Image from www.weapons-universe.com & scene from the movie American Ninja
We are all familiar with the Ninja-to. The ninja sword with its square guard & short blade. History however has no record of the ninja-to & more importantly the Shinobi seldom used to run about in black garb from head to toe. While such a uniform might have been used by Ninpei or Shinobi-soldiers who specialized in scouting & reconnaissance behind enemy lines (they were the lowest form of Shinobi relying on Innin to gather information and intelligence).
Ashigaru literally means light foot & the Ashigaru were essentially peasants conscripted into a local landowning samurai. These land owning samurai Shugo-Daimyo fought each other Onin Wars and during this time many kuni ikki (provincial uprisings) allowed the rise of Sengoku-Daimyowho arise from the position of Kokujin. Kokujin (also known as Ji-Samurai or country samurai) often held the position of shugo-dai the landlords of the Shugo-Daimyo; who resided in Kiyoto and later Edo; modern day Tokyo, was a class of lords who arose out of the Shugo; governors of the Muromachi period, where the governors of lands were required to live in the Imperial Capital. Later adopting the title of Shugo-Daimyo or Governor Lords the Shugo-Daimyo administered through the Kokujin.
The shinobi during the Sengoku period were ji-samurai and many were Sengoku-Daimyo. Localized lords who claimedtheir domains from Shugo-Daimyo unable to retain control of their own forces from distance. The common sword of conscripted peasants, low-level professional soldiers and even mercenaries who became the Ashigaru. The Shinobi agents trained in innin (even if they were true masters of the complete transmissions of the art of ninjutsu, including yonin skills) would not doubt use a common lesser quality sword rather then carry a finely crafted weapon for a particular samurai or clan. Doing such would in fact, betray the shinobi.
Let us use reason, I wish to infiltrate an enemy town posing as a merchant. Do I carry the finely craft Katana of a Samurai of my station or a plain, cheap sword befitting the image of a lowly merchant peddling hi wears? Would a high level Shinobi such as Hattori Hanzo be in the field as a scout with his finely crafted Katana or would he order out low level ashigaru/ninpei to bring him back information of enemy positions and dispositions?
The Ninja-to does not exist in history, yet it exists as a common sword made people of low status. The sword was not restricted from being worn by all but the samurai as many believe. Instead two sword, the Daito & Wakasashi were restricted to only be worn by the Samurai class.