TCP/IP governs the transport and routing of data over the Internet. Other protocols, such as the HTTP, LDAP, or IMAP use TCP/IP to support typical application tasks such as displaying web pages or running mail servers.
The SSL protocol runs above TCP/IP and below higher-level protocols such as HTTP or IMAP. It uses TCP/IP on behalf of the higher-level protocols, and in the process allows an SSL-enabled server to authenticate itself to an SSL-enabled client, allows the client to authenticate itself to the server, and allows both machines to establish an encrypted connection.
SSL addresses the following concerns about communication over the Internet and other TCP/IP networks:
- SSL server authentication allows a user to confirm a server’s identity.
SSL-enabled client software can use standard techniques of public-key cryptography to check that a server’s certificate and public ID are valid and have been issued by a certificate authority (CA) listed in the client’s list of trusted CAs. This confirmation might be important if the user, for example, is sending a credit card number over the network and wants to check the receiving server’s identity.
- SSL client authentication allows a server to confirm a user’s identity.
Using the same techniques as those used for server authentication, SSL-enabled server software can check that a client’s certificate and public ID are valid and have been issued by a certificate authority (CA) listed in the server’s list of trusted CAs. This confirmation might be important if the server, for example, is a bank sending confidential financial information to a customer and wants to check the recipient’s identity.
- An encrypted SSL connection requires all information sent between a client and a server to be encrypted by the sending software and decrypted by the receiving software, thus providing a high degree of confidentiality.
Confidentiality is important for both parties to any private transaction. In addition, all data sent over an encrypted SSL connection is protected with a mechanism for detecting tampering—that is, for automatically determining whether the data has been altered in transit.
The SSL protocol includes two sub-protocols: the SSL record protocol and the SSL handshake protocol.
The SSL record protocol defines the format used to transmit data. The SSL handshake protocol involves using the SSL record protocol to exchange a series of messages between an SSL-enabled server and an SSL-enabled client when they first establish an SSL connection. This exchange of messages is designed to facilitate the following actions:
Authenticate the server to the client.
Allow the client and server to select the cryptographic algorithms, or ciphers, that they both support.
Optionally authenticate the client to the server.
Use public-key encryption techniques to generate shared secrets.
Establish an encrypted SSL connection.
SSL Handshake
The SSL protocol uses a combination of public-key and symmetric key encryption. Symmetric key encryption is much faster than public-key encryption, but public-key encryption provides better authentication techniques. An SSL session always begins with an exchange of messages called the SSL handshake. The handshake allows the server to authenticate itself to the client by using public-key techniques, and then allows the client and the server to cooperate in the creation of symmetric keys used for rapid encryption, decryption, and tamper detection. Optionally, the handshake also allows the client to authenticate itself to the server.
Messages Exchanged During SSL Handshake
The following steps describes the sequence of messages exchanged during an SSL handshake. These step describe the programmatic details of the messages exchanged during the SSL handshake.
The client sends the server the client’s SSL version number, cipher settings, randomly generated data, and other information the server needs to communicate with the client using SSL.
The server sends the client the server’s SSL version number, cipher settings, randomly generated data, and other information the client needs to communicate with the server over SSL. The server also sends its own certificate and, if the client is requesting a server resource that requires client authentication, requests the client’s certificate.
The client can use some of the information sent by the server to authenticate the server. For details, see Server Authentication During SSL Handshake. If the server cannot be authenticated, the user is warned of the problem and informed that an encrypted and authenticated connection cannot be established. If the server can be successfully authenticated, the client goes on to Step 4.
Using all data generated in the handshake so far, the client, with the cooperation of the server, depending on the cipher being used, creates the pre-master secret for the session, encrypts it with the server’s public key, obtained from the server’s certificate, sent in Step 2, and sends the encrypted pre-master secret to the server.
If the server has requested client authentication (an optional step in the handshake), the client also signs another piece of data that is unique to this handshake and known by both the client and server. In this case the client sends both the signed data and the client’s own certificate to the server along with the encrypted pre-master secret.
If the server has requested client authentication, the server attempts to authenticate the client. For details, see Server Authentication During SSL Handshake. If the client cannot be authenticated, the session is terminated. If the client can be successfully authenticated, the server uses its private key to decrypt the pre-master secret, then performs a series of steps (which the client also performs, starting from the same pre-master secret) to generate the master secret.
Both the client and the server use the master secret to generate the session keys, which are symmetric keys used to encrypt and decrypt information exchanged during the SSL session and to verify its integrity—that is, to detect changes in the data between the time it was sent and the time it is received over the SSL connection.
The client sends a message to the server informing it that future messages from the client are encrypted with the session key. It then sends a separate (encrypted) message indicating that the client portion of the handshake is finished.
The server sends a message to the client informing it that future messages from the server are encrypted with the session key. It then sends a separate (encrypted) message indicating that the server portion of the handshake is finished.
The SSL handshake is now complete, and the SSL session has begun. The client and the server use the session keys to encrypt and decrypt the data they send to each other and to validate its integrity.
