Source File
	ccm.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/rand"
	"encoding/binary"
	"fmt"

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

// CCMTagLen is the length of Authentication Tag.
type CCMTagLen int

// CCM Enums.
const (
	CCMTagLength8  CCMTagLen = 8
	CCMTagLength   CCMTagLen = 16
	ccmNonceLength           = 12
)

// CCM Provides an API to Encrypt/Decrypt DTLS 1.2 Packets.
type CCM struct {
	localCCM, remoteCCM         ccm.CCM
	localWriteIV, remoteWriteIV []byte
	tagLen                      CCMTagLen
}

// NewCCM creates a DTLS GCM Cipher.
func NewCCM(tagLen CCMTagLen, localKey, localWriteIV, remoteKey, remoteWriteIV []byte) (*CCM, error) {
	localBlock, err := aes.NewCipher(localKey)
	if err != nil {
		return nil, err
	}
	localCCM, err := ccm.NewCCM(localBlock, int(tagLen), ccmNonceLength)
	if err != nil {
		return nil, err
	}

	remoteBlock, err := aes.NewCipher(remoteKey)
	if err != nil {
		return nil, err
	}
	remoteCCM, err := ccm.NewCCM(remoteBlock, int(tagLen), ccmNonceLength)
	if err != nil {
		return nil, err
	}

	return &CCM{
		localCCM:      localCCM,
		localWriteIV:  localWriteIV,
		remoteCCM:     remoteCCM,
		remoteWriteIV: remoteWriteIV,
		tagLen:        tagLen,
	}, nil
}

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

	nonce := append(append([]byte{}, c.localWriteIV[:4]...), make([]byte, 8)...)
	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 := c.localCCM.Seal(nil, nonce, payload, additionalData)

	encryptedPayload = append(nonce[4:], encryptedPayload...)
	raw = append(raw, encryptedPayload...)

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

	return raw, nil
}

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

	nonce := append(append([]byte{}, c.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)-int(c.tagLen))
	} else {
		additionalData = generateAEADAdditionalData(&header, len(out)-int(c.tagLen))
	}
	var err error
	out, err = c.remoteCCM.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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