Cryptography for Network and Information Security

Digital Signatures

Just as handwritten signatures or physical thumbprints are commonly used to uniquely identify people for legal proceedings or transactions, so digital signatures ("digital thumbprints") are commonly used to identify electronic entities for online transactions. A digital signature uniquely identifies the originator of digitally signed data and also ensures the integrity of the signed data against tampering or corruption.

One possible method for creating a digital signature is for the originator of data to create the signature by encrypting all of the data with the originator's private key and enclosing the signature with the original data. Anyone with the originator's public key can decrypt the signature and compare the decrypted message to the original message. Because only someone with the private key can create the signature, the integrity of the message is verified when the decrypted message matches the original. If an intruder alters the original message during transit, the intruder cannot also create a new valid signature. If an intruder alters the signature during transit, the signature does not verify properly and is invalid.

However, encrypting all data to provide a digital signature is impractical for three reasons:

The ciphertext signature is the same size as the corresponding plaintext, so message sizes are doubled, consuming large amounts of bandwidth and storage space.
Public key encryption is slow and places heavy computational loads on computer processors, so network and computer performance can be significantly degraded.
Encrypting the entire contents of information produces large amounts of ciphertext, which can be used for cryptanalysis attacks, especially known plaintext attacks (where certain parts of the encrypted data, such as e-mail headers, are known beforehand to the attacker).

Digital signature algorithms use more efficient methods to create digital signatures. The most common types of digital signatures today are created by signing message digests with the originator's private key to create a digital thumbprint of the data. Because only the message digest is signed, the signature is usually much shorter than the data that was signed. Therefore, digital signatures place a relatively low load on computer processors during the signing process, consume insignificant amounts of bandwidth, and produce small amounts of ciphertext for cryptanalysis. Two of the most widely used digital signature algorithms today are the RSA digital signature process and the Digital Signature Algorithm (DSA).

RSA Data Security Digital Signature Process

In the RSA digital signature process, the private key is used to encrypt only the message digest. The encrypted message digest becomes the digital signature and is attached to the original data. Figure 14.7 illustrates the basic RSA Data Security digital signature process.

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Figure 14.7 Basic RSA Data Security Digital Signature Process

To verify the contents of digitally signed data, the recipient generates a new message digest from the data that was received, decrypts the original message digest with the originator's public key, and compares the decrypted digest with the newly generated digest. If the two digests match, the integrity of the message is verified. The identify of the originator also is confirmed because the public key can decrypt only data that has been encrypted with the corresponding private key.

Digital Signature Security Standard

Another widely used technology for creating digital signatures is the Digital Signature Security Standard (DSS) that was developed by the National Security Agency and adopted by the United States government as its digital-signature standard. DSS defines the Digital Signature Algorithm (DSA), which functions in a manner similar to RSA. Although similar to RSA, DSA does not encrypt message digests with the private key or decrypt the message digest with the public key. Instead, DSA uses special mathematical functions to generate a digital signature composed of two 160-bit numbers that are derived from the message digest and the private key. DSA uses the public key to verify the signature, but the verification process is more complex than RSA.

The digital signature processes for DSA and RSA are generally considered to be of equal strength. However, DSA requires the use of the SHA-1 message digest function to ensure strong digital signatures. RSA can be used with other message digest functions (besides SHA-1) that might produce weaker digital signatures. Because the DSA signature verification process increases computer processor load significantly, relative to the verification process for RSA (all other conditions being equal), the RSA digital signature process generally provides better overall performance.

Because DSA is used only for digital signatures and makes no provisions for data encryption (for example, to provide secure secret key exchange), DSA is usually not subject to the export or import restrictions commonly imposed on RSA cryptography technology. Therefore, DSS digital signature technology can often be used when RSA digital signature technology cannot be used because of government-imposed export or import restrictions.

Uses for Digital Signatures

Anyone with the public key can use it to perform a validity check of digital signatures created by the private key. Only a digital signature created by the appropriate private key decrypts and validates properly with the public key. If a different private key was used to sign the data, the validity check fails. If the contents of digitally signed data or the digital signature have been tampered with or are corrupted, the validity check also fails. Valid digital signatures can be used to perform the following functions:

Authenticate online entities.
Verify the authorship or origin of digital data.
Ensure the integrity of digital data against tampering.

Many security technologies use digital signatures. For example, Microsoft® Authenticode® can be used to digitally sign software programs, safeguarding them when they are distributed on the intranet or Internet to help counter the threat of software tampering and the spread of viruses and other malicious code. Likewise, the S/MIME protocol can be used to digitally sign e-mail messages to ensure the integrity of mail communications.