Server Authentication During SSL Handshake
SSL-enabled client software always requires server authentication, or cryptographic validation by a client of the server’s identity. The server sends the client a certificate to authenticate itself. The client uses the certificate to authenticate the identity the certificate claims to represent.
To authenticate the binding between a public key and the server identified by the certificate that contains the public key, an SSL-enabled client must receive a yes answer to the four questions shown in the following figure.
An SSL-enabled client goes through the following steps to authenticate a server’s identity:
Is today’s date within the validity period?
The client checks the server certificate’s validity period. If the current date and time are outside of that range, the authentication process won’t go any further. If the current date and time are within the certificate’s validity period, the client goes on to the next step.
Is the issuing CA a trusted CA?
Each SSL-enabled client maintains a list of trusted CA certificates, represented by the shaded area on the right—hand side of diagram This list determines which server certificates the client accepts. If the distinguished name (DN) of the issuing CA matches the DN of a CA on the client’s list of trusted CAs, the answer to this question is yes, and the client goes on to the next step. If the issuing CA is not on the list, the server is not authenticated unless the client can verify a certificate chain ending in a CA that is on the list.
Does the issuing CA’s public key validate the issuer’s digital signature?
The client uses the public key from the CA’s certificate (which it found in its list of trusted CAs in step 2) to validate the CA’s digital signature on the server certificate being presented. If the information in the server certificate has changed since it was signed by the CA or if the CA certificate’s public key doesn’t correspond to the private key used by the CA to sign the server certificate, the client won’t authenticate the server’s identity. If the CA’s digital signature can be validated, the server treats the user’s certificate as a valid “letter of introduction” from that CA and proceeds. At this point, the client has determined that the server certificate is valid.
Does the domain name in the server’s certificate match the domain name of the server itself?
This step confirms that the server is actually located at the same network address specified by the domain name in the server certificate. Although step 4 is not technically part of the SSL protocol, it provides the only protection against a form of security attack known as man-in-the-middle. Clients must perform this step and must refuse to authenticate the server or establish a connection if the domain names don’t match. If the server’s actual domain name matches the domain name in the server certificate, the client goes on to the next step.
The server is authenticated.
The client proceeds with the SSL handshake. If the client doesn’t get to step 5 for any reason, the server identified by the certificate cannot be authenticated, and the user is warned of the problem and informed that an encrypted and authenticated connection cannot be established. If the server requires client authentication, the server performs the steps described in Client Authentication During SSL Handshake.
After the steps described here, the server must successfully use its private key to decrypt the pre-master secret sent by the client.
Man-In-the-Middle Attack
The man-in-the-middle is a rogue program that intercepts all communication between the client and a server with which the client is attempting to communicate via SSL. The rogue program intercepts the legitimate keys that are passed back and forth during the SSL handshake, substitutes its own, and makes it appear to the client that it is the server, and to the server that it is the client.
The encrypted information exchanged at the beginning of the SSL handshake is actually encrypted with the rogue program’s public key or private key, rather than the client’s or server’s real keys. The rogue program ends up establishing one set of session keys for use with the real server, and a different set of session keys for use with the client. This allows the rogue program not only to read all the data that flows between the client and the real server, but also to change the data without being deleted. Therefore, it is extremely important for the client to check that the domain name in the server certificate corresponds to the domain name of the server with which a client is attempting to communicate—in addition to checking the validity of the certificate by performing the other steps described in Server Authentication During SSL Handshake.
Client Authentication During SSL Handshake this is optional
Digital Signatures
Digital signatures can be used by Directory Server to maintain integrity of information. If encryption and message digests are applied to the information being sent, the recipient can determine that the information was not tampered with during transit.
Tamper detection and related authentication techniques rely on a mathematical function called a one-way hash. This function is also called a message digest. A one-way hash is a number of fixed length with the following characteristics:
The value of the hash is unique for the hashed data. Any change in the data, even deleting or altering a single character, results in a different value.
The content of the hashed data cannot, for all practical purposes, be deduced from the hash — which is why it is called one-way.
It is possible to use a private key for encryption and a public key for decryption. Although this is not desirable when you are encrypting sensitive information, it is a crucial part of digitally signing any data. Instead of encrypting the data itself, the signing software creates a one-way hash of the data, then uses your private key to encrypt the hash. The encrypted hash, along with other information, such as the hashing algorithm, is known as a digital signature. Diagram shows two items transferred to the recipient of some signed data.
In Diagram, the original data and the digital signature, which is basically a one-way hash (of the original data) that has been encrypted with the signer's private key. To validate the integrity of the data, the receiving software first uses the signer’s public key to decrypt the hash. It then uses the same hashing algorithm that generated the original hash to generate a new one-way hash of the same data. (Information about the hashing algorithm used is sent with the digital signature, although this isn’t shown in the figure.) Finally, the receiving software compares the new hash against the original hash. If the two hashes match, the data has not changed since it was signed. If they don’t match, the data may have been tampered with since it was signed, or the signature may have been created with a private key that doesn’t correspond to the public key presented by the signer.
