org.cryptimeleon.math.prf.zn

Class HashThenPrfToZn

java.lang.Object

org.cryptimeleon.math.prf.zn.HashThenPrfToZn

All Implemented Interfaces:

Representable, StandaloneRepresentable

public class HashThenPrfToZn
extends java.lang.Object
implements StandaloneRepresentable

Get pseudorandom Zn Elements by a hash-then-prf construction.

We use a PRF that outputs bitstrings and map these to Zp as follows: We divide the output interval of size 2^n (with elements in [0,2^n-1]) into a 'good' and a 'bad' interval, where the good interval is [0,x*p[ with x*p<2^n chosen to be maximal and the bad interval [x*p,2^n-1]. Since x is maximal, the bad interval has size at most p-1.

To get a ZnElement, we hash, use the PRF and get some output value o. If o is in the good interval, we output o mod p as our ZnElement. This is random for random bit strings, since the good interval's size is a multiple of p. If o is in the bad interval, we reject and throw an exception. We don't want this to happen, hence we increase the good interval by using a longer PRF, this can be influenced by the so called 'oversubscription'. Since the bad interval's size is bound by p, increasing the total interval reduces the probability of landing in the bad interval. More precisely, the reject rate is bound by (1/2)^oversubscription.

Constructor Summary

Constructors

Constructor and Description
HashThenPrfToZn(int aesKeyLength, Zn zn, HashFunction hashFunction, int oversubscription) Instantiate HashThenPrfToZn
HashThenPrfToZn(Representation repr)

Method Summary

Modifier and Type	Method and Description
boolean	equals(java.lang.Object o)
PrfKey	generateKey() Generates a PRF key that can be used to hash-then-prf to Zn
HashFunction	getHashFunction()
LongAesPseudoRandomFunction	getLongAesPseudoRandomFunction()
Representation	getRepresentation() The representation of this object.
int	hashCode()
Zn.ZnElement	hashThenPrfToZn(PrfKey prfKey, UniqueByteRepresentable hashInput)
Zn.ZnElement	hashThenPrfToZn(PrfKey prfKey, UniqueByteRepresentable hashInput, java.lang.String prefix)
RingElementVector	hashThenPrfToZnVector(PrfKey prfKey, UniqueByteRepresentable hashInput, int vectorSize)
RingElementVector	hashThenPrfToZnVector(PrfKey prfKey, UniqueByteRepresentable hashInput, int vectorSize, java.lang.String prefix) Generate pseudorandom ZnVectors of variable size using unique prefixes for the vectorSize and index.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

HashThenPrfToZn

public HashThenPrfToZn(int aesKeyLength,
                       Zn zn,
                       HashFunction hashFunction,
                       int oversubscription)

Instantiate HashThenPrfToZn

Parameters:

aesKeyLength - bit length of AES

zn - target ring

hashFunction - hash function to use, output size should be larger than AES input size

oversubscription - parameter that binds the probability of failing by (1/2)^oversubscription. Probability can be lower due to rounding

HashThenPrfToZn

public HashThenPrfToZn(Representation repr)

Method Detail

generateKey

public PrfKey generateKey()

Generates a PRF key that can be used to hash-then-prf to Zn

Returns:

a PRF key

hashThenPrfToZnVector

public RingElementVector hashThenPrfToZnVector(PrfKey prfKey,
                                               UniqueByteRepresentable hashInput,
                                               int vectorSize,
                                               java.lang.String prefix)

Generate pseudorandom ZnVectors of variable size using unique prefixes for the vectorSize and index.

Parameters:

prfKey - the PRF key

hashInput - input to hash

vectorSize - target vector size

prefix - prefix to allow using the same vectorSize and preImage several times

Returns:

a pseudorandom Vector of Zn elements

hashThenPrfToZnVector

public RingElementVector hashThenPrfToZnVector(PrfKey prfKey,
                                               UniqueByteRepresentable hashInput,
                                               int vectorSize)

hashThenPrfToZn

public Zn.ZnElement hashThenPrfToZn(PrfKey prfKey,
                                    UniqueByteRepresentable hashInput)

hashThenPrfToZn

public Zn.ZnElement hashThenPrfToZn(PrfKey prfKey,
                                    UniqueByteRepresentable hashInput,
                                    java.lang.String prefix)

getLongAesPseudoRandomFunction

public LongAesPseudoRandomFunction getLongAesPseudoRandomFunction()

getHashFunction

public HashFunction getHashFunction()

getRepresentation

public Representation getRepresentation()

Description copied from interface: Representable

The representation of this object. Used for serialization. A convenient way to implement this is using @link ReprUtil

Specified by:

getRepresentation in interface Representable

Returns:

a Representation or null if an equal object can be recreated without any information.

See Also:

Representation

equals

public boolean equals(java.lang.Object o)

Overrides:

equals in class java.lang.Object

hashCode

public int hashCode()

Overrides:

hashCode in class java.lang.Object