Class SRtpUtils

Static class that provides static functions for performing various Secure RTP (SRTP) calculations.

Inheritance

object

SRtpUtils

Inherited Members

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object.Equals(object, object)

object.GetHashCode()

object.GetType()

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Namespace: SipLib.RtpCrypto

Assembly: SipLib.dll

Syntax

public static class SRtpUtils

Fields

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n_a

Default size of the authentication key in bits. See Section 5.2 of RFC 3711.

Declaration

public const int n_a = 160

Field Value

Type	Description
int

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n_aB

Default size of the authentication key in bytes. See Section 5.2 of RFC 3711.

Declaration

public const int n_aB = 20

Field Value

Type	Description
int

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n_b

Bit size of the block for the block cipher

Declaration

public const int n_b = 128

Field Value

Type	Description
int

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n_e

Bit size of the encryption key.

Declaration

public const int n_e = 128

Field Value

Type	Description
int

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n_eB

Byte size of the encryption key

Declaration

public const int n_eB = 16

Field Value

Type	Description
int

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n_s

Bit size of the session salting key

Declaration

public const int n_s = 112

Field Value

Type	Description
int

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n_sB

Byte size of the session salting key

Declaration

public const int n_sB = 14

Field Value

Type	Description
int

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n_tag

Default bit size of the authentication tag.

Declaration

public const int n_tag = 80

Field Value

Type	Description
int

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n_tagB

Default byte size of the authentication tag.

Declaration

public const int n_tagB = 10

Field Value

Type	Description
int

Methods

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AuthRtpPacket(byte[], byte[], uint, byte[], byte[])

Authenticates an SRTP or an SRTCP packet. See Section 4.2 of RFC 3711. This function computes the authentication tag for the packet using the HMAC-SHA1 algorithm and compares it to the authentication tag that was sent with the SRTP or SRTCP packet.

Declaration

public static bool AuthRtpPacket(byte[] HdrBytes, byte[] PacketPayload, uint ROCorSRTCPIndex, byte[] k_a, byte[] AuthTag)

Parameters

Type	Name	Description
byte[]	HdrBytes	Byte array containing the RTP or RTCP packet header.
byte[]	PacketPayload	Byte array containing the payload of the RTP or RTCP packet. This is the encrypted portion of the packet.
uint	ROCorSRTCPIndex	Roll Over Counter (ROC) value or the SRTCP Index.
byte[]	k_a	Byte array containing the session authentication key.
byte[]	AuthTag	Byte array containing the authentication tag read from the SRTP packet.

Returns

Type	Description
bool	Returns true if the packet is successfully authenticated, i.e. the computed authentication tag matches the received authentication tag.

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CalcAesCmIV(byte[], uint, ulong)

Calculates the Initialization Vector (IV) for AES in Counter Mode (AES-CM) as specified in Section 4.1.1 AES in Counter Mode of RFC 3711.

Declaration

public static byte[] CalcAesCmIV(byte[] k_s, uint SSRC, ulong i)

Parameters

Type	Name	Description
byte[]	k_s	Session salting key. Must be at least n_sB bytes in length.
uint	SSRC	SSRC value from the RTCP packet.
ulong	i	Packet index for the RTP packet. Calculated as using the PacketIndex() function as specifed in Section 3.3.1 and Appendix A of RFC 3711.

Returns

Type	Description
byte[]	Returns the IV array, which is n_eB (16) bytes in length.

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CalcF8SRTCPIV(RtcpHeader, uint, uint)

Calculates the Initialization Vector (IV) for the F8 AES encryption algorithm for a RTCP packet given the RTCP packet and the SRTCP packet index. See Section 4.1.2.3 of RFC 3711.

Declaration

public static byte[] CalcF8SRTCPIV(RtcpHeader RtcpHdr, uint SrtcpIndex, uint SSRC)

Parameters

Type	Name	Description
RtcpHeader	RtcpHdr	RTCP packet header to compute the IV for.
uint	SrtcpIndex	SRTCP packet index as read from the the SRTCP packet. Includes the "E" bit. See Figure 2 of RFC 3711.
uint	SSRC	SSRC for the sender of the RTCP packet.

Returns

Type	Description
byte[]	Returns the 16-byte long IV to use for the IV for an RTCP packet.

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CalcF8SRTPIV(RtpPacket, uint)

Calculates the Initialization Vector (IV) for the F8 AES encryption algorithm for a RTP packet given the RTP packet header and the Roll Over Counter (ROC). See Section 4.1.2.2 of RFC 3711.

Declaration

public static byte[] CalcF8SRTPIV(RtpPacket RtpPckt, uint ROC)

Parameters

Type	Name	Description
RtpPacket	RtpPckt	RTP packet header.
uint	ROC	Current Roll Over Counter value.

Returns

Type	Description
byte[]	Returns the 16-byte long IV to use for the IV for an RTP packet.

