AES hardware accelerator (AES) RM0444
500/1390 RM0444 Rev 5
standard also states that, on MSB bits of the first message block (B1), the associated
data length expressed in bytes (a) must be encoded as follows:
– If 0 < a < 2
16
-2
8
, then it is encoded as [a]
16
, that is, on two bytes.
– If 2
16
-2
8
< a < 2
32
, then it is encoded as 0xff || 0xfe || [a]
32
, that is, on six bytes.
– If 2
32
< a < 2
64
, then it is encoded as 0xff || 0xff || [a]
64
, that is, on ten bytes.
• 16-byte blocks (B) associated to the plaintext message P, which is both authenticated
and encrypted as ciphertext C, with a known length Len(P). This length can be a non-
multiple of 16 bytes (see Figure 96).
• Encrypted MAC (T) of length Len(T) appended to the ciphertext C of overall length
Len(C).
When a part of the message (A or P) has a length that is a non-multiple of 16-bytes, a
special padding scheme is required.
Note: CCM chaining mode can also be used with associated data only (that is, no payload).
As an example, the C.1 section in NIST Special Publication 800-38C gives the following
values (hexadecimal numbers):
N: 10111213 141516 (Len(N)= 56 bits or 7 bytes)
A: 00010203 04050607 (Len(A)= 64 bits or 8 bytes)
P: 20212223 (Len(P)= 32 bits or 4 bytes)
T: 6084341B (Len(T)= 32 bits or t = 4)
B0: 4F101112 13141516 00000000 00000004
B1: 00080001 02030405 06070000 00000000
B2: 20212223 00000000 00000000 00000000
CTR0: 0710111213 141516 00000000 00000000
CTR1: 0710111213 141516 00000000 00000001
Generation of formatted input data blocks Bx (especially B0 and B1) must be managed by
the application.
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