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SRTCP packet, the SRTCP index must be 0, and this index will be increased (by 1) as
packets are sent. However, when the re-keying mechanism is applied, the index cannot
be reset to 0. E-flat indicates whether the current SRTCP packet is encrypted. The
functions of MKI and authentication tag in SRTCP are similar to those in SRTP.
4.2.3
Keys and Parameters of SRTP
In SRTP/SRTCP, there are different types of keys: session keys, master keys, and
master salt keys. The master key is a random bit string of various lengths (determined
by the key management protocol) which must be random and kept secret [16]. The
master key is used to generate session keys. Session keys are utilized for message
authentication and encryption. The salt is the input parameter used when generating
session keys. The master salt key is used in the key derivation procedure to produce
session keys. Master keys and master salt keys are established by an external key
management mechanism (such as MIKEY – see section 4.3 on page 33). The key
derivation functions will calculate session keys for authentication and encryption.
Figure 4-3 describes the key derivation in SRTP.
Figure 4-3: SRTP keys derivation
A cryptographic context is the cryptographic state information that the SRTP
sender and receiver set up for each session. The cryptographic context should be
identified based upon the synchronization source (SSRC), destination network
addresses, and destination transport port number. There are two types of parameters in a
cryptographic context: Transform-independent parameters and transform-dependent
parameters. Transform-dependent parameters are some parameters about encryption,
authentication and key derivation, such as block size of ciphers, session keys, data for
the Initialization Vector (IV) formation, etc. [16] Details of the significant
transform-independent parameters are introduced below.
A 32-bit unsigned rollover counter (ROC) records how many times the RTP
sequence number (SEQ) has been reset to zero when it reaches at 65535. SEQ is
extracted from the RTP packet header. The SRTP index is calculated as i 2
. A 16-bit sequence number is used to record the highest RTP sequence
number. It is recommended that the sequence number field be authenticated. When
authentication and replay protection are provided, a replay list is needed to record the
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