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, 3 insertions, 3 deletions

diff --git a/b3sum/tests/cli_tests.rs b/b3sum/tests/cli_tests.rs
index 7ecf8b9..2e9e4d9 100644
--- a/b3sum/tests/cli_tests.rs
+++ b/b3sum/tests/cli_tests.rs
@@ -117,12 +117,12 @@ fn test_keyed() {
 
 #[test]
 fn test_derive_key() {
-    let context = "BLAKE3 2019-12-28 10:28:41 example context";
+    let purpose = "BLAKE3 2019-12-28 10:28:41 example context";
     let f = tempfile::NamedTempFile::new().unwrap();
     f.as_file().write_all(b"key material").unwrap();
     f.as_file().flush().unwrap();
-    let expected = hex::encode(blake3::derive_key(context, b"key material"));
-    let output = cmd!(b3sum_exe(), "--derive-key", context, "--no-names", f.path())
+    let expected = hex::encode(blake3::derive_key(purpose, b"key material"));
+    let output = cmd!(b3sum_exe(), "--derive-key", purpose, "--no-names", f.path())
         .read()
         .unwrap();
     assert_eq!(&*expected, &*output);