[asterisk-bugs] [JIRA] (ASTERISK-26207) [patch] sRTP: Count a roll-over of the sequence number even on lost packets.

[Alexander Traud (JIRA)]

Alexander Traud created ASTERISK-26207:
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             Summary: [patch] sRTP: Count a roll-over of the sequence number even on lost packets.
                 Key: ASTERISK-26207
                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-26207
             Project: Asterisk
          Issue Type: Bug
      Security Level: None
          Components: Resources/res_rtp_asterisk
    Affects Versions: 13.9.1, 11.22.0
            Reporter: Alexander Traud
            Severity: Minor


See the sRTP FAQ, [Question 6…|http://srtp.sourceforge.net/faq.html#Q6]

With RTP media, each packet gets its own sequence number to detect lost  and late packets. That sequence number is limited to a value of {{0xffff}}. To avoid any replay-attacks, sRTP introduced a counter which is incremented whenever the sequence number rolls over (ROC). The ROC is not transmitted. Therefore, both sender and receiver must note each roll over of the sequence number.

The initial sequence number is created randomly. However, when
- that random value is near the maximum value *and*
- the first RTP packets get lost (or arrive late),

the receiver might not notice the roll-over of the sequence number. The receiver might think, the RTP sequence started at 0,1,2,3…. Consequently, the ROC is of the receiver is 0. The ROC of the sender is 1. Both parties lost ROC synchronization. Therefore, the receiver is not able to decrypt the sRTP packets anymore. No media is the result.

The mentioned webpage therefore suggests: The SRTP sender should randomly select an initial sequence number that is between {{0x0000}} and {{0x7fff}}. The attached patch does this.



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