Reading the developer docu about Code Signing, it says in section Digital Signatures and Signed Code
:
To create a digital signature, the signing software computes a special type of checksum called a hash (or digest) based on a piece of data or code and encrypts that hash with the signer’s private key. This encrypted hash is called a signature.
To verify that signature, the verifying software computes a hash of the data or code. It then uses the signer’s public key to decrypt the signature, thus obtaining the original hash as computed by the signer. If the two hashes match, the data has not been modified since it was signed by someone in possession of the signer’s private key.
Signed code contains several digital signatures:
- If the code is universal, the object code for each slice (architecture) is signed separately. This signature is stored within the binary file itself.
- Various components of the application bundle (such as the Info.plist file, if there is one) are also signed. These signatures are stored in a file called _CodeSignature/CodeResources within the bundle.
After some fiddling, I know the following:
- Inside the binary, I got the
CodeDirectory
(0xfade0c02
) containing all SHA-1 hashes or checksums of the Bundle, e.g.Info.plist
etc. - Inside the bundle, I got the
_CodeSignature
directory with theCodeResources
file, which - as the cited paragraph above mentions - should contain all signatures. However, theCodeResources
only contains base64 encoded SHA-1 hashes of several files. - Inside the binary, I've got some certificates, which I can extract using
codesign -d --extract-certificates <binary>
- generatingcodesign0
,codesign1
etc. These certificates actually areCMS Signatures
(0xfade0b01
) that do contain certificate signatures. Looking upCMS
I get Cryptographic Message Syntax, which is used to sign data.
Now the paragraph tells me that some hash has to be stored as a signature (encrypted hash) in the binary or in the CodeResources
file.
I suspect that codesign
uses one of the certificates/CMS Signatures
to extract the public key, then decrypts a signature (encrypted hash) and compares the output to the original hash of a given file, e.g. Info.plist
.
So my question is: Where is this signature (encrypted hash)? Is it the signature inside a certificate/CMS Signature
? Or is it somewhere else?