CryptoContext(3)

NAME
CryptoContext -

The implementation for a SRTP cryptographic context.

SYNOPSIS

#include <CryptoContext.h>

  Public Member Functions

CryptoContext (uint32 ssrc)
Constructor for empty SRTP cryptographic context.
CryptoContext (uint32 ssrc, int32 roc, int64 keyDerivRate, const int32 ealg, const int32 aalg, uint8 *masterKey, int32 masterKeyLength, uint8 *masterSalt, int32 masterSaltLength, int32 ekeyl, int32 akeyl, int32 skeyl, int32 tagLength)
Constructor for an active SRTP cryptographic context.
~CryptoContext ()
Destructor.
void setRoc (uint32 r)
Set the Roll-Over-Counter.
uint32 getRoc () const
Get the Roll-Over-Counter.
void srtpEncrypt (RTPPacket *rtp, uint64 index, uint32 ssrc)
Perform SRTP encryption.
void srtpAuthenticate (RTPPacket *rtp, uint32 roc, uint8 *tag)
Compute the authentication tag.
void deriveSrtpKeys (uint64 index)
Perform key derivation according to SRTP specification.
uint64 guessIndex (uint16 newSeqNumber)
Compute (guess) the new SRTP index based on the sequence number of a received RTP packet.
bool checkReplay (uint16 newSeqNumber)
Check for packet replay.
void update (uint16 newSeqNumber)
Update the SRTP packet index.
int32 getTagLength () const
Get the length of the SRTP authentication tag in bytes.
int32 getMkiLength () const
Get the length of the MKI in bytes.
uint32 getSsrc () const
Get the SSRC of this SRTP Cryptograhic context.
CryptoContext * newCryptoContextForSSRC (uint32 ssrc, int roc, int64 keyDerivRate)
Derive a new Crypto Context for use with a new SSRC.

Detailed Description

The implementation for a SRTP cryptographic context.

This class holds data and provides functions that implement a cryptographic context for SRTP, Refer to RFC 3711, chapter 3.2 for some more detailed information about the SRTP cryptographic context.

Each SRTP cryptographic context maintains a RTP source identified by its SSRC. Thus you can independently protect each source inside a RTP session.

Key management mechanisms negotiate the parameters for the SRTP cryptographic context, such as master key, key length, authentication length and so on. The key management mechanisms are not part of SRTP. Refer to MIKEY (RFC 3880) or to Phil Zimmermann's ZRTP protocol (draft-zimmermann-avt-zrtp-01). After key management negotiated the data the application can setup the SRTP cryptographic context and enable SRTP processing.

Currently this implementation supports RTP only, not RTCP.

Author:
    Israel Abad i_abad@terra.es

    Erik Eliasson eliasson@it.kth.se

    Johan Bilien jobi@via.ecp.fr

    Joachim Orrblad joachim@orrblad.com

    Werner Dittmann Werner.Dittmann@t-online.de

Constructor & Destructor Documentation

  CryptoContext::CryptoContext (uint32ssrc)

Constructor for empty SRTP cryptographic context. This constructor creates an empty SRTP cryptographic context were all algorithms are set to the null algorithm, that is no SRTP processing is performed.

Parameters:
    ssrc The RTP SSRC that this SRTP cryptographic context protects.

  CryptoContext::CryptoContext (uint32ssrc, int32roc, int64keyDerivRate, const int32ealg, const int32aalg, uint8 *masterKey, int32masterKeyLength, uint8 *masterSalt, int32masterSaltLength, int32ekeyl, int32akeyl, int32skeyl, int32tagLength)

Constructor for an active SRTP cryptographic context. This constructor creates an active SRTP cryptographic context were algorithms are enabled, keys are computed and so on. This SRTP cryptographic context can protect a RTP SSRC stream.

Parameters:
    ssrc The RTP SSRC that this SRTP cryptographic context protects.
    roc The initial Roll-Over-Counter according to RFC 3711. These are the upper 32 bit of the overall 48 bit SRTP packet index. Refer to chapter 3.2.1 of the RFC.
    keyDerivRate The key derivation rate defines when to recompute the SRTP session keys. Refer to chapter 4.3.1 in the RFC.
    ealg The encryption algorithm to use. Possible values are SrtpEncryptionNull, SrtpEncryptionAESCM, SrtpEncryptionAESF8 . See chapter 4.1.1 for AESCM (Counter mode) and 4.1.2 for AES F8 mode.
    aalg The authentication algorithm to use. Possible values are SrtpEncryptionNull, SrtpAuthenticationSha1Hmac. The only active algorithm here is SHA1 HMAC, a SHA1 based hashed message authentication code as defined in RFC 2104.
    masterKey Pointer to the master key for this SRTP cryptographic context. Must point to masterKeyLength bytes. Refer to chapter 3.2.1 of the RFC about the role of the master key.
    masterKeyLength The length in bytes of the master key in bytes. The length must match the selected encryption algorithm. Because SRTP uses AES based encryption only, then master key length may be 16 or 32 bytes (128 or 256 bit master key)
    masterSalt SRTP uses the master salt to computer the initialization vector that in turn is input to compute the session key, session authentication key and the session salt.
    masterSaltLength The length in bytes of the master salt data in bytes. SRTP uses AES as encryption algorithm. AES encrypts 16 byte blocks (independent of the key length). According to RFC3711 the standard value for the master salt length should be 112 bit (14 bytes).
    ekeyl The length in bytes of the session encryption key that SRTP shall compute and use. Usually the same length as for the master key length. But you may use a different length as well. Be carefull that the key management mechanisms supports different key lengths.
    akeyl The length in bytes of the session authentication key. SRTP computes this key and uses it as input to the authentication algorithm. The standard value is 160 bits (20 bytes).
    skeyl The length in bytes of the session salt. SRTP computes this salt key and uses it as input during encryption. The length usually is the same as the master salt length.
    tagLength The length is bytes of the authentication tag that SRTP appends to the RTP packet. Refer to chapter 4.2. in the RFC 3711.

