1: 一 2: 二 3: 三 4: 四 5: 五 6: 六 7: 七 8: 八 9: 九 10: 十
For financial documents, a more complicated form of symbols is used:
1: 壹 2: 貳 3: 參 4: 肆 5: 伍 6: 陸 7: 柒 8: 捌 9: 玖 10: 拾
The best security for thousands of years has been simple - be a suspicious bastard.
You can also easily change a one into a ten, for example. Note also that it's not exactly a positional number system like Arabic. E.g. the number 123 would be written 1 hundred 2 ten 3. IIRC you can also easily change a ten into a hundred or thousand, so for larger values with a few zeroes in between you could shift a meaningful place into the zeroes to the left sometimes.
See https://en.wikipedia.org/wiki/Japanese_numerals#Formal_numbe...