If encryption takes place at a high layer of the communications architecture, like the applications layer or the presentation layer, then it can be independent of the type of communication network used. It is still end-to-end encryption, but the encryption implementation does not have to bother about line codes, synchronization between modems, physical interfaces, and so forth. In the early days of electro- mechanical cryptography, encryption and decryption took place entirely offline; this is only one step r e-moved from that.

Encryption at these high layers interacts with the user software. This software is different for different computer archite c-tures, and so the encryption must be optimized for different computer systems. Encryption can occur in the software itself or in specialized hardware. In the latter case, the computer will send the data to the specialized hardware for encryption before sending it to lower layers of the communication architecture for transmission. This process requires some intelligence and is not suitable for dumb terminals. Additionally, there may be compatibility problems with different types of computers. The major disadvantage of end-to-end encryption is that it allows traffic analysis. Traffic analysis is the analysis of encrypted messages: where they come from, where they go to, how long they are, when they are sent, how frequent or infrequent they are, whether they coincide with outside events like meetings, and more. A lot of good information is buried in that data, and a cryptanalyst will want to get his hands on it. Table 10.3 presents the positive and negative aspects of end-to-end encryption.

Combining the Two

Table 10.4, primarily from [1244], compares link-by-link and end-to-end encryption. Combining the two, while most expe n-sive, is the most effective way of securing a network. Encryption of each physical link makes any analysis of the routing information impossible, while end-to-end encryption reduces the threat of unencrypted data at the various nodes in the network. Key ma n-agement for the two schemes can be completely separate: The network managers can take care of encryption at the physical level, while the individual users have responsibility for end-to-end encryption.

10.4 ENCRYPTING DATA FOR STORAGE

Encrypting data for storage and later retrieval can also be thought of in the Alice and Bob model. Alice is still sending a me s-sage to Bob, but in this case "Bob" is Alice at some future time. However, the problem is fundamentally different. In communic a-tions channels, messages in transit have no intrinsic value. If Bob doesnt receive a particular message, Alice can always resend it. This is not true for data encrypted for storage. If Alice cant decrypt her message, she cant go back in time and re-encrypt it. She has lost it forever. This means that encryption applications for data storage should have some mechanisms to prevent unrecove r-able errors from creeping into the ciphertext. The encryption key has the same value as the message, only it is smaller. In effect, cryptography converts large secrets into smaller ones. Being smaller, they can be easily lost. Key management procedures should assume that the same keys will be used again and again, and that data may sit on a disk for years before being decrypted. Fu r-thermore, the keys will be around for a long time. A key used on a communications link should, ideally, exist only for the length of the communication. A key used for data storage might be needed for years, and hence must be stored securely for years.

Other problems particular to encrypting computer data for storage were listed in [357]:

-The data may also exist in plaintext form, either on another disk, in another computer, or on paper. There is much more opportunity for a cryptanalyst to perform a known-plaintext attack.

-In database applications, pieces of data may be smaller than the block size of most algorithms. This will cause the ciphe r-text to be considerably larger than the plaintext.

-The speed of I/O devices demands fast encryption and decryption, and will probably require encryption hardware. In some applications, special high-speed algorithms may be required.

-Safe, long-term storage for keys is required.

-Key management is much more complicated, since different people need access to different files, different portions of the same file, and so forth. If the encrypted files are not structured as records and fields, such as text files, retrieval is easier: The entire file is decrypted before use. If the encrypted files are database files, this solution is problematic. Decrypting the entire dat abase to access a single record is inefficient, but encrypting records independently might be susceptible to a block-replay kind of attack. In addition, you must make sure the unencrypted file is erased after encryption (see Section 10.9). For further details and insights, consult [425,569].

Dereferencing Keys

When encrypting a large hard drive, you have two options. You can encrypt all the data using a single key. This gives a cryptanalyst a large amount of ciphertext to analyze and makes it impossible to allow multiple users to see only parts of the drive. Or, you can encrypt each file with a different key, forcing users to memorize a different key for each file.

The solution is to encrypt each file with a separate key, and to encrypt the keys with another key known by the users. Each user only has to remember that one key. Different users can have different subsets of the file-encryption keys encrypted with their key. And there can even be a master key under which every file-encryption key is encrypted. This is even more secure because the file-encryption keys are random and less susceptible to a dictionary attack.

Driver-Level vs. File-Level Encryption

There are two ways to encrypt a hard drive: at the file level and at the driver level. Encryption at the file level means that every file is encrypted separately. To use a file thats been encrypted, you must first decrypt the file, then use it, and then re- e n-crypt it.

Driver-level encryption maintains a logical drive on the users machine that has all data on it encrypted. If done well, this can provide security that, beyond choosing good passwords, requires little worry on the part of the user. The driver must be consider ably more complex than a simple file-encryption program, however, because it must deal with the issues of being an installed d e-vice driver, allocation of new sectors to files, recycling of old sectors from files, random-access read and update requests for any data on the logical disk, and so on.

Typically, the driver prompts the user for a password before starting up. This is used to generate the master decryption key, which may then be used to decrypt actual decryption keys used on different data.

Providing Random Access to an Encrypted Drive

Most systems expect to be able to access individual disk sectors randomly. This adds some complication for using many stream ciphers and block ciphers in any chaining mode. Several solutions are possible.

Use the sector address to generate a unique IV for each sector being encrypted or decrypted. The drawback is that each se c-tor will always be encrypted with the same IV. Make sure this is not a security problem.

For the master key, generate a pseudo-random block as large as one sector. You can do this by running an algorithm in OFB mode, for example.) To encrypt any sec- tor, first XOR in this pseudo-random block, then encrypt normally with a block cipher in ECB mode. This is called ECB+OFB (see Section 15.4).

Since CBC and CFB are error-recovering modes, you can use all but the first block or two in the sector to generate the IV for that sector. For example, the IV for sector 3001 may be the hash of the all but the first 128 bits of the sectors data. After genera ting the IV, encrypt normally in CBC mode. To decrypt the sector, you use the second 64-bit block of the sector as an IV, and d e-crypt the remainder of the sector. Then, using the decrypted data, you regenerate the IV and decrypt the first 128 bits.

You can use a block cipher with a large enough block size that it can encrypt the whole sector at once. Crab See Section 14.6) is an example.

10.5 HARDWARE ENCRYPTION VERSUS SOFTWARE ENCRYPTION

Hardware

Until very recently, all encryption products were in the form of specialized hardware. These encryption/decryption boxes plugged into a communications line and encrypted all the data going across that line. Although software encryption is becoming more prevalent today, hardware is still the embodiment of choice for military and serious commercial applications. The NSA, for example, only authorizes encryption in hardware. There are several reasons why this is so.