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Takes a RSA public key modulus and exponent in base64 encoding and produces a public key file in PEM format
CC = clang
OBJ = modexp2pubkey.o
LIBS = -lssl -lcrypto
%.o: %.c $(DEPS)
$(CC) -c -o $@ $< $(CFLAGS)
modexp2pubkey: $(OBJ)
$(CC) -o $@ $^ $(CFLAGS) $(LIBS)
.PHONY: clean
rm -f *.o
#include <string.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>
#include <openssl/bn.h>
#include <openssl/pem.h>
// cheating, .. ignoring deprecation warnings
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
unsigned char *base64_decode(const char* base64data, int* len) {
BIO *b64, *bmem;
size_t length = strlen(base64data);
unsigned char *buffer = (unsigned char *)malloc(length);
b64 = BIO_new(BIO_f_base64());
BIO_set_flags(b64, BIO_FLAGS_BASE64_NO_NL);
bmem = BIO_new_mem_buf((void*)base64data, length);
bmem = BIO_push(b64, bmem);
*len = BIO_read(bmem, buffer, length);
return buffer;
BIGNUM* bignum_base64_decode(const char* base64bignum) {
int len;
unsigned char* data = base64_decode(base64bignum, &len);
if (len) {
bn = BN_bin2bn(data, len, NULL);
return bn;
EVP_PKEY* RSA_fromBase64(const char* modulus_b64, const char* exp_b64) {
BIGNUM *n = bignum_base64_decode(modulus_b64);
BIGNUM *e = bignum_base64_decode(exp_b64);
if (!n) printf("Invalid encoding for modulus\n");
if (!e) printf("Invalid encoding for public exponent\n");
if (e && n) {
EVP_PKEY* pRsaKey = EVP_PKEY_new();
RSA* rsa = RSA_new();
rsa->e = e;
rsa->n = n;
EVP_PKEY_assign_RSA(pRsaKey, rsa);
return pRsaKey;
} else {
if (n) BN_free(n);
if (e) BN_free(e);
return NULL;
void assert_syntax(int argc, char** argv) {
if (argc != 4) {
fprintf(stderr, "Description: %s takes a RSA public key modulus and exponent in base64 encoding and produces a public key file in PEM format.\n", argv[0]);
fprintf(stderr, "syntax: %s <modulus_base64> <exp_base64> <output_file>\n", argv[0]);
int main(int argc, char** argv) {
assert_syntax(argc, argv);
const char* modulus = argv[1];
const char* exp = argv[2];
const char* filename = argv[3];
EVP_PKEY* pkey = RSA_fromBase64(modulus, exp);
if (pkey == NULL) {
fprintf(stderr, "an error occurred :(\n");
return 2;
} else {
printf("success decoded into RSA public key\n");
FILE* file = fopen(filename, "w");
PEM_write_PUBKEY(file, pkey);
printf("written to file: %s\n", filename);
return 0;
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