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In a x509 digital certificate there is a "certificate fingerprint" section. It contains md5, sha1 and sha256. How are these obtained, and during the SSL connection, how are these values checked for?

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3  
Can you clarify exactly which kind of digital certificate you are referring to? I'd guess X.509 since you mention SSL but it is as well to be clear. –  Graham Hill Apr 30 '12 at 14:43
    
@GrahamHill - x509 –  Ashwin May 1 '12 at 2:29

1 Answer 1

up vote 18 down vote accepted

The fingerprint, as displayed in the Fingerprints section when looking at a certificate with Firefox or the thumbprint in IE is the hash of the entire certificate in DER form.

If your certificate is in PEM format, convert it to DER with OpenSSL:

openssl x509 -in cert.crt -outform DER -out cert.cer

Then, perform a SHA-1 hash on it (e.g. with sha1sum1):

sha1sum cert.cer

This should produce the same result as what you see in the browser. These values are not part of the certificate, rather they are computed from the certificate.

One application of these fingerprints is to validate EV certificates. In this case, the SHA-1 fingerprint of the root EV CA certificate is hard-coded in the browser (note that (a) it's the fingerprint of the root cert and (b) it has to match exactly the trust anchors shipped with the version of the browser compiled with those values).

Apart from this, these fingerprints are mostly used for identifying the certificates (for organising them).

It's the actual public keys that are used for the verification of other certificates in the chain. The digest used for signing the certificate is actually not in the certificate (only the resulting signature). See certificate structure:

   Certificate  ::=  SEQUENCE  {
        tbsCertificate       TBSCertificate,
        signatureAlgorithm   AlgorithmIdentifier,
        signatureValue       BIT STRING  }

   TBSCertificate  ::=  SEQUENCE  {
        version         [0]  EXPLICIT Version DEFAULT v1,
        serialNumber         CertificateSerialNumber,
        signature            AlgorithmIdentifier,
        issuer               Name,
        validity             Validity,
        subject              Name,
        ...

In this case, the signature value is computed from the DER encoded tbsCertificate (i.e. its content). When the signature algorithm is SHA1 with RSA (for example), a SHA-1 digest is computed and then signed using the RSA private key of the issuer. This SHA-1 digest has nothing to do with the fingerprint has shown by openssl x509 -fingerprint or within the browser, since it's that of the tbsCertificate section only.

There are also a couple of unrelated extensions that can make use of digests, of the public keys this time: the Subject Key Identifier and the Authority Key Identifier. These are optional (and within the TBS content of the certificate).

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How to see the signature value. Why is it not present in the certificate? –  Ashwin May 1 '12 at 1:42
    
The signature is in the certificate, not its fingerprint. You're changing topic here. You could use the Signature class to verify it by hand using the cert's tbsCertificate, its signature and the issuer's public key. If you want RFC 3280 compliance, look into the Java PKI programmer guide. More generally, if this cert is an SSL/TLS client cert, let the X509TrustManager do this. –  Bruno May 1 '12 at 9:27
    
@Ashwin, what I'm saying is that this question (on this page) is about fingerprint (you wrote it). What you're asking now in a comment is a different question, and its answer will not fit in a comment. –  Bruno May 2 '12 at 10:23
    
Can you answer post the answer in this answer itself(I mean after the answer) or do you want me to post another question? –  Ashwin May 2 '12 at 10:44
1  
@Ashwin, I was using sha1sum, a standalone SHA-1 utility. You can use openssl sha1 instead. –  Bruno May 29 '12 at 8:00

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