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/c/test.py b/c/test.py
index b0b1929..3c3aca1 100755
--- a/c/test.py
+++ b/c/test.py
@@ -36,7 +36,7 @@ def main():
         input_len = case["input_len"]
         input = make_test_input(input_len)
         hex_key = hexlify(TEST_VECTORS["key"].encode())
-        context_string = TEST_VECTORS["context_string"]
+        purpose_string = TEST_VECTORS["purpose_string"]
         expected_hash_xof = case["hash"]
         expected_hash = expected_hash_xof[:64]
         expected_keyed_hash_xof = case["keyed_hash"]
@@ -76,7 +76,7 @@ def main():
                     input_len, expected_keyed_hash_xof, line)
 
         # Test the default derive key.
-        test_derive_key = run_blake3(["--derive-key", context_string], input)
+        test_derive_key = run_blake3(["--derive-key", purpose_string], input)
         for line in test_derive_key.splitlines():
             assert expected_derive_key == line, \
                 "derive_key({}): {} != {}".format(
@@ -85,7 +85,7 @@ def main():
         # Test the extended derive key.
         xof_len = len(expected_derive_key_xof) // 2
         test_derive_key_xof = run_blake3(
-            ["--derive-key", context_string, "--length",
+            ["--derive-key", purpose_string, "--length",
              str(xof_len)], input)
         for line in test_derive_key_xof.splitlines():
             assert expected_derive_key_xof == line, \