.PFX

Introduction

A PFX or P12 file carries a private key together with its certificate, and usually the chain of certificates above it, in one password-protected file. It is the usual way to move a certificate and its key between machines: exporting a certificate with its key from Windows, from macOS Keychain Access or from a browser gives a file in this format.

The format is PKCS #12, first published by RSA Laboratories and now RFC 7292. It grew out of Microsoft’s earlier PFX format, which is why Windows uses .pfx and most other systems .p12; the contents are the same.

Structure

The file is DER-encoded ASN.1, so it starts with the byte 30. The top level is a PFX SEQUENCE holding a version number, 3, the authenticated content, and usually a MAC computed from the password, which proves the file has not been changed.

The content is a list of SafeContents, each either plain or encrypted with a key derived from the password. Inside are SafeBags: a certificate bag for each certificate, and a shrouded key bag holding the private key encrypted as PKCS #8. Bags can carry a friendly name and a local key ID that ties a key to its certificate. Older files use weak encryption such as 40-bit RC2 or triple DES; newer ones use AES with PBKDF2.

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