The Mechanism of Apple's DRM

bassi · 05-01-2003, 10:45 AM

From the 'Fairplay' thread which just got shutdown I was enjoying the discussion on how Apple may be applying their *DRM* strategy. Whether you're a believer in *DRM* or not, I'm just incredibly curious about the mechanism and I thought we could have an open discussion about it. I'm not interested in cracking their technology just trying to understand where this may lead in the future for other content delivery systems on our Macs.

Craig R. Arko · 05-01-2003, 10:59 AM

From what I have read so far, they must be taking a unique identifier for the machine, (either serial number or MAC address) hashing it with the AppleID and then writing a key into the .m4p (protected) AAC encoded file itself. Kind of like PGP's public and private key system. Apple retains the private key and you get the public key.

Then if you burn it to CD the resulting .m4a AAC file does not contain a key, which is how you can listen to it on any device.

The new iPods (and the firmware update for the old ones) may gain the ability to deal with .m4p, whereas most other devices can't (yet).

My question: does the iPod support .m4p, or are the files transferred .m4a?? I don't have an iPod, so I can't try this.

Jacques · 05-01-2003, 11:13 AM

It's logical that they must be transferred .m4p to iPods. Otherwise the protection mech would be too easy to break - the industry wouldn't like that!

Anyone have the new iPod yet, to test?

bassi · 05-01-2003, 11:29 AM

Craig, that's an elegant enhancement of what petey, hayne and I discussed on the last thread.
If the m4p file is decompressed to AIFF format on a CD the key is removed, but if you try to rip it back to MP3 or m4a the sound quality is supposed to drop. Is that correct? If it is then the AIFF file from the m4p must have some data embedded in it to cause this 'corruption'.
The confusing part is the iPod stuff, they probably handle m4a. In the transfer process the 'key' is stripped and the file may be converted to m4a. You could check this via terminal and look for the file extensions.
If any iPod you own can keep and play those files then validation has to be dropped somewhere. If you manage your own playlists then you can simply drag m4p files over to your iPod, or any iPod right? You can't drag them off the iPod library back to your computer. I wish I could "test" this.

edit: Jacques, I don't think you need a new iPod for this test, the new firmware update should enable all iPods, I think. you're right about protection mechanism, in retrospect, iPod Viewer or some other program could just open up the files and allow transfer to any computer.

petey · 05-01-2003, 01:01 PM

bassi,

my guesses:

- any iPod will play any M4P.

- you can only transfer an M4P to an iPod from an authorized computer.

- (i'm really guessing here, but i bet the above item only applies to doing the transfer using iTunes. i imagine 3rd party tools will transfer any M4P to any iPod from any computer.)

- the file remains M4P on the iPod, so if you use a 3rd party tool to transfer the M4P from the iPod to a different computer, the DRM remains in the file.

- the quality loss in transcoding AAC to MP3 has nothing to do with the DRM. it's inherent in recompressing any format. going M4A to MP3 would have the same issues.

hayne · 05-01-2003, 01:04 PM

Quote:


	If the m4p file is decompressed to AIFF format on a CD the key is removed, but if you try to rip it back to MP3 or m4a the sound quality is supposed to drop. Is that correct? If it is then the AIFF file from the m4p must have some data embedded in it to cause this 'corruption'.

I believe the loss in quality that has been refered to is merely the usual result of re-compressing a file that was expanded from a different compressed form. Each of these lossy compression schemes (AAC, MP3) drops some information from the original and each scheme works well (gives quality sound) only when the original is pristine.

bassi · 05-02-2003, 11:36 AM

So if I were to convert an AAC file to AIFF for a CD, then rip that AIFF back to AAC at the same bit rate etc. of the original file, the quality of the new AAC should be lower. Is that correct? If the same engine is being used to compress the file and 'expand' it to AIFF the m4 files should be identical, talking from a mathematical standpoint. This may be an old argument, I don't know if it is.

From a m4a and m4p standpoint it might be important.

hayne · 05-02-2003, 12:19 PM

Quote:


	If the same engine is being used to compress the file and 'expand' it to AIFF the m4 files should be identical

I don't know about identical, but I think it is correct that the transition AAC->AIFF->AAC won't lose much, but AAC->AIFF->MP3 will lose significantly in quality.

Here's an analogy:
Text compression method AOA saves space by omitting all the a's and o's in the text (with the idea that you can still understand the text even without the a's and o's) and then compressing further by some other non-lossy algorithm.
Text compression EEE saves space by omitting all the e's in the text (with the idea that you can still understand the text even without the e's) and then compressing further by some other non-lossy algorithm.

