Substitution

A substitution cipher uses a key to substitute characters or character blocks with different characters or character blocks. The Caesar cipher and the Vigenère cipher are two of the earliest forms of substitution ciphers. Figure 3.1 shows the ROT13, which is a Caesar cipher. It rotates the alphabet 13 positions. Therefore, the message “Hello” encrypts to the ciphertext URYYB.

One of the issues with substitution ciphers is if the message is of sufficient length, patterns in the encryption begin to become noticeable, which makes it vulnerable to a frequency attack. A frequency attack is when the attacker uses these recurring patterns to reverse engineer the message. For this reason, the polyalphabetic algorithm was created.

Polyalphabetic

To increase the difficulty of performing a frequency attack, polyalphabetic algorithms were created. They use multiple instances of the alphabet shifted in a 26 × 26 table called a tableau, shown in Figure 3.2 . The figure shows the Vigenère cipher, an example of a polyalphabetic cipher.

As an example of a message on which the Vigenère cipher is applied, let’s use the security key SYBEX and the plaintext message of WE ATTACK AT FIVE. The first letter in the plaintext message is W, and the first letter in the key is S. We should locate the letter W across the headings for the columns. We follow that column down until it intersects with the row that starts with the letter S, resulting in the letter O. The second letter of the plaintext message is E, and the second letter in the key is Y. Using the same method, we obtain the letter C. We continue in this same manner until we run out of key letters, and then we start over with the key, which would result in the second A in the plaintext message working with the letter S of the key.

So, applying this technique to the entire message of WE ATTACK AT FIVE, the plaintext message converts to the OCBXQSALEQXGWI ciphertext message.

Transposition

A transposition cipher scrambles the letters of the original message in a different order. The key determines the positions to which the letters are moved.

The following is an example of a simple transposition cipher:

Original message            SNOWFLAKES WILL FALL
Broken into groups          SNOW   FLAK    ESWI   FALL
Key                         4231   2314    4231   2314
Ciphertext message          WONS   LAFK    IWSE   ALFL

With this example, the original message is SNOWFLAKES WILL FALL, and the key is 4231 2314. The ciphertext message is WONS LAFK IWSE ALFL. So, you take the first four letters of the plaintext message (SNOW) and use the first four numbers (4231) as the key for transposition. The key describes the relative potions of the same characters in the ciphertext. In the new ciphertext, the letters would be WONS. Then you take the next four letters of the plaintext message (FLAK) and use the next four numbers (2314) as the key for transposition. In the new ciphertext, the letters would be LAFK. Then you take the next four letters of the original message and apply the first four numbers of the key because you do not have any more numbers in the key. Continue this pattern until complete.

Symmetric

Symmetric algorithms use a private or secret key that must remain secret between the two parties. Each party pair requires a separate private key. Therefore, a single user would need a unique secret key for every user with whom she communicates.

Consider an example where there are 10 unique users. Each user needs a separate private key to communicate with the other users. To calculate the number of keys that would be needed in this example, you would use the following formula:

# of users × (# of users – 1) / 2

Using our example, you would calculate 10 × (10 – 1) / 2, or 45 needed keys.

With symmetric algorithms, the encryption key must remain secure. To obtain the secret key, the users must find a secure out-of-band method for communicating the secret key, including courier or direct physical contact between the users.

A special type of symmetric key called a session key encrypts messages between two users during one communication session. Symmetric algorithms can be referred to as single-key, secret-key, private-key, or shared-key cryptography.

Symmetric systems provide confidentiality but not authentication or nonrepudiation. If both users use the same key, determining where the message originated is impossible. Symmetric algorithms include DES, AES, 3DES, and RC4. Table 3.1 lists the strengths and weaknesses of symmetric algorithms.

The two broad types of symmetric algorithms are stream-based ciphers and block ciphers. Initialization vectors (IVs) are an important part of block ciphers. These three components will be discussed in the next sections.

3DES

Because of the need to quickly replace DES, Triple DES (3DES), a version of DES that increases security by using three 56-bit keys, was developed. Although 3DES is resistant to attacks, it is up to three times slower than DES. 3DES did serve as a temporary replacement to DES. However, the National Institute of Standards and Technology (NIST) has actually designated the Advanced Encryption Standard (AES) as the replacement for DES, even though 3DES is still in use today.

