1. A Technical Review of ROC, Cryptographic Context, Indices, and Sliding Windows

2. SIPS (or SIP over TLS)  Per Hop Encryption of Transport SRTP SRTP  End-to-End Protection of Media Content  Provides Confidentiality, Message Authentication, and Replay Protection  Encryption for Confidentiality  Keyed Hash Function for Message Authentication  Counters for Replay Protection

3.  Packet stored by an adversary, and then injected back into the network.  Example - storing video of a surveillance camera and injecting it to the monitoring station to avoid surveillance  Message authentication provides integrity but is not enough

4.  Sequence Number  ROC (Rollover Counter)  Cryptographic Context  Implicit Index  Replay List  Sliding Window Algorithm

5.  Sequence Number (SEQNUM) = 16- bits  Incremented up to 65,535 (64-bits)  Defined in the SRTP header

7.  32-Bit Unsigned Counter  Number of Times Sequence Number Reset to Zero (After Passing Through 65,535)  Incremented By “1” When Wrapped  Maintained By SRTP (Not in the Header)

8.  SRTP Creates Implicit Index from Values in the Cryptographic Context  Includes State Information to Define Proper Security Measures  16-bit Sequence Number  Also the Highest Received SRTP Sequence Number

9.  Implicit (Not Carried in the Packet)  ROC + Sequence Number  48-bit (SEQNUM 16 + ROC 32)  Per Packet Basis  Also Used to Create Session Key for Encryption and Authentication  Index = 2^16 * ROC + SEQNUM

10.  Receiver Calculates Implicit Index  Determines if Unique Before Accepting the Packet  Only Accepts if Within Sliding Window  Compares Index and Last Index (contained in Cryptographic Context)

11.  Maintained Only by Receiver  SRTP Has to Provide Authentication and Replay Protection  Contains Indices of Recently Received and Authenticated SRTP Packets

12. Default Window Size is 64 - If the attacker chooses a sequence number at random, and the window size is 64, there is a 99.9-percent likelihood (1–64/2 16 ) that the packet will be discarded

13.  Only packets with index ahead of the window, or, inside the window but not already received, SHALL be accepted.  Packets with sequence numbers < 64 packets behind the highest-numbered packet will be discarded  Packets > 64 packets ahead of the window are discarded  Discarded if “RECEIVED?” Bit = Set  Packets Within the Window Accepted  Packets Higher Than the Window Causes It to Advanced

15.  Receiver’s Window Size (RWS)  Packets Arrive Out of Order  Sequence Number May Have Wrapped  32,768 (half of 16 bit)  Also Packet Loss and Bit Errors

16. Karen Lugo April 8, 2013 CSCI e 139