash and add it to the hash buffer.
+		if n != 0 {
+			hash := sb.saltedDigest(dbuf)
+			hblk.Write(hash[:])
+			dcnt++
+		}
+		// Handle the read errors.
+		switch err {
+		case nil:
+		case io.ErrUnexpectedEOF, io.EOF:
+			// io.ReadFull returns io.ErrUnexpectedEOF after a partial read,
+			// and io.EOF if no bytes were read. In both cases it's possible
+			// to end up with a partially filled hash block.
+			if hblk.Len() != 0 {
+				// Return a zero-padded hash block if any hashes were written
+				// to it, and signal that no more blocks can be built.
+				res := hblk.Bytes()
+				return res[:cap(res)], dcnt, io.EOF
+			}
+			// Return nil if the block doesn't contain any hashes.
+			return nil, 0, io.EOF
+		default:
+			// Wrap unhandled read errors.
+			return nil, 0, fmt.Errorf("while computing a hash block: %w", err)
+		}
+	}
+	// Return a completely filled hash block.
+	res := hblk.Bytes()
+	return res[:cap(res)], dcnt, nil
+}
+
+// WriteTo writes a verity superblock to a given writer object.
+// It returns the count of bytes written, and a write error, if
+// encountered.
+func (sb *superblock) WriteTo(w io.Writer) (int64, error) {
+	// Write the superblock.
+	if err := binary.Write(w, binary.LittleEndian, sb); err != nil {
+		return -1, fmt.Errorf("while writing a header: %w", err)
+	}
+
+	// Get the padding size by substracting current offset from a hash block
+	// size.
+	co := int(binary.Size(sb))
+	pbc := int(sb.hashBlockSize) - int(co)
+	if pbc <= 0 {
+		return int64(co), fmt.Errorf("hash device block size smaller than dm-verity superblock")
+	}
+
+	// Write the padding bytes at the end of the block.
+	n, err := w.Write(bytes.Repeat([]byte{0}, pbc))
+	co += n
+	if err != nil {
+		return int64(co), fmt.Errorf("while writing padding: %w", err)
+	}
+	return int64(co), nil
+}
+
+// computeLevel produces a verity hash tree level based on data read from
+// a given reader object.
+// It returns a byte slice containing one or more hash blocks, or an
+// error.
+// BUG(mz): Current implementation requires a 1/128th of the data image
+// size to be allocatable on the heap.
+func (sb *superblock) computeLevel(r io.Reader) ([]byte, error) {
+	// hbuf will store all the computed hash blocks.
+	var hbuf bytes.Buffer
+	// Compute one or more hash blocks, reading all data available in the
+	// 'r' reader object, and write them into hbuf.
+	for {
+		hblk, _, err := sb.computeHashBlock(r)
+		if err != nil && err != io.EOF {
+			return nil, fmt.Errorf("while building a hash tree level: %w", err)
+		}
+		if hblk != nil {
+			_, err := hbuf.Write(hblk)
+			if err != nil {
+				return nil, fmt.Errorf("while writing to hash block buffer: %w", err)
+			}
+		}
+		if err == io.EOF {
+			break
+		}
+	}
+	return hbuf.Bytes(), nil
+}
+
+// hashTree stores hash tree levels, each level comprising one or more
+// Verity hash blocks. Levels are ordered from bottom to top.
+type hashTree [][]byte
+
+// push appends a level to the hash tree.
+func (ht *hashTree) push(nl []byte) {
+	*ht = append(*ht, nl)
+}
+
+// top returns the topmost level of the hash tree.
+func (ht *hashTree) top() []byte {
+	if len(*ht) == 0 {
+		return nil
+	}
+	return (*ht)[len(*ht)-1]
+}
+
+// WriteTo writes a verity-formatted hash tree to the given writer
+// object.
+// It returns a write error, if encountered.
+func (ht *hashTree) WriteTo(w io.Writer) (int64, error) {
+	// t keeps the count of bytes written to w.
+	var t int64
+	// Write the hash tree levels from top to bottom.
+	for l := len(*ht) - 1; l >= 0; l-- {
+		level := (*ht)[l]
+		// Call w.Write until a whole level is written.
+		for len(level) != 0 {
+			n, err := w.Write(level)
+			if err != nil {
+				return t, fmt.Errorf("while writing a level: %w", err)
+			}
+			level = level[n:]
+			t += int64(n)
+		}
+	}
+	return t, nil
+}
+
+// MappingTable aggregates data needed to generate a complete Verity
+// mapping table.
+type MappingTable struct {
+	// superblock defines the following elements of the mapping table:
+	// - data device block size
+	// - hash device block size
+	// - total count of data blocks
+	// - hash algorithm used
+	// - cryptographic salt used
+	superblock *superblock
+	// dataDevicePath is the filesystem path of the data device used as part
+	// of the Verity Device Mapper target.