DES can operate in a number of different modes, but the two most common are Electronic Code Book (ECB) and Cipher Block Chaining (CBC). In ECB, 64-bit blocks of data are processed by the algorithm using the key. The ciphertext produced can be padded to ensure that the result is a 64-bit block. If an encryption error occurs, only one block of the message is affected. ECB operations run in parallel, making it a fast method.

Although ECB is the easiest and fastest mode to use, it has security issues because every 64-bit block is encrypted with the same key. If an attacker discovers the key, all the blocks of data can be read. If an attacker discovers both versions of the 64-bit block (plaintext and ciphertext), the key can be determined. For these reasons, the mode should not be used when encrypting a large amount of data because patterns would emerge. ECB is a good choice if an organization needs encryption for its databases because ECB works well with the encryption of short messages.

Figure 3.3 shows the ECB encryption process.

In CBC, each 64-bit block is chained together because each resultant 64-bit ciphertext block is applied to the next block. So, plaintext message block 1 is processed by the algorithm using an IV. The resultant ciphertext message block 1 is XORed with plaintext message block 2, resulting in ciphertext message 2. This process continues until the message is complete.

Unlike ECB, CBC encrypts large files without having any patterns within the resulting ciphertext. If a unique IV is used with each message encryption, the resultant ciphertext will be different every time even in cases where the same plaintext message is used.

Figure 3.4 shows the CBC encryption process.

Asymmetric

Asymmetric algorithms use both a public key and a private or secret key. The public key is known by all parties, and the private key is known only by its owner. One of these keys encrypts the message, and the other decrypts the message.

In asymmetric cryptography, determining a user’s private key is virtually impossible even if the public key is known, although both keys are mathematically related. However, if a user’s private key is discovered, the system can be compromised.

Asymmetric algorithms can be referred to as dual-key or public-key cryptography.

Asymmetric systems provide confidentiality, integrity, authentication, and nonrepudiation. Because both users have one unique key that is part of the process, determining where the message originated is possible.

If confidentiality is the primary concern for an organization, a message should be encrypted with the receiver’s public key, which is referred to as a secure message format. If authentication is the primary concern for an organization, a message should be encrypted with the sender’s private key, which is referred to as an open message format. When using open message format, the message can be decrypted by anyone with the public key.

Perhaps the most widely known and used asymmetric algorithm is RSA. Oher asymmetric algorithms include RSA, El Gamal, DSA, and Elliptic Curve Cryptography (ECC).

Private Key

The private key that is generated as part of the key pair is made available only to the user or device to which it was issued. This key may be stored on software in the user’s computer, or it might be stored on a smart card if it is to be used for authentication. At any rate, the key concept here is that it is available only to the user or device to which it was issued.

Public Key

The public key that is generated as part of the key pair is made available to anyone to whom the certificate is presented because it is part of the information contained in this digital document. In some cases, public keys may be kept in a repository so they can be requested by an entity if required. Regardless of the method used to obtain the public key, the key concept here is that it is available to anyone.

Putting It Together

These keys work together to perform both encryption and digital signatures. To provide encryption, the data is encrypted with the receiver’s public key, which results in ciphertext that only the receiver’s private key can decrypt. Figure 3.8 shows this process.

To digitally sign a document, the sender creates what is called a hash value of the data being sent, encrypts that value with the sender’s his private key, and sends this value along with the message. The receiver decrypts the hash using the sender’s public key. The receiver then, using the same hashing algorithm, hashes the message. The sender then compares the decrypted hash value to the one just generated. If they are the same, the signature (and the integrity of the data) has been verified. Figure 3.9 shows this process.

Revocation

Certificates have a defined lifetime. When the validity period ends, the certificate must be renewed to continue to be valid. There are cases when a certificate must be revoked before its lifetime ends. Reasons for certificate revocation include the following:

• Compromise of the associated keys
• Improper issuance
• Compromise of the issuing CA
• Owner of the certificate no longer owning the domain for which it was issued
• Owner of the certificate ceasing operations entirely
• Original certificate being replaced with a different certificate from a different issuer

A certificate revocation list (CRL) is a list of digital certificates that a CA has revoked. To find out whether a digital certificate has been revoked, either the browser must check the CRL or the CA must push out the CRL values to clients. This can become quite daunting when you consider that the CRL contains every certificate that has ever been revoked.

One concept to keep in mind is the revocation request grace period. This period is the maximum amount of time between when the revocation request is received by the CA and when the revocation actually occurs. A shorter revocation period provides better security but often results in a higher implementation cost.

