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package srtp
import ( //nolint:gci
"crypto/aes"
"crypto/cipher"
"crypto/hmac"
"crypto/sha1" //nolint:gosec
"crypto/subtle"
"encoding/binary"
"hash"
"github.com/pion/rtp"
)
type srtpCipherAesCmHmacSha1 struct {
srtpSessionSalt []byte
srtpSessionAuth hash.Hash
srtpBlock cipher.Block
srtcpSessionSalt []byte
srtcpSessionAuth hash.Hash
srtcpBlock cipher.Block
}
func newSrtpCipherAesCmHmacSha1(masterKey, masterSalt []byte) (*srtpCipherAesCmHmacSha1, error) {
s := &srtpCipherAesCmHmacSha1{}
srtpSessionKey, err := aesCmKeyDerivation(labelSRTPEncryption, masterKey, masterSalt, 0, len(masterKey))
if err != nil {
return nil, err
} else if s.srtpBlock, err = aes.NewCipher(srtpSessionKey); err != nil {
return nil, err
}
srtcpSessionKey, err := aesCmKeyDerivation(labelSRTCPEncryption, masterKey, masterSalt, 0, len(masterKey))
if err != nil {
return nil, err
} else if s.srtcpBlock, err = aes.NewCipher(srtcpSessionKey); err != nil {
return nil, err
}
if s.srtpSessionSalt, err = aesCmKeyDerivation(labelSRTPSalt, masterKey, masterSalt, 0, len(masterSalt)); err != nil {
return nil, err
} else if s.srtcpSessionSalt, err = aesCmKeyDerivation(labelSRTCPSalt, masterKey, masterSalt, 0, len(masterSalt)); err != nil {
return nil, err
}
authKeyLen, err := ProtectionProfileAes128CmHmacSha1_80.authKeyLen()
if err != nil {
return nil, err
}
srtpSessionAuthTag, err := aesCmKeyDerivation(labelSRTPAuthenticationTag, masterKey, masterSalt, 0, authKeyLen)
if err != nil {
return nil, err
}
srtcpSessionAuthTag, err := aesCmKeyDerivation(labelSRTCPAuthenticationTag, masterKey, masterSalt, 0, authKeyLen)
if err != nil {
return nil, err
}
s.srtcpSessionAuth = hmac.New(sha1.New, srtcpSessionAuthTag)
s.srtpSessionAuth = hmac.New(sha1.New, srtpSessionAuthTag)
return s, nil
}
func (s *srtpCipherAesCmHmacSha1) authTagLen() int {
return 10
}
func (s *srtpCipherAesCmHmacSha1) aeadAuthTagLen() int {
return 0
}
func (s *srtpCipherAesCmHmacSha1) encryptRTP(dst []byte, header *rtp.Header, payload []byte, roc uint32) (ciphertext []byte, err error) {
// Grow the given buffer to fit the output.
dst = growBufferSize(dst, header.MarshalSize()+len(payload)+s.authTagLen())
// Copy the header unencrypted.
n, err := header.MarshalTo(dst)
if err != nil {
return nil, err
}
// Encrypt the payload
counter := generateCounter(header.SequenceNumber, roc, header.SSRC, s.srtpSessionSalt)
stream := cipher.NewCTR(s.srtpBlock, counter)
stream.XORKeyStream(dst[n:], payload)
n += len(payload)
// Generate the auth tag.
authTag, err := s.generateSrtpAuthTag(dst[:n], roc)
if err != nil {
return nil, err
}
// Write the auth tag to the dest.
copy(dst[n:], authTag)
return dst, nil
}
func (s *srtpCipherAesCmHmacSha1) decryptRTP(dst, ciphertext []byte, header *rtp.Header, roc uint32) ([]byte, error) {
// Split the auth tag and the cipher text into two parts.
actualTag := ciphertext[len(ciphertext)-s.authTagLen():]
ciphertext = ciphertext[:len(ciphertext)-s.authTagLen()]
// Generate the auth tag we expect to see from the ciphertext.
expectedTag, err := s.generateSrtpAuthTag(ciphertext, roc)
if err != nil {
return nil, err
}
// See if the auth tag actually matches.
