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explicitly document the properties of short outputs 

Suggested by @joshtriplett at:
https://github.com/BLAKE3-team/BLAKE3/issues/168#issuecomment-829609667
This commit is contained in:
Jack O'Connor 2021-04-30 19:05:56 -04:00 committed by Jack O'Connor
parent 4b7babbe99
commit 7cd208afcf
2 changed files with 26 additions and 4 deletions
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19
c/README.md
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@ -92,10 +92,21 @@ void blake3_hasher_finalize(
size_t out_len);
```
Finalize the hasher and emit an output of any length. This doesn't
modify the hasher itself, and it's possible to finalize again after
adding more input. The constant `BLAKE3_OUT_LEN` provides the default
output length, 32 bytes.
Finalize the hasher and return an output of any length, given in bytes.
This doesn't modify the hasher itself, and it's possible to finalize
again after adding more input. The constant `BLAKE3_OUT_LEN` provides
the default output length, 32 bytes, which is recommended for most
callers.
Outputs shorter than the default length of 32 bytes (256 bits) provide
less security. An N-bit BLAKE3 output is intended to provide N bits of
first and second preimage resistance and N/2 bits of collision
resistance, for any N up to 256. Longer outputs don't provide any
additional security.
Shorter BLAKE3 outputs are prefixes of longer ones. Explicitly
requesting a short output is equivalent to truncating the default-length
output. (Note that this is different between BLAKE2 and BLAKE3.)
## Less Common API Functions

11
src/lib.rs
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@ -1337,6 +1337,17 @@ impl std::io::Write for Hasher {
/// An incremental reader for extended output, returned by
/// [`Hasher::finalize_xof`](struct.Hasher.html#method.finalize_xof).
///
/// Outputs shorter than the default length of 32 bytes (256 bits)
/// provide less security. An N-bit BLAKE3 output is intended to provide
/// N bits of first and second preimage resistance and N/2 bits of
/// collision resistance, for any N up to 256. Longer outputs don't
/// provide any additional security.
///
/// Shorter BLAKE3 outputs are prefixes of longer ones. Explicitly
/// requesting a short output is equivalent to truncating the
/// default-length output. (Note that this is different between BLAKE2
/// and BLAKE3.)
#[derive(Clone)]
pub struct OutputReader {
inner: Output,