Digital Signatures - Red Hat CERTIFICATE SYSTEM 8 - DEPLOYMENT Deployment Manual

Chapter 1. Introduction to Public-Key Cryptography
Because it is relatively trivial to break an RSA key, an RSA public-key encryption cipher must have a
very long key — at least 1024 bits — to be considered cryptographically strong. On the other hand,
symmetric-key ciphers are reckoned to be equivalently strong using a much shorter key length, as little
as 80 bits for most algorithms.

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.
Section 1.1.2, "Public-Key
As mentioned in
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"
can be used to validate the integrity of signed data.
Figure 1.3. Using a Digital Signature to Validate Data Integrity
Figure 1.3, "Using a Digital Signature to Validate Data Integrity"
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
4
Encryption", it is possible to use a private key for
illustrates the way a digital signature
shows two items transferred to
Section 1.3, "Certificates and Authentication".