Unable to get correct output from AES-128-GCM - c++

Question

The following test code should theoretically give me the result from the NIST test suite of 58e2fccefa7e3061367f1d57a4e7455a , however a hexdump of the output yields 9eeaed13b5f591104e2cda197fb99eeaed13b5f591104e2cda197fb9 instead ?
#include <iostream>
#include <cstdio>
#include <polarssl/md.h>
#include <polarssl/entropy.h>
#include <polarssl/ctr_drbg.h>
#include <polarssl/cipher.h>
#include <cstdlib>
#include <fstream>
int main(int argc, char** argv) {
const cipher_info_t *cipher_info;
cipher_info = cipher_info_from_string( "AES-128-GCM" );
cipher_context_t cipher_ctx;
cipher_init_ctx (&cipher_ctx,cipher_info);
std::cout<<"KEYLEN"<<std::endl;
std::cout<<cipher_info->key_length<<std::endl;
std::cout<<"IVLEN"<<std::endl;
std::cout<<cipher_info->iv_size<<std::endl;
unsigned char key[cipher_info->key_length/8];
unsigned char iv[cipher_info->iv_size];
memset(key,0x00,cipher_info->key_length/8);
memset(iv,0x00,cipher_info->iv_size);
unsigned char iBuffer[10];
unsigned char oBuffer[1024];
size_t ilen, olen;
std::ofstream oFile2;
oFile2.open("testOut",std::ofstream::out | std::ofstream::trunc | std::ofstream::binary);
cipher_setkey( &cipher_ctx,key,cipher_info->key_length,POLARSSL_ENCRYPT);
cipher_set_iv( &cipher_ctx, iv, 16 );
cipher_reset( &cipher_ctx );
cipher_update( &cipher_ctx, iBuffer, sizeof(iBuffer), oBuffer, &olen );
oFile2 << oBuffer;
cipher_finish( &cipher_ctx, oBuffer, &olen );
oFile2 << oBuffer;
oFile2.close();
}
This is the nIST test :
Variable
Value
K 00000000000000000000000000000000
P
IV 000000000000000000000000
H 66e94bd4ef8a2c3b884cfa59ca342b2e
Yo 00000000000000000000000000000001
E ( K,Yo) 58e2fccefa7e3061367f1d57a4e7455a
len(A)||len(C) 00000000000000000000000000000000
GHASH (H,A,C) 00000000000000000000000000000000
C
T 58e2fccefa7e3061367f1d57a4e7455a
(test case No. 1 http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-revised-spec.pdf)

Answer 1

I can see two immediate mistakes:
the plain text size is set to 10 bytes instead of no bytes at all - this makes the ciphertext too large and the authentication tag incorrect;
the IV is 12 bytes set to 0 instead of 16 bytes set to 0 - 12 is the default for GCM mode - this makes the ciphertext if any and authentication tag incorrect.
These issues are in the following lines:
unsigned char iBuffer[10];
...
cipher_update( &cipher_ctx, iBuffer, sizeof(iBuffer), oBuffer, &olen );
and
cipher_set_iv( &cipher_ctx, iv, 16 );
Furthermore, it seems like the API requires you to retrieve the tag separately using the ...write_tag... method. Currently you are only seeing the CTR ciphertext, not the authentication tag.