1.2. Digital Signatures

Tamper detection relies on a mathematical function called a one-way hash (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 be deduced from the hash.
As mentioned in Section 1.1.2, “Public-Key Encryption”, it is possible to use a private key for encryption and the corresponding public key for decryption. Although not recommended when 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 the private key to encrypt the hash. The encrypted hash, along with other information such as the hashing algorithm, is known as a digital signature.
Figure 1.3, “Using a Digital Signature to Validate Data Integrity” illustrates the way a digital signature can be used to validate the integrity of signed data.
Using a Digital Signature to Validate Data Integrity

Figure 1.3. Using a Digital Signature to Validate Data Integrity

Figure 1.3, “Using a Digital Signature to Validate Data Integrity” shows two items transferred to the recipient of some signed data: the original data and the digital signature, which is a one-way hash of the original data encrypted with the signer's private key. To validate the integrity of the data, the receiving software first uses the 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.) 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 do not match, the data may have been tampered with since it was signed, or the signature may have been created with a private key that does not 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 also requires some way of confirming that the public key belongs to a particular entity. For more information on authenticating users, see Section 1.3, “Certificates and Authentication”.
A digital signature is similar to a handwritten signature. Once data have been signed, it is difficult to deny doing so later, assuming the private key has not been compromised. This quality of digital signatures provides a high degree of nonrepudiation; digital signatures make it difficult for the signer to deny having signed the data. In some situations, a digital signature is as legally binding as a handwritten signature.