Added support for NULL ciphers

Added support for NULL ciphers. When they are used, created SRTP and
SRTCP packets are authenticated only (no encryption).

Received SRTP/SRTCP packets are checked if their authentication tag
is valid, and extra SRTP protocol fields are removed before returning
then to application.

Fixed processing of SRTCP packets with E (encryption) bit cleared,
previously duplicate check and tag valiation was not performed, and
whole packet was returned as-is (with extra fields) from decryptRTCP.

Use of NULL ciphers can be enabled independently for SRTP and SRTCP
using SRTPNoEncryption and SRTCPNoEncryption options. They can be used
with key exchange protocols which allows to configure them separately.

Added support for SRTP_NULL_HMAC_SHA1_80 and SRTP_NULL_HMAC_SHA1_32
cipher suites. They use key and salt of the same length as AES_CM_128
ones.

Added new tests to verify test vectors from RFCs.

context.go

@@ -62,6 +62,9 @@ type Context struct {
 
 	sendMKI []byte                // Master Key Identifier used for encrypting RTP/RTCP packets. Set to nil if MKI is not enabled.
 	mkis    map[string]srtpCipher // Master Key Identifier to cipher mapping. Used for decrypting packets. Empty if MKI is not enabled.
+
+	encryptSRTP  bool
+	encryptSRTCP bool
 }
 
 // CreateContext creates a new SRTP Context.
@@ -83,6 +86,8 @@ func CreateContext(masterKey, masterSalt []byte, profile ProtectionProfile, opts
 		[]ContextOption{ // Default options
 			SRTPNoReplayProtection(),
 			SRTCPNoReplayProtection(),
+			SRTPEncryption(),
+			SRTCPEncryption(),
 		},
 		opts..., // User specified options
 	) {
@@ -91,7 +96,7 @@ func CreateContext(masterKey, masterSalt []byte, profile ProtectionProfile, opts
 		}
 	}
 
-	c.cipher, err = c.createCipher(c.sendMKI, masterKey, masterSalt)
+	c.cipher, err = c.createCipher(c.sendMKI, masterKey, masterSalt, c.encryptSRTP, c.encryptSRTCP)
 	if err != nil {
 		return nil, err
 	}
@@ -102,6 +107,26 @@ func CreateContext(masterKey, masterSalt []byte, profile ProtectionProfile, opts
 	return c, nil
 }
 
+// createContextWithCipher creates a new SRTP Context with a pre-created cipher. This is used for testing purposes only.
+func createContextWithCipher(profile ProtectionProfile, cipher srtpCipher) (*Context, error) {
+	c := &Context{
+		srtpSSRCStates:  map[uint32]*srtpSSRCState{},
+		srtcpSSRCStates: map[uint32]*srtcpSSRCState{},
+		profile:         profile,
+		mkis:            map[string]srtpCipher{},
+		cipher:          cipher,
+	}
+	err := SRTPNoReplayProtection()(c)
+	if err != nil {
+		return nil, err
+	}
+	err = SRTCPNoReplayProtection()(c)
+	if err != nil {
+		return nil, err
+	}
+	return c, nil
+}
+
 // AddCipherForMKI adds new MKI with associated masker key and salt. Context must be created with MasterKeyIndicator option
 // to enable MKI support. MKI must be unique and have the same length as the one used for creating Context.
 // Operation is not thread-safe, you need to provide synchronization with decrypting packets.
@@ -116,15 +141,15 @@ func (c *Context) AddCipherForMKI(mki, masterKey, masterSalt []byte) error {
 		return errMKIAlreadyInUse
 	}
 
-	cipher, err := c.createCipher(mki, masterKey, masterSalt)
+	cipher, err := c.createCipher(mki, masterKey, masterSalt, c.encryptSRTP, c.encryptSRTCP)
 	if err != nil {
 		return err
 	}
 	c.mkis[string(mki)] = cipher
 	return nil
 }
 
