key_derivation.go

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

package srtp

import (
	"crypto/aes"
	"encoding/binary"
)

func aesCmKeyDerivation(label byte, masterKey, masterSalt []byte, indexOverKdr int, outLen int) ([]byte, error) {
	if indexOverKdr != 0 {
		// 24-bit "index DIV kdr" must be xored to prf input.
		return nil, errNonZeroKDRNotSupported
	}

	// https://tools.ietf.org/html/rfc3711#appendix-B.3
	// The input block for AES-CM is generated by exclusive-oring the master salt with the
	// concatenation of the encryption key label 0x00 with (index DIV kdr),
	// - index is 'rollover count' and DIV is 'divided by'

	nMasterSalt := len(masterSalt)

	prfIn := make([]byte, 16)
	copy(prfIn[:nMasterSalt], masterSalt)

	prfIn[7] ^= label

	// The resulting value is then AES encrypted using the master key to get the cipher key.
	block, err := aes.NewCipher(masterKey)
	if err != nil {
		return nil, err
	}

	nBlockSize := block.BlockSize()
	out := make([]byte, ((outLen+nBlockSize-1)/nBlockSize)*nBlockSize)
	var i uint16
	for n := 0; n < outLen; n += nBlockSize {
		binary.BigEndian.PutUint16(prfIn[len(prfIn)-2:], i)
		block.Encrypt(out[n:n+nBlockSize], prfIn)
		i++
	}

	return out[:outLen], nil
}

// Generate IV https://tools.ietf.org/html/rfc3711#section-4.1.1
// where the 128-bit integer value IV SHALL be defined by the SSRC, the
// SRTP packet index i, and the SRTP session salting key k_s, as below.
// - ROC = a 32-bit unsigned rollover counter (ROC), which records how many
// -       times the 16-bit RTP sequence number has been reset to zero after
// -       passing through 65,535
// i = 2^16 * ROC + SEQ
// IV = (salt*2 ^ 16) | (ssrc*2 ^ 64) | (i*2 ^ 16).
func generateCounter(
	sequenceNumber uint16,
	rolloverCounter uint32,
	ssrc uint32, sessionSalt []byte,
) (counter [16]byte) {
	copy(counter[:], sessionSalt)

	counter[4] ^= byte(ssrc >> 24)
	counter[5] ^= byte(ssrc >> 16)
	counter[6] ^= byte(ssrc >> 8)
	counter[7] ^= byte(ssrc)
	counter[8] ^= byte(rolloverCounter >> 24)
	counter[9] ^= byte(rolloverCounter >> 16)
	counter[10] ^= byte(rolloverCounter >> 8)
	counter[11] ^= byte(rolloverCounter)
	counter[12] ^= byte(sequenceNumber >> 8)
	counter[13] ^= byte(sequenceNumber)

	return counter
}