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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) []byte {
counter := make([]byte, 16)
binary.BigEndian.PutUint32(counter[4:], ssrc)
binary.BigEndian.PutUint32(counter[8:], rolloverCounter)
binary.BigEndian.PutUint32(counter[12:], uint32(sequenceNumber)<<16)
for i := range sessionSalt {
counter[i] ^= sessionSalt[i]
}
return counter
}
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