AES-GCM Gives Wrong Tag Value?

[Li, David]

Hi Experts,

First time I am using AES-GCM mode to run the NIST test vectors.  The API is:

void AES_gcm128_encrypt(GCM128_CONTEXT *ctx,
                const unsigned char *in, unsigned char *out,
                size_t len)


After initialization and encryption, my cipher text matched the one from NIST 
vector but tag value didn't.  I obtained the tag value by:

Unsigned char tag[16];
memcpy(tag, ctx.Xi.c, 16);

Anyone can think of a reason what might have gone wrong?


-          David



The test vectors from NIST look like:
============================
[Keylen = 256]
[IVlen = 128]
[PTlen = 1024]
[AADlen = 1024]
[Taglen = 120]

Count = 0
Key = ec566324ad9d4cd015821e2cd4ed4d3d507bdb3c65bd50acc85f690ef06740fa
PT = 
348d35768d7192415cbb92c5625f10edd79f24c56d4b821aaf80d7dc83e901ede6be94d1efe11a3acd16ac00aea8d0d4875c47522332fed11cdf0816b26978de431c89d2fe6d122b2d4980f1d53a97edc15e490a44e73cba9394ca4bbb871675c729c39de80d6678c71b1bd220e4647bfd20a7ddbefe2b7eec7276b87c92ba77
AAD = 
fa6f38f8e562a54bb2281dc9a7cbe0b981292fb00dc0053185550a300661852179d0f2beb4e7759b81316fbfead5c858e6fce73f3cd2c2462925dbb199a4e6c121d051b1b5ebf60e16d1e30f6973b19cf31830da30588fdfff6115a4a1f6d977a72583379a56055724581be5232b0d1b0ae88bab5d4a031b058bc8d03078dcd5
IV = 95c8a544c4b94e9fbfd76e66f40bb975
CT = 
8b4da79f3ae1ea35a80af2f52fc640055e6a3b92617ddfa79fe5d8a49f28ddf36a82a17ca0b3cdf1726700f7ffc09ae5b412d064fd52a90a76bacc74a0b89e38dc474e880a2b768ffa91fef34c47759a7b8fd7faa32a4fcb258349495e4438c7b2055a8f462729fa4e7223aa9b47087695e3aabf43afb32e272d536b257b748a
Tag = b1faec277697add8f756391dd9c7f4