Subject: [rohc] doubt in calculating the SHA-1 in case of TCP

From: Rasheed Ahammad S <[email-masked]>
Date: Wed, 21 Jun 2006 16:46:26 +0530

Hi,

I have following doubt regarding the calculation of SHA-1 id in case the 
transport is stream based. Please clarify 

RFC 4077 section 3.1 says

SHA-1 Hash of failed message" contains the full 20-byte SHA-1
      hash of the SigComp message that could not be decompressed.  This
      information allows the NACK recipient to locate the message that
      failed to decompress so that adjustments to the correct
      compartment can be performed.  When performing this hash, the
      entire SigComp message is used, from the header byte (binary
      11111xxx) to the end of the input.  Any lower-level protocol
      headers (such as UDP or IP) and message delimiters (the 0xFFFF
      that marks message boundaries in stream protocols) are not
      included in the hash.  When used over a stream based protocol, any
      0xFFxx escape sequences are un-escaped before performing the hash
      operation.

What does the above colored and bold statment means:

Does it mean that we need to calculate the SHA-1 after removing the record 
marking or before removing the record marking at Decompressor dispatcher..

For example:

Suppose the received Sigcomp message is

01 02 FF 05 FF FF FF FF FF 03 09 07 0A FF FF (Actual message before 
performing record marking at the sending side is 01 02 FF FF FF FF FF FF 
03 09 07 0A)

On which string the SHA-1 need to be calculated

01 02 FF 05 FF FF FF FF FF 03 09 07 0A

                (or)

01 02 FF FF FF FF FF FF 03 09 07 0A

Regards,
Rasheed
Ext: 9113

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From: Cristian Constantin <[email-masked]>
Date: Wed, 21 Jun 2006 13:30:19 +0000

> 01 02 FF FF FF FF FF FF 03 09 07 0A

cristian: at both ends you compute the SHA-1 over the unescaped message.

so: 01 02 FF FF FF FF FF FF 03 09 07 0A.
the message is anyway broken bec. its first 5 bits are 00000 and not
11111.

bye now!
cristian