  CryptoContext::~CryptoContext ()

Destructor. Cleans the SRTP cryptographic context.

Member Function Documentation

  bool CryptoContext::checkReplay (uint16newSeqNumber)

Check for packet replay. The method check if a received packet is either to old or was already received.

The method supports a 64 packet history relative the the given sequence number.

Parameters:
    newSeqNumber The sequence number of the received RTP packet in host order.

Returns:
    true if no replay, false if packet is too old ar was already received.

  void CryptoContext::deriveSrtpKeys (uint64index)

Perform key derivation according to SRTP specification. This method computes the session key, session authentication key and the session salt key. This method must be called at least once after the SRTP Cryptograhic context was set up.

Parameters:
    index The 48 bit SRTP packet index. See the guessIndex method.

  int32 CryptoContext::getMkiLength () const [inline]

Get the length of the MKI in bytes.

Returns:
    the length of the MKI.

  uint32 CryptoContext::getRoc () const [inline]

Get the Roll-Over-Counter. Ths method get the upper 32 bit of the 48 bit SRTP packet index (the roll-over-part)

Returns:
    The roll-over-counter

  uint32 CryptoContext::getSsrc () const [inline]

Get the SSRC of this SRTP Cryptograhic context.

Returns:
    the SSRC.

  int32 CryptoContext::getTagLength () const [inline]

Get the length of the SRTP authentication tag in bytes.

Returns:
    the length of the authentication tag.

  uint64 CryptoContext::guessIndex (uint16newSeqNumber)

Compute (guess) the new SRTP index based on the sequence number of a received RTP packet. The method uses the algorithm show in RFC3711, Appendix A, to compute the new index.

Parameters:
    newSeqNumber The sequence number of the received RTP packet in host order.

Returns:
    The new SRTP packet index

  CryptoContext* CryptoContext::newCryptoContextForSSRC (uint32ssrc, introc, int64keyDerivRate)

Derive a new Crypto Context for use with a new SSRC. This method returns a new Crypto Context initialized with the data of this crypto context. Replacing the SSRC, Roll-over-Counter, and the key derivation rate the application cab use this Crypto Context to encrypt / decrypt a new stream (Synchronization source) inside one RTP session.

Before the application can use this crypto context it must call the deriveSrtpKeys method.

Parameters:
    ssrc The SSRC for this context
    roc The Roll-Over-Counter for this context
    keyDerivRate The key derivation rate for this context

Returns:
    a new CryptoContext with all relevant data set.

  void CryptoContext::setRoc (uint32r) [inline]

Set the Roll-Over-Counter. Ths method sets the upper 32 bit of the 48 bit SRTP packet index (the roll-over-part)

Parameters:
    r The roll-over-counter

  void CryptoContext::srtpAuthenticate (RTPPacket *rtp, uint32roc, uint8 *tag)

Compute the authentication tag. Compute the authentication tag according the the parameters in the SRTP Cryptograhic context.

Parameters:
    rtp The RTP packet that contains the data to authenticate.
    roc The 32 bit SRTP roll-over-counter.
    tag Points to a buffer that hold the computed tag. This buffer must be able to hold tagLength bytes.

  void CryptoContext::srtpEncrypt (RTPPacket *rtp, uint64index, uint32ssrc)

Perform SRTP encryption. This method encrypts and decrypts SRTP payload data. Plain data gets encrypted, encrypted data get decrypted.

Parameters:
    rtp The RTP packet that contains the data to encrypt.
    index The 48 bit SRTP packet index. See the guessIndex method.
    ssrc The RTP SSRC data in host order.

  void CryptoContext::update (uint16newSeqNumber)

Update the SRTP packet index. Call this method after all checks were successful. See chapter 3.3.1 in the RFC when to update the ROC and ROC processing.

Parameters:
    newSeqNumber The sequence number of the received RTP packet in host order.

Author

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