If the two hashes match, the recipient can be certain that the public key used to decrypt the digital signature corresponds to the private key used to create the digital signature. Confirming the identity of the signer, however, also requires some way of confirming that the public key really belongs to a particular person or other entity.
The significance of a digital signature is comparable to the significance of a handwritten signature. Once you have signed some data, it is difficult to deny doing so later — assuming that the private key has not been compromised or out of the owner’s control. This quality of digital signatures provides a high degree of non-repudiation — that is, digital signatures make it difficult for the signer to deny having signed the data. In some situations, a digital signature may be as legally binding as a handwritten signature.
Key Encryption
With most modern cryptography, the ability to keep encrypted information secret is based not on the cryptographic algorithm, which is widely known, but on a key. A key is a number that must be used with the algorithm to produce an encrypted result or to decrypt previously encrypted information. For information about encryption and decryption with keys, see the following sections:
Symmetric-Key Encryption
With symmetric-key encryption, the encryption key can be calculated from the decryption key, and vice versa. With most symmetric algorithms, the same key is used for both encryption and decryption. The following figure shows a symmetric-key encryption.
Figure 5–12 Symmetric-Key Encryption
Implementations of symmetric-key encryption can be highly efficient, so that users do not experience any significant time delay as a result of the encryption and decryption. Symmetric-key encryption also provides a degree of authentication, since information encrypted with one symmetric key cannot be decrypted with any other symmetric key. Thus, as long as the symmetric key is kept secret by the two parties using it to encrypt communications, each party can be sure that it is communicating with the other as long as the decrypted messages continue to make sense.
Symmetric-key encryption is effective only if the symmetric key is kept secret by the two parties involved. If anyone else discovers the key, it affects both confidentiality and authentication. A person with an unauthorized symmetric key not only can decrypt messages sent with that key, but can encrypt new messages and send them as if they came from one of the two parties who were originally using the key.
Symmetric-key encryption plays an important role in the SSL protocol, which is widely used for authentication, tamper detection, and encryption over TCP/IP networks. SSL also uses techniques of public-key encryption, which is described in the next section.
Public-Key Encryption
The most commonly used implementations of public-key encryption are based on algorithms patented by RSA Data Security. Therefore, this section describes the RSA approach to public-key encryption.
Public-key encryption (also called asymmetric encryption) involves a pair of keys—a public key and a private key—associated with an entity that needs to authenticate its identity electronically or to sign or encrypt data. Each public key is published, and the corresponding private key is kept secret. The following figure shows a simplified view of the way public-key encryption works.
Figure 5–13 Public-Key Encryption
Public—key encryption lets you distribute a public key, and only you can read data encrypted by this key. In general, to send encrypted data to someone, you encrypt the data with that person’s public key, and the person receiving the encrypted data decrypts it with the corresponding private key.
Compared with symmetric-key encryption, public-key encryption requires more computation and is therefore not always appropriate for large amounts of data. However, it’s possible to use public-key encryption to send a symmetric key, which can then be used to encrypt additional data. This is the approach used by the SSL protocol.
As it happens, the reverse of the scheme shown in Diagram also works: data encrypted with your private key can be decrypted with your public key only. This would not be a desirable way to encrypt sensitive data, however, because it means that anyone with your public key, which is by definition published, could decrypt the data. Nevertheless, private-key encryption is useful, because it means you can use your private key to sign data with your digital signature—an important requirement for electronic commerce and other commercial applications of cryptography. Client software can then use your public key to confirm that the message was signed with your private key and that it hasn’t been tampered with since being signed. Digital Signatures and subsequent sections describe how this confirmation process works.
Key Length and Encryption Strength
The strength of encryption is related to the difficulty of discovering the key, which in turn depends on both the cipher used and the length of the key. For example, the difficulty of discovering the key for the RSA cipher most commonly used for public-key encryption depends on the difficulty of factoring large numbers, a well-known mathematical problem.
Encryption strength is often described in terms of the size of the keys used to perform the encryption: in general, longer keys provide stronger encryption. Key length is measured in bits. For example, 128-bit keys for use with the RC4 symmetric-key cipher supported by SSL provide significantly better cryptographic protection than 40-bit keys for use with the same cipher. Roughly speaking, 128-bit RC4 encryption is 3 x 1026 times stronger than 40-bit RC4 encryption.
Different ciphers may require different key lengths to achieve the same level of encryption strength. The RSA cipher used for public-key encryption, for example, can use only a subset of all possible values for a key of a given length, due to the nature of the mathematical problem on which it is based. Other ciphers, such as those used for symmetric key encryption, can use all possible values for a key of a given length, rather than a subset of those values. Thus a 128-bit key for use with a symmetric-key encryption cipher would provide stronger encryption than a 128-bit key for use with the RSA public-key encryption cipher. This difference explains why the RSA public-key encryption cipher must use a 512-bit key (or longer) to be considered cryptographically strong, whereas symmetric key ciphers can achieve approximately the same level of strength with a 64-bit key. Even this level of strength may be vulnerable to attacks in the near future.
