PFX is commonly used as a synonym of PKCS#12 certificate stores. However, I've never seen an explanation why these stores are called PFX (on Microsoft platforms) in multiple articles, including those of Microsoft. I can think up some names for what is seemingly an acronym, but "protected file X.509" doesn't seem to work well.

Does anybody have any idea what it actually stands for?

EDIT: I did know, but I forgot.


On my Windows 7 machine the .pfx file-type is called:

Personal Information Exchange

I'm not sure why this turned into .pfx, but guess that they wanted to avoid .pix, because it sounds like an image format.

The acronym can also be found in MSDN where it describes a set of functions in an API for creating PFX files.

  • $\begingroup$ Do you know if - originally - is could handle more data than just PKCS#12? Because it seems a bit strange to call this "exchange"; if it contains the private key it should probably not be exchanged with anybody, even if encrypted using a password. $\endgroup$ – Maarten Bodewes Dec 18 '17 at 13:29
  • $\begingroup$ Maybe it is intended for exchanging private keys between applications, e.g. a key storage application and an S/MIME mail client that for hysterical raisins can't talk to the key storage application to sign on its behalf? $\endgroup$ – Squeamish Ossifrage Dec 18 '17 at 16:11
  • $\begingroup$ Could well be, it is certainly possible, and it is old enough for those kind of shenanigans. $\endgroup$ – Maarten Bodewes Dec 19 '17 at 8:50

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