dump signature generated by `openssl dgst -sign private.pem -sha256 myfile > signature.sig`

dump_sign.c

#include <openssl/pem.h>
#include <openssl/rsa.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>

int usage(const char *cmd) {
    fprintf(stderr, "Error: %s private.pem signature.sig\n", cmd);
    return 1;
}

int load_data(const char *name, unsigned char *buf, size_t buf_len) {
    FILE *fp;
    size_t read_len;
    fp = fopen(name, "rb");
    if (fp == NULL) {
        return -1;
    }
    read_len = fread(buf, 1, buf_len, fp);
    fclose(fp);
    return read_len;
}

void dump(const unsigned char *data, size_t len) {
    for (size_t i = 0; i < len; i ++) {
        printf("%02x", data[i]);
    }
    printf("\n");
}

int main(int argc, char *argv[]) {
	FILE* fp;
    RSA *key;
    /*
    EVP_PKEY *key = NULL;
    EVP_PKEY_CTX *ctx = NULL;
    */
    unsigned char buf[2048 / 8];
    unsigned char out[2048 / 8];
    size_t outlen;
    memset(out, 0, sizeof(out));

    if (argc != 3) {
        return usage(argv[0]);
    }
    fp = fopen(argv[1], "rb");
    key = PEM_read_RSAPrivateKey(fp, NULL, NULL, NULL);

    if (load_data("test.sig", buf, sizeof(buf)) < 0) {
        return usage(argv[0]);
    }

    outlen = RSA_public_decrypt(sizeof(buf), buf, out, key, RSA_NO_PADDING);
    if (outlen < 0) {
        return usage(argv[0]);
    }
    dump(out, outlen);
    memset(out, 0, sizeof(out));
    outlen = RSA_public_decrypt(sizeof(buf), buf, out, key, RSA_PKCS1_PADDING);
    if (outlen < 0) {
        return usage(argv[0]);
    }
    dump(out, outlen);
}