Part 1: General - Computer Security Resource Center - National ...

March, 2007
Asymmetric key algorithms, commonly known as public key algorithms, use two related keys
(i.e., a key pair) to perform their functions: a public key and a private key. The public key may
be known by anyone; the private key should 1 be under the sole control of the entity that “owns”
the key pair. Even though the public and private keys of a key pair are related, knowledge of the
public key does not reveal the private key. Asymmetric algorithms are used, for example,
1. To compute digital signatures (Section 4.2.4);
2. To establish cryptographic keying material (Section 4.2.5); and
3. To generate random numbers (see Section 4.2.7).
4.2 Cryptographic Algorithm Functionality
Security services are fulfilled using a number of different algorithms. In many cases, the same
algorithm may be used to provide multiple services.
4.2.1 Hash Functions
Many algorithms and schemes that provide a security service use a hash function as a component
of the algorithm. Hash functions can be found in digital signature algorithms (see [FIPS186-3]),
Keyed-Hash Message Authentication Codes (HMAC) (see [FIPS198]), key derivation functions
(see [SP800-56]), and random number generators [ANSX9.82]. Approved hash functions are
defined in [FIPS180-2].
A hash function takes an input of arbitrary length and outputs a fixed size value. Common names
for the output of a hash function include hash value, hash, message digest, and digital fingerprint.
The maximum number of input and output bits is determined by the design of the hash function.
All Approved hash functions are cryptographic hash functions. With a well-designed
cryptographic hash function, it is not feasible to find a message that will produce a given hash
value (pre-image resistance), nor is it feasible to find two messages that produce the same hash
value (collision resistance).
Five hash functions are Approved for Federal Government use and are defined in [FIPS180-2]:
SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512 2 . The hash size produced by SHA-1 is 160
bits, 224 bits for SHA-224, 256 bits for SHA-256, 384 bits for SHA-384, and 512 bits for SHA-
512. Algorithms standards must specify either the appropriate size hash function or the hash
function size selection criteria if the algorithm can be configured to use different size hash
functions.
4.2.2 Symmetric Key Algorithms used for Encryption and Decryption
Encryption is used to provide confidentiality for data. The data to be protected is called plaintext
when in its original form. Encryption transforms the data into ciphertext. Ciphertext can be
transformed back into plaintext using decryption. The Approved algorithms for
encryption/decryption are symmetric key algorithms: AES and TDEA. Each of these algorithms
1 Sometimes a key pair is generated by a party that is trusted by the key owner.
2 In general the notation SHA-n indicates a hash function specified in [FIPS 180-2] that provides an n-bit hash value.
However, SHA-1 indicates a hash function with a 160-bit hash value that was originally specified in FIPS 180-1.
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