4 ms·

I'm not sure I understand your criticism, but I should've clarified "content addressing using long, cryptographically secure hashes". Most of the major projects

by btrask 11y ago

I'm not sure I understand your criticism, but I should've clarified "content addressing using long, cryptographically secure hashes". Most of the major projects are using SHA-256.

jjtheblunt 11y ago
The point is that hash functions of fixed bit length b have 2^b possible values, but are used to "represent" bit sequences of length longer than b, of which there are more than 2^b values. Collisions necessarily occur, but finding colliding values isn't necessarily easy (cf. cryptographic hashes), which means if you want to tamper with a message, which has an associated hash, without being detected, you're constrained by the hash function to tamperings which produce the same hash.
- drdaeman 11y ago
  I guess, the most widely used content-addressing system is BitTorrent. Wonder, if anyone had already did a research how many different torrents with the same `info_hash` one can find out there.
  - deleted 11y ago
    [deleted]
  - versteegen 11y ago
    I had to look it up, and found info_hash is the SHA-1 of the 'info' section of the metainfo file, which lists the files and the SHA-1 hashes of each of their pieces. So you are asking how many torrents have exactly the same file data but different metadata. So yes, interesting question. If two torrents differ only in metadata, will any bittorrent clients actually allow the data be shared (without manual intervention like symlinking files)?
    - akvadrako 11y ago
      That would be an SHA-1 collision which has never been reported and would break lots of encryption in common use. You guys are worrying about the wrong thing. Any hash considered secure will have zero collisions, at least in your light cone for trillions of years. If current hashes are shown to have weaknesses they will be deprecated, probably before even a single collision takes place.
      - versteegen 11y ago
        No, read what I wrote again. Same files, different metadata (eg. tracker URL, comments, creation date)
  - dchest 11y ago
    None. Accidental collision resistance of SHA-1 is 2^80. SHA-1 is "cryptographically broken": it's estimated that you can produce collision with about 2^60 operations, however nobody has produced them yet.
    - versteegen 11y ago
      The question is about duplicate torrents (ie content addressing), not hash collisions.
- dchest 11y ago
  Which basically means that it guarantees the integrity of message, where "guarantee" is defined as computational infeasibility to produce different messages with the same hash value. You know this stuff, why are you arguing semantics?
  - jjtheblunt 11y ago
    I hear you regarding infeasibility, though right now there's another article about content hash collisions being used to distribute a tampered-with Mint Linux image (MD5 collisions). It's "easier" to get the computational resources to fabricate a collision there, of course, but who is to say how much compute power state level adversaries bring to bear?
    - dchest 11y ago
      To be fair, MD5 was broken in 1996. If your content addressable system uses crypto broken 10 years ago, of course, you can't guarantee integrity :-) who is to say how much compute power state level adversaries bring to bear Not enough to break modern hash functions.

by btrask 11y ago

I'm not sure I understand your criticism, but I should've clarified "content addressing using long, cryptographically secure hashes". Most of the major projects are using SHA-256.

jjtheblunt 11y ago

The point is that hash functions of fixed bit length b have 2^b possible values, but are used to "represent" bit sequences of length longer than b, of which there are more than 2^b values. Collisions necessarily occur, but finding colliding values isn't necessarily easy (cf. cryptographic hashes), which means if you want to tamper with a message, which has an associated hash, without being detected, you're constrained by the hash function to tamperings which produce the same hash.

drdaeman 11y ago
I guess, the most widely used content-addressing system is BitTorrent. Wonder, if anyone had already did a research how many different torrents with the same `info_hash` one can find out there.
- deleted 11y ago
  [deleted]
- versteegen 11y ago
  I had to look it up, and found info_hash is the SHA-1 of the 'info' section of the metainfo file, which lists the files and the SHA-1 hashes of each of their pieces. So you are asking how many torrents have exactly the same file data but different metadata. So yes, interesting question. If two torrents differ only in metadata, will any bittorrent clients actually allow the data be shared (without manual intervention like symlinking files)?
  - akvadrako 11y ago
    That would be an SHA-1 collision which has never been reported and would break lots of encryption in common use. You guys are worrying about the wrong thing. Any hash considered secure will have zero collisions, at least in your light cone for trillions of years. If current hashes are shown to have weaknesses they will be deprecated, probably before even a single collision takes place.
    - versteegen 11y ago
      No, read what I wrote again. Same files, different metadata (eg. tracker URL, comments, creation date)
- dchest 11y ago
  None. Accidental collision resistance of SHA-1 is 2^80. SHA-1 is "cryptographically broken": it's estimated that you can produce collision with about 2^60 operations, however nobody has produced them yet.
  - versteegen 11y ago
    The question is about duplicate torrents (ie content addressing), not hash collisions.
dchest 11y ago
Which basically means that it guarantees the integrity of message, where "guarantee" is defined as computational infeasibility to produce different messages with the same hash value. You know this stuff, why are you arguing semantics?
- jjtheblunt 11y ago
  I hear you regarding infeasibility, though right now there's another article about content hash collisions being used to distribute a tampered-with Mint Linux image (MD5 collisions). It's "easier" to get the computational resources to fabricate a collision there, of course, but who is to say how much compute power state level adversaries bring to bear?
  - dchest 11y ago
    To be fair, MD5 was broken in 1996. If your content addressable system uses crypto broken 10 years ago, of course, you can't guarantee integrity :-) who is to say how much compute power state level adversaries bring to bear Not enough to break modern hash functions.