mirror of https://github.com/zcash/zips.git
Add note about endianness of repr_J.
Signed-off-by: Daira Hopwood <daira@jacaranda.org>
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@ -4264,6 +4264,12 @@ Define $\reprJ \typecolon \GroupJ \rightarrow \bitseq{\ellJ}$ such
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that $\reprJ(u, \varv) = \ItoLEBSP{256}(\varv + 2^{255} \smult \tilde{u})$, where
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that $\reprJ(u, \varv) = \ItoLEBSP{256}(\varv + 2^{255} \smult \tilde{u})$, where
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$\tilde{u} = u \bmod 2$.
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$\tilde{u} = u \bmod 2$.
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\todo{Representing this as a bit string is problematic because we normally encode
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most-significant-bit first within a byte, so that would result in the wrong
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(i.e. non-standard) encoding as a byte sequence. It's a tricky specification
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problem that we get away with elsewhere in the spec mostly by luck. Maybe keep
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the representation as an integer?}
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Let $\abstJ \typecolon \bitseq{\ellJ} \rightarrow \GroupJ \union \setof{\bot}$
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Let $\abstJ \typecolon \bitseq{\ellJ} \rightarrow \GroupJ \union \setof{\bot}$
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be the left inverse of $\reprJ$ such that if $S$ is not in the range of
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be the left inverse of $\reprJ$ such that if $S$ is not in the range of
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$\reprJ$, then $\abstJ(S) = \bot$.
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$\reprJ$, then $\abstJ(S) = \bot$.
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