  Data-dependent access patterns of an application to an untrusted storage
system are notorious for leaking sensitive information about the user's data.
Previous research has shown how an adversary capable of monitoring both read
and write requests issued to the memory can correlate them with the application
to learn its sensitive data. However, information leakage through only the
write access patterns is less obvious and not well studied in the current
literature. In this work, we demonstrate an actual attack on power-side-channel
resistant Montgomery's ladder based modular exponentiation algorithm commonly
used in public key cryptography. We infer the complete 512-bit secret exponent
in $\sim3.5$ minutes by virtue of just the write access patterns of the
algorithm to the main memory. In order to learn the victim algorithm's write
access patterns under realistic settings, we exploit a compromised DMA device
to take frequent snapshots of the application's address space, and then run a
simple differential analysis on these snapshots to find the write access
sequence. The attack has been shown on an Intel Core(TM) i7-4790 3.60GHz
processor based system. We further discuss a possible attack on McEliece
public-key cryptosystem that also exploits the write-access patterns to learn
the secret key.
