Provided by: libccrtp-doc_2.0.9-2.2_all 
NAME
CryptoContextCtrl - The implementation for a SRTCP cryptographic context.
SYNOPSIS
#include <CryptoContextCtrl.h>
Public Member Functions
CryptoContextCtrl (uint32 ssrc)
Constructor for empty SRTP cryptographic context.
CryptoContextCtrl (uint32 ssrc, 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.
~CryptoContextCtrl ()
Destructor.
void srtcpEncrypt (uint8 *rtp, size_t len, uint64 index, uint32 ssrc)
Perform SRTP encryption.
void srtcpAuthenticate (uint8 *rtp, size_t len, uint32 roc, uint8 *tag)
Compute the authentication tag.
void deriveSrtcpKeys ()
Perform key derivation according to SRTP specification.
bool checkReplay (uint32 newSeqNumber)
Check for packet replay.
void update (uint32 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.
CryptoContextCtrl * newCryptoContextForSSRC (uint32 ssrc)
Derive a new Crypto Context for use with a new SSRC.
Detailed Description
The implementation for a SRTCP 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 SRTCP cryptographic context and enable SRTCP processing.
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
CryptoContextCtrl::CryptoContextCtrl (uint32 ssrc)
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.
CryptoContextCtrl::CryptoContextCtrl (uint32 ssrc, 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. 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.
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.
CryptoContextCtrl::~CryptoContextCtrl ()
Destructor. Cleans the SRTP cryptographic context.
Member Function Documentation
bool CryptoContextCtrl::checkReplay (uint32 newSeqNumber)
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 CryptoContextCtrl::deriveSrtcpKeys ()
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 CryptoContextCtrl::getMkiLength () const [inline]
Get the length of the MKI in bytes.
Returns:
the length of the MKI.
uint32 CryptoContextCtrl::getSsrc () const [inline]
Get the SSRC of this SRTP Cryptograhic context.
Returns:
the SSRC.
int32 CryptoContextCtrl::getTagLength () const [inline]
Get the length of the SRTP authentication tag in bytes.
Returns:
the length of the authentication tag.
CryptoContextCtrl* CryptoContextCtrl::newCryptoContextForSSRC (uint32 ssrc)
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 CryptoContextCtrl::srtcpAuthenticate (uint8 * rtp, size_t len, uint32 roc, 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 CryptoContextCtrl::srtcpEncrypt (uint8 * rtp, size_t len, uint64 index, uint32 ssrc)
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 CryptoContextCtrl::update (uint32 newSeqNumber)
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 RTCP packet in host order.
Author
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