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31.4.11.2 CBC Decryption
In CBC decryption, CipherText (C
from the previous block C
to form the PlainText P
i-1
Because each block requires both C
can be performed in as a repeated sequence by having C
updating the write pointer (using the DATA1TOBUF instruction). Then the cipher output is XOR'ed with C
DATA0TOBUFXOR instruction.
In
Figure 31.8 CBC Decryption Operation on page 1043
page 1043
each step in the loop is explained.
CBC Decryption
DATA0
DATA1
BUF
STEP
STEP0
STEP1
STEP2
STEP3
STEP4
silabs.com | Building a more connected world.
) is used as input to the Cipher Core. The output from the Cipher Core is XOR'ed with the CipherText
i
(an initialization vector IV is used as C
i
and C
, decryption is somewhat more complex than encryption. Nevertheless, CBC decryption
i-1
i
i-1
the CBC decryption is illustrated and in
Loop 0
Init
S
C
S
S
0,1
0
0,2
IV
S
C
IV
S
0
1
2
3
Steps
Figure 31.8. CBC Decryption Operation
Table 31.10. CBC Decryption Steps
ACTION
BUFTODATA0
DATA1TOBUF
DATA0TODATA1
CIPHER Operation
DATA0TOBUFXOR
from the previous block stored in DATA1 and then writing it to buffer without
E(C
a
)
a
E(C
a
X
)
a
C
0,3
0
0
4
8
0
C
a
a
C
a
1,3
0
1
5
0
E(C
)
0
a
a
a
C
a
XOR
2,3
2
6
10
1
IV
4
5
1
Description
Moves data (CipherText, C
DATA1 (CipherText, C
) is moved to buffer. BUFC Write pointer is not in-
i-1
cremented!
Value of DATA0 is copied to DATA1.
The AES Cipher Core operates on DATA0
The cipher output is XOR'ed with C
form the PlainText, P
.
i
CRYPTO - Crypto Accelerator
during the first block).
i-1
in the buffer by using the
i-1
Table 31.10 CBC Decryption Steps on
Loop 1
S
a
S
a
E(C
a
a
)
a
a
12
1
0,2
16
20
0,3
24
0
1
28
a
C
a
a
a
a
13
17
1
25
1
29
E(C
a
C
S
a
a
a
a
XOR
14
18
0
2,3
26
2
30
C
2
3
4
5
Steps
) from buffer to DATA0
i
(placed in the buffer at Step 2) to
i-1
Reference Manual
4
5
)
1
6
0
Rev. 1.1 | 1043
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