She sells seashells by the sea shore
AOA -> She sells sehells by the se shre
-> uf2wheuifwqfoiufh

uf2wheuifwqfoiufh
reverse AOA ->She sells sehells by the se shre
followed by EEE -> Sh slls shlls by the s shr

You get the idea.

Jacques · 05-02-2003, 12:58 PM

Assuming your actions are legal..

Going from the MP3/AAC/Ogg burn to analog and then ripping back to compressed digital may yield better results.

Hmm?

bassi · 05-02-2003, 06:34 PM

Maybe I didn't make myself very clear on this. What Jacques said is right, that is the flow of information I was originally talking about.

If it's true that an m4p file, when decompressed and burned on to a CD as an AIFF file, cannot be recompressed back to m4a without loss of integrity, then the file being burned on the CD is not a true AIFF file. It's AIFFp.

I'm not talking about converting it back to MP3 at all. Just the AAC format for now.

m4p >> AIFF >> m4a

Am I missing something here.

[I don't want to copy the m4p files and redistribute them, I live outside the US so I can't do it if I could. Just curious]

hayne · 05-02-2003, 07:54 PM

Quote:


	If it's true that an m4p file, when decompressed and burned on to a CD as an AIFF file, cannot be recompressed back to m4a without loss of integrity, then the file being burned on the CD is not a true AIFF file. It's AIFFp

I don't think there is any such thing as an AIFF file that will not lose integrity upon lossy compression. AAC is a lossy compression method. While it may be true that if you go from
AAC-1 -> AIFF-1 -> AAC-2
there is relatively little difference between AAC-1 and AAC-2, I think there is always going to be some small difference. This difference is nothing to do with anything special (your 'p') in the AIFF file, it is merely a fact of life with lossy compression.

In other words I don't think there is some subset of AIFF files (the ones you create by decoding from AAC files) which undergo non-lossy compression. I think there is always some loss, but the amount of loss is less for some AIFFs than others.

mervTormel · 05-02-2003, 09:13 PM

Quote:


	Originally posted by hayne ... While it may be true that if you go from AAC-1 -> AIFF-1 -> AAC-2 there is relatively little difference between AAC-1 and AAC-2, I think there is always going to be some small difference. This difference is nothing to do with anything special (your 'p') in the AIFF file, it is merely a fact of life with lossy compression. In other words I don't think there is some subset of AIFF files (the ones you create by decoding from AAC files) which undergo non-lossy compression. I think there is always some loss, but the amount of loss is less for some AIFFs than others.

hmm, i've got no proof, but i just don't buy this AAC-1 -> AIFF -> AAC-2 as not losing a lot of quality. it's like a copy of a copy. an interpretation that has been scrubbed and cooked twice.

here's the start of the experiment, a three minute song:

I'm Eighteen 2:58 Alice Cooper Alice Cooper's Greatest Hits Rock

$ file /Volumes/Alice\ Cooper\'s\ Greatest\ Hits/1\ I\'m\ Eighteen.aiff
/Volumes/Alice Cooper's Greatest Hits/1 I'm Eighteen.aiff: IFF data, AIFF-C compressed audio

$ ls -s /Volumes/Alice\ Cooper\'s\ Greatest\ Hits/1\ I\'m\ Eighteen.aiff
30771 /Volumes/Alice Cooper's Greatest Hits/1 I'm Eighteen.aiff

okay, so there's a .aiff file, from the CD, i presume in it's pristine format, 30MB large.

now then, rip it to AAC, and you get what?

$ file /Volumes/flivver/cd_Ripper/Alice\ Cooper/Alice\ Cooper\'s\ Greatest\ Hits/I\'m\ Eighteen.m4a
/Volumes/flivver/cd_Ripper/Alice Cooper/Alice Cooper's Greatest Hits/I'm Eighteen.m4a: data

$ ls -s /Volumes/flivver/cd_Ripper/Alice\ Cooper/Alice\ Cooper\'s\ Greatest\ Hits/I\'m\ Eighteen.m4a
2844 /Volumes/flivver/cd_Ripper/Alice Cooper/Alice Cooper's Greatest Hits/I'm Eighteen.m4a

now, burn it to an Audio CD and you're going to get back a ~30MB .aiff file (???), with a lot of missing data, i call "air".

now, if you then encode that CD track to ACC or what have you, wouldn't it repeat it's interpretation on the file? not knowing that it was inflated from an AAC source, or any other source?

so, we have the data washed and cooked twice now, introducing more interpretation, even of the air we created?

i can't complete the experiment, can't create this Audio CD for the test at this time.

but i would like to see/hear results of the AAC to AIFF back to some compressed audio format.

