without any changes. It is mandatory for this cipher mode to be implemented in any SRTP-compatible system. As such, it can be used when the confidentiality guarantees ensured by SRTP are not required, while other SRTP features, such as authentication and message integrity, may be used. Through SRTP can easily accommodate new encryption algorithms, the SRTP standard states that new encryption algorithms may only be introduced through publication of a new companion standard track RFC which must clearly define the new algorithm. Authentication, integrity and replay protection The above-listed encryption algorithms do not alone secure message integrity, an attacker will not be able to decrypt data but may be able to forge or replay previously transmitted data. Hence the SRTP standard also provides the means to secure the integrity of data and safety from replay. To authenticate the message and protect its integrity, the HMAC-SHA1 algorithm is used. This produces a 160-bit result, which is then truncated to 80 or 32 bits to become the authentication tag appended to each packet. The HMAC is calculated over the packet payload and material from the packet header, including the packet sequence number. To protect against replay attacks, the receiver maintains the sequence numbers of previously received messages, compares them with the sequence number in each new received message and admits the new message only if it has not been previously received. This approach relies on the integrity protection to make it impossible to modify the sequence number without detection. Key derivation A key derivation function is used to derive the different keys used in a crypto context (SRTP and SRTCP encryption keys and salts, SRTP and SRTCP authentication keys) from one single master key in a cryptographically secure way. Thus, the key management protocol needs to exchange only one master key, all the necessary session keys are generated by applying the key derivation function. Periodic application of the key derivation function prevents an attacker from collecting large amounts of ciphertext encrypted with one single session key. This provides protection against certain attacks which are easier to carry out when a large amount of ciphertext is available. Furthermore, multiple applications of the key derivation function provides backwards and forward security in the sense that a compromised session key does not compromise other session keys derived from the same master key. This means that even if an attacker managed to recover a session key,