[Bitcoin-development] var_int ambiguous serialization consequences

Wladimir laanwj at gmail.com
Sun Feb 1 10:44:46 UTC 2015


On Sun, 1 Feb 2015, Tamas Blummer wrote:

> I wonder of consequences if var_int is used in its longer than necessary forms (e.g encoding 1 as 0xfd0100 instead of 0x01)

In serialize.h lingo you are talking about CompactSize, not VarInt.

CompactSizes indeed have redundancy in their representation, i.e. the same 
number can be represented as up to four different byte sequences.

VARINTs have a different format that (AFAIK) isn't used anywhere in 
the block chain. See WriteVarInt / ReadVarInt. These were designed to 
not have any redundancy in their representation.

> This is already of interest if applying size limit to a block, since transaction count is var_int but is not part of the hashed header or the
> merkle tree.

Are you sure that this is a current concern? Non-canonical CompactSizes 
are forbidden - in serialize.h this is flagged in ReadCompactSize.

Wladimir





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