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 (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.
CryptoContext::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. 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 (uint16 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 CryptoContext::deriveSrtpKeys (uint64 index)
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 (uint16 newSeqNumber)
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 (uint32 ssrc, int
roc, int64 keyDerivRate)
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 (uint32 r) [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, uint32 roc, uint8 *
tag)
Compute the authentication tag. Compute the authentication tag
according the the paramters 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, 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 CryptoContext::update (uint16 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 RTP packet in host
order.
Author
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