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java.lang.Object
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+--java.security.MessageDigestSpi
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+--java.security.MessageDigest
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+--iaik.security.md.SHA
This class implements the SHA message digest algorithm according to FIPS PUB 180-1.
A message digest algorithm represents the functionality of an one-way hash function for computing a fixed sized data value (message digest, hash) from input data of arbitrary size. The length of the resulting hash value usually is shorter than the length of the input data. Using a one-way hash function will make it easy to compute the hash from the given data, but hard to go the reverse way for calculating the input data when only the hash is known. Furthermore, a proper hash function should avoid any collision, meaning that it has to be hard to find two different messages producing the same hash value.
Through its basic structure, the Secure Hash Algorithm (SHA) resembles the MD5 message digest algorithm, but provides a higher degree of security since producing a longer (160-Bit) hash value than the MD5 algorithm, which gives a 128-Bit hash.
Like MD5, the SHA algorithm processes the data in blocks of 512-Bit length. First the input data is properly padded and subsequently extended by a 64-Bit representation of the data´s original length to become a multiple of 512 Bit. The resulting text is devided into 512-Bit blocks, each of it again devided into sixteen 32-Bit sub-blocks. Each of the 512-Bit blocks is processed in a main loop. Input of the main loop are five 32-Bit variables, which are properly initialized with constant values for the first main loop run processing the first 512-Bit block. Each main loop run is featured with a new 512-Bit block as long as there blocks available. Each main loop run produces an output of five 32-Bit variables which are added to the initial five 32-bit variables to be fed into the next main loop run for pocessing the next 512-Bit block. When no further 512-Bit block is available, the algorithm leaves the main loop and calculates the final 160-Bit hash output by concatenating the five 32-Bit variables resulting from the last main loop run. The main loop consists of four rounds based on a certain nonlinear function (see "Applied Cryptography", Bruce Schneier, ISBN 0-471-59756-2).
For digital signature peocessing, the SHA algorithm is recommended by FIPS to be used together with the Digital Signature Algorithm (DSA) for computing a 160-Bit hash value of data input of any length shorter than 264 Bits, which subsequently is processed using the DSA algorithm to produce the signature.
This class extends the java.security.MessageDigest class and
applications should use one of the getInstance methods presented
there to create a MessageDigest-SHA object. Generally, an application wishing
to compute the message digest of some data has to perform three steps:
getInstance method, e.g.:
MessageDigest sha = MessageDigest.getInstance("SHA");
update methods, e.g:
sha.update(m1);
sha.update(m2);
...
digest
methods:
byte[] hash_value = sha.digest();
There are several ways for combining update and digest
methods for computing a message digest. Since this class implements the
Cloneable interface, SHA MessageDigest objects may be used for
compute intermediate hashes through cloning (see
http://java.sun.com/products/JDK/1.1/docs/guide/security/CryptoSpec.html).
When the hash value successfully has been computed, the SHA MessageDigest object automatically resets for being able to be supplied with new data to be hashed.
The SHA algorithm may be combined with the RSA public-key algorithm to be
used as digital signature algorithm ShaRSASignature).
ShaRSASignature,
iaik.security.dsa.DSA,
MessageDigest,
Md5, Serialized Form| Fields inherited from interface iaik.security.md.Padding |
padding |
| Constructor Summary | |
SHA()
Creates a new SHA message digest object. |
|
| Method Summary | |
java.lang.Object |
clone()
Returns a clone of this SHA Object. |
protected byte[] |
engineDigest()
SPI: Returns the result of this hash computation and resets this SHA message digest object for being supplied with new data to be hashed. |
void |
engineReset()
SPI: Resets this SHA message digest object for being supplied with new data. |
protected void |
engineUpdate(byte input)
SPI: Updates the data to be hashed with the specified byte. |
protected void |
engineUpdate(byte[] input,
int offset,
int len)
SPI: Updates the data to be hashed with the specified number of bytes, beginning at the specified offset within the given byte array. |
| Methods inherited from class java.security.MessageDigest |
digest,
digest,
digest,
getAlgorithm,
getDigestLength,
getInstance,
getInstance,
getProvider,
isEqual,
reset,
toString,
update,
update,
update |
| Methods inherited from class java.security.MessageDigestSpi |
engineDigest,
engineGetDigestLength |
| Methods inherited from class java.lang.Object |
equals,
finalize,
getClass,
hashCode,
notify,
notifyAll,
wait,
wait,
wait |
| Constructor Detail |
public SHA()
MessageDigest.getInstance("SHA");
MessageDigest.getInstance(java.lang.String)| Method Detail |
public void engineReset()
This method is called by engineDigest
after the actual hash computation has been finished to automatically reset the
message digest object for being supplied with new data for starting a new hash
computation.
engineDigest()public java.lang.Object clone()
protected void engineUpdate(byte input)
input - the byte to be used for updating.
protected void engineUpdate(byte[] input,
int offset,
int len)
input - the byte array holding the data to be used for this update
operation.offset - the offset, indicating the start position within the given
byte array.len - the number of bytes to be obtained from the given byte array,
starting at the given position.protected byte[] engineDigest()
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