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CalcRtpPacketAuthTag(byte[], byte[], uint, byte[], int)

Calculates the authentication tag for an RTP or an RTCP packet. See Section 4.2 of RFC 3711.

Declaration

public static byte[] CalcRtpPacketAuthTag(byte[] HdrBytes, byte[] Payload, uint ROCorSRTCPIndex, byte[] k_a, int TagLength)

Parameters

Type	Name	Description
byte[]	HdrBytes	Byte array containing the RTP packet header.///
byte[]	Payload	Byte array containing the payload of the RTP or RTCP packet. This is the encrypted portion of the packet.
uint	ROCorSRTCPIndex	Roll Over Counter (ROC) value.
byte[]	k_a	Byte array containing the session authentication key.
int	TagLength	Length in bytes of the authentication tag to return. This must be either 10 (for 80 bits) or 4 (for 32 bits) depending upon the encryption profile being used.

Returns

Type	Description
byte[]	The authentication tag

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DeriveRtcpSessionKeys(ulong, MasterKeys, CryptoContext)

Derives the RTCP session keys for a CryptoContext. See Section 4.3.2 of RFC 3711.

Declaration

public static void DeriveRtcpSessionKeys(ulong PI, MasterKeys Mks, CryptoContext Context)

Parameters

Type	Name	Description
ulong	PI	Packet Index
MasterKeys	Mks	Master keys to use to derive the session keya
CryptoContext	Context	CryptoContext to write the session keys to

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DeriveRtpSessionKeys(ulong, MasterKeys, CryptoContext)

Derives the RTP session keys for a CryptoContext. See Section 4.3.1 of RFC 3711

Declaration

public static void DeriveRtpSessionKeys(ulong PI, MasterKeys Mks, CryptoContext Context)

Parameters

Type	Name	Description
ulong	PI	Packet Index
MasterKeys	Mks	Master keys to use to derive the session keya
CryptoContext	Context	CryptoContext to write the session keys to

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DeriveSrtpSessionKey(ulong, ulong, SrtpLabelItem, byte[], byte[], byte[])

Derives a session key given the master key and the master salt for SRTP. See Section 4.3.1 Key Derivation Algorithm and Section 4.3.2 SRTCP Key Derivation of RFC 3711.

Declaration

public static byte[] DeriveSrtpSessionKey(ulong PI, ulong kdr, SrtpLabelItem Label, byte[] MasterSalt, byte[] MasterKey, byte[] InputZeroArray)

Parameters

Type	Name	Description
ulong	PI	Packet index for the SRTP packet as calculated by the PacketIndex() function is for SRTP. Pass in 32-bit value 0 \|\| SRTCP Index from the SRTCP packet as described in Section 4.3.2 of RFC 3711.
ulong	kdr	Key Derivation Rate (KDR)
SrtpLabelItem	Label	Identifies the type of key to derive.
byte[]	MasterSalt	Master salt array.
byte[]	MasterKey	Master Key array.
byte[]	InputZeroArray	Array of zero values to use as the input for the key derivation. The length of this array determines the length of the array returned by this function.

Returns

Type	Description
byte[]	Returns a byte array containing the derived key, salting key or the authentication key.

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ExtractDelimStr(char, char, string)

Extracts a string that is delimited by two characters. The delimiter characters may be different.

Declaration

public static string? ExtractDelimStr(char D1, char D2, string strInput)

Parameters

Type	Name	Description
char	D1	First delimiter character.
char	D2	Second delimiter character.
string	strInput	Input string.

Returns

Type	Description
string	Returns the delimited string. Returns null if the delimiters are not present or the extracted string length is zero.

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GetValueOfNameValuePair(string, char)

Gets the value portion of a name/value pair from a string that is formatted as a name followed by a 1 character separator followed by a value. For example Param=Value or Param:Value

Declaration

public static string? GetValueOfNameValuePair(string Input, char Sep)

Parameters

Type	Name	Description
string	Input	Input string containing the name/value.
char	Sep	Separator character such as '=' or ':'

Returns

Type	Description
string	Returns the value portion or null if the input string is not in the proper format.

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PacketIndex(ushort, ushort, RocVals)

Calculates the packet index for an RTP packet. See Section 3.3.1 and Appendix A of RFC 3711. This algorithm accounds for rollover of the sequence number and the impact of packet loss.

Declaration

public static ulong PacketIndex(ushort SEQ, ushort s_l, RocVals Rv)

Parameters

Type	Name	Description
ushort	SEQ	Sequence number (SEQ) read from the RTP packet header.
ushort	s_l	Highest received SEQ number so far.
RocVals	Rv	Stored ROC values.

Returns

Type	Description
ulong	Returns the packet index as a 48-bit number right justified in the lower 48 bits of a 64-bit unsigned integer.