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Something doesn't add up in this story. Encryption and hashing are different processes with different algorithms - SHA vs MD5. For example, IPSec VPNs hash a pa

Something doesn't add up in this story. Encryption and hashing are different processes with different algorithms - SHA vs MD5. For example, IPSec VPNs hash a packet with MD5 to prevent tampering, then encrypt the hash with SHA256 to prevent viewing. (Because the message could be modified while encrypted, were it not also hashed.) Isn't the point of encryption that it doesn't create a reliable hash - that 2 identical files will appear different while encrypted, as part of the larger encrypted drive? Or are encrypted-hash collisions possible when small files are encrypted individually?

Neverbolt 9y ago
There are several misconceptions in this comment, first and foremost that SHA is encryption, which it is not. It is a hashing algorithm, not unlike MD5, though "stronger". Secondly, when you have two files that are exactly the same and encrypt both with the same key, method and parameters then both will have the same hash. ( Though I could imagine Apple doing stuff with padding, and other parameters to make this not happen)
- chatmasta 9y ago
  Right... so for the authorities to "compare" the hash of an encrypted file with that of a known original, they would need to encrypt the original with the same private key used to encrypt the encrypted file. If they had that private key, wouldn't it be sufficient to unlock the drive? They wouldn't need his cooperation to decrypt the drive if they had a private key. So it seems like a catch-22 compelling him to decrypt the drive based on a hash collision.
  - deleted 9y ago
    [deleted]
  - weinzierl 9y ago
    Exactly, but the original quote doesn't say that they compared decrypted content with known hashes. It doesn't say anything about how they learned about the "content stored on the encrypted hard drive". "Investigators said content stored on the encrypted hard drive matched file hashes for known child pornography content." I read it like this: They figured out that the disk had some incriminating files, as I described in another comment of this thread. To make this work hashes are of no use, they need the original files. For various reasons they might not want to admit that they are in possession of the original files, hence the cryptic and vague phrasing.
    - foldr 9y ago
      If they're in possession of the original files they can just look at the files to see what they contain.
      - beisner 9y ago
        There could be a hash collision, which might be enough to provide reasonable doubt for a jury.
        johncolanduoni 9y ago
        The chances of a hash collision are drastically lower than the false positive rate of a DNA test, and US courts have accepted the latter for a long time.
        foldr 9y ago
        No, I mean if they have the image files, they can look at the images. It would be irrelevant what the image hashes to.
      - weinzierl 9y ago
        Sure. My point was more that they might possibly not want to openly admit that they are in the possession of the original files. I'm sure law enforcement has lists with hashes of incriminating files, but I'm not sure if they are allowed to keep the original files. Even if they are, maybe they just want to avoid public discussion about it.
        foldr 9y ago
        It's logically impossible for them to have the hashes of the files without having had the files at some point. If they no longer have the files, you might just as well take their word for it as to the content of the files as take their word for it what the hashes of the files are.
- hndamien 9y ago
  With the same key is a very important distinction there.
- wyager 9y ago
  All modern encryption schemes inject randomness into the encryption process (via an initialization vector) so no, there is no way to check if two encrypted files are the same file. The investigators' claim doesn't make sense.
  - Xorlev 9y ago
    This. The padding changes as well. You have to work hard to produce the same ciphertext from a given plaintext. The claim is bogus.
djsumdog 9y ago
I would assume they have evidence, maybe via a warrant to his ISP, that showed the transfer of files with hashes of known illegal content. They might have evidence content was transferred to a computer on an account in his name, but they most likely do not have evidence of his actual possession of illegal content.
- icebraining 9y ago
  How would the ISP have such hashes? Are they hashing and recording every file one downloads? And the sources of this illegal content weren't even using HTTPS, let alone Tor?
dozzie 9y ago
> Encryption and hashing are different processes with different algorithms - SHA vs MD5. For example, IPSec VPNs hash a packet with MD5 to prevent tampering, then encrypt the hash with SHA256 to prevent viewing. You do realize that MD5, SHA-1, and SHA-256 are all hashing algorithms? I highly doubt you meant a cipher built from a hash function (easy to do with Feistel net, though nobody does that), especially that IPsec doesn't define any such thing, from what I remember. Given that you apparently don't understand how encryption and hashing work and relate to each other, you're not in a position to question the technical aspects of the story.

