Source File
	gcm.go

Belonging Package
	github.com/pion/dtls/v3/pkg/crypto/ciphersuite

// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

package ciphersuite

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/binary"
	"fmt"

	"github.com/pion/dtls/v3/pkg/protocol"
	"github.com/pion/dtls/v3/pkg/protocol/recordlayer"
)

const (
	gcmTagLength   = 16
	gcmNonceLength = 12
)

// GCM Provides an API to Encrypt/Decrypt DTLS 1.2 Packets.
type GCM struct {
	localGCM, remoteGCM         cipher.AEAD
	localWriteIV, remoteWriteIV []byte
}

// NewGCM creates a DTLS GCM Cipher.
func NewGCM(localKey, localWriteIV, remoteKey, remoteWriteIV []byte) (*GCM, error) {
	localBlock, err := aes.NewCipher(localKey)
	if err != nil {
		return nil, err
	}
	localGCM, err := cipher.NewGCM(localBlock)
	if err != nil {
		return nil, err
	}

	remoteBlock, err := aes.NewCipher(remoteKey)
	if err != nil {
		return nil, err
	}
	remoteGCM, err := cipher.NewGCM(remoteBlock)
	if err != nil {
		return nil, err
	}

	return &GCM{
		localGCM:      localGCM,
		localWriteIV:  localWriteIV,
		remoteGCM:     remoteGCM,
		remoteWriteIV: remoteWriteIV,
	}, nil
}

// Encrypt encrypt a DTLS RecordLayer message.
func (g *GCM) Encrypt(pkt *recordlayer.RecordLayer, raw []byte) ([]byte, error) {
	payload := raw[pkt.Header.Size():]
	raw = raw[:pkt.Header.Size()]

	nonce := make([]byte, gcmNonceLength)
	copy(nonce, g.localWriteIV[:4])
	if _, err := rand.Read(nonce[4:]); err != nil {
		return nil, err
	}

	var additionalData []byte
	if pkt.Header.ContentType == protocol.ContentTypeConnectionID {
		additionalData = generateAEADAdditionalDataCID(&pkt.Header, len(payload))
	} else {
		additionalData = generateAEADAdditionalData(&pkt.Header, len(payload))
	}
	encryptedPayload := g.localGCM.Seal(nil, nonce, payload, additionalData)
	r := make([]byte, len(raw)+len(nonce[4:])+len(encryptedPayload))
	copy(r, raw)
	copy(r[len(raw):], nonce[4:])
	copy(r[len(raw)+len(nonce[4:]):], encryptedPayload)

	// Update recordLayer size to include explicit nonce
	binary.BigEndian.PutUint16(r[pkt.Header.Size()-2:], uint16(len(r)-pkt.Header.Size())) //nolint:gosec //G115

	return r, nil
}

// Decrypt decrypts a DTLS RecordLayer message.
func (g *GCM) Decrypt(header recordlayer.Header, in []byte) ([]byte, error) {
	err := header.Unmarshal(in)
	switch {
	case err != nil:
		return nil, err
	case header.ContentType == protocol.ContentTypeChangeCipherSpec:
		// Nothing to encrypt with ChangeCipherSpec
		return in, nil
	case len(in) <= (8 + header.Size()):
		return nil, errNotEnoughRoomForNonce
	}

	nonce := make([]byte, 0, gcmNonceLength)
	nonce = append(append(nonce, g.remoteWriteIV[:4]...), in[header.Size():header.Size()+8]...)
	out := in[header.Size()+8:]

	var additionalData []byte
	if header.ContentType == protocol.ContentTypeConnectionID {
		additionalData = generateAEADAdditionalDataCID(&header, len(out)-gcmTagLength)
	} else {
		additionalData = generateAEADAdditionalData(&header, len(out)-gcmTagLength)
	}
	out, err = g.remoteGCM.Open(out[:0], nonce, out, additionalData)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", errDecryptPacket, err) //nolint:errorlint
	}

	return append(in[:header.Size()], out...), nil
}

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