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Most of the functions related to points on the Edwards25519 curve
check that input points are not in a small-order subgroup.
They don't check that points are on the prime-order subgroup,
because this is expensive, and not always necessary.
However, applications may require such a check in order to
ensure that a public key is valid, and that a secret key counterpart
exists.
Many functions in the public API of libsodium related to arithmetic
over Edwards25519 also do that check unconditionally. This is
expensive, but a good way to catch bugs in protocols and
implementations.
So, add a `rejectUnexpectedSubgroup()` function to achieve this.
The documentation on the edwards25519->curve25519 conversion
function was also updated, in order to explain how to match
libsodium's behavior if necessary.
We use an addition chain to multiply the point by the order of
the prime group.
An alternative we may implement later is Pornin's point halving
technique: https://eprint.iacr.org/2022/1164.pdf
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The majority of these are in comments, some in doc comments which might
affect the generated documentation, and a few in parameter names -
nothing that should be breaking, however.
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std/crypto: use finer-grained error sets in function signatures
Returning the `crypto.Error` error set for all crypto operations
was very convenient to ensure that errors were used consistently,
and to avoid having multiple error names for the same thing.
The flipside is that callers were forced to always handle all
possible errors, even those that could never be returned by a
function.
This PR makes all functions return union sets of the actual errors
they can return.
The error sets themselves are all limited to a single error.
Larger sets are useful for platform-specific APIs, but we don't have
any of these in `std/crypto`, and I couldn't find any meaningful way
to build larger sets.
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