petey · 05-02-2003, 10:29 PM

bassi, the correct analogy is generational loss in making copies of cassette tapes. everytime you compress into a lossy codec, you are going to lose information. go AAC -> AIFF -> AAC -> AIFF -> AAC enough times, and you'll end up with some form of white noise, the same as if you made a copy of a copy of a copy of a cassette enough times.

there seems to be a widespread impression that AAC -> AIFF -> AAC will provide better quality than AAC -> AIFF -> MP3. as far as i can tell, this impression is mistaken. i haven't performed tests, but i do have some experience with codec issues, and i've never run across the particular piece of wisdom that recompression with the same lossy codec produces better results than using a different lossy codec.

and i'm with mervTormel that the AAC -> AIFF -> AAC conversion will produce rather noticeable degradation.

however, the definition of "noticeable" depends on the sensitivity of the ear of the listener. when i watch Digital Cable TV, i constantly notice motion artifacts from the low bitrate MPEG-2 compression they use. my girlfriend can be sitting next to me and not see any of it. neither one of us is wrong, we just have different thresholds for when the degradation is visually noticeable.

with that disclaimer said, played side by side, i think AAC -> AIFF -> AAC degradation will be noticeable by almost anyone who isn't legally deaf.

the real issue is whether or not the recompressed AAC will be of acceptable quality. for me, i'd easily say no. for some people, it should be acceptable.

mervTormel · 05-02-2003, 10:35 PM

thanks, petey. i will look/listen for corroboration.

considering the music i prefer, i am illegally deaf

petey · 05-02-2003, 10:50 PM

no, mervTormel. you're deaf if you DON'T like alice cooper. in fact, i believe that is the actual legal definition of deafness. (killer is his best album, y'know...)

mervTormel · 05-02-2003, 11:13 PM

ha! yeah. killer slays me.

bassi · 05-03-2003, 04:49 AM

Hayne, mervTormel and petey, thanks for the clarification.

mervTormel's experiment is worth a try but subjectivity is another key. Need to do a double blind test on this.

For what it's worth, I just ripped a song from a store bought CD into AAC at 128 kbps/160 kbps and 192 kbps. The differences between 128, 160 and 192 are noticable, but below it all is the washed out feel of the lower frequencies compared to my normal VBR ~224 kbps MP3. I wonder if they filter out the frequencies below 10 hz to save space. I want to feel the brown note

Personal taste aside, the main question still remains. What has Apple done to the m4p files and how do they manage the DRM w.r.t. our Apple ID etc. The answers so far are quite good and I imagine only Apple can give the skinny on this. More conjecture and hand waving needed here, just for fun.

Plus, Craig's question about the format of the m4 files from the music store on the iPod is worth answering. Anyone?

petey · 05-03-2003, 05:28 AM

bassi,

as related previously, here's my educated guess:

- the files are transferred to the iPod as M4P.

- any iPod can play any M4P.

- the DRM comes in because iTunes will only let you transfer an M4P to an iPod if it's authorized on the transferring computer.

the work around would be to use a 3rd party app to load up your iPod with M4P's traded through the net. this fits in with the whole Fairplay idea of making piracy too much trouble for most people, but not absolutely impossible.

i arrived at this because no other scheme seems to work. i don't think apple can store the files as M4A on the iPod, because they'd be too easy to access and then trade across the net. i'm guessing apple would prefer any net trading be in M4P, rather than M4A.

and since they're not making people authorize iPods, i can't see how they can prevent one of the two forms of trading.

anyone see anything different than this?

petey · 05-03-2003, 05:35 AM

bassi,

one more thing: apple WON'T be giving the skinny on how this works.

the first rule of DRM is don't talk about DRM.

bassi · 05-03-2003, 06:47 PM

petey, I can understand why the first rule is necessary, from a business standpoint. From a user standpoint I just don't like it.

As an analogy, I just went to see a movie and paid around $9 to get in. I sat through 20 minutes of adverts which was really annoying because I don't like to pay $9 to watch advertising and a movie.

(note: one can argue that the movie itself may be advertising of one sort but that's a whole other issue)

05-01-2003, 10:45 AM	#1
bassi Major Leaguer Join Date: Jan 2002 Location: Paris, France Posts: 498	The Mechanism of Apple's DRM From the 'Fairplay' thread which just got shutdown I was enjoying the discussion on how Apple may be applying their DRM strategy. Whether you're a believer in DRM or not, I'm just incredibly curious about the mechanism and I thought we could have an open discussion about it. I'm not interested in cracking their technology just trying to understand where this may lead in the future for other content delivery systems on our Macs.

