Manually verifying a digital signature of SSL cert in hex format-CodePudding

I'm creating a program that, given a purely hexidecimal format of a cert, will extract and verify the digital signature.

I have the following example cert in hex format:

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

From here, I am able to successfully derive the digital signature (this is located on the end of the cert hex above)

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

I then take the public key that issued this cert, and use it to decrypt the Digital Signature Digest, which looks like this:

3031300d0609608648016503040201050004200bf3dcf2340b972e97fe3c8493e11eeee01f298939734690d0b4e79e1f5701b4

At this point is where I am trying to verify the Digital Signature by creating a hash of the entire SSL cert (this cert uses SHA256 Hash for digest):

hash('sha256', hex2bin($CertWithoutSignature))

Where $CertWithoutSignature is the same hex above (1st hex string above) WITHOUT the digital signature (2nd hex string above).

At this point I'm a bit confused, because not only does the length of the sha256 hash not match, neither does the data. I know I am decrypting the digest correctly, because otherwise I would get an error if the key were invalid, etc. But that extracted value is 102 characters long, and does not match what I will ever get from sha256 since the length of the string is totally different. Basically I know it is valid if I can get them to match because that means the document is the exact same and therefore will get the same hash. Any help is appreciated. Thanks.

CodePudding user response：

You are forgetting that the hash is encoded and then padded in the signature, assuming that you are using PKCS#1 v1.5 padding within the certificate.

Hashing everything up to the signature is not correct either, you need to hash the TBSCertificate, where TBS means To Be Signed.

For more information, I would strongly recommend to read the X.509 specifications in RFC 5280. And, since ASN.1 encoders / decoders are rather complex you might want to use a library function instead of programming it yourself.

CodePudding user response：

You're going to need to use an actual SSL library in order to validate the signature properly, and it's not simply comparing a couple hashes. You're going to also need the signing cert at the least.

To get you started, you're going to want to convert that cert to PEM format so that OpenSSL will like it.

// hex-encoded binary format
$cert_hex = '308206cd...';

// convert to PEM format
$cert_pem = sprintf(
    "-----BEGIN CERTIFICATE-----\n%s\n-----END CERTIFICATE-----\n",
    implode("\n", str_split(base64_encode(hex2bin($cert_hex)), 64))
);

// read in as an OpenSSL resource
$cert = openssl_x509_read($cert_pem);

// check the fingerprint just to verify it loaded
$fp = openssl_x509_fingerprint($cert, 'sha256');

var_dump($fp);

Output:

string(64) "d74157547fb287694b95b2533588c71f8706b0960e023fc4317f4f9a49ad2721"

After this you're likely going to want to load the signing cert in the same way and feed them both into openssl_x509_verify(), eg:

openssl_x509_verify($cert, $signing_cert);