Field25519

java.lang.Object
- com.google.crypto.tink.internal.Field25519

```
@Alpha
public final class Field25519
extends java.lang.Object
```
Defines field 25519 function based on curve25519-donna C implementation (mostly identical).
Field elements are written as an array of signed, 64-bit limbs (an array of longs), least significant first. The value of the field element is:
```
 x[0] + 2^26·x[1] + 2^51·x[2] + 2^77·x[3] + 2^102·x[4] + 2^128·x[5] + 2^153·x[6] + 2^179·x[7] +
 2^204·x[8] + 2^230·x[9],
 
```
i.e. the limbs are 26, 25, 26, 25, ... bits wide.

Field Summary

Fields
Modifier and Type	Field	Description
`static int`	`FIELD_LEN`	During Field25519 computation, the mixed radix representation may be in different forms: Reduced-size form: the array has size at most 10.
`static int`	`LIMB_CNT`

Method Summary

All Methods Static Methods Concrete Methods
Modifier and Type	Method	Description
`static byte[]`	`contract(long[] inputLimbs)`	Takes a fully reduced mixed radix form number and contract it into a little-endian, 32-byte array.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Field Detail
  - FIELD_LEN
```
public static final int FIELD_LEN
```
    During Field25519 computation, the mixed radix representation may be in different forms:
    
    Reduced-size form: the array has size at most 10.
    Non-reduced-size form: the array is not reduced modulo 2^255 - 19 and has size at most 19.
    TODO(quannguyen):
    
    Clarify ill-defined terminologies.
    The reduction procedure is different from DJB's paper (http://cr.yp.to/ecdh/curve25519-20060209.pdf). The coefficients after reducing degree and reducing coefficients aren't guaranteed to be in range {-2^25, ..., 2^25}. We should check to see what's going on.
    Consider using method mult() everywhere and making product() private.
    See Also:
    
    Constant Field Values
  - LIMB_CNT
```
public static final int LIMB_CNT
```
    See Also:
    
    Constant Field Values
- Method Detail
  - contract
```
public static byte[] contract(long[] inputLimbs)
```
    Takes a fully reduced mixed radix form number and contract it into a little-endian, 32-byte array.
    On entry: |input_limbs[i]| < 2^26