// We use a constant time comparison to prevent timing attacks.
if subtle.ConstantTimeCompare(actualTag, expectedTag) != 1 {
return nil, errFailedToVerifyAuthTag
}
// Write the plaintext header to the destination buffer.
copy(dst, ciphertext[:header.PayloadOffset])
// Decrypt the ciphertext for the payload.
counter := generateCounter(header.SequenceNumber, roc, header.SSRC, s.srtpSessionSalt)
stream := cipher.NewCTR(s.srtpBlock, counter)
stream.XORKeyStream(dst[header.PayloadOffset:], ciphertext[header.PayloadOffset:])
return dst, nil
}
func (s *srtpCipherAesCmHmacSha1) encryptRTCP(dst, decrypted []byte, srtcpIndex uint32, ssrc uint32) ([]byte, error) {
dst = allocateIfMismatch(dst, decrypted)
// Encrypt everything after header
stream := cipher.NewCTR(s.srtcpBlock, generateCounter(uint16(srtcpIndex&0xffff), srtcpIndex>>16, ssrc, s.srtcpSessionSalt))
stream.XORKeyStream(dst[8:], dst[8:])
// Add SRTCP Index and set Encryption bit
dst = append(dst, make([]byte, 4)...)
binary.BigEndian.PutUint32(dst[len(dst)-4:], srtcpIndex)
dst[len(dst)-4] |= 0x80
authTag, err := s.generateSrtcpAuthTag(dst)
if err != nil {
return nil, err
}
return append(dst, authTag...), nil
}
func (s *srtpCipherAesCmHmacSha1) decryptRTCP(out, encrypted []byte, index, ssrc uint32) ([]byte, error) {
tailOffset := len(encrypted) - (s.authTagLen() + srtcpIndexSize)
out = out[0:tailOffset]
expectedTag, err := s.generateSrtcpAuthTag(encrypted[:len(encrypted)-s.authTagLen()])
if err != nil {
return nil, err
}
actualTag := encrypted[len(encrypted)-s.authTagLen():]
if subtle.ConstantTimeCompare(actualTag, expectedTag) != 1 {
return nil, errFailedToVerifyAuthTag
}
stream := cipher.NewCTR(s.srtcpBlock, generateCounter(uint16(index&0xffff), index>>16, ssrc, s.srtcpSessionSalt))
stream.XORKeyStream(out[8:], out[8:])
return out, nil
}
func (s *srtpCipherAesCmHmacSha1) generateSrtpAuthTag(buf []byte, roc uint32) ([]byte, error) {
// https://tools.ietf.org/html/rfc3711#section-4.2
// In the case of SRTP, M SHALL consist of the Authenticated
// Portion of the packet (as specified in Figure 1) concatenated with
// the ROC, M = Authenticated Portion || ROC;
//
// The pre-defined authentication transform for SRTP is HMAC-SHA1
// [RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL
// be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to
// the session authentication key and M as specified above, i.e.,
// HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag
// left-most bits.
// - Authenticated portion of the packet is everything BEFORE MKI
// - k_a is the session message authentication key
// - n_tag is the bit-length of the output authentication tag
s.srtpSessionAuth.Reset()
if _, err := s.srtpSessionAuth.Write(buf); err != nil {
return nil, err
}
// For SRTP only, we need to hash the rollover counter as well.
rocRaw := [4]byte{}
binary.BigEndian.PutUint32(rocRaw[:], roc)
_, err := s.srtpSessionAuth.Write(rocRaw[:])
if err != nil {
return nil, err
}
// Truncate the hash to the first 10 bytes.
return s.srtpSessionAuth.Sum(nil)[0:s.authTagLen()], nil
}
func (s *srtpCipherAesCmHmacSha1) generateSrtcpAuthTag(buf []byte) ([]byte, error) {
// https://tools.ietf.org/html/rfc3711#section-4.2
//
// The pre-defined authentication transform for SRTP is HMAC-SHA1
// [RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL
// be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to
// the session authentication key and M as specified above, i.e.,
// HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag
// left-most bits.
// - Authenticated portion of the packet is everything BEFORE MKI
// - k_a is the session message authentication key
// - n_tag is the bit-length of the output authentication tag
s.srtcpSessionAuth.Reset()
if _, err := s.srtcpSessionAuth.Write(buf); err != nil {
return nil, err
}
return s.srtcpSessionAuth.Sum(nil)[0:s.authTagLen()], nil
}
func (s *srtpCipherAesCmHmacSha1) getRTCPIndex(in []byte) uint32 {
tailOffset := len(in) - (s.authTagLen() + srtcpIndexSize)
srtcpIndexBuffer := in[tailOffset : tailOffset+srtcpIndexSize]
return binary.BigEndian.Uint32(srtcpIndexBuffer) &^ (1 << 31)
}
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