Figure 2-4 Anatomy of a digital certificate
A digital certificate is a collection of data used to verify the identity of the holder or sender of the certificate. For example, an X.509 certificate contains such information as:
Version
Serial number
Certificate issuer
Certificate holder
Validity period (the certificate is not valid before or after this period)
Attributes, known as certificate extensions, that contain additional information such as allowable uses for this certificate
Digital signature from the certification authority to ensure that the certificate has not been altered and to indicate the identity of the issuer
Public key of the owner of the certificate
Message digest algorithm used to create the signature
The careful reader will have noticed that a digital signature indicates the certificate of the signer, and a certificate contains a digital signature, which indicates another certificate. In general, each certificate is verified through the use of another certificate, creating a chain of certificates that ends with the root certificate. The issuer of a certificate is called a certification authority (CA). The owner of the root certificate is the root certification authority. Figure 2-4 illustrates the anatomy of a digital certificate.
Figure 2-4 Anatomy of a digital certificate
The root certificate is self-signed, meaning the signature of the root certificate was created by the root certification authority themselves. Figure 2-5 and Figure 2-6 illustrate how a chain of certificates is created and used. Figure 2-5 shows how the root certification authority creates its own certificate and then creates a certificate for a secondary certification authority.
Figure 2-6 shows how the secondary certification authority creates a certificate for an end user and how the end user uses it to sign a document.
In Figure 2-6, the creator of the document has signed the document. The signature indicates the certificate of the document’s creator (labeled “User” in the figure). The document’s creator signs the document with a private key, and the signing certificate contains the corresponding public key, which can be used to decrypt the message digest to verify the signature. This certificate—together with the private and public keys—was provided by a certification authority (CA). In order to verify the validity of the user’s certificate, the certificate is signed using the certificate of the CA. The certificate of the CA includes the public key needed to decrypt the message digest of the user’s certificate. Continuing the certificate chain, the certificate of the CA is signed using the certificate of the authority who issued that certificate. The chain can go on through any number of intermediate certificates, but in Figure 2-5 the issuer of the CA’s certificate is the root certification authority. Note that the certificate of the root CA, unlike the others, is “self signed.” That is, it does not refer to a further certification authority but is signed using the root CA’s own private key.
When a CA creates a certificate, it uses its private key to encrypt the certificate’s message digest. The signature of every certificate the CA issues refers to its own signing certificate. The CA’s public key is in this certificate, and the application verifying the signature must extract this key to verify the certificate of the CA. So it continues, on down the certificate chain, to the certificate of the root CA. When a root CA issues a certificate, it, too, signs the certificate. However, this signing certificate was not issued by another CA; the chain stops here.
The certificate of the root CA can be verified by creating a digest and comparing it with one widely available. Typically, the root certificate and root CA’s public key are already stored in the application or on the computer that needs to verify the signature.
The confidence you can have in a given certificate depends on the confidence you have in the certificate authorities and in their procedures for ensuring that subsequent certificate recipients in the certificate chain are fully authenticated. For this reason, it is always a good idea to examine the certificate that comes with a digital signature, even when the signature appears to be valid. In Mac OS X, all certificates you receive are stored in your keychain. You can use the Keychain Access utility to view them.
Certain attributes of a digital certificate (known as certificate extensions) are said to establish a level of trust for a digital certificate. A trust policy is a set of rules that specify the appropriate uses for a certificate that has a specific level of trust. In other words, the level of trust for a certificate is used to answer the question “Should I trust this certificate for this action?”
