I found an earlier reference for an algorithm for the optimized
computation of n0 that is very similar to the one in the "Montgomery
Multiplication" paper cited in the comments. Add a reference to it.
Henry S. Warren, Jr. pointed out that his "Montgomery Multiplication"
paper is not a chapter of his book, but a supplement to the book.
Correct the reference to it.
Change-Id: Iadeb148c61ce646d1262ccba0207a31ebdad63e9
Reviewed-on: https://boringssl-review.googlesource.com/10480
Reviewed-by: Adam Langley <agl@google.com>
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