Class Ed25519FieldElement

Method Details

• isNonZero

  public boolean isNonZero()

  Gets a value indicating whether or not the field element is non-zero.

  Specified by:

  isNonZero in class FieldElement

  Returns:

  1 if it is non-zero, 0 otherwise.

• add

  public FieldElement add​(FieldElement val)

  $h = f + g$

  TODO-CR BR: $h$ is allocated via new, probably not a good idea. Do we need the copying into temp variables if we do that?

  Preconditions:

  • $|f|$ bounded by $1.1*2^{25},1.1*2^{24},1.1*2^{25},1.1*2^{24},$ etc.
  • $|g|$ bounded by $1.1*2^{25},1.1*2^{24},1.1*2^{25},1.1*2^{24},$ etc.

  Postconditions:

  • $|h|$ bounded by $1.1*2^{26},1.1*2^{25},1.1*2^{26},1.1*2^{25},$ etc.

  Specified by:

  add in class FieldElement

  Parameters:

  val - The field element to add.

  Returns:

  The field element this + val.

• subtract

  public FieldElement subtract​(FieldElement val)

  $h = f - g$

  Can overlap $h$ with $f$ or $g$.

  TODO-CR BR: See above.

  Preconditions:

  • $|f|$ bounded by $1.1*2^{25},1.1*2^{24},1.1*2^{25},1.1*2^{24},$ etc.
  • $|g|$ bounded by $1.1*2^{25},1.1*2^{24},1.1*2^{25},1.1*2^{24},$ etc.

  Postconditions:

  • $|h|$ bounded by $1.1*2^{26},1.1*2^{25},1.1*2^{26},1.1*2^{25},$ etc.

  Specified by:

  subtract in class FieldElement

  Parameters:

  val - The field element to subtract.

  Returns:

  The field element this - val.

• negate

  public FieldElement negate()

  $h = -f$

  TODO-CR BR: see above.

  Preconditions:

  • $|f|$ bounded by $1.1*2^{25},1.1*2^{24},1.1*2^{25},1.1*2^{24},$ etc.

  Postconditions:

  • $|h|$ bounded by $1.1*2^{25},1.1*2^{24},1.1*2^{25},1.1*2^{24},$ etc.

  Specified by:

  negate in class FieldElement

  Returns:

  The field element (-1) * this.

• multiply

  public FieldElement multiply​(FieldElement val)

  $h = f * g$

  Can overlap $h$ with $f$ or $g$.

  Preconditions:

  • $|f|$ bounded by $1.65*2^{26},1.65*2^{25},1.65*2^{26},1.65*2^{25},$ etc.
  • $|g|$ bounded by $1.65*2^{26},1.65*2^{25},1.65*2^{26},1.65*2^{25},$ etc.

  Postconditions:

  • $|h|$ bounded by $1.01*2^{25},1.01*2^{24},1.01*2^{25},1.01*2^{24},$ etc.

  Notes on implementation strategy:

  Using schoolbook multiplication. Karatsuba would save a little in some cost models.

  Most multiplications by 2 and 19 are 32-bit precomputations; cheaper than 64-bit postcomputations.

  There is one remaining multiplication by 19 in the carry chain; one *19 precomputation can be merged into this, but the resulting data flow is considerably less clean.

  There are 12 carries below. 10 of them are 2-way parallelizable and vectorizable. Can get away with 11 carries, but then data flow is much deeper.

  With tighter constraints on inputs can squeeze carries into int32.

  Specified by:

  multiply in class FieldElement

  Parameters:

  val - The field element to multiply.

  Returns:

  The (reasonably reduced) field element this * val.

• square

  public FieldElement square()

  $h = f * f$

  Can overlap $h$ with $f$.

  Preconditions:

  • $|f|$ bounded by $1.65*2^{26},1.65*2^{25},1.65*2^{26},1.65*2^{25},$ etc.

  Postconditions:

  • $|h|$ bounded by $1.01*2^{25},1.01*2^{24},1.01*2^{25},1.01*2^{24},$ etc.

  See multiply(FieldElement) for discussion of implementation strategy.

  Specified by:

  square in class FieldElement

  Returns:

  The (reasonably reduced) square of this field element.

• squareAndDouble

  public FieldElement squareAndDouble()

  $h = 2 * f * f$

  Can overlap $h$ with $f$.

  Preconditions:

  • $|f|$ bounded by $1.65*2^{26},1.65*2^{25},1.65*2^{26},1.65*2^{25},$ etc.

  Postconditions:

  • $|h|$ bounded by $1.01*2^{25},1.01*2^{24},1.01*2^{25},1.01*2^{24},$ etc.

  See multiply(FieldElement) for discussion of implementation strategy.

  Specified by:

  squareAndDouble in class FieldElement

  Returns:

  The (reasonably reduced) square of this field element times 2.

• invert

  public FieldElement invert()

  Invert this field element.

  The inverse is found via Fermat's little theorem:
  $a^p \cong a \mod p$ and therefore $a^{(p-2)} \cong a^{-1} \mod p$

  Specified by:

  invert in class FieldElement

  Returns:

  The inverse of this field element.

• pow22523

  public FieldElement pow22523()

  Gets this field element to the power of $(2^{252} - 3)$. This is a helper function for calculating the square root.

  TODO-CR BR: I think it makes sense to have a sqrt function.

  Specified by:

  pow22523 in class FieldElement

  Returns:

  This field element to the power of $(2^{252} - 3)$.

• cmov

  public FieldElement cmov​(FieldElement val, int b)

  Constant-time conditional move. Well, actually it is a conditional copy. Logic is inspired by the SUPERCOP implementation at: https://github.com/floodyberry/supercop/blob/master/crypto_sign/ed25519/ref10/fe_cmov.c

  Specified by:

  cmov in class FieldElement

  Parameters:

  val - the other field element.

  b - must be 0 or 1, otherwise results are undefined.

  Returns:

  a copy of this if $b == 0$, or a copy of val if $b == 1$.

  Since:

  0.9.36

• hashCode

  public int hashCode()

  Overrides:

  hashCode in class Object

• equals

  public boolean equals​(Object obj)

  Overrides:

  equals in class Object

• toString

  public String toString()

  Overrides:

  toString in class Object