-func (c *Context) createCipher(mki, masterKey, masterSalt []byte) (srtpCipher, error) {
+func (c *Context) createCipher(mki, masterKey, masterSalt []byte, encryptSRTP, encryptSRTCP bool) (srtpCipher, error) {
 	keyLen, err := c.profile.KeyLen()
 	if err != nil {
 		return nil, err
@@ -143,9 +168,11 @@ func (c *Context) createCipher(mki, masterKey, masterSalt []byte) (srtpCipher, e
 
 	switch c.profile {
 	case ProtectionProfileAeadAes128Gcm, ProtectionProfileAeadAes256Gcm:
-		return newSrtpCipherAeadAesGcm(c.profile, masterKey, masterSalt, mki)
+		return newSrtpCipherAeadAesGcm(c.profile, masterKey, masterSalt, mki, encryptSRTP, encryptSRTCP)
 	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80:
-		return newSrtpCipherAesCmHmacSha1(c.profile, masterKey, masterSalt, mki)
+		return newSrtpCipherAesCmHmacSha1(c.profile, masterKey, masterSalt, mki, encryptSRTP, encryptSRTCP)
+	case ProtectionProfileNullHmacSha1_32, ProtectionProfileNullHmacSha1_80:
+		return newSrtpCipherAesCmHmacSha1(c.profile, masterKey, masterSalt, mki, false, false)
 	default:
 		return nil, fmt.Errorf("%w: %#v", errNoSuchSRTPProfile, c.profile)
 	}

option.go

@@ -84,3 +84,37 @@ func MasterKeyIndicator(mki []byte) ContextOption {
 		return nil
 	}
 }
+
+// SRTPEncryption enables SRTP encryption.
+func SRTPEncryption() ContextOption { // nolint:revive
+	return func(c *Context) error {
+		c.encryptSRTP = true
+		return nil
+	}
+}
+
+// SRTPNoEncryption disables SRTP encryption. This option is useful when you want to use NullCipher for SRTP and keep authentication only.
+// It simplifies debugging and testing, but it is not recommended for production use.
+func SRTPNoEncryption() ContextOption { // nolint:revive
+	return func(c *Context) error {
+		c.encryptSRTP = false
+		return nil
+	}
+}
+
+// SRTCPEncryption enables SRTCP encryption.
+func SRTCPEncryption() ContextOption {
+	return func(c *Context) error {
+		c.encryptSRTCP = true
+		return nil
+	}
+}
+
+// SRTCPNoEncryption disables SRTCP encryption. This option is useful when you want to use NullCipher for SRTCP and keep authentication only.
+// It simplifies debugging and testing, but it is not recommended for production use.
+func SRTCPNoEncryption() ContextOption {
+	return func(c *Context) error {
+		c.encryptSRTCP = false
+		return nil
+	}
+}

protection_profile.go

@@ -15,19 +15,24 @@ type ProtectionProfile uint16
 // in RFC 5764. They were in earlier draft of this RFC: https://datatracker.ietf.org/doc/html/draft-ietf-avt-dtls-srtp-03#section-4.1.2
 // Their IDs are now marked as reserved in the IANA registry. Despite this Chrome supports them:
 // https://chromium.googlesource.com/chromium/deps/libsrtp/+/84122798bb16927b1e676bd4f938a6e48e5bf2fe/srtp/include/srtp.h#694
+//
+// Null profiles disable encryption, they are used for debugging and testing. They are not recommended for production use.
+// Use of them is equivalent to using ProtectionProfileAes128CmHmacSha1_NN profile with SRTPNoEncryption and SRTCPNoEncryption options.
 const (
 	ProtectionProfileAes128CmHmacSha1_80 ProtectionProfile = 0x0001
 	ProtectionProfileAes128CmHmacSha1_32 ProtectionProfile = 0x0002
 	ProtectionProfileAes256CmHmacSha1_80 ProtectionProfile = 0x0003
 	ProtectionProfileAes256CmHmacSha1_32 ProtectionProfile = 0x0004
+	ProtectionProfileNullHmacSha1_80     ProtectionProfile = 0x0005
+	ProtectionProfileNullHmacSha1_32     ProtectionProfile = 0x0006
 	ProtectionProfileAeadAes128Gcm       ProtectionProfile = 0x0007
 	ProtectionProfileAeadAes256Gcm       ProtectionProfile = 0x0008
 )
 
