Hello,
I am trying to do a GCM and CCM encryption application on the M4F core using the DTHE symmetric encryption registers.
I am following the AES Module Programming Guide in Section 6.7.3.8 of the "AWR2944 Hardware Security Module Addendum"
For GCM I getting an accurate ciphertext output but the tag output is incorrect, here are some details
- I have no AAD for this test, so the S_AUTH_LENGTH is set to zero
- for the CTRL register I set the following value: 0003005E, this would indicate:
- INPUT_READ = 1
- DIRECTION = 1, meaning encryption
- KEY_SIZE = 0x11, meaning I am using a 256 bit key
- CTR = 1, meaning that coutner mode is enabled, since GCM is a counter mode type
- GCM = 0x11, meaning that I am using Autonomous GHASH
- for the IV registers, I set my 12 byte IV to the first 3 words (S_Iv_In_0 to S_IV_IN_2) and I set S_IV_IN_3 = 1
- S_Iv_In_0 = 67CC3955
- S_Iv_In_0 = BE348B0A
- S_Iv_In_0 = EBEA863B
- S_Iv_In_0 = 01000000
- RESULTS: I get the correct cipherText but the wrong TAG
What am I doing wrong?
For CCM:
- I have no AAD for this test, so the S_AUTH_LENGTH is set to zero
- for the CTRL register I set the following value: 01D4005E, this would indicate:
- INPUT_READ = 1
- DIRECTION = 1, meaning encryption
- KEY_SIZE = 0x11, meaning I am using a 256 bit key
- CTR = 1, meaning that coutner mode is enabled, since GCM is a counter mode type
- CCM = 1, meaning I've selected CCM mode
- CCM_L = 0x010 = 2, which means that ccm_L = 2, which means my number of bytes to store the length of the input data is (2*-1) = 1 byte, this is fine since the input length is only 16 bytes
- CCM_M = 0X111 = 7, SO ccm_M = 7, so the tag length requested is (7 -1) * 2 = 16 bytes, which is what I want
- for the IV registers, I set just like GCM...I set my 12 byte IV to the first 3 words (S_Iv_In_0 to S_IV_IN_2) and I set S_IV_IN_3 = 1
- S_Iv_In_0 = 67CC3955
- S_Iv_In_0 = BE348B0A
- S_Iv_In_0 = EBEA863B
- S_Iv_In_0 = 01000000
- Results: wrong ciphertext and wrong tag... HOWEVER, the ciphertext is the same as that given by GCM
What am I doing wrong here?
What should I be doing differently?
If I were to add AAD, I know I would update the S_AUTH_LENGTH register but how would I feed it to the accelerator? would it be put into the DATA_IN registers? If so would the AAD be added before the plaintext or after? What do I do if the AAD length is not a full AES block (16 bytes)?
Please let me know,
Thanks in advance,
Mark
