IEEE Consumer Electronics Magazine - January/February 2022 - 85

Figure 4. Simplified architecture of the sha256crypt
accelerating core by Zhang and Liu8.
Figure 3. Architecture of the sha256crypt
computing unit in John the Ripper12.
accelerator in John the Ripper12 also benefited
from multicore design. Their accelerator is built
upon the ZTEX 1.15y board, featuring four
XC6SLX150 FPGAs with each FPGA containing 23
computing units and each computing unit consisting
of 3 SHA256 cores and a simple customized
CPU (see Figure 3). In total, the whole system has
276 cores and can process 1104 passwords in parallel.
As the cores in a multicore-based design
show weak dependence among themselves,
scheduling the cores can be simple. Scaling of the
accelerator is also straightforward. However, as
the control unit is needed for every core, the overall
system will likely dedicate a lot hardware
resources for control purposes than those for
computing, which imposes a backlash on the
resource efficiency of themulticore-based design.
Pipeline-Based Design
Deep-pipeline-based designs, on the other
hand, have better resource efficiency than the
multicore-based designs. By using a deep pipeline,
the accelerator could process multiple
passwords at the same time with low hardware
requirement for control logic. Take the sha256crypt
accelerator proposed by Zhang and Liu8 as
an example, the accelerating core has a 64-stage
pipeline (see Figure 4), and each core can process
at most 64 passwords at the same time. In
this case, all the computing logic in the pipeline
January/February 2022
could share the same control logic, freeing up
precious hardware resources for computing.
The experimental result shows that the energy
efficiency and resource efficiency are 1.64 and
1.69 of the accelerator in John the Ripper12,
respectively.
The disadvantage of a deep-pipeline-based
design comes from its pipeline schedule. The
first problem lies in the data dependence of the
PHF. A 64-stage pipeline takes 64 cycles to finish
its computation. However, in a PHF, the new
data depend on the computed result of the old
data, which means the pipeline can not be fed
with the new data until the computation of the
old data is finished. Therefore, bubbles must be
inserted into the pipeline stages, which will significantly
reduce the speed of the accelerator,
as shown in Figure 5(a). This problem has been
solved by applying the group schedule technique.2,8,13
Because there is no data dependence
between different passwords, a group of passwords
can be processed together. Thereby, the
pipeline can be fed with useful data at every
cycle, as shown in Figure 5(b).
Another problem is that in some PHFs, such
as sha256crypt, the number of hash computations
Figure 5. (a) Pipeline without applying group
schedule. (b) Pipeline with group schedule.
85

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