ASN.1 integers are encoded as two's complement numbers. This
means that the most significant bit of a positive number must be zero
and the most significant bit of a negative integer must be set to one.
Additionally prepended 0x00 octets for positive and 0xFF octets for
negative numbers, respectively are discarded. Here are some examples
I used in my ASN.1 lecture a couple of years back
http://www.strongsec.com/zhw/KSy_ASN1.pdf (slide 22)
BER coding of two‘s complement integers
• -129: 1111 1111 0111 1111 = 02 02 FF 7F
• -128: 1111 1111 1000 0000 = 02 01 80
• -127: 1111 1111 1000 0001 = 02 01 81
• -1: 1111 1111 1111 1111 = 02 01 FF
• 0: 0000 0000 0000 0000 = 02 00
• 1: 0000 0000 0000 0001 = 02 01 01
• 127: 0000 0000 0111 1111 = 02 01 7F
• 128: 0000 0000 1000 0000 = 02 02 00 80
• 129: 0000 0000 1000 0001 = 02 02 00 81
Best regards
Andreas
Alessandro Premoli wrote:
> Ludovic Rousseau ha scritto:
>> Do we have ASN.1 experts on this list?
>
> Yes. You are right, these are the rules:
>
> If the contents octets of an integer value encoding consist of more than
> one octet, then the bits of the first octet and bit 8 of the second octet:
> a) shall not all be ones; and
> b) shall not all be zero.
> NOTE – These rules ensure that an integer value is always encoded in the
> smallest possible number of octets.
>
> The correct encodings of 128 and -128 are:
>
> (128) 02:02:00:80
> (-128) 02:01:80
>
> Anything else is wrong.
>
> --
> Alessandro Premoli
======================================================================
Andreas Steffen [EMAIL PROTECTED]
strongSwan - the Linux VPN Solution! www.strongswan.org
Institute for Internet Technologies and Applications
University of Applied Sciences Rapperswil
CH-8640 Rapperswil (Switzerland)
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