-// KeyLen returns length of encryption key in bytes.
+// KeyLen returns length of encryption key in bytes. For all profiles except NullHmacSha1_32 and NullHmacSha1_80 is is also the length of the session key.
 func (p ProtectionProfile) KeyLen() (int, error) {
 	switch p {
-	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAeadAes128Gcm:
+	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAeadAes128Gcm, ProtectionProfileNullHmacSha1_32, ProtectionProfileNullHmacSha1_80:
 		return 16, nil
 	case ProtectionProfileAeadAes256Gcm, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80:
 		return 32, nil
@@ -36,10 +41,10 @@ func (p ProtectionProfile) KeyLen() (int, error) {
 	}
 }
 
-// SaltLen returns length of salt key in bytes.
+// SaltLen returns length of salt key in bytes. For all profiles except NullHmacSha1_32 and NullHmacSha1_80 is is also the length of the session salt.
 func (p ProtectionProfile) SaltLen() (int, error) {
 	switch p {
-	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80:
+	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80, ProtectionProfileNullHmacSha1_32, ProtectionProfileNullHmacSha1_80:
 		return 14, nil
 	case ProtectionProfileAeadAes128Gcm, ProtectionProfileAeadAes256Gcm:
 		return 12, nil
@@ -51,9 +56,9 @@ func (p ProtectionProfile) SaltLen() (int, error) {
 // AuthTagRTPLen returns length of RTP authentication tag in bytes for AES protection profiles. For AEAD ones it returns zero.
 func (p ProtectionProfile) AuthTagRTPLen() (int, error) {
 	switch p {
-	case ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_80:
+	case ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_80, ProtectionProfileNullHmacSha1_80:
 		return 10, nil
-	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_32:
+	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileNullHmacSha1_32:
 		return 4, nil
 	case ProtectionProfileAeadAes128Gcm, ProtectionProfileAeadAes256Gcm:
 		return 0, nil
@@ -65,7 +70,7 @@ func (p ProtectionProfile) AuthTagRTPLen() (int, error) {
 // AuthTagRTCPLen returns length of RTCP authentication tag in bytes for AES protection profiles. For AEAD ones it returns zero.
 func (p ProtectionProfile) AuthTagRTCPLen() (int, error) {
 	switch p {
-	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80:
+	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80, ProtectionProfileNullHmacSha1_32, ProtectionProfileNullHmacSha1_80:
 		return 10, nil
 	case ProtectionProfileAeadAes128Gcm, ProtectionProfileAeadAes256Gcm:
 		return 0, nil
@@ -77,7 +82,7 @@ func (p ProtectionProfile) AuthTagRTCPLen() (int, error) {
 // AEADAuthTagLen returns length of authentication tag in bytes for AEAD protection profiles. For AES ones it returns zero.
 func (p ProtectionProfile) AEADAuthTagLen() (int, error) {
 	switch p {
-	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80:
+	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80, ProtectionProfileNullHmacSha1_32, ProtectionProfileNullHmacSha1_80:
 		return 0, nil
 	case ProtectionProfileAeadAes128Gcm, ProtectionProfileAeadAes256Gcm:
 		return 16, nil
@@ -89,7 +94,7 @@ func (p ProtectionProfile) AEADAuthTagLen() (int, error) {
 // AuthKeyLen returns length of authentication key in bytes for AES protection profiles. For AEAD ones it returns zero.
 func (p ProtectionProfile) AuthKeyLen() (int, error) {
 	switch p {
-	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80:
+	case ProtectionProfileAes128CmHmacSha1_32, ProtectionProfileAes128CmHmacSha1_80, ProtectionProfileAes256CmHmacSha1_32, ProtectionProfileAes256CmHmacSha1_80, ProtectionProfileNullHmacSha1_32, ProtectionProfileNullHmacSha1_80:
 		return 20, nil
 	case ProtectionProfileAeadAes128Gcm, ProtectionProfileAeadAes256Gcm:
 		return 0, nil
@@ -113,6 +118,10 @@ func (p ProtectionProfile) String() string {
 		return "SRTP_AEAD_AES_128_GCM"
 	case ProtectionProfileAeadAes256Gcm:
 		return "SRTP_AEAD_AES_256_GCM"
+	case ProtectionProfileNullHmacSha1_80:
+		return "SRTP_NULL_HMAC_SHA1_80"
+	case ProtectionProfileNullHmacSha1_32:
+		return "SRTP_NULL_HMAC_SHA1_32"
 	default:
 		return fmt.Sprintf("Unknown SRTP profile: %#v", p)
 	}