05-01-2003, 11:29 AM	#4
bassi Major Leaguer Join Date: Jan 2002 Location: Paris, France Posts: 498	Craig, that's an elegant enhancement of what petey, hayne and I discussed on the last thread. If the m4p file is decompressed to AIFF format on a CD the key is removed, but if you try to rip it back to MP3 or m4a the sound quality is supposed to drop. Is that correct? If it is then the AIFF file from the m4p must have some data embedded in it to cause this 'corruption'. The confusing part is the iPod stuff, they probably handle m4a. In the transfer process the 'key' is stripped and the file may be converted to m4a. You could check this via terminal and look for the file extensions. If any iPod you own can keep and play those files then validation has to be dropped somewhere. If you manage your own playlists then you can simply drag m4p files over to your iPod, or any iPod right? You can't drag them off the iPod library back to your computer. I wish I could "test" this. edit: Jacques, I don't think you need a new iPod for this test, the new firmware update should enable all iPods, I think. you're right about protection mechanism, in retrospect, iPod Viewer or some other program could just open up the files and allow transfer to any computer. Last edited by bassi; 05-01-2003 at 11:33 AM.

05-01-2003, 01:01 PM	#5
petey All Star Join Date: Feb 2002 Location: peteyville Posts: 794	bassi, my guesses: - any iPod will play any M4P. - you can only transfer an M4P to an iPod from an authorized computer. - (i'm really guessing here, but i bet the above item only applies to doing the transfer using iTunes. i imagine 3rd party tools will transfer any M4P to any iPod from any computer.) - the file remains M4P on the iPod, so if you use a 3rd party tool to transfer the M4P from the iPod to a different computer, the DRM remains in the file. - the quality loss in transcoding AAC to MP3 has nothing to do with the DRM. it's inherent in recompressing any format. going M4A to MP3 would have the same issues. Last edited by petey; 05-01-2003 at 01:21 PM.

05-02-2003, 12:58 PM	#9
Jacques All Star Join Date: Jan 2002 Location: New York, NY Posts: 972	Assuming your actions are legal.. Going from the MP3/AAC/Ogg burn to analog and then ripping back to compressed digital may yield better results. Hmm? Last edited by Jacques; 05-02-2003 at 01:04 PM.

05-02-2003, 10:29 PM	#13
petey All Star Join Date: Feb 2002 Location: peteyville Posts: 794	lossy! timmy's trapped in the well! bassi, the correct analogy is generational loss in making copies of cassette tapes. everytime you compress into a lossy codec, you are going to lose information. go AAC -> AIFF -> AAC -> AIFF -> AAC enough times, and you'll end up with some form of white noise, the same as if you made a copy of a copy of a copy of a cassette enough times. there seems to be a widespread impression that AAC -> AIFF -> AAC will provide better quality than AAC -> AIFF -> MP3. as far as i can tell, this impression is mistaken. i haven't performed tests, but i do have some experience with codec issues, and i've never run across the particular piece of wisdom that recompression with the same lossy codec produces better results than using a different lossy codec. and i'm with mervTormel that the AAC -> AIFF -> AAC conversion will produce rather noticeable degradation. however, the definition of "noticeable" depends on the sensitivity of the ear of the listener. when i watch Digital Cable TV, i constantly notice motion artifacts from the low bitrate MPEG-2 compression they use. my girlfriend can be sitting next to me and not see any of it. neither one of us is wrong, we just have different thresholds for when the degradation is visually noticeable. with that disclaimer said, played side by side, i think AAC -> AIFF -> AAC degradation will be noticeable by almost anyone who isn't legally deaf. the real issue is whether or not the recompressed AAC will be of acceptable quality. for me, i'd easily say no. for some people, it should be acceptable. Last edited by petey; 05-02-2003 at 10:35 PM.

05-01-2003, 10:59 AM	#2
Craig R. Arko Site Admin Join Date: Dec 2001 Location: Minneapolis, MN Posts: 3,988	From what I have read so far, they must be taking a unique identifier for the machine, (either serial number or MAC address) hashing it with the AppleID and then writing a key into the .m4p (protected) AAC encoded file itself. Kind of like PGP's public and private key system. Apple retains the private key and you get the public key. Then if you burn it to CD the resulting .m4a AAC file does not contain a key, which is how you can listen to it on any device. The new iPods (and the firmware update for the old ones) may gain the ability to deal with .m4p, whereas most other devices can't (yet). My question: does the iPod support .m4p, or are the files transferred .m4a?? I don't have an iPod, so I can't try this.

05-01-2003, 11:13 AM	#3
Jacques All Star Join Date: Jan 2002 Location: New York, NY Posts: 972	It's logical that they must be transferred .m4p to iPods. Otherwise the protection mech would be too easy to break - the industry wouldn't like that! Anyone have the new iPod yet, to test?

