I wonder how secure the encryption on Mac is when creating an encrypted disk image. Is there any information about the "security level" when creating such an image? I'm curious because if I create an image, it is so fast.

If I create an encrypted image with VeraCrypt, for example, it takes much more time than creating one with Apple. Why is that?

  • What is your threat scenario...i.e. how will potential attackers gain access to the encrypted image? There may be other factors which have to be overcome before the encryption mechanism's resistance to attack becomes 'critical'. – R15 Feb 21 '17 at 8:18

According the documentation it states that

After you click the ‘Format’ button in the Volume Creation Wizard window (the last step), there will be a short delay while your system is being polled for additional random data.

So that might take some time. Also it depends on what options you use when creating the VeraCrypt volume especially if you don't use Quick format which is unchecked by default which will fill the volume with random data. I don't think Mac does this or may do it in the background.

As long as "security level" of what Apple provides, this is not really known since their code is not open source and there is little technical documentation around their encrypted volume so there is no way to know if they are following security best practices.

Here is an article with some technical details on encrypted dmg format and how it can be cracked.

According to linked article "

AES256-encrypted DMG uses 250,000 rounds of PBKDF2-HMAC-SHA-1 to generate the encryption key

So the "security level" is pretty good when it comes to the encryption algorithm (AES) but the key derivation (PBKDF2-HMAC-SHA-1) is known to have weaknesses. According to Wikipedia

One weakness of PBKDF2 is that while its number of iterations can be adjusted to make it take an arbitrarily large amount of computing time, it can be implemented with a small circuit and very little RAM, which makes brute-force attacks using application-specific integrated circuits or graphics processing units relatively cheap.The bcrypt key derivation function requires a larger amount of RAM (but still not tunable separately, i. e. fixed for a given amount of CPU time) and is slightly stronger against such attacks,while the more modern scrypt key derivation function can use arbitrarily large amounts of memory and is therefore more resistant to ASIC and GPU attacks.

  • Any more information about the security level of the encrypted .dmg images? – user284148 Oct 22 '16 at 13:50
  • I updated the answer with some more information about the security level – ARau Oct 26 '16 at 2:47
  • Thanks but I'm curious because Apple is in the US. I'm so afraid because Lavabit were in the US as well. – user284148 Nov 20 '16 at 16:58
  • Apple's level of disk encryption has some open questions for sure. Use VeraCrypt which has been recently audited and it looks to be a good solution. – ARau Nov 24 '16 at 3:15

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