+	dataDevicePath string
+	// hashDevicePath is the filesystem path of the hash device used as part
+	// of the Verity Device Mapper target.
+	hashDevicePath string
+	// hashStart marks the starting block of the Verity hash tree.
+	hashStart int
+	// rootHash stores a cryptographic hash of the top hash tree block.
+	rootHash []byte
+}
+
+// VerityParameterList returns a list of Verity target parameters, ordered
+// as they would appear in a parameter string.
+func (t *MappingTable) VerityParameterList() []string {
+	return []string{
+		"1",
+		t.dataDevicePath,
+		t.hashDevicePath,
+		strconv.FormatUint(uint64(t.superblock.dataBlockSize), 10),
+		strconv.FormatUint(uint64(t.superblock.hashBlockSize), 10),
+		strconv.FormatUint(uint64(t.superblock.dataBlocks), 10),
+		strconv.FormatInt(int64(t.hashStart), 10),
+		t.superblock.algorithmName(),
+		hex.EncodeToString(t.rootHash),
+		hex.EncodeToString(t.superblock.salt()),
+	}
+}
+
+// TargetParameters returns the mapping table as a list of Device Mapper
+// target parameters, ordered as they would appear in a parameter string
+// (see: String).
+func (t *MappingTable) TargetParameters() []string {
+	return append(
+		[]string{
+			"0",
+			strconv.FormatUint(t.Length(), 10),
+			"verity",
+		},
+		t.VerityParameterList()...,
+	)
+}
+
+// String returns a string-formatted mapping table for use with Device
+// Mapper.
+// BUG(mz): unescaped whitespace can appear in block device paths
+func (t *MappingTable) String() string {
+	return strings.Join(t.TargetParameters(), " ")
+}
+
+// Length returns the data device length, represented as a number of
+// 512-byte sectors.
+func (t *MappingTable) Length() uint64 {
+	return t.superblock.dataBlocks * uint64(t.superblock.dataBlockSize) / 512
+}
+
+// encoder transforms data blocks written into it into a verity hash
+// tree. It writes out the hash tree only after Close is called on it.
+type encoder struct {
+	// out is the writer object Encoder will write to.
+	out io.Writer
+	// writeSb, if true, will cause a Verity superblock to be written to the
+	// writer object.
+	writeSb bool
+	// sb contains the most of information needed to build a mapping table.
+	sb *superblock
+	// bottom stands for the bottom level of the hash tree. It contains
+	// complete hash blocks of data written to the encoder.
+	bottom bytes.Buffer
+	// dataBuffer stores incoming data for later processing.
+	dataBuffer bytes.Buffer
+	// rootHash stores the verity root hash set on Close.
+	rootHash []byte
+}
+
+// computeHashTree builds a complete hash tree based on the encoder's
+// state. Levels are appended to the returned hash tree starting from the
+// bottom, with the top level written last.
+// e.sb.dataBlocks is set according to the bottom level's length, which
+// must be divisible by e.sb.hashBlockSize.
+// e.rootHash is set on success.
+// It returns an error, if encountered.
+func (e *encoder) computeHashTree() (*hashTree, error) {
+	// Put b at the bottom of the tree. Don't perform a deep copy.
+	ht := hashTree{e.bottom.Bytes()}
+
+	// Other levels are built by hashing the hash blocks comprising a level
+	// below.
+	for {
+		if len(ht.top()) == int(e.sb.hashBlockSize) {
+			// The last level to compute has a size of exactly one hash block.
+			// That's the root level. Its hash serves as a cryptographic root of
+			// trust and is saved into a encoder for later use.
+			// In case the bottom level consists of only one hash block, no more
+			// levels are computed.
+			sd := e.sb.saltedDigest(ht.top())
+			e.rootHash = sd[:]
+			return &ht, nil
+		}
+
+		// Create the next level by hashing the previous one.
+		nl, err := e.sb.computeLevel(bytes.NewReader(ht.top()))
+		if err != nil {
+			return nil, fmt.Errorf("while computing a level: %w", err)
+		}
+		// Append the resulting next level to a tree.
+		ht.push(nl)
+	}
+}
+
+// processDataBuffer processes data blocks contained in e.dataBuffer
+// until no more data is available to form a completely filled hash block.
+// If 'incomplete' is true, all remaining data in e.dataBuffer will be
+// processed, producing a terminating incomplete block.
+// It returns the count of data blocks processed, or an error, if
+// encountered.
+func (e *encoder) processDataBuffer(incomplete bool) (uint64, error) {
+	// tdcnt stores the total count of data blocks processed.
