1 files changed, 84 insertions, 0 deletions
diff --git a/katzenpost/dispatcher/frame.go b/katzenpost/dispatcher/frame.go
new file mode 100644
index 0000000..43f547c
--- /dev/null
+++ b/katzenpost/dispatcher/frame.go
@@ -0,0 +1,84 @@
+package dispatcher
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+
+	"github.com/fxamacker/cbor/v2"
+)
+
+const (
+	ProtocolVersion  = 1
+	MaxSphinxPayload = 2000
+	FrameDataBytes   = 1500
+	MaxFrames        = (MaxEnvelopeBytes + FrameDataBytes - 1) / FrameDataBytes
+)
+
+type Frame struct {
+	Version uint8    `cbor:"1,keyasint"`
+	ID      [16]byte `cbor:"2,keyasint"`
+	Digest  [32]byte `cbor:"3,keyasint"`
+	Index   uint16   `cbor:"4,keyasint"`
+	Total   uint16   `cbor:"5,keyasint"`
+	Data    []byte   `cbor:"6,keyasint"`
+}
+
+func SplitEnvelope(encoded []byte) ([][]byte, error) {
+	if len(encoded) == 0 || len(encoded) > MaxEnvelopeBytes {
+		return nil, errors.New("invalid encoded envelope size")
+	}
+
+	var id [16]byte
+	if _, err := rand.Read(id[:]); err != nil {
+		return nil, fmt.Errorf("generate message id: %w", err)
+	}
+	digest := sha256.Sum256(encoded)
+	total := (len(encoded) + FrameDataBytes - 1) / FrameDataBytes
+	if total > MaxFrames {
+		return nil, errors.New("encoded envelope requires too many frames")
+	}
+
+	frames := make([][]byte, 0, total)
+	for index, start := 0, 0; start < len(encoded); index, start = index+1, start+FrameDataBytes {
+		end := min(start+FrameDataBytes, len(encoded))
+		frame := Frame{
+			Version: ProtocolVersion,
+			ID:      id,
+			Digest:  digest,
+			Index:   uint16(index),
+			Total:   uint16(total),
+			Data:    encoded[start:end],
+		}
+		payload, err := cbor.Marshal(frame)
+		if err != nil {
+			return nil, fmt.Errorf("encode frame %d: %w", index, err)
+		}
+		if len(payload) > MaxSphinxPayload {
+			return nil, fmt.Errorf("frame %d exceeds Sphinx payload limit", index)
+		}
+		frames = append(frames, payload)
+	}
+	return frames, nil
+}
+
+func DecodeFrame(payload []byte) (Frame, error) {
+	if len(payload) == 0 || len(payload) > MaxSphinxPayload {
+		return Frame{}, errors.New("invalid frame size")
+	}
+	var frame Frame
+	if err := cbor.Unmarshal(payload, &frame); err != nil {
+		return Frame{}, fmt.Errorf("decode frame: %w", err)
+	}
+	if frame.Version != ProtocolVersion {
+		return Frame{}, errors.New("unsupported frame version")
+	}
+	if frame.Total == 0 || int(frame.Total) > MaxFrames || frame.Index >= frame.Total {
+		return Frame{}, errors.New("invalid frame sequence")
+	}
+	if len(frame.Data) == 0 || len(frame.Data) > FrameDataBytes {
+		return Frame{}, errors.New("invalid frame data size")
+	}
+	return frame, nil
+}