Can zipping a file break it?
I just asked someone to send me a zipped psd file.
They declined, citing that zipping a file can break the fonts.
I assumed zipping a file is perfectly reversible, hence why it is commonly used. I think the other person is incorrect.
Is there any truth about zipping breaking its files' contents?
No, zipping a file cannot break it. Providing your zip file is not corrupted, it will reproduce the identical file when unzipped.
In this case, difference between fonts installed on the two different systems may cause issues but that is completely unrelated to the zip/unzip process.
In general usage, zip is lossless (assuming a bug-freeimplementation), but there is one scenario that could apply to data-loss: NTFS Alternate Data Streams. This little-used feature allows a single file to have multiple independent sets of contents. Most code will only ever see the unnamed stream, but others can exist.
So; if a program decided to store the data in an NTFS Alternate Data Stream, your zip client won't see that portion (it needs to explicitly ask for it, and RAR is the only one that does this currently).
But to emphasise: this is used very rarely, and not normally with things like PSD. I suspect your friend/associate is simply wrong.
There are circumstances in which a Mac font may not be identical if it is zipped and then unzipped. This may not break it, but contrary to some statements above, the process may not provide an identical file.
The circumstances are discussed here:
http://xahlee.org/UnixResource_dir/macosx.html
http://ask.metafilter.com/59789/How-to-email-my-font
But in short:
If they are much older fonts that contain resource forks and the user has an older version of Mac OS X, typically 10.4 or earlier. Legacy fonts like this work on OS X though they were originally intended for OS 9 and earlier versions of the Macintosh operating system. It is entirely likely (and, in my experience, common) that some folks are still using a font library they built as long as 20 years ago. Typically these are artists and art director types. For example, I have a few fonts with creation dates of 1993 and hundreds with creation dates of 1998, most with resource forks. Certainly I should have converted these to more modern formats or stopped using them, but let's face it: once you buy the Adobe Font Library, you never want to buy it again. In my years working with art directors in advertising, I learned to respect the font folder as if it were an art director's diary, commonplace book, or superego.
Some metadata will be stripped in certain versions of the operating system. Metadata may be things added to the information field of the file. This will not break the file, but again, nor will the roundtrip zip-unzip produce an identical file.
PS: I am assuming here that if one is zipping a PSD file for delivery to another person, that it has not been flattened and that the font has not been converted to outline, which means that one would also deliver the font files with the PSD so that the person on the receiving end could make their own changes to the file. This is a common practice.
ZIP uses checksum to check if the unpacked file is exactly the same as it was before packing.
So if it was changed in some reason (broken archive, for example) - it would not even be unpacked.