The security services provided by TCAPsec are:

• data integrity;
• data origin authentication;
• anti-replay protection;
• confidentiality (optional).

An SS7-SEG shall maintain the following databases:

• SPD-SEG: A database containing TCAPsec policy information (see clause 5.3);
• SAD-SEG: A database containing TCAPsec SA information. SS7-SEG shall monitor the SA hard expiry time and expired SAs shall be deleted from the database (see clause 5.4).

SS7-SEG shall be able to perform the following operations:

• Secure TCAP user signalling (i.e. send/receive protected or unprotected messages) according to information in SPD-SEG and SAD-SEG. The structure of protected messages is defined in clause 5.5 and the protection algorithms are defined in clause 5.6.

The security policies for TCAPsec key management are specified in the SS7-SEG's SPD. SPD entries define per peer network whether protection shall be applied, and if protection shall be applied then which protection mode shall be used. SPD entries of different SS7-SEGs within the same network shall be consistent. Fallback to unprotected mode:

• The "fallback to unprotected mode" (enabled/disabled) is a parameter for the receiving direction per network, if enabled it allows the receiving network to accept unprotected traffic as well as protected traffic. If disabled, only protected traffic is to be accepted
• The use of the fallback indicator is specified in Annex B;
• The security measures specified in this TS are only fully useful for a particular network if it disallows fallback to unprotected mode for TCAP user messages received from any other network.

Explicit policy configuration:

• The SPD shall contain an entry for each network the SS7-SEG is allowed to communicate with.
  Protection granularity:
• SPD administration shall be allowed on TCAP user application part level for each network the SS7-SEG is allowed to communicate with.

Migration support between protection modes:

• An SPD entry may contain two protection modes for the same network. If this is the case then both protection modes shall be acceptable for incoming messages, but only one (preferred) protection mode shall be used for outgoing messages.

The TCAPsec security association shall contain the following data elements which can be classified in two groups

1. SA Identification attributes i.e. Network Ids and SPI:
   In sending direction the SA identification is based on Destination Network Id. Per Destination Network Id more than one SA may exist. In the case where more than one valid SA is available at the SAD, the sending SS7-SEG shall choose the SA with the earliest soft expiry time.
   In receiving direction the used SPI from within the TCAPsec security header can be used to retrieve the Origin Network Id.
2. Assigned cryptographic parameters per SPI:

Key and algorithm identifiers and SA lifetime. SA Identification attributes:

• Destination Network Id:
  The Destination Network Id is a concatenation of the Country Code (CC) and National Destination Code (NDC) of the receiving network. The Destination Network-Id is used to identify which SAD-entry shall be used when traffic protection is needed.
• Security Parameters Index (SPI):
  SPI is a 32-bit value that is used in combination with Destination Network Id to uniquely identify a TCAPsec SA. The SPI is used to identify which SAD entry shall be used when de-protecting traffic. Therefore the SPI needs to be assigned by the destination network.
• Origin Network Id:
  The Origin Network Id is a concatenation of the Country Code (CC) and National Destination Code (NDC) of the sending network.

Cryptographic parameters per SPI:

• SS7 Security Gateway Encryption Algorithm identifier (SEA):
  Identifies the encryption algorithm. Mode of operation of algorithm is implicitly defined by the algorithm identifier. Mapping of algorithm identifiers is defined in clause 5.6.
• SS7 Security Gateway Encryption Key (SEK):
  Contains the encryption key. The length of SEK is defined according to the algorithm identifier.
• SS7 Security Gateway Integrity Algorithm identifier (SIA):
  Identifies the integrity algorithm. Mode of operation of algorithm is implicitly defined by the algorithm identifier. Mapping of algorithm identifiers is defined in clause 5.6.
• SS7 Security Gateway Integrity Key (SIK):
  Contains the integrity key. The length of SIK is defined according to the algorithm identifier.
• SA Hard Expiry Time:
  Defines the actual expiry time of the SA. The Hard Expiry Time shall be given in UTC time with format YYYY-MM-DDThh:mm:ssTZD as described by W3C DTF [7].
• SA Soft Expiry Time:
  Defines Soft Expiry Time of the SA for outbound traffic. The format of the Soft Expiry Time is the same as the Hard Expiry Time. The SA Soft Expiry Time is determined by the Originating Network and shall expire before the SA Hard Expiry Time.

After the Hard Expiry Time has been reached, the SA shall no longer be used for inbound or outbound traffic. When the Soft Expiry Time is reached, the SA shall not be used any longer for the outbound traffic unless no other valid SA exists.

TCAPsec provides following protection modes and these are defined as follows:

• Protection Mode 1: Integrity, Authenticity
• Protection Mode 2: Confidentiality, Integrity, and Authenticity

In case a TCAP message does not require protection (as indicated by the SPD) then the message shall be routed unchanged by the SS7-SEG. TCAP messages protected by means of TCAPsec include a Security Header and a Protected Payload. Secured TCAP user messages have the following structure:

Security Header

Protected Payload

For detailed message structure see TS 29.204. In all protection modes, the security header is transmitted in cleartext. The intermediate unprotected TCAP message representation is the cleartext concatenation of DialoguePortion and ComponentPortion of the original TCAP message (after message reassembly if this applies). For protection mode 1, the protected payload is the concatenation of the intermediate unprotected TCAP message representation and the message authentication code. For protection mode 2, the protected payload is the concatenation of the result of applying the encryption function to the intermediate unprotected TCAP message representation, and the message authentication code. For integrity and authenticity in protection mode 1, the message authentication code is calculated on the concatenation of the security header and the intermediate unprotected TCAP message representation. The message authentication code in protection mode 2 is calculated on concatenation of the security header and the result of applying the encryption function to the intermediate unprotected TCAP message representation.