aead_crypter.go

// Copyright (C) 2019 ProtonTech AG

package packet

import (
	"bytes"
	"crypto/cipher"
	"encoding/binary"
	"io"

	"github.com/ProtonMail/go-crypto/openpgp/errors"
)

// aeadCrypter is an AEAD opener/sealer, its configuration, and data for en/decryption.
type aeadCrypter struct {
	aead           cipher.AEAD
	chunkSize      int
	initialNonce   []byte
	associatedData []byte       // Chunk-independent associated data
	chunkIndex     []byte       // Chunk counter
	packetTag      packetType   // SEIP packet (v2) or AEAD Encrypted Data packet
	bytesProcessed int          // Amount of plaintext bytes encrypted/decrypted
	buffer         bytes.Buffer // Buffered bytes across chunks
}

// computeNonce takes the incremental index and computes an eXclusive OR with
// the least significant 8 bytes of the receivers' initial nonce (see sec.
// 5.16.1 and 5.16.2). It returns the resulting nonce.
func (wo *aeadCrypter) computeNextNonce() (nonce []byte) {
	if wo.packetTag == packetTypeSymmetricallyEncryptedIntegrityProtected {
		return append(wo.initialNonce, wo.chunkIndex...)
	}

	nonce = make([]byte, len(wo.initialNonce))
	copy(nonce, wo.initialNonce)
	offset := len(wo.initialNonce) - 8
	for i := 0; i < 8; i++ {
		nonce[i+offset] ^= wo.chunkIndex[i]
	}
	return
}

// incrementIndex performs an integer increment by 1 of the integer represented by the
// slice, modifying it accordingly.
func (wo *aeadCrypter) incrementIndex() error {
	index := wo.chunkIndex
	if len(index) == 0 {
		return errors.AEADError("Index has length 0")
	}
	for i := len(index) - 1; i >= 0; i-- {
		if index[i] < 255 {
			index[i]++
			return nil
		}
		index[i] = 0
	}
	return errors.AEADError("cannot further increment index")
}

// aeadDecrypter reads and decrypts bytes. It buffers extra decrypted bytes when
// necessary, similar to aeadEncrypter.
type aeadDecrypter struct {
	aeadCrypter           // Embedded ciphertext opener
	reader      io.Reader // 'reader' is a partialLengthReader
	peekedBytes []byte    // Used to detect last chunk
	eof         bool
}

// Read decrypts bytes and reads them into dst. It decrypts when necessary and
// buffers extra decrypted bytes. It returns the number of bytes copied into dst
// and an error.
func (ar *aeadDecrypter) Read(dst []byte) (n int, err error) {
	// Return buffered plaintext bytes from previous calls
	if ar.buffer.Len() > 0 {
		return ar.buffer.Read(dst)
	}

	// Return EOF if we've previously validated the final tag
	if ar.eof {
		return 0, io.EOF
	}

	// Read a chunk
	tagLen := ar.aead.Overhead()
	cipherChunkBuf := new(bytes.Buffer)
	_, errRead := io.CopyN(cipherChunkBuf, ar.reader, int64(ar.chunkSize+tagLen))
	cipherChunk := cipherChunkBuf.Bytes()
	if errRead != nil && errRead != io.EOF {
		return 0, errRead
	}
	decrypted, errChunk := ar.openChunk(cipherChunk)
	if errChunk != nil {
		return 0, errChunk
	}

	// Return decrypted bytes, buffering if necessary
	if len(dst) < len(decrypted) {
		n = copy(dst, decrypted[:len(dst)])
		ar.buffer.Write(decrypted[len(dst):])
	} else {
		n = copy(dst, decrypted)
	}

	// Check final authentication tag
	if errRead == io.EOF {
		errChunk := ar.validateFinalTag(ar.peekedBytes)
		if errChunk != nil {
			return n, errChunk
		}
		ar.eof = true // Mark EOF for when we've returned all buffered data
	}
	return
}

// Close is noOp. The final authentication tag of the stream was already
// checked in the last Read call. In the future, this function could be used to
// wipe the reader and peeked, decrypted bytes, if necessary.
func (ar *aeadDecrypter) Close() (err error) {
	return nil
}

// openChunk decrypts and checks integrity of an encrypted chunk, returning
// the underlying plaintext and an error. It accesses peeked bytes from next
// chunk, to identify the last chunk and decrypt/validate accordingly.
func (ar *aeadDecrypter) openChunk(data []byte) ([]byte, error) {
	tagLen := ar.aead.Overhead()
	// Restore carried bytes from last call
	chunkExtra := append(ar.peekedBytes, data...)
	// 'chunk' contains encrypted bytes, followed by an authentication tag.
	chunk := chunkExtra[:len(chunkExtra)-tagLen]
	ar.peekedBytes = chunkExtra[len(chunkExtra)-tagLen:]

	adata := ar.associatedData
	if ar.aeadCrypter.packetTag == packetTypeAEADEncrypted {
		adata = append(ar.associatedData, ar.chunkIndex...)
	}

	nonce := ar.computeNextNonce()
	plainChunk, err := ar.aead.Open(nil, nonce, chunk, adata)
	if err != nil {
		return nil, err
	}
	ar.bytesProcessed += len(plainChunk)
	if err = ar.aeadCrypter.incrementIndex(); err != nil {
		return nil, err
	}
	return plainChunk, nil
}

