[sf-lug] Ex 11.7 Exponentiation (understandable) & RSA algorithm (not so!)

Asheesh Laroia asheesh at asheesh.org
Mon Jan 12 09:09:48 PST 2009

On Mon, 12 Jan 2009, jim wrote:

> i'd like verification of my understanding of this:
> "Let d be the reciprocal of e mod r."

> my arithmetic:
> if e is 35 and r is 9111,
> then d = 1/(35%9111) ## == 1/35 == 0.028571429

No, sorry! Reciprocal isn't defined that way in modulo arithmetic. I'm 
glad you asked.

Quoth Wikipedia: "In modular arithmetic, the modular multiplicative 
inverse of x is also defined: it is the number a such that a*x ≡ 1 (mod 
n). This multiplicative inverse exists if and only if a and n are coprime. 
For example, the inverse of 3 modulo 11 is 4 because it is the solution to 
3*x ≡ 1 (mod 11). The extended Euclidean algorithm may be used to compute 

In modular arithmetic, 1/a mod b is an *integer* (iff a and b are 
relatively prime).

There are some Java (not JavaScript) calculators for this on the web, e.g. 
http://www.mtholyoke.edu/~mpeterso/Applets/CalculatorApplet.html . I 
haven't found any working ones in JavaScript yet; perhaps you or another 
list member can.

-- Asheesh.

You're definitely on their list.  The question to ask next is what list it is.

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