Neverbolt 9y ago

There are several misconceptions in this comment, first and foremost that SHA is encryption, which it is not. It is a hashing algorithm, not unlike MD5, though "stronger". Secondly, when you have two files that are exactly the same and encrypt both with the same key, method and parameters then both will have the same hash. ( Though I could imagine Apple doing stuff with padding, and other parameters to make this not happen)

chatmasta 9y ago
Right... so for the authorities to "compare" the hash of an encrypted file with that of a known original, they would need to encrypt the original with the same private key used to encrypt the encrypted file. If they had that private key, wouldn't it be sufficient to unlock the drive? They wouldn't need his cooperation to decrypt the drive if they had a private key. So it seems like a catch-22 compelling him to decrypt the drive based on a hash collision.
- deleted 9y ago
  [deleted]
- weinzierl 9y ago
  Exactly, but the original quote doesn't say that they compared decrypted content with known hashes. It doesn't say anything about how they learned about the "content stored on the encrypted hard drive". "Investigators said content stored on the encrypted hard drive matched file hashes for known child pornography content." I read it like this: They figured out that the disk had some incriminating files, as I described in another comment of this thread. To make this work hashes are of no use, they need the original files. For various reasons they might not want to admit that they are in possession of the original files, hence the cryptic and vague phrasing.
  - foldr 9y ago
    If they're in possession of the original files they can just look at the files to see what they contain.
    - beisner 9y ago
      There could be a hash collision, which might be enough to provide reasonable doubt for a jury.
      - johncolanduoni 9y ago
        The chances of a hash collision are drastically lower than the false positive rate of a DNA test, and US courts have accepted the latter for a long time.
      - foldr 9y ago
        No, I mean if they have the image files, they can look at the images. It would be irrelevant what the image hashes to.
    - weinzierl 9y ago
      Sure. My point was more that they might possibly not want to openly admit that they are in the possession of the original files. I'm sure law enforcement has lists with hashes of incriminating files, but I'm not sure if they are allowed to keep the original files. Even if they are, maybe they just want to avoid public discussion about it.
      - foldr 9y ago
        It's logically impossible for them to have the hashes of the files without having had the files at some point. If they no longer have the files, you might just as well take their word for it as to the content of the files as take their word for it what the hashes of the files are.
hndamien 9y ago
With the same key is a very important distinction there.
wyager 9y ago
All modern encryption schemes inject randomness into the encryption process (via an initialization vector) so no, there is no way to check if two encrypted files are the same file. The investigators' claim doesn't make sense.
- Xorlev 9y ago
  This. The padding changes as well. You have to work hard to produce the same ciphertext from a given plaintext. The claim is bogus.

djsumdog 9y ago

I would assume they have evidence, maybe via a warrant to his ISP, that showed the transfer of files with hashes of known illegal content. They might have evidence content was transferred to a computer on an account in his name, but they most likely do not have evidence of his actual possession of illegal content.

icebraining 9y ago
How would the ISP have such hashes? Are they hashing and recording every file one downloads? And the sources of this illegal content weren't even using HTTPS, let alone Tor?

dozzie 9y ago

> Encryption and hashing are different processes with different algorithms - SHA vs MD5. For example, IPSec VPNs hash a packet with MD5 to prevent tampering, then encrypt the hash with SHA256 to prevent viewing. You do realize that MD5, SHA-1, and SHA-256 are all hashing algorithms? I highly doubt you meant a cipher built from a hash function (easy to do with Feistel net, though nobody does that), especially that IPsec doesn't define any such thing, from what I remember. Given that you apparently don't understand how encryption and hashing work and relate to each other, you're not in a position to question the technical aspects of the story.