Question

I know that it is done by signing assembly with private key.

So here how I see the process …
When we have the private/public key pair file we can build assembly signing it using this keys.
So what in reallity is done is that compiler opens the ‘sk'(or pfx) file and retreives the private key (which I understand is impossible for human) and after signing the assembly with the private key it adds the public key into assembly manifest and that is it I have the strongly named assembly.

So what when I run the application which is referencing that assemly ?
What does CLR to be sure that the assebly is not replaced and nothing was changed?

Answer 1

A quote from CLR via C#

Signing an assembly with a private key
ensures that the holder of the
corresponding public key produced the
assembly. When the assembly is
installed into the GAC, the system
hashes the contents of the file
containing the manifest and compares
the hash value with the RSA digital
signature value embedded within the PE
file (after unsigning it with the
public key). If the values are
identical, the file’s contents haven’t
been tampered with, and you know that
you have the public key that
corresponds to the publisher’s private
key. In addition, the system hashes
the contents of the assembly’s other
files and compares the hash values
with the hash values stored in the
manifest file’s FileDef table. If any
of the hash values don’t match, at
least one of the assembly’s files has
been tampered with, and the assembly
will fail to install into the GAC.

Well, here how it works.

When you compile the assembly noting that you want to sign it with already generated public/private key pair file the compiler computes the hash of the assembly (also computes hashes for each file in the assembly and stores the values along with file names in FileDef table) then it signs the hash value with private key and embeds public key in manifest for that assembly.

Now in runtime when the application (assembly) tries to load that signed assembly the assembly is again hashed then CLR gets the public key from the assembly manifest and decrypts the RSA sign and compares the hash value with the sign value. If they are the same than nothing was changed.