05-02-2003, 11:36 AM	#7
bassi Major Leaguer Join Date: Jan 2002 Location: Paris, France Posts: 498	So if I were to convert an AAC file to AIFF for a CD, then rip that AIFF back to AAC at the same bit rate etc. of the original file, the quality of the new AAC should be lower. Is that correct? If the same engine is being used to compress the file and 'expand' it to AIFF the m4 files should be identical, talking from a mathematical standpoint. This may be an old argument, I don't know if it is. From a m4a and m4p standpoint it might be important.

05-02-2003, 06:34 PM	#10
bassi Major Leaguer Join Date: Jan 2002 Location: Paris, France Posts: 498	Maybe I didn't make myself very clear on this. What Jacques said is right, that is the flow of information I was originally talking about. If it's true that an m4p file, when decompressed and burned on to a CD as an AIFF file, cannot be recompressed back to m4a without loss of integrity, then the file being burned on the CD is not a true AIFF file. It's AIFFp. I'm not talking about converting it back to MP3 at all. Just the AAC format for now. m4p >> AIFF >> m4a Am I missing something here. [I don't want to copy the m4p files and redistribute them, I live outside the US so I can't do it if I could. Just curious]

05-02-2003, 10:35 PM	#14
mervTormel League Commissioner Join Date: Jan 2002 Posts: 5,536	thanks, petey. i will look/listen for corroboration. considering the music i prefer, i am illegally deaf __________________ On a clear disk, you can seek forever.

05-02-2003, 10:50 PM	#15
petey All Star Join Date: Feb 2002 Location: peteyville Posts: 794	no, mervTormel. you're deaf if you DON'T like alice cooper. in fact, i believe that is the actual legal definition of deafness. (killer is his best album, y'know...)

05-02-2003, 11:13 PM	#16
mervTormel League Commissioner Join Date: Jan 2002 Posts: 5,536	ha! yeah. killer slays me.

05-03-2003, 04:49 AM	#17
bassi Major Leaguer Join Date: Jan 2002 Location: Paris, France Posts: 498	Hayne, mervTormel and petey, thanks for the clarification. mervTormel's experiment is worth a try but subjectivity is another key. Need to do a double blind test on this. For what it's worth, I just ripped a song from a store bought CD into AAC at 128 kbps/160 kbps and 192 kbps. The differences between 128, 160 and 192 are noticable, but below it all is the washed out feel of the lower frequencies compared to my normal VBR ~224 kbps MP3. I wonder if they filter out the frequencies below 10 hz to save space. I want to feel the brown note Personal taste aside, the main question still remains. What has Apple done to the m4p files and how do they manage the DRM w.r.t. our Apple ID etc. The answers so far are quite good and I imagine only Apple can give the skinny on this. More conjecture and hand waving needed here, just for fun. Plus, Craig's question about the format of the m4 files from the music store on the iPod is worth answering. Anyone?

05-03-2003, 05:28 AM	#18
petey All Star Join Date: Feb 2002 Location: peteyville Posts: 794	bassi, as related previously, here's my educated guess: - the files are transferred to the iPod as M4P. - any iPod can play any M4P. - the DRM comes in because iTunes will only let you transfer an M4P to an iPod if it's authorized on the transferring computer. the work around would be to use a 3rd party app to load up your iPod with M4P's traded through the net. this fits in with the whole Fairplay idea of making piracy too much trouble for most people, but not absolutely impossible. i arrived at this because no other scheme seems to work. i don't think apple can store the files as M4A on the iPod, because they'd be too easy to access and then trade across the net. i'm guessing apple would prefer any net trading be in M4P, rather than M4A. and since they're not making people authorize iPods, i can't see how they can prevent one of the two forms of trading. anyone see anything different than this? Last edited by petey; 05-03-2003 at 05:31 AM.

05-03-2003, 05:35 AM	#19
petey All Star Join Date: Feb 2002 Location: peteyville Posts: 794	bassi, one more thing: apple WON'T be giving the skinny on how this works. the first rule of DRM is don't talk about DRM.

05-03-2003, 06:47 PM	#20
bassi Major Leaguer Join Date: Jan 2002 Location: Paris, France Posts: 498	petey, I can understand why the first rule is necessary, from a business standpoint. From a user standpoint I just don't like it. As an analogy, I just went to see a movie and paid around $9 to get in. I sat through 20 minutes of adverts which was really annoying because I don't like to pay $9 to watch advertising and a movie. (note: one can argue that the movie itself may be advertising of one sort but that's a whole other issue)