Derive the key

Cargo.toml

@@ -19,5 +19,7 @@ i-understand-and-accept-the-risks = []
 
 [dependencies]
 base64ct = { version = "1.6.0", default-features = false, features = ["std"] }
+blake3 = { version = "1.5.0", default-features = false, features = ["std", "traits-preview"] }
+digest = { version = "0.10.7", default-features = false, features = ["std"] }
 getrandom = { version = "0.2.12", default-features = false }
 thiserror = { version = "1.0.57", default-features = false }

src/encryption.rs

@@ -1,3 +1,4 @@
+use crate::key::derive_key;
 use crate::{Error, InputKeyMaterialList};
 
 pub fn encrypt(
@@ -6,6 +7,8 @@ pub fn encrypt(
 	data: impl AsRef<[u8]>,
 	data_context: &[impl AsRef<[u8]>],
 ) -> Result<String, Error> {
+	let ikm = ikml.get_latest_ikm()?;
+	let key = derive_key(ikm, key_context);
 	unimplemented!("encrypt");
 }
 

src/kdf.rs

@@ -0,0 +1,9 @@
+pub(crate) fn blake3_derive(context: &[u8], ikm: &[u8]) -> Vec<u8> {
+	// TODO: remove this hack as soon as `blake3::derive_key` accepts bytes
+	use std::fmt::Write;
+	let context: String = context.iter().fold(String::new(), |mut output, b| {
+		let _ = write!(output, "{b:02x}");
+		output
+	});
+	blake3::derive_key(&context, ikm).to_vec()
+}

src/key.rs

@@ -0,0 +1,69 @@
+use crate::ikm::InputKeyMaterial;
+use crate::scheme::Scheme;
+use digest::Digest;
+
+fn canonicalize(scheme: Scheme, key_context: &[impl AsRef<[u8]>]) -> Vec<u8> {
+	match key_context.len() {
+		0 => Vec::new(),
+		1 => key_context[0].as_ref().into(),
+		n => {
+			let mut ret = Vec::with_capacity(n * scheme.key_size());
+			for ctx_elem in key_context {
+				let mut elem_hasher = scheme.canonicalization_hasher();
+				elem_hasher.update(ctx_elem.as_ref());
+				ret.extend_from_slice(&elem_hasher.finalize());
+			}
+			ret
+		}
+	}
+}
+
+pub(crate) fn derive_key(ikm: &InputKeyMaterial, key_context: &[impl AsRef<[u8]>]) -> Vec<u8> {
+	let key_context = canonicalize(ikm.scheme, key_context);
+	let kdf = ikm.scheme.get_kdf();
+	kdf(&key_context, &ikm.content)
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use crate::scheme::Scheme;
+
+	const EMPTY_CTX: &[[u8; 0]] = &[];
+
+	#[test]
+	fn canonicalize_empty() {
+		let canon = canonicalize(Scheme::XChaCha20Poly1305WithBlake3, EMPTY_CTX);
+		assert_eq!(canon, vec![]);
+	}
+
+	#[test]
+	fn canonicalize_one() {
+		let s = "test";
+		let canon = canonicalize(Scheme::XChaCha20Poly1305WithBlake3, &[s]);
+		assert_eq!(canon, s.as_bytes());
+	}
+
+	#[test]
+	fn blake3_canonicalize_many() {
+		let canon = canonicalize(
+			Scheme::XChaCha20Poly1305WithBlake3,
+			&["test", "bis", "ter", ""],
+		);
+		assert_eq!(
+			canon,
+			[
+				0x48, 0x78, 0xca, 0x04, 0x25, 0xc7, 0x39, 0xfa, 0x42, 0x7f, 0x7e, 0xda, 0x20, 0xfe,
+				0x84, 0x5f, 0x6b, 0x2e, 0x46, 0xba, 0x5f, 0xe2, 0xa1, 0x4d, 0xf5, 0xb1, 0xe3, 0x2f,
+				0x50, 0x60, 0x32, 0x15, 0x62, 0x01, 0x8f, 0x96, 0x98, 0x6e, 0x0e, 0x48, 0x8e, 0x07,
+				0x4d, 0xa6, 0xb7, 0x28, 0xbc, 0x24, 0x2b, 0xf0, 0xcc, 0x6e, 0x5a, 0x5d, 0x4e, 0x78,
+				0x02, 0x14, 0x0a, 0x52, 0xbf, 0xe1, 0x58, 0x86, 0x91, 0x9e, 0x10, 0xc3, 0xe4, 0xbf,
+				0x61, 0x2f, 0x36, 0x39, 0xbe, 0x86, 0xb9, 0x34, 0x6b, 0xc3, 0x4a, 0x8c, 0x89, 0x14,
+				0xbd, 0x78, 0x9b, 0x47, 0x79, 0xf7, 0xc9, 0x83, 0x32, 0x80, 0x1c, 0x1b, 0xaf, 0x13,
+				0x49, 0xb9, 0xf5, 0xf9, 0xa1, 0xa6, 0xa0, 0x40, 0x4d, 0xea, 0x36, 0xdc, 0xc9, 0x49,
+				0x9b, 0xcb, 0x25, 0xc9, 0xad, 0xc1, 0x12, 0xb7, 0xcc, 0x9a, 0x93, 0xca, 0xe4, 0x1f,
+				0x32, 0x62
+			]
+		);
+	}
+}

src/lib.rs

@@ -3,6 +3,10 @@ mod encryption;
 mod error;
 #[cfg(any(feature = "encryption", feature = "ikm-management"))]
 mod ikm;
+#[cfg(feature = "encryption")]
+mod kdf;
+#[cfg(feature = "encryption")]
+mod key;
 #[cfg(any(feature = "encryption", feature = "ikm-management"))]
 mod scheme;
 

src/scheme.rs

@@ -5,6 +5,26 @@ pub enum Scheme {
 	XChaCha20Poly1305WithBlake3 = 1,
 }
 
+impl Scheme {
+	pub(crate) fn canonicalization_hasher(&self) -> impl digest::Digest {
+		match self {
+			Scheme::XChaCha20Poly1305WithBlake3 => blake3::Hasher::new(),
+		}
+	}
+
+	pub(crate) fn get_kdf(&self) -> impl Fn(&[u8], &[u8]) -> Vec<u8> {
+		match self {
+			Scheme::XChaCha20Poly1305WithBlake3 => crate::kdf::blake3_derive,
+		}
+	}
+
+	pub(crate) fn key_size(&self) -> usize {
+		match self {
+			Scheme::XChaCha20Poly1305WithBlake3 => 32,
+		}
+	}
+}
+
 impl TryFrom<u32> for Scheme {
 	type Error = Error;