C语言实现ntru算法

NTRU算法是一种公钥加密算法，它的实现不仅需要数学基础，还需要一定的编程技巧。下面是一个简单的C语言实现，仅供参考。

首先，需要定义一些常量和数据结构：

#define NTRU_N 503
#define NTRU_Q 2048
#define NTRU_P 3
#define NTRU_K 8
#define NTRU_QINV 1025
#define NTRU_KEYBYTES (sizeof(int) + sizeof(poly) + sizeof(poly) + sizeof(poly))
#define NTRU_CIPHERTEXTBYTES (sizeof(poly) + sizeof(poly))

typedef struct {
    int coeffs[NTRU_N];
} poly;

typedef struct {
    poly f;
    poly g;
    poly h;
} ntru_keypair;

typedef struct {
    poly c;
    poly e;
} ntru_ciphertext;

其中，`poly`表示一个多项式，`ntru_keypair`表示NTRU算法的密钥对，`ntru_ciphertext`表示NTRU算法的密文。

接下来，实现一些基本的函数：

static inline int mod(int x, int q) {
    int r = x % q;
    return r < 0 ? r + q : r;
}

static inline int ntru_rand(void) {
    return rand() % NTRU_Q;
}

void poly_add(poly *result, const poly *a, const poly *b) {
    for (int i = 0; i < NTRU_N; ++i) {
        result->coeffs[i] = mod(a->coeffs[i] + b->coeffs[i], NTRU_Q);
    }
}

void poly_sub(poly *result, const poly *a, const poly *b) {
    for (int i = 0; i < NTRU_N; ++i) {
        result->coeffs[i] = mod(a->coeffs[i] - b->coeffs[i], NTRU_Q);
    }
}

void poly_mul(poly *result, const poly *a, const poly *b) {
    for (int i = 0; i < NTRU_N; ++i) {
        result->coeffs[i] = 0;
        for (int j = 0; j < NTRU_N; ++j) {
            result->coeffs[i] += a->coeffs[j] * b->coeffs[mod(i - j, NTRU_N)];
        }
        result->coeffs[i] = mod(result->coeffs[i], NTRU_Q);
    }
}

以上函数分别实现了模运算、随机数生成和多项式加、减、乘运算。

接下来，实现NTRU算法的核心函数：

void ntru_gen_keypair(ntru_keypair *kp) {
    poly t, p, q, f, g, h, invq;
    int w, success;

    while (1) {
        success = 0;

        while (!success) {
            for (int i = 0; i < NTRU_N; ++i) {
                t.coeffs[i] = ntru_rand() % NTRU_P - NTRU_P / 2;
            }

            for (int i = 0; i < NTRU_N; ++i) {
                p.coeffs[i] = ntru_rand() % 2;
            }

            w = 0;
            for (int i = 0; i < NTRU_N; ++i) {
                w += p.coeffs[i];
            }

            if (w == NTRU_K && p.coeffs[0] == 1 && p.coeffs[NTRU_N - 1] == 0) {
                success = 1;
            }
        }

        poly_mul(&q, &p, &t);

        for (int i = 0; i < NTRU_N; ++i) {
            q.coeffs[i] = mod(q.coeffs[i] + NTRU_Q / 2, NTRU_Q);
        }

        for (int i = 0; i < NTRU_N; ++i) {
            invq.coeffs[i] = mod(NTRU_Q - q.coeffs[i], NTRU_Q);
        }

        poly_mul(&f, &invq, &p);
        poly_mul(&g, &invq, &t);

        for (int i = 0; i < NTRU_N; ++i) {
            h.coeffs[i] = mod(NTRU_Q - f.coeffs[i], NTRU_Q);
        }

        if (poly_add(&t, &g, &h), &f) {
            break;
        }
    }

    kp->f = f;
    kp->g = g;
    kp->h = h;
}

void ntru_encrypt(ntru_ciphertext *ct, const poly *m, const ntru_keypair *kp) {
    poly e, c;

    for (int i = 0; i < NTRU_N; ++i) {
        e.coeffs[i] = ntru_rand() % NTRU_Q - NTRU_Q / 2;
    }

    poly_mul(&c, &e, &kp->f);
    poly_add(&c, m, &c);
    poly_mul(&e, &e, &kp->h);

    ct->c = c;
    ct->e = e;
}

void ntru_decrypt(poly *m, const ntru_ciphertext *ct, const ntru_keypair *kp) {
    poly r;

    poly_mul(&r, &ct->c, &kp->g);
    poly_sub(m, &ct->e, &r);

    for (int i = 0; i < NTRU_N; ++i) {
        m->coeffs[i] = mod(m->coeffs[i] * NTRU_QINV, NTRU_Q);
    }
}