+	var tdcnt uint64
+	// Compute the count of bytes needed to produce a complete hash block.
+	bph := e.sb.dataBlocksPerHashBlock() * uint64(e.sb.dataBlockSize)
+
+	// Iterate until no more data is available in e.dbuf.
+	for uint64(e.dataBuffer.Len()) >= bph || incomplete && e.dataBuffer.Len() != 0 {
+		hb, dcnt, err := e.sb.computeHashBlock(&e.dataBuffer)
+		if err != nil && err != io.EOF {
+			return 0, fmt.Errorf("while processing a data buffer: %w", err)
+		}
+		// Increment the total count of data blocks processed.
+		tdcnt += dcnt
+		// Write the resulting hash block into the level-zero buffer.
+		e.bottom.Write(hb[:])
+	}
+	return tdcnt, nil
+}
+
+// NewEncoder returns a fully initialized encoder, or an error. The
+// encoder will write to the given io.Writer object.
+// A verity superblock will be written, preceding the hash tree, if
+// writeSb is true.
+func NewEncoder(out io.Writer, writeSb bool) (*encoder, error) {
+	sb, err := newSuperblock()
+	if err != nil {
+		return nil, fmt.Errorf("while creating a superblock: %w", err)
+	}
+
+	e := encoder{
+		out:     out,
+		writeSb: writeSb,
+		sb:      sb,
+	}
+	return &e, nil
+}
+
+// Write hashes raw data to form the bottom hash tree level.
+// It returns the number of bytes written, and an error, if encountered.
+func (e *encoder) Write(data []byte) (int, error) {
+	// Copy the input into the data buffer.
+	n, _ := e.dataBuffer.Write(data)
+	// Process only enough data to form a complete hash block. This may
+	// leave excess data in e.dbuf to be processed later on.
+	dcnt, err := e.processDataBuffer(false)
+	if err != nil {
+		return n, fmt.Errorf("while processing the data buffer: %w", err)
+	}
+	// Update the superblock with the count of data blocks written.
+	e.sb.dataBlocks += dcnt
+	return n, nil
+}
+
+// Close builds a complete hash tree based on cached bottom level blocks,
+// then writes it to a preconfigured io.Writer object. A Verity superblock
+// is written, if e.writeSb is true. No data, nor the superblock is written
+// if the encoder is empty.
+// It returns an error, if one was encountered.
+func (e *encoder) Close() error {
+	// Process all buffered data, including data blocks that may not form
+	// a complete hash block.
+	dcnt, err := e.processDataBuffer(true)
+	if err != nil {
+		return fmt.Errorf("while processing the data buffer: %w", err)
+	}
+	// Update the superblock with the count of data blocks written.
+	e.sb.dataBlocks += dcnt
+
+	// Don't write anything if nothing was written to the encoder.
+	if e.bottom.Len() == 0 {
+		return nil
+	}
+
+	// Compute remaining hash tree levels based on the bottom level: e.bottom.
+	ht, err := e.computeHashTree()
+	if err != nil {
+		return fmt.Errorf("while encoding a hash tree: %w", err)
+	}
+
+	// Write the Verity superblock if the encoder was configured to do so.
+	if e.writeSb {
+		if _, err = e.sb.WriteTo(e.out); err != nil {
+			return fmt.Errorf("while writing a superblock: %w", err)
+		}
+	}
+	// Write the hash tree.
+	_, err = ht.WriteTo(e.out)
+	if err != nil {
+		return fmt.Errorf("while writing a hash tree: %w", err)
+	}
+
+	// Reset the encoder.
+	e, err = NewEncoder(e.out, e.writeSb)
+	if err != nil {
+		return fmt.Errorf("while resetting an encoder: %w", err)
+	}
+	return nil
+}
+
+// MappingTable returns a string-convertible Verity target mapping table
+// for use with Device Mapper, or an error. Close must be called on the
+// encoder before calling this function.
+func (e *encoder) MappingTable(dataDevicePath, hashDevicePath string) (*MappingTable, error) {
+	if e.rootHash == nil {
+		if e.bottom.Len() != 0 {
+			return nil, fmt.Errorf("encoder wasn't closed.")
+		}
+		return nil, fmt.Errorf("encoder is empty.")
+	}
+
+	var hs int
+	if e.writeSb {
+		// Account for the superblock by setting the hash tree starting block
+		// to 1 instead of 0.
+		hs = 1
+	}
+	return &MappingTable{
+		superblock:     e.sb,
+		dataDevicePath: dataDevicePath,
+		hashDevicePath: hashDevicePath,
+		hashStart:      hs,
+		rootHash:       e.rootHash,
+	}, nil
+}