3 files changed, 199 insertions, 0 deletions
diff --git a/internal/api/api.go b/internal/api/api.go
new file mode 100644
index 0000000..7c8472a
--- /dev/null
+++ b/internal/api/api.go
@@ -0,0 +1,64 @@
+// Package api contains type definitions for the gob API.
+package api
+
+import "encoding/gob"
+
+type ErrorCode int
+
+//go:generate go tool stringer -type=ErrorCode
+const (
+	Ok               ErrorCode = iota
+	Unauthenticated            // missing credentials
+	PermissionDenied           // invalid credentials, or login required
+	BadRequest                 // incorrect request format (try gob)
+	NotFound                   // requested resource was not found
+	Internal                   // server encountered an unexpected error
+)
+
+func (c ErrorCode) Error() string {
+	return c.String()
+}
+
+type Response struct {
+	Status ErrorCode
+	Err    string
+	Ok     any
+}
+
+type LoginResponse struct {
+	Token string
+}
+
+type ListNotesRequest struct{}
+
+type ListNotesResponse struct {
+	Notes []string
+}
+
+type CreateNoteRequestStream struct {
+	Chunk []byte
+}
+
+type CreateNoteResponse struct {
+	Name string
+}
+
+type ReadNoteRequest struct {
+	Note string
+}
+
+type ReadNoteResponseStream struct {
+	Chunk []byte
+}
+
+type DeleteNoteRequest struct{}
+
+type DeleteNoteResponse struct{}
+
+func init() {
+	gob.Register(&LoginResponse{})
+	gob.Register(&ListNotesResponse{})
+	gob.Register(&CreateNoteResponse{})
+	gob.Register(&ReadNoteResponseStream{})
+	gob.Register(&DeleteNoteResponse{})
+}
diff --git a/internal/api/errorcode_string.go b/internal/api/errorcode_string.go
new file mode 100644
index 0000000..2b4a294
--- /dev/null
+++ b/internal/api/errorcode_string.go
@@ -0,0 +1,28 @@
+// Code generated by "stringer -type=ErrorCode"; DO NOT EDIT.
+
+package api
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[Ok-0]
+	_ = x[Unauthenticated-1]
+	_ = x[PermissionDenied-2]
+	_ = x[BadRequest-3]
+	_ = x[NotFound-4]
+	_ = x[Internal-5]
+}
+
+const _ErrorCode_name = "OkUnauthenticatedPermissionDeniedBadRequestNotFoundInternal"
+
+var _ErrorCode_index = [...]uint8{0, 2, 17, 33, 43, 51, 59}
+
+func (i ErrorCode) String() string {
+	if i < 0 || i >= ErrorCode(len(_ErrorCode_index)-1) {
+		return "ErrorCode(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _ErrorCode_name[_ErrorCode_index[i]:_ErrorCode_index[i+1]]
+}
diff --git a/internal/cryptoutil/cryptoutil.go b/internal/cryptoutil/cryptoutil.go
index 4ed600d..09ed996 100644
--- a/internal/cryptoutil/cryptoutil.go
+++ b/internal/cryptoutil/cryptoutil.go
@@ -13,6 +13,10 @@ import (
 	"crypto/subtle"
 	"encoding/hex"
 	"errors"
+	"fmt"
+	"hash"
+	"io"
+	"slices"
 )
 
 const (
@@ -169,3 +173,106 @@ func (k EncryptionKey) Decrypt(msg EncryptedMessage) ([]byte, error) {
 	cipher.NewCTR(block, nonce).XORKeyStream(msg, msg)
 	return msg, nil
 }
+
+// An EncryptingWriter encrypts the output and writes it to the underlying
+// writer. It's very important to call .Flush() to write the MAC after the data
+// has been written.
+type EncryptingWriter struct {
+	w      io.Writer
+	stream cipher.Stream
+	mac    hash.Hash
+	buf    []byte
+}
+
+// Writer returns a new writer that encrypts its output. Don't forget to call
+// .Flush() to write the MAC.
+func (k EncryptionKey) Writer(w io.Writer) (*EncryptingWriter, error) {
+	aesKey, hmacKey := k[:aesKeySize], k[aesKeySize:]
+	block, err := aes.NewCipher(aesKey)
+	if err != nil {
+		return nil, err
+	}
+	mac := hmac.New(sha512.New, hmacKey)
+	nonce := make([]byte, nonceSize)
+	rand.Read(nonce)
+	mac.Write(nonce)
+	if _, err := w.Write(nonce); err != nil {
+		return nil, err
+	}
+	return &EncryptingWriter{
+		w:      w,
+		stream: cipher.NewCTR(block, nonce),
+		mac:    mac,
+	}, nil
+}
+
+// Write encrypts buf and writes it to the underlying writer.
+func (w *EncryptingWriter) Write(buf []byte) (int, error) {
+	if len(w.buf) < len(buf) {
+		w.buf = slices.Grow(w.buf, len(buf)-len(w.buf))
+	}
+	w.buf = w.buf[:len(buf)]
+	// Encrypt-then-MAC
+	w.stream.XORKeyStream(w.buf, buf)
+	w.mac.Write(w.buf)
+	return w.w.Write(w.buf)
+}
+
+// Flush writes the MAC for the encrypted message. Flush must be called after
+// all data has been written.
+func (w *EncryptingWriter) Flush() error {
+	_, err := w.w.Write(w.mac.Sum(nil))
+	return err
+}
+
+// A DecryptingReader decrypts data from an underlying reader.
+type DecryptingReader struct {
+	r cipher.StreamReader
+}
+
+// Reader returns a new DecryptingReader. The data is processed in two passes,
+// first to verify the MAC, then the io.Seeker interface is used to reset the
+// reader for decryption.
+func (k EncryptionKey) Reader(r io.ReadSeeker) (*DecryptingReader, error) {
+	aesKey, hmacKey := k[:aesKeySize], k[aesKeySize:]
+	block, err := aes.NewCipher(aesKey)
+	if err != nil {
+		return nil, err
+	}
+	totalLen, err := r.Seek(0, io.SeekEnd)
+	if err != nil {
+		return nil, err
+	}
+	if totalLen < certSize {
+		return nil, errors.New("file too short")
+	}
+	dataLen := totalLen - certSize
+	if _, err := r.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+	mac := hmac.New(sha512.New, hmacKey)
+	if _, err := io.Copy(mac, &io.LimitedReader{R: r, N: dataLen}); err != nil {
+		return nil, err
+	}
+	expectedMAC := mac.Sum(nil)
+	sig := make([]byte, certSize)
+	if _, err := io.ReadFull(r, sig); err != nil {
+		return nil, err
+	}
+	if !hmac.Equal(sig, expectedMAC) {
+		return nil, fmt.Errorf("invalid mac (got %x, want %x)", sig, expectedMAC)
+	}
+	if _, err := r.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+	nonce := make([]byte, nonceSize)
+	if _, err := io.ReadFull(r, nonce); err != nil {
+		return nil, err
+	}
+	return &DecryptingReader{cipher.StreamReader{S: cipher.NewCTR(block, nonce), R: &io.LimitedReader{R: r, N: dataLen - nonceSize}}}, nil
+}
+
+// Read reads and decrypts data from the underlying reader.
+func (r *DecryptingReader) Read(buf []byte) (int, error) {
+	return r.r.Read(buf)
+}