vendor.github.com.pion.srtp.v2.key_derivation.go Maven / Gradle / Ivy
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'
nMasterKey := len(masterKey)
nMasterSalt := len(masterSalt)
prfIn := make([]byte, nMasterKey)
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
}
out := make([]byte, ((outLen+nMasterKey)/nMasterKey)*nMasterKey)
var i uint16
for n := 0; n < outLen; n += nMasterKey {
binary.BigEndian.PutUint16(prfIn[nMasterKey-2:], i)
block.Encrypt(out[n:n+nMasterKey], 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
}