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server/vendor/github.com/pion/srtp/v2/key_derivation.go

@@ -0,0 +1,69 @@
+// 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
+}