70 lines
2.2 KiB
Go
70 lines
2.2 KiB
Go
// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
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// SPDX-License-Identifier: MIT
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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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nMasterSalt := len(masterSalt)
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prfIn := make([]byte, 16)
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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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nBlockSize := block.BlockSize()
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out := make([]byte, ((outLen+nBlockSize-1)/nBlockSize)*nBlockSize)
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var i uint16
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for n := 0; n < outLen; n += nBlockSize {
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binary.BigEndian.PutUint16(prfIn[len(prfIn)-2:], i)
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block.Encrypt(out[n:n+nBlockSize], 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) (counter [16]byte) {
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copy(counter[:], sessionSalt)
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counter[4] ^= byte(ssrc >> 24)
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counter[5] ^= byte(ssrc >> 16)
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counter[6] ^= byte(ssrc >> 8)
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counter[7] ^= byte(ssrc)
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counter[8] ^= byte(rolloverCounter >> 24)
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counter[9] ^= byte(rolloverCounter >> 16)
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counter[10] ^= byte(rolloverCounter >> 8)
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counter[11] ^= byte(rolloverCounter)
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counter[12] ^= byte(sequenceNumber >> 8)
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counter[13] ^= byte(sequenceNumber)
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return counter
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}
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