The NTRU Cryptosystem

  A Java implementation of the NTRU public-key cryptosystem, consisting of
  the encryption scheme NTRUEncrypt and the signature scheme NTRUSign.
  
  NTRU's main strengths are high performance and resistance to quantum
  computer attacks.
  NTRU keys are longer than ECC keys; they can be longer or shorter than RSA
  keys depending on the security level.
  The NTRUSign algorithm has been broken and is included only for historical
  interest.
  NTRUEncrypt remains unbroken; its main drawback is that it is patent
  encumbered. It is safe to use NTRUEncrypt under the GPL (see
  https://github.com/NTRUOpenSourceProject/ntru-crypto#is-ntru-patented) but
  if you choose any other license, you should talk to the patent holder first.
  
  http://en.wikipedia.org/wiki/NTRUEncrypt
  http://en.wikipedia.org/wiki/NTRUSign
  
  The implementation follows IEEE P1363.1 for NTRUEncrypt and the EESS
  (http://grouper.ieee.org/groups/1363/lattPK/submissions/EESS1v2.pdf)
  for NTRUSign.
  

NtruEncrypt Usage

  The first step is always to create an NtruEncrypt instance by calling the
  constructor with an EncryptionParameters object representing the desired
  algorithm parameters.
  It is recommended to use one of the predefined parameter sets which are
  available as constants in EncryptionParameters, but new ones can be created
  as well.
  
  After an NtruEncrypt instance has been created, it can be used to generate
  new key pairs, and encrypt / decrypt messages.
  
  Encrypting a message is done by calling encrypt() which takes the following
  parameters:

    1. the message itself as a byte array. Strings can be encrypted after
       converting them to byte[] via getBytes()
    2. an EncryptionPublicKey, which can be generated via
       NtruEncrypt.generateKeyPair() or an existing key can be reconstructed
       from a byte array by calling new EncryptionPublicKey(byte[])
  
  The encrypted message is returned as a byte array.

  Decrypting a message is done by calling decrypt() which takes the message,
  an EncryptionKeyPair containing the public and private keys, and the
  encryption parameters. The parameters used for decrypting must be the same
  as the ones used to encrypt the message; the same goes for the public key.

  Like all public-key encryption schemes, NtruEncrypt can only encrypt a
  limited number of data. To find out how long a NTRU message can be, use the
  method EncryptionParameters.getMaxMessageLength().
  To encrypt larger amounts of data, use symmetric encryption and encrypt the
  symmetric key with NTRU. The sample program AesExample shows how to do this.


NtruSign Usage
  
  The NtruSign algorithm was broken in 2012 by Ducas and Nguyen; it should not
  be used. See
  http://www.di.ens.fr/~ducas/NTRUSign_Cryptanalysis/DucasNguyen_Learning.pdf
  for details.
  

Key Import / Export

  Encryption keys and signature keys can be written to a file via the writeTo
  methods by supplying a FileOutputStream. The key can then be read from the
  file by passing a FileInputStream to the appropriate constructor.
  
  Keys are encoded raw, so the array contains no information about the
  parameters used. It is advisable to first write the parameters, then the
  key. Parameters can be written to an OutputStream just like keys, and just
  like keys they have a constructor that takes an InputStream.

  Keys can also be converted to and from byte arrays.
  
  NTRUEncrypt keys (but not NTRUSign keys) can be created from a passphrase
  by calling generateKeyPair(char[], byte[]) with a passphrase and a salt
  value. The passphrase is a char array rather than a string so it can be
  cleared from memory when it is no longer needed.
  The salt parameter makes attacks using precomputed keys harder. It should be
  a random value that is generated once and stored. The method generateSalt()
  can be used to generate a salt value.


Parameter Sets

  It is generally recommended to use the APR2011_439_FAST or APR2011_743_FAST
  parameter set for encryption. The security levels are 128 and 256 bits,
  respectively.


Error Conditions

  Errors cause a NtruException with an appropriate message and/or cause to be
  thrown. NtruException is an unchecked exception.


Sample Programs

  The net.sf.ntru.demo package contains several small console programs:

  SimpleExample    A minimal example showing how to use NTRUEncrypt and
                   NTRUSign
  AesExample       Demonstrates how to encrypt arbitrary-length messages
                   using NTRUEncrypt and AES
  Benchmark        Benchmarks NTRUEncrypt against RSA and ECC
  Timings          Similar to Benchmark but only NTRUEncrypt and NTRUSign are
                   benchmarked, and the output is in table format.

  The src/main/android directory contains a simple Android app similar
  to SimpleExample. It has been tested with Android 4.0.3.
  To build and run the app, follow these steps:
   1) Start Eclipse and make sure you have the ADT plugin installed
   2) Create a new Android project. Enter net.sf.ntru.demo for the package
      name and NtruActivity for the activity.
   3) Replace the generated AndroidManifest.xml with the NTRU version from
      src/main/android/AndroidManifest.xml
   4) Replace the generated NtruActivity.java with the NTRU version from
      src/main/android/net/sf/ntru/demo/NtruActivity.java
   5) Create a folder named libs in your Eclipse project and copy the NTRU
      .jar into the new folder, then refresh the project in Eclipse.
   6) In the package explorer, right click on your project and select
      Run As -> Android Application.


Maven Artifact

  NTRU is available from the Maven central repository.

  <dependency>
    <groupId>net.sf.ntru</groupId>
    <artifactId>ntru</artifactId>
    <version>1.2</version>
  </dependency>


Other NTRU implementations

  * As of Bouncy Castle 1.47 (http://bouncycastle.org/), it contains a fork of
    this library in the bprov-ext and lcrypto jars.
  * There is a C implementation of NTRUEncrypt at
    https://github.com/tbuktu/libntru
  * and another one at
    https://github.com/NTRUOpenSourceProject/ntru-crypto


Further reading

  Original NTRUEncrypt paper: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.25.8422&rep=rep1&type=pdf
  Follow-up NTRUEncrypt paper: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.64.6834&rep=rep1&type=pdf
  Original NTRUSign paper: http://www.math.brown.edu/~jpipher/NTRUSign_RSA.pdf
  Follow-up NTRUSign paper: http://grouper.ieee.org/groups/1363/WorkingGroup/presentations/NTRUSignParams-2005-08.pdf
  NTRU articles (technical and mathematical): http://www.securityinnovation.com/security-lab/crypto.html
  Jeffrey Hoffstein et al: An Introduction to Mathematical Cryptography, Springer-Verlag, ISBN 978-0-387-77993-5
  EESS: http://grouper.ieee.org/groups/1363/lattPK/submissions/EESS1v2.pdf