Fully in line...look for NVM not for SRAM.
so rather look into M7 (which has both large quantity of embedded flash, and encryption capability).
PS: it would be worth to have floor plan analysis of M7 as well, and get evaluation of technology used and embedded flash size.
Apple and tsmc rescue Apple iPhone 5S fingerprint identification chip.
The article was published in DigiTimes on August 12, 2013 in Chinese and the partial abstract is translated in English and listed below for your reference:
1) Fingerprint ID chip production was not ready until July, 2013 by assembly issues in Xintec. The original mass production schedule of the ID chip was in May, 2013.
2) Apple sent technical people to Taiwan and work with tsmc engineers to solve assembly and test issues in Xintec (wholly own OSAT by tsmc).
3) Apple iPhone 5S fingerprint ID chip gets 30,000 8" wafer per month capacity from tsmc.
4) The schedule delay may be related to a) the incompatibility between IOS7 software and ID chip hardware; b) ID chip is interfered by Sapphire blue paint and c) the new assembly methodology that results in very low yield.
5) Both Apple and tsmc have confidence that the low yield problem will be solved and the mass production will be started by the end of August.
This may imply that the 3MB SRAM is not for storing fingerprint data and the separated fingerprint ID chip may have EEPROM on 8" wafer.
Apple patented the fingerdata verification techinique for mobiles in very recently infact in 2012 they filed for the patent. I do not know where they store this information but 3MB looks rather too much for just few fingerprint.