Matching to latest test vectors with metadata mode (#3)

src/ciphersuite.rs

@@ -5,7 +5,14 @@
 
 //! Defines the CipherSuite trait to specify the underlying primitives for VOPRF
 
-use crate::{group::Group, hash::Hash};
+use crate::{errors::InternalError, group::Group, hash::Hash};
+
+static STR_VOPRF: &[u8] = b"VOPRF07-";
+
+pub enum Mode {
+    Base = 0,
+    Verifiable = 1,
+}
 
 /// Configures the underlying primitives used in VOPRF
 pub trait CipherSuite {
@@ -14,4 +21,17 @@ pub trait CipherSuite {
     type Group: Group;
     /// The main hash function to use (for HKDF computations and hashing transcripts).
     type Hash: Hash;
+
+    /// Generates the contextString parameter as defined in
+    /// <https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-07.html>
+    fn get_context_string(mode: Mode) -> Result<alloc::vec::Vec<u8>, InternalError> {
+        use crate::serialization::i2osp;
+
+        Ok([
+            STR_VOPRF,
+            &i2osp(mode as usize, 1)?,
+            &i2osp(Self::Group::SUITE_ID, 2)?,
+        ]
+        .concat())
+    }
 }