Using AES-GCM encryption and the results of ciphertext and tag are incorrect.

[WayneTao]

Hello,

I am trying to use AES-GCM from the STM32 cryptographic library V4.2.0 to encrypt data.

When running the example program, encryption and decryption work smoothly.

However, when I integrate the library and function into my existing program, the encrypted ciphertext differs from expected_ciphertext.

Additionally, using this incorrect ciphertext and incorrect tag still decrypts to the correct plaintext.

Could you please advise where the error might be causing this result?

const uint8_t Key[] =
{
  0x46, 0x3b, 0x41, 0x29, 0x11, 0x76, 0x7d, 0x57, 0xa0, 0xb3, 0x39, 0x69, 0xe6, 0x74, 0xff, 0xe7,
  0x84, 0x5d, 0x31, 0x3b, 0x88, 0xc6, 0xfe, 0x31, 0x2f, 0x3d, 0x72, 0x4b, 0xe6, 0x8e, 0x1f, 0xca
};
const uint8_t IV[] =
{
  0x61, 0x1c, 0xe6, 0xf9, 0xa6, 0x88, 0x07, 0x50, 0xde, 0x7d, 0xa6, 0xcb
};
const uint8_t Plaintext[] =
{
  0xe7, 0xd1, 0xdc, 0xf6, 0x68, 0xe2, 0x87, 0x68, 0x61, 0x94, 0x0e, 0x01, 0x2f, 0xe5, 0x2a, 0x98,
  0xda, 0xcb, 0xd7, 0x8a, 0xb6, 0x3c, 0x08, 0x84, 0x2c, 0xc9, 0x80, 0x1e, 0xa5, 0x81, 0x68, 0x2a,
  0xd5, 0x4a, 0xf0, 0xc3, 0x4d, 0x0d, 0x7f, 0x6f, 0x59, 0xe8, 0xee, 0x0b, 0xf4, 0x90, 0x0e, 0x0f,
  0xd8, 0x50, 0x42
};
const uint8_t AddData[] =
{
  0x0a, 0x68, 0x2f, 0xbc, 0x61, 0x92, 0xe1, 0xb4, 0x7a, 0x5e, 0x08, 0x68, 0x78, 0x7f, 0xfd, 0xaf,
  0xe5, 0xa5, 0x0c, 0xea, 0xd3, 0x57, 0x58, 0x49, 0x99, 0x0c, 0xdd, 0x2e, 0xa9, 0xb3, 0x59, 0x77,
  0x49, 0x40, 0x3e, 0xfb, 0x4a, 0x56, 0x68, 0x4f, 0x0c, 0x6b, 0xde, 0x35, 0x2d, 0x4a, 0xee, 0xc5
};
const uint8_t Expected_Ciphertext[] =
{
  0x88, 0x86, 0xe1, 0x96, 0x01, 0x0c, 0xb3, 0x84, 0x9d, 0x9c, 0x1a, 0x18, 0x2a, 0xbe, 0x1e, 0xea,
  0xb0, 0xa5, 0xf3, 0xca, 0x42, 0x3c, 0x36, 0x69, 0xa4, 0xa8, 0x70, 0x3c, 0x0f, 0x14, 0x6e, 0x8e,
  0x95, 0x6f, 0xb1, 0x22, 0xe0, 0xd7, 0x21, 0xb8, 0x69, 0xd2, 0xb6, 0xfc, 0xd4, 0x21, 0x6d, 0x7d,
  0x4d, 0x37, 0x58,
};
const uint8_t Expected_Tag[] =
{
  0x24, 0x69, 0xce, 0xcd, 0x70, 0xfd, 0x98, 0xfe, 0xc9, 0x26, 0x4f, 0x71, 0xdf, 0x1a, 0xee, 0x9a
};

incorrect_ciphertext:
{
	0xA3, 0x0A, 0x42, 0x8A, 0xCA, 0x35, 0x1C, 0x9A, 0x40, 0x45, 0x06, 0xF0, 0xA1, 0xB9, 0x29, 0x6C,
	0x20, 0xE8, 0x43, 0x35, 0x4D, 0xF2, 0xFD, 0x12, 0x98, 0x5E, 0xE3, 0x22, 0xC0, 0xF6, 0xD8, 0xEF,
	0x47, 0x17, 0x6C, 0x36, 0xC7, 0x09, 0x4D, 0x3B, 0xCE, 0xC0, 0xAD, 0xC0, 0xB9, 0xD2, 0x06, 0xC3,
	0x6D, 0x4F, 0x8C,
}

incorrect_tag:
{
	0x8D, 0xF4, 0x2D, 0x51, 0x9A, 0x60, 0x48, 0xD6, 0x9C, 0x93, 0xD7, 0xA3, 0xF1, 0x84, 0xAA, 0xB4,
}

retval = cmox_aead_encrypt(CMOX_AES_GCM_ENC_ALGO,                  /* Use AES GCM algorithm */
                             Plaintext, sizeof(Plaintext),           /* Plaintext to encrypt */
                             sizeof(Expected_Tag),                   /* Authentication tag size */
                             Key, sizeof(Key),                       /* AES key to use */
                             IV, sizeof(IV),                         /* Initialization vector */
                             AddData, sizeof(AddData),               /* Additional authenticated data */
                             Computed_Ciphertext, &computed_size);   /* Data buffer to receive generated ciphertext
                                                                        and authentication tag */

The model used in the example program is NUCLEO-L476RG.

My development board model is STM32L431CBT6.

Using KEIL5.

[Tesla DeLorean]

Make sure CRC Peripheral clock is enabled when using ST libraries 

[BarryWhit]

Can you do a comparison with a plaintext which is an exact multiple of block-size?  (that won't work, since the padding scheme might then add an extra block which could again differ in contents)

This could be due to a difference in default padding scheme between the STM32 lib and whatever impl you're using as a reference.