93 lines
2.9 KiB
C
93 lines
2.9 KiB
C
/* Arithmetic mod p = 2^255-19
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* Daniel Beer <dlbeer@gmail.com>, 8 Jan 2014
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*
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* This file is in the public domain.
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*/
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#ifndef F25519_H_
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#define F25519_H_
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#include <stdint.h>
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#include <string.h>
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/* Field elements are represented as little-endian byte strings. All
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* operations have timings which are independent of input data, so they
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* can be safely used for cryptography.
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*
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* Computation is performed on un-normalized elements. These are byte
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* strings which fall into the range 0 <= x < 2p. Use f25519_normalize()
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* to convert to a value 0 <= x < p.
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*
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* Elements received from the outside may greater even than 2p.
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* f25519_normalize() will correctly deal with these numbers too.
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*/
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#define F25519_SIZE 32
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/* Identity constants */
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extern const uint8_t f25519_zero[F25519_SIZE];
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extern const uint8_t f25519_one[F25519_SIZE];
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/* Load a small constant */
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void f25519_load(uint8_t *x, uint32_t c);
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/* Copy two points */
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static inline void f25519_copy(uint8_t *x, const uint8_t *a)
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{
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memcpy(x, a, F25519_SIZE);
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}
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/* Normalize a field point x < 2*p by subtracting p if necessary */
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void f25519_normalize(uint8_t *x);
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/* Compare two field points in constant time. Return one if equal, zero
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* otherwise. This should be performed only on normalized values.
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*/
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uint8_t f25519_eq(const uint8_t *x, const uint8_t *y);
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/* Conditional copy. If condition == 0, then zero is copied to dst. If
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* condition == 1, then one is copied to dst. Any other value results in
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* undefined behaviour.
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*/
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void f25519_select(uint8_t *dst,
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const uint8_t *zero, const uint8_t *one,
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uint8_t condition);
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/* Add/subtract two field points. The three pointers are not required to
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* be distinct.
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*/
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void f25519_add(uint8_t *r, const uint8_t *a, const uint8_t *b);
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void f25519_sub(uint8_t *r, const uint8_t *a, const uint8_t *b);
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/* Unary negation */
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void f25519_neg(uint8_t *r, const uint8_t *a);
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/* Multiply two field points. The __distinct variant is used when r is
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* known to be in a different location to a and b.
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*/
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void f25519_mul(uint8_t *r, const uint8_t *a, const uint8_t *b);
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void f25519_mul__distinct(uint8_t *r, const uint8_t *a, const uint8_t *b);
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/* Multiply a point by a small constant. The two pointers are not
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* required to be distinct.
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*
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* The constant must be less than 2^24.
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*/
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void f25519_mul_c(uint8_t *r, const uint8_t *a, uint32_t b);
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/* Take the reciprocal of a field point. The __distinct variant is used
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* when r is known to be in a different location to x.
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*/
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void f25519_inv(uint8_t *r, const uint8_t *x);
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void f25519_inv__distinct(uint8_t *r, const uint8_t *x);
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/* Compute one of the square roots of the field element, if the element
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* is square. The other square is -r.
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*
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* If the input is not square, the returned value is a valid field
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* element, but not the correct answer. If you don't already know that
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* your element is square, you should square the return value and test.
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*/
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void f25519_sqrt(uint8_t *r, const uint8_t *x);
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#endif
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