srtcp.go

@@ -12,9 +12,9 @@ import (
 
 const maxSRTCPIndex = 0x7FFFFFFF
 
-func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
-	out := allocateIfMismatch(dst, encrypted)
+const srtcpHeaderSize = 8
 
+func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
 	authTagLen, err := c.cipher.AuthTagRTCPLen()
 	if err != nil {
 		return nil, err
@@ -24,12 +24,10 @@ func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
 		return nil, err
 	}
 	mkiLen := len(c.sendMKI)
-	tailOffset := len(encrypted) - (authTagLen + mkiLen + srtcpIndexSize)
 
-	if tailOffset < aeadAuthTagLen {
+	// Verify that encrypted packet is long enough
+	if len(encrypted) < (srtcpHeaderSize + aeadAuthTagLen + srtcpIndexSize + mkiLen + authTagLen) {
 		return nil, fmt.Errorf("%w: %d", errTooShortRTCP, len(encrypted))
-	} else if isEncrypted := encrypted[tailOffset] >> 7; isEncrypted == 0 {
-		return out, nil
 	}
 
 	index := c.cipher.getRTCPIndex(encrypted)
@@ -51,6 +49,8 @@ func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
 		}
 	}
 
+	out := allocateIfMismatch(dst, encrypted)
+
 	out, err = cipher.decryptRTCP(out, encrypted, index, ssrc)
 	if err != nil {
 		return nil, err
@@ -74,7 +74,7 @@ func (c *Context) DecryptRTCP(dst, encrypted []byte, header *rtcp.Header) ([]byt
 }
 
 func (c *Context) encryptRTCP(dst, decrypted []byte) ([]byte, error) {
-	if len(decrypted) < 8 {
+	if len(decrypted) < srtcpHeaderSize {
 		return nil, fmt.Errorf("%w: %d", errTooShortRTCP, len(decrypted))
 	}
 

srtp.go

@@ -5,6 +5,8 @@
 package srtp
 
 import (
+	"fmt"
+
 	"github.com/pion/rtp"
 )
 
@@ -13,9 +15,15 @@ func (c *Context) decryptRTP(dst, ciphertext []byte, header *rtp.Header, headerL
 	if err != nil {
 		return nil, err
 	}
+	aeadAuthTagLen, err := c.cipher.AEADAuthTagLen()
+	if err != nil {
+		return nil, err
+	}
+	mkiLen := len(c.sendMKI)
 
-	if len(ciphertext) < headerLen+len(c.sendMKI)+authTagLen {
-		return nil, errTooShortRTP
+	// Verify that encrypted packet is long enough
+	if len(ciphertext) < (headerLen + aeadAuthTagLen + mkiLen + authTagLen) {
+		return nil, fmt.Errorf("%w: %d", errTooShortRTP, len(ciphertext))
 	}
 
 	s := c.getSRTPSSRCState(header.SSRC)

srtp_cipher.go

@@ -49,3 +49,13 @@ It is *before* the ESRTCP word (Encrypted-flag and SRTCP index).
 
 See https://tools.ietf.org/html/rfc7714 for the full specifications.
 */
+
+// deriveSessionKeys should be used in tests only. RFCs test vectors specifes derived keys to use, this struct is used to inject them into the cipher in tests.
+type derivedSessionKeys struct {
+	srtpSessionKey      []byte
+	srtpSessionSalt     []byte
+	srtpSessionAuthTag  []byte
+	srtcpSessionKey     []byte
+	srtcpSessionSalt    []byte
+	srtcpSessionAuthTag []byte
+}