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DSA and ECDSA signatures are about 340 bits, regardless of key size. An RSA signature is the same size as the key. So if you use a 1,024-bit RSA key pair, each time you send a digital signature you add 1,024 bits to the message. Again, if transmission size is important, you may want to look at DSA or ECDSA.
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The story s the same with signatures as with key distribution. RSA is almost ubiquitous and has become the de facto standard. DSA was promoted by the U.S. government and has become a part of most cryptographic packages. So if you sign using RSA or DSA, other parties will
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almost certainly be able to verify it, whether or not they use the same application you do. ECC is less prevalent.
Protecting Private Keys
3 shows how to protect symmetric keys, and 4 explains that you protect a private key in a similar way. Tokens such as smart cards add a dimension to protection, but for the most part, the way you protect one key is the way you protect any key. Many protocols (discussed in s 7 and 8) require that you have two keys: a digital envelope (or key exchange key) and a separate signing key. So you ll likely have to protect two private keys. But if you lose your private key, there are ways to revoke, or cancel, the public key affiliated with it. If Pao-Chi claims that someone obtained his private key and is signing under his name, he can have his public key revoked. After the effective date of the revocation, any signatures verified with Pao-Chi s public key are invalid because the public key is invalid. Now Pao-Chi has to generate a new key pair, this time protecting the private key more diligently. 6 talks about revoking keys. For now, note that if attackers steal your signing key, they can do a lot more damage than if they steal other types of keys because your signing key lets them pose as you. By stealing your digital envelope or key exchange private key, attackers can get at secrets, but they cannot act on your behalf. If you don t protect your signing key or don t protect it well enough, you re making yourself much more vulnerable.
Introduction to Certificates
Throughout s 4 and 5, we ve talked about other individuals using someone else s public key. To send a secure message to Gwen, Pao-Chi found her public key and created a digital envelope. To verify Pao-Chi s message, Daniel acquired Pao-Chi s public key and verified the digital signature. But how can anyone truly know whether a public key belongs to the purported individual Pao-Chi has in his possession a public key, which is purportedly Gwen s. The key works; he is able to create a digital envelope. But what if
The Digital Signature
Satomi somehow substituted her public key for Gwen s While Pao-Chi was out to lunch, Satomi may have broken into his laptop, found a file called Gwen s public key and edited it so that this file contained her public key, not Gwen s. Then when Pao-Chi sends the digital envelope, Satomi will be able to intercept and read it. Gwen won t be able to open it because she does not have access to the private key partner to the public key used. Suppose the company Pao-Chi and Daniel work for has a centralized directory where everyone s public key is stored. When Daniel wants to verify Pao-Chi s signature, he goes to the directory and find s Pao-Chi s key. But what if Satomi broke into that directory and replaced Pao-Chi s public key with hers Now she can send a fake message to Daniel with a valid digital signature. Daniel will think it came from Pao-Chi because he verifies the signature against what he thinks is Pao-Chi s public key. The most common way to know whether or not a public key does belong to the purported entity is through a digital certificate. A digital certificate binds a name to a public key. An analogy would be a passport, which binds a photo to a name and number. A passport is supposed to be produced in such a way that it is detectable if someone takes an existing passport and replaces the true photo with an imposter s photo. It may be a valid passport, but not for the person in the photo. Immigration officials will not honor that passport. A digital certificate is produced in such a way that it is detectable if someone takes an existing certificate and replaces the public key or name with an imposter s. Anyone examining that certificate will know that something is wrong. Maybe the name or public key is wrong , so you don t trust that name/key pair combination. Here s how it works. Take a name and public key. Consider those two things to be a message, and sign the message. The certificate is the name, public key, and signature (see Figure 5-13). The only thing left to determine is who will sign the certificate. Signing is almost always done by a certificate authority, also known as a CA. More on that later. Gwen originally generated her key pair, protected the private key, and contacted her CA requesting a certificate. Depending on the CA s policy, Gwen may be required to show up in person. The CA verifies Gwen is who she claims to be by examining her passport, driver s license, company ID badge, or whatever method the CA uses to determine identity. Then Gwen uses her private key to sign something (the certificate request, probably). In that way, the CA knows that Gwen does indeed have access to the private key partner to the public key presented, and that the public key has not been replaced. The CA combines Gwen s name and public key into a
Figure 5-13
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