64 lines
2.0 KiB
Go
64 lines
2.0 KiB
Go
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package srtp
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import (
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"crypto/aes"
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"encoding/binary"
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)
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func aesCmKeyDerivation(label byte, masterKey, masterSalt []byte, indexOverKdr int, outLen int) ([]byte, error) {
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if indexOverKdr != 0 {
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// 24-bit "index DIV kdr" must be xored to prf input.
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return nil, errNonZeroKDRNotSupported
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}
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// https://tools.ietf.org/html/rfc3711#appendix-B.3
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// The input block for AES-CM is generated by exclusive-oring the master salt with the
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// concatenation of the encryption key label 0x00 with (index DIV kdr),
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// - index is 'rollover count' and DIV is 'divided by'
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nMasterKey := len(masterKey)
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nMasterSalt := len(masterSalt)
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prfIn := make([]byte, nMasterKey)
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copy(prfIn[:nMasterSalt], masterSalt)
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prfIn[7] ^= label
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// The resulting value is then AES encrypted using the master key to get the cipher key.
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block, err := aes.NewCipher(masterKey)
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if err != nil {
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return nil, err
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}
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out := make([]byte, ((outLen+nMasterKey)/nMasterKey)*nMasterKey)
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var i uint16
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for n := 0; n < outLen; n += nMasterKey {
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binary.BigEndian.PutUint16(prfIn[nMasterKey-2:], i)
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block.Encrypt(out[n:n+nMasterKey], prfIn)
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i++
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}
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return out[:outLen], nil
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}
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// Generate IV https://tools.ietf.org/html/rfc3711#section-4.1.1
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// where the 128-bit integer value IV SHALL be defined by the SSRC, the
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// SRTP packet index i, and the SRTP session salting key k_s, as below.
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// - ROC = a 32-bit unsigned rollover counter (ROC), which records how many
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// - times the 16-bit RTP sequence number has been reset to zero after
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// - passing through 65,535
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// i = 2^16 * ROC + SEQ
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// IV = (salt*2 ^ 16) | (ssrc*2 ^ 64) | (i*2 ^ 16)
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func generateCounter(sequenceNumber uint16, rolloverCounter uint32, ssrc uint32, sessionSalt []byte) []byte {
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counter := make([]byte, 16)
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binary.BigEndian.PutUint32(counter[4:], ssrc)
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binary.BigEndian.PutUint32(counter[8:], rolloverCounter)
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binary.BigEndian.PutUint32(counter[12:], uint32(sequenceNumber)<<16)
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for i := range sessionSalt {
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counter[i] ^= sessionSalt[i]
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}
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return counter
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}
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