Created
April 15, 2021 22:38
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Google OAuth JWT in C++
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#include <stdio.h> | |
#include <string.h> | |
#include <stdlib.h> | |
#include <openssl/ssl.h> | |
#include <openssl/err.h> | |
#define CPPHTTPLIB_OPENSSL_SUPPORT | |
#include "httplib.h" | |
char get_base64_char(char byte) { | |
if (byte < 26) { | |
return 'A' + byte; | |
} else if (byte < 52) { | |
return 'a' + byte - 26; | |
} else if (byte < 62) { | |
return '0' + byte - 52; | |
} else if (byte == 62) { | |
return '-'; | |
} else if (byte == 63) { | |
return '_'; | |
} else { | |
fprintf(stderr, "BAD BYTE: %02x\n", byte); | |
exit(1); | |
return 0; | |
} | |
} | |
// To execute C, please define "int main()" | |
void base64encode(char *output, const char *input, size_t input_length) { | |
size_t input_index = 0; | |
size_t output_index = 0; | |
for (; input_index < input_length; ++output_index) { | |
switch (output_index % 4) { | |
case 0: | |
output[output_index] = get_base64_char((0xfc & input[input_index]) >> 2); | |
break; | |
case 1: | |
output[output_index] = get_base64_char(((0x03 & input[input_index]) << 4) | ((0xf0 & input[input_index + 1]) >> 4)); | |
++input_index; | |
break; | |
case 2: | |
output[output_index] = get_base64_char(((0x0f & input[input_index]) << 2) | ((0xc0 & input[input_index + 1]) >> 6)); | |
++input_index; | |
break; | |
case 3: | |
output[output_index] = get_base64_char(0x3f & input[input_index]); | |
++input_index; | |
break; | |
default: | |
exit(1); | |
} | |
} | |
output[output_index] = '\0'; | |
} | |
int main(int argc, char **argv) { | |
const char *header = "{\"alg\":\"RS256\",\"typ\":\"JWT\"}"; | |
const int iat = time(NULL); | |
const int exp = iat + 60 * 60 - 1; | |
char claim_set[1024]; | |
sprintf(claim_set, "{\ | |
\"iss\":\"%s\",\ | |
\"scope\":\"%s\",\ | |
\"aud\":\"https://oauth2.googleapis.com/token\",\ | |
\"exp\":%d,\ | |
\"iat\":%d\ | |
}", argv[1], argv[2], argc > 3 ? atoi(argv[3]) : exp, argc > 4 ? atoi(argv[4]) : iat); | |
printf("%s\n", claim_set); | |
char header_64[1024]; | |
base64encode(header_64, header, strlen(header)); | |
char claim_set_64[1024]; | |
base64encode(claim_set_64, claim_set, strlen(claim_set)); | |
char input[1024]; | |
int input_length = sprintf(input, "%s.%s", header_64, claim_set_64); | |
printf("input = %s\ninput_length = %d\n", input, input_length); | |
unsigned char *digest = SHA256((const unsigned char *)input, input_length, NULL); | |
char digest_str[1024]; | |
for (int i = 0; i < SHA256_DIGEST_LENGTH; ++i) { | |
sprintf(digest_str + i * 2, "%02x", digest[i]); | |
} | |
digest_str[SHA256_DIGEST_LENGTH * 2] = '\0'; | |
printf("digest_str = %s\n", digest_str); | |
FILE *key_file = fopen("key.pem", "r"); | |
RSA *rsa = PEM_read_RSAPrivateKey(key_file, NULL, NULL, NULL); | |
int rsa_size = RSA_size(rsa); | |
printf("rsa_size = %d\n", rsa_size); | |
if (rsa != NULL) { | |
unsigned char sigret[rsa_size]; | |
unsigned int siglen; | |
if (RSA_sign(NID_sha256, digest, SHA256_DIGEST_LENGTH, sigret, &siglen, rsa)) { | |
printf("siglen = %d\n", siglen); | |
if (RSA_verify(NID_sha256, digest, SHA256_DIGEST_LENGTH, sigret, siglen, rsa)) { | |
char signature_64[1024]; | |
base64encode(signature_64, (const char *)sigret, siglen); | |
char jwt[1024]; | |
sprintf(jwt, "%s.%s", input, signature_64); | |
printf("jwt = %s\n", jwt); | |
httplib::Client cli("https://oauth2.googleapis.com"); | |
httplib::Params params; | |
params.emplace("grant_type", "urn:ietf:params:oauth:grant-type:jwt-bearer"); | |
params.emplace("assertion", jwt); | |
auto result = cli.Post("/token", params); | |
printf("status = %d\nbody = %s\n", result->status, result->body.c_str()); | |
} else { | |
printf("Could not verify RSA signature."); | |
} | |
} else { | |
unsigned long err = ERR_get_error(); | |
printf("RSA_sign failed: %lu, %s\n", err, ERR_error_string(err, NULL)); | |
} | |
RSA_free(rsa); | |
} | |
return 0; | |
} |
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