Uses

Certificates can be used for variety of operations. This can include authentication, encryption, digital signatures, and email to name a few. VeriSign first introduced the following digital certificate classes:

• Class 1: For individuals intended for email. These certificates get saved by web browsers.
• Class 2: For organizations that must provide proof of identity.
• Class 3: For servers and software signing in which independent verification and identity and authority checking is done by the issuing CA.
• Class 4: For online business transactions between companies.
• Class 5: For private organizations or governmental security.

Application: SSL/TLS

Certificates are often used when using SSL/TLS. Most modern systems today use TLS, but the term SSL is often still used to refer to the connection. SSL is used to protect many types of applications, the most common being HTTPS (as HTTP is called when used with SSL).

An SSL session is formed between a web server and the web browser of the client. Figure 3.10 depicts the process.

Default Certificate

The ASA has a self-signed default certificate that can be used for the operations listed in the previous section. The issue with a self-signed certificate is that no browsers or devices will have the ASA listed as a trusted CA. Because of this, any HTTPS connections to the ASA will generate a warning message that the certificate being presented is not trusted. To avoid this issue, you can install a root certificate of the CA whose certificate is found in the browsers and devices that will interact with the ASA (either that you own or a public CA).

Viewing and Adding Certificates in the ASDM

To view the current certificates in the ADSM, select Configuration at the top of the ADSM console and Device Management from the tabs on the left side of the console, as shown in Figure 3.13 . As you can see, this ASA currently has no certificates installed other than the default.

To add a certificate, follow these steps:

1. In the Cisco ASDM Configuration Tool, select Configuration ➢ Device Management ➢ Certificate Management ➢ CA Certificates.
2. Click Add. The Install Certificate dialog box appears. You have three options: install from a file, paste the information, or use SCEP. If the root CA represented by the root certificate supports SCEP, choose that option. Otherwise, use the next two steps.
3. Enter a trustpoint name or use the default name that appears in the box.
4. Click the Install From A File radio button and browse to the location of the Root.crt file that you are installing.
5. Click the More Options button, and here you can configure how certificate revocation will be checked, the protocols to be used for certificate verification, and other settings.

SCEP

Simple Certificate Enrollment Protocol is a protocol used for enrollment and other PKI operations. It is supported on most Cisco devices. It simplifies the process of obtaining and installing both the root and the identity certificates. The process to use SCEP is as follows:

1. Choose Configuration ➢ Device Management ➢ Certificate Management ➢ Identity Certificates and click Add.
2. Click the Add A New Identity Certificate radio button and click the Advanced button.
3. In the Advanced box, on the Enrollment Mode tab, select Request From A CA and then enter the IP address of the CA that supports SCEP. Click OK.
4. In the Add A New Identity Certificate dialog box, select Add Certificate. If the enrollment is successful, you will receive an Enrollment Succeeded message.

Ciphertext-Only Attack

In a ciphertext-only attack, an attacker uses several encrypted messages (ciphertext) to figure out the key used in the encryption process. Although it is a common type of attack, it is usually not successful because so little is known about the encryption used.

Known Plaintext Attack

In a known plaintext attack, an attacker uses the plaintext and ciphertext versions of a message to discover the key used. This type of attack implements reverse engineering, frequency analysis, or brute force to determine the key so that all messages can be deciphered.

Chosen Plaintext Attack

In a chosen plaintext attack, an attacker chooses the plaintext to get encrypted to obtain the ciphertext. The attacker sends a message hoping that the user will forward that message as ciphertext to another user. The attacker captures the ciphertext version of the message and tries to determine the key by comparing the plaintext version he originated with the captured ciphertext version. Once again, key discovery is the goal of this attack.

Chosen Ciphertext Attack

A chosen ciphertext attack is the opposite of a chosen plaintext attack. In a chosen ciphertext attack, an attacker chooses the ciphertext to be decrypted to obtain the plaintext. This attack is more difficult because control of the system that implements the algorithm is needed.

Brute Force

As with a brute-force attack against passwords, a brute-force attack executed against a cryptographic algorithm uses all possible keys until a key is discovered that successfully decrypts the ciphertext. This attack requires considerable time and processing power and is difficult to complete.

Birthday Attack

A birthday attack uses the premise that finding two messages that result in the same hash value is easier than matching a message and its hash value. Most hash algorithms can resist simple birthday attacks.

Meet-in-the-Middle Attack

In a meet-in-the middle attack, an attacker tries to break the algorithm by encrypting from one end and decrypting from the other to determine the mathematical problem used.