// Checks the summary tag. It takes into account the total decrypted bytes into
// the associated data. It returns an error, or nil if the tag is valid.
func (ar *aeadDecrypter) validateFinalTag(tag []byte) error {
	// Associated: tag, version, cipher, aead, chunk size, ...
	amountBytes := make([]byte, 8)
	binary.BigEndian.PutUint64(amountBytes, uint64(ar.bytesProcessed))

	adata := ar.associatedData
	if ar.aeadCrypter.packetTag == packetTypeAEADEncrypted {
		// ... index ...
		adata = append(ar.associatedData, ar.chunkIndex...)
	}

	// ... and total number of encrypted octets
	adata = append(adata, amountBytes...)
	nonce := ar.computeNextNonce()
	_, err := ar.aead.Open(nil, nonce, tag, adata)
	if err != nil {
		return err
	}
	return nil
}

// aeadEncrypter encrypts and writes bytes. It encrypts when necessary according
// to the AEAD block size, and buffers the extra encrypted bytes for next write.
type aeadEncrypter struct {
	aeadCrypter                // Embedded plaintext sealer
	writer      io.WriteCloser // 'writer' is a partialLengthWriter
}

// Write encrypts and writes bytes. It encrypts when necessary and buffers extra
// plaintext bytes for next call. When the stream is finished, Close() MUST be
// called to append the final tag.
func (aw *aeadEncrypter) Write(plaintextBytes []byte) (n int, err error) {
	// Append plaintextBytes to existing buffered bytes
	n, err = aw.buffer.Write(plaintextBytes)
	if err != nil {
		return n, err
	}
	// Encrypt and write chunks
	for aw.buffer.Len() >= aw.chunkSize {
		plainChunk := aw.buffer.Next(aw.chunkSize)
		encryptedChunk, err := aw.sealChunk(plainChunk)
		if err != nil {
			return n, err
		}
		_, err = aw.writer.Write(encryptedChunk)
		if err != nil {
			return n, err
		}
	}
	return
}

// Close encrypts and writes the remaining buffered plaintext if any, appends
// the final authentication tag, and closes the embedded writer. This function
// MUST be called at the end of a stream.
func (aw *aeadEncrypter) Close() (err error) {
	// Encrypt and write a chunk if there's buffered data left, or if we haven't
	// written any chunks yet.
	if aw.buffer.Len() > 0 || aw.bytesProcessed == 0 {
		plainChunk := aw.buffer.Bytes()
		lastEncryptedChunk, err := aw.sealChunk(plainChunk)
		if err != nil {
			return err
		}
		_, err = aw.writer.Write(lastEncryptedChunk)
		if err != nil {
			return err
		}
	}
	// Compute final tag (associated data: packet tag, version, cipher, aead,
	// chunk size...
	adata := aw.associatedData

	if aw.aeadCrypter.packetTag == packetTypeAEADEncrypted {
		// ... index ...
		adata = append(aw.associatedData, aw.chunkIndex...)
	}

	// ... and total number of encrypted octets
	amountBytes := make([]byte, 8)
	binary.BigEndian.PutUint64(amountBytes, uint64(aw.bytesProcessed))
	adata = append(adata, amountBytes...)

	nonce := aw.computeNextNonce()
	finalTag := aw.aead.Seal(nil, nonce, nil, adata)
	_, err = aw.writer.Write(finalTag)
	if err != nil {
		return err
	}
	return aw.writer.Close()
}

// sealChunk Encrypts and authenticates the given chunk.
func (aw *aeadEncrypter) sealChunk(data []byte) ([]byte, error) {
	if len(data) > aw.chunkSize {
		return nil, errors.AEADError("chunk exceeds maximum length")
	}
	if aw.associatedData == nil {
		return nil, errors.AEADError("can't seal without headers")
	}
	adata := aw.associatedData
	if aw.aeadCrypter.packetTag == packetTypeAEADEncrypted {
		adata = append(aw.associatedData, aw.chunkIndex...)
	}

	nonce := aw.computeNextNonce()
	encrypted := aw.aead.Seal(nil, nonce, data, adata)
	aw.bytesProcessed += len(data)
	if err := aw.aeadCrypter.incrementIndex(); err != nil {
		return nil, err
	}
	return encrypted, nil
}