以上函数分别实现了密钥对生成、加密和解密操作。

最后，提供一个完整的代码示例：

#include <stdio.h>
#include <stdlib.h>

#define NTRU_N 503
#define NTRU_Q 2048
#define NTRU_P 3
#define NTRU_K 8
#define NTRU_QINV 1025
#define NTRU_KEYBYTES (sizeof(int) + sizeof(poly) + sizeof(poly) + sizeof(poly))
#define NTRU_CIPHERTEXTBYTES (sizeof(poly) + sizeof(poly))

typedef struct {
    int coeffs[NTRU_N];
} poly;

typedef struct {
    poly f;
    poly g;
    poly h;
} ntru_keypair;

typedef struct {
    poly c;
    poly e;
} ntru_ciphertext;

static inline int mod(int x, int q) {
    int r = x % q;
    return r < 0 ? r + q : r;
}

static inline int ntru_rand(void) {
    return rand() % NTRU_Q;
}

void poly_add(poly *result, const poly *a, const poly *b) {
    for (int i = 0; i < NTRU_N; ++i) {
        result->coeffs[i] = mod(a->coeffs[i] + b->coeffs[i], NTRU_Q);
    }
}

void poly_sub(poly *result, const poly *a, const poly *b) {
    for (int i = 0; i < NTRU_N; ++i) {
        result->coeffs[i] = mod(a->coeffs[i] - b->coeffs[i], NTRU_Q);
    }
}

void poly_mul(poly *result, const poly *a, const poly *b) {
    for (int i = 0; i < NTRU_N; ++i) {
        result->coeffs[i] = 0;
        for (int j = 0; j < NTRU_N; ++j) {
            result->coeffs[i] += a->coeffs[j] * b->coeffs[mod(i - j, NTRU_N)];
        }
        result->coeffs[i] = mod(result->coeffs[i], NTRU_Q);
    }
}

void ntru_gen_keypair(ntru_keypair *kp) {
    poly t, p, q, f, g, h, invq;
    int w, success;

    while (1) {
        success = 0;

        while (!success) {
            for (int i = 0; i < NTRU_N; ++i) {
                t.coeffs[i] = ntru_rand() % NTRU_P - NTRU_P / 2;
            }

            for (int i = 0; i < NTRU_N; ++i) {
                p.coeffs[i] = ntru_rand() % 2;
            }

            w = 0;
            for (int i = 0; i < NTRU_N; ++i) {
                w += p.coeffs[i];
            }

            if (w == NTRU_K && p.coeffs[0] == 1 && p.coeffs[NTRU_N - 1] == 0) {
                success = 1;
            }
        }

        poly_mul(&q, &p, &t);

        for (int i = 0; i < NTRU_N; ++i) {
            q.coeffs[i] = mod(q.coeffs[i] + NTRU_Q / 2, NTRU_Q);
        }

        for (int i = 0; i < NTRU_N; ++i) {
            invq.coeffs[i] = mod(NTRU_Q - q.coeffs[i], NTRU_Q);
        }

        poly_mul(&f, &invq, &p);
        poly_mul(&g, &invq, &t);

        for (int i = 0; i < NTRU_N; ++i) {
            h.coeffs[i] = mod(NTRU_Q - f.coeffs[i], NTRU_Q);
        }

        if (poly_add(&t, &g, &h), &f) {
            break;
        }
    }

    kp->f = f;
    kp->g = g;
    kp->h = h;
}

void ntru_encrypt(ntru_ciphertext *ct, const poly *m, const ntru_keypair *kp) {
    poly e, c;

    for (int i = 0; i < NTRU_N; ++i) {
        e.coeffs[i] = ntru_rand() % NTRU_Q - NTRU_Q / 2;
    }

    poly_mul(&c, &e, &kp->f);
    poly_add(&c, m, &c);
    poly_mul(&e, &e, &kp->h);

    ct->c = c;
    ct->e = e;
}

void ntru_decrypt(poly *m, const ntru_ciphertext *ct, const ntru_keypair *kp) {
    poly r;

    poly_mul(&r, &ct->c, &kp->g);
    poly_sub(m, &ct->e, &r);

    for (int i = 0; i < NTRU_N; ++i) {
        m->coeffs[i] = mod(m->coeffs[i] * NTRU_QINV, NTRU_Q);
    }
}

void print_poly(const char *name, const poly *p) {
    printf("%s: [", name);
    for (int i = 0; i < NTRU_N; ++i) {
        printf("%d,", p->coeffs[i]);
    }
    printf("]\n");
}

int main() {
    ntru_keypair kp;
    ntru_ciphertext ct;
    poly m, m2;

    srand(time(NULL));

    ntru_gen_keypair(&kp);

    for (int i = 0; i < NTRU_N; ++i) {
        m.coeffs[i] = ntru_rand() % NTRU_Q - NTRU_Q / 2;
    }

    print_poly("m", &m);

    ntru_encrypt(&ct, &m, &kp);

    print_poly("c", &ct.c);
    print_poly("e", &ct.e);

    ntru_decrypt(&m2, &ct, &kp);

    print_poly("m2", &m2);

    return 0;
}

以上代码实现了NTRU算法的密钥对生成、加密和解密操作，并对加密、解密前后的多项式进行了打印。