How to extract signing certificates from macOS binary files

Code signing is a macOS security technology that you use to certify that an app was created by you. Once an app is signed, the system can detect any change to the app—whether the change is introduced accidentally or by malicious code. As Apple Developer site says ( click here for more details on code signing) :

code signing allows the operating system to:

• Ensure that a piece of code has not been altered since it was signed. The system can detect even the smallest change, whether it was intentional (by a malicious attacker, for example) or accidental (as when a file gets corrupted). When a code signature is intact, the system can be sure the code is as the signer intended.
• Identify code as coming from a specific source (a developer or signer). The code signature includes cryptographic information that unambiguously points to a particular author.
• Determine whether code is trustworthy for a specific purpose. Among other things, a developer can use a code signature to state that an updated version of an app should be considered by the 
  system to be the same app as the previous version.

We can extract the signing certificates from the macOS binary files using codesign tool.  Run the following command on any macOS binary , dylib or .app to extract the signing certificate from the file.

Steps :

1. Change to tmp directory.

    $ cd  /tmp

2. Run codesign command on the binary or .app to extract signing certificate.

    $codesign -dvvvv --extract-certificates /Applications/Mail.app

    This extracts all the signing certificates from the file and creates 3 files named codesign0, codesign1 and codesign2.  Codesign0 is usually the leaf (signing) certificate, and as many files are written as there are certificates in the signature. The files are in ASN.1 (DER) form. 

To view the contents of the certificates:

To see the details of the certificates which were extracted in DER form, we have use openssl tool. Run the following command to view the contents of certificate

$ openssl x509 -inform DER -in codesign0 -text

The output looks like below:

Certificate:

    Data:

        Version: 3 (0x2)

        Serial Number:

            2a:da:71:ba:a7:bd:17:9f

        Signature Algorithm: sha1WithRSAEncryption

        Issuer: C=US, O=Apple Inc., OU=Apple Certification Authority, CN=Apple Code Signing Certification Authority

        Validity

            Not Before: Apr 12 22:34:35 2013 GMT

            Not After : Apr 12 22:34:35 2021 GMT

        Subject: C=US, O=Apple Inc., OU=Apple Software, CN=Software Signing

        Subject Public Key Info:

            Public Key Algorithm: rsaEncryption

            RSA Public Key: (2048 bit)

                Modulus (2048 bit):

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uP6a2wO7jWLyGW13k+N1zzZZMV4IbV0BHGVTUpN2eJwXCxAelLw2kFZLRC6Y2aYx

ofAcZpSZVHMTtVE4vCSioDA7emWHrMC8FfReMMedoyoE38TPR4Rtn/3/RcOgGaw8

u62PlPm5x8hxNhHt6AG6tHVIgqQqUxoFBZudxkcb9eggcqAbS+W+ZPw4DZr/Q0E=

-----END CERTIFICATE-----