We analyze optical encryption systems using the techniques of conventional cryptography. All conventional block encryption algorithms are vulnerable to attack, and often they employ secure modes of operation as one way to increase security. We introduce the concept of conventional secure modes to optical encryption and analyze the results in the context of known conventional and optical attacks. We consider only the optical system "double random phase encoding," which forms the basis for a large number of optical encryption, watermarking, and multiplexing systems. We consider all attacks proposed to date in one particular scenario. We analyze only the mathematical algorithms themselves and do not consider the additional security that arises from employing these algorithms in physical optical systems.

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22 23 Few very small drumlins are typically mapped in previously glaciated landscapes, which might 24 be an important signature of subglacial processes or an observational artefact. newly 25 emergent drumlins, recently sculpted by the Múlajökull glacier, have been mapped using high-26 resolution LiDAR and aerial photographs in addition to field surveying. In this paper, these are 27 used as evidence that few small drumlins (e.g. height H ≲ 4 m, width W ≲ 40 m, length L ≲ 100 28 m) are produced; at least, few survive to pass outside the ice margin in this actively forming 29 drumlin field. Specifically, the lack of a multitude of small features seen in other landforms (e.g. 30 volcanoes) is argued not to be due to i) Digital Elevation Model (DEM) resolution or quality, ii) 31 mapper ability in complex (i.e. anthropogenically cluttered or vegetated) landscapes, or iii) 32 post-glacial degradation at this site. So, whilst detection ability must still be at least 33 acknowledged in drumlin mapping, and ideally corrected for in quantitative analyses, this 34 observation can now be firmly taken as a constraint upon drumlin formation models (i.e. 35 statistical, conceptual, or numerical ice flow). Our preferred explanation for the scarcity of 36 small drumlins, at least at sites similar to Múlajökull (i.e. ice lobes with near-margin drumlin 37 genesis), is that they form stochastically during multiple surge cycles, evolving from wide and 38 gentle pre-existing undulations by increasing rapidly in amplitude before significant 39 streamlining occurs.