doc: justify lack of seed keyword in hash construction.

* txr.1: The hash function doesn't support a keyword
like :seed <n> to specify the hash seed. Adding an explanation
of why. Spoiler: security reason.

1 files changed, 8 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 67e007e7..34fac7dc 100644
--- a/txr.1
+++ b/txr.1
@@ -40971,6 +40971,14 @@ specifies the user data for the hash table, which can be retrieved using the
 .code hash-userdata
 function.
 
+Note: there doesn't exist a keyword for specifying the seed.
+This omission is deliberate. These hash construction keywords may appear in the
+hash literal
+.code #H
+syntax. A seed keyword would allow literals to specify their own seed, which
+would allow malicious hash literals to be crafted that perpetrate a hash
+collision attack against the parser.
+
 .coNP Functions @, hash-construct @ hash-from-pairs and @ hash-from-alist
 .synb
 .mets (hash-construct < hash-args << key-val-pairs )