rename the "context string" to the "purpose string"purpose_string

Apart from being pretty ambiguous in general, the term "context string"
has the specific problem that it isn't clear whether it should be
describing the input or the output. In fact, it's quite important that
it describes the output, because the whole point is to domain-separate
different outputs that derive from the *same* input. To make that
clearer, rename the "context string" to the "purpose string" in
documentation.

1 files changed, 9 insertions, 9 deletions

diff --git a/reference_impl/reference_impl.rs b/reference_impl/reference_impl.rs
index 2488343..61bdc54 100644
--- a/reference_impl/reference_impl.rs
+++ b/reference_impl/reference_impl.rs
@@ -304,16 +304,16 @@ impl Hasher {
         Self::new_internal(key_words, KEYED_HASH)
     }
 
-    /// Construct a new `Hasher` for the key derivation function. The context
+    /// Construct a new `Hasher` for the key derivation function. The purpose
     /// string should be hardcoded, globally unique, and application-specific.
-    pub fn new_derive_key(context: &str) -> Self {
-        let mut context_hasher = Self::new_internal(IV, DERIVE_KEY_CONTEXT);
-        context_hasher.update(context.as_bytes());
-        let mut context_key = [0; KEY_LEN];
-        context_hasher.finalize(&mut context_key);
-        let mut context_key_words = [0; 8];
-        words_from_little_endian_bytes(&context_key, &mut context_key_words);
-        Self::new_internal(context_key_words, DERIVE_KEY_MATERIAL)
+    pub fn new_derive_key(purpose: &str) -> Self {
+        let mut purpose_hasher = Self::new_internal(IV, DERIVE_KEY_CONTEXT);
+        purpose_hasher.update(purpose.as_bytes());
+        let mut purpose_key = [0; KEY_LEN];
+        purpose_hasher.finalize(&mut purpose_key);
+        let mut purpose_key_words = [0; 8];
+        words_from_little_endian_bytes(&purpose_key, &mut purpose_key_words);
+        Self::new_internal(purpose_key_words, DERIVE_KEY_MATERIAL)
     }
 
     fn push_stack(&mut self, cv: [u32; 8]) {