Removed byteorder dependency

Cargo.toml

@@ -28,7 +28,6 @@ std = []
 [dependencies]
 aead = "0.5"
 aes-gcm = "0.10"
-byteorder = { version = "1.4", default-features = false }
 chacha20poly1305 = "0.10"
 generic-array = { version = "0.14", default-features = false }
 digest = "0.10"

src/aead.rs

@@ -4,14 +4,13 @@ use crate::{
     kdf::{Kdf as KdfTrait, LabeledExpand, SimpleHkdf},
     kem::Kem as KemTrait,
     setup::ExporterSecret,
-    util::{enforce_equal_len, enforce_outbuf_len, full_suite_id, FullSuiteId},
+    util::{enforce_equal_len, enforce_outbuf_len, full_suite_id, write_u64_be, FullSuiteId},
     Deserializable, HpkeError, Serializable,
 };
 
 use core::{default::Default, marker::PhantomData};
 
 use aead::{AeadCore as BaseAeadCore, AeadInPlace as BaseAeadInPlace, KeyInit as BaseKeyInit};
-use byteorder::{BigEndian, ByteOrder};
 use generic_array::GenericArray;
 use zeroize::Zeroize;
 
@@ -113,7 +112,7 @@ fn mix_nonce<A: Aead>(base_nonce: &AeadNonce<A>, seq: &Seq) -> AeadNonce<A> {
     // because this is a big-endian number.
     let seq_size = core::mem::size_of::<Seq>();
     let nonce_size = base_nonce.0.len();
-    BigEndian::write_u64(&mut seq_buf.0[nonce_size - seq_size..], seq.0);
+    write_u64_be(&mut seq_buf.0[nonce_size - seq_size..], seq.0);
 
     // XOR the base nonce bytes with the sequence bytes
     let new_nonce_iter = base_nonce

src/kdf.rs

@@ -1,6 +1,7 @@
 //! Traits and structs for key derivation functions
 
-use byteorder::{BigEndian, ByteOrder};
+use crate::util::write_u16_be;
+
 use digest::{core_api::BlockSizeUser, Digest, OutputSizeUser};
 use generic_array::GenericArray;
 use hmac::SimpleHmac;
@@ -154,7 +155,7 @@ where
 
         // Encode the output length in the info string
         let mut len_buf = [0u8; 2];
-        BigEndian::write_u16(&mut len_buf, out.len() as u16);
+        write_u16_be(&mut len_buf, out.len() as u16);
 
         // Call HKDF-Expand() with the info string set to the concatenation of all of the above
         let labeled_info = [&len_buf, VERSION_LABEL, suite_id, label, info];

src/util.rs

@@ -1,14 +1,34 @@
 use crate::{aead::Aead, kdf::Kdf as KdfTrait, kem::Kem as KemTrait, HpkeError, Serializable};
 
-use byteorder::{BigEndian, ByteOrder};
-
 /// Represents a ciphersuite context. That's "KEMXX", where `XX` is the KEM ID
 pub(crate) type KemSuiteId = [u8; 5];
 
 /// Represents a ciphersuite context. That's "HPKEXXYYZZ", where `XX` is the KEM ID, `YY` is the
 /// KDF ID, and `ZZ` is the AEAD ID
 pub(crate) type FullSuiteId = [u8; 10];
 
+/// Writes a u16 to a bytestring in big-endian order. `buf.len()` MUST be 2
+#[rustfmt::skip]
+pub(crate) fn write_u16_be(buf: &mut [u8], n: u16) {
+    assert_eq!(buf.len(), 2);
+    buf[0] = ((n & 0xff00) >> 8) as u8;
+    buf[1] =  (n & 0x00ff)       as u8;
+}
+
+/// Writes a u64 to a bytestring in big-endian order. `buf.len()` MUST be 8
+#[rustfmt::skip]
+pub(crate) fn write_u64_be(buf: &mut [u8], n: u64) {
+    assert_eq!(buf.len(), 8);
+    buf[0] = ((n & 0xff00000000000000) >> 56) as u8;
+    buf[1] = ((n & 0x00ff000000000000) >> 48) as u8;
+    buf[2] = ((n & 0x0000ff0000000000) >> 40) as u8;
+    buf[3] = ((n & 0x000000ff00000000) >> 32) as u8;
+    buf[4] = ((n & 0x00000000ff000000) >> 24) as u8;
+    buf[5] = ((n & 0x0000000000ff0000) >> 16) as u8;
+    buf[6] = ((n & 0x000000000000ff00) >>  8) as u8;
+    buf[7] =  (n & 0x00000000000000ff)        as u8;
+}
+
 // RFC 9180 §5.1
 // suite_id = concat(
 //   "HPKE",
@@ -28,9 +48,9 @@ where
     let mut suite_id = *b"HPKEXXYYZZ";
 
     // Write the ciphersuite identifiers to the buffer. Forgive the explicit indexing.
-    BigEndian::write_u16(&mut suite_id[4..6], Kem::KEM_ID);
-    BigEndian::write_u16(&mut suite_id[6..8], Kdf::KDF_ID);
-    BigEndian::write_u16(&mut suite_id[8..10], A::AEAD_ID);
+    write_u16_be(&mut suite_id[4..6], Kem::KEM_ID);
+    write_u16_be(&mut suite_id[6..8], Kdf::KDF_ID);
+    write_u16_be(&mut suite_id[8..10], A::AEAD_ID);
 
     suite_id
 }
@@ -44,7 +64,7 @@ pub(crate) fn kem_suite_id<Kem: KemTrait>() -> KemSuiteId {
     let mut suite_id = *b"KEMXX";
 
     // Write the KEM ID to the buffer. Forgive the explicit indexing.
-    BigEndian::write_u16(&mut suite_id[3..5], Kem::KEM_ID);
+    write_u16_be(&mut suite_id[3..5], Kem::KEM_ID);
 
     suite_id
 }