From ea91c3a517eef38c0b957a4340d1e4e9c59a9634 Mon Sep 17 00:00:00 2001
From: Paul Duncan <pabs@pablotron.org>
Date: Sun, 3 Sep 2023 01:58:46 -0400
Subject: sha3.[hc]: add kmac128(), kmac_params_t, and stub test_kmac128()

---
 sha3.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 sha3.h | 11 +++++++++--
 2 files changed, 70 insertions(+), 2 deletions(-)

diff --git a/sha3.c b/sha3.c
index b77d5ce..eef281f 100644
--- a/sha3.c
+++ b/sha3.c
@@ -669,6 +669,62 @@ void cshake256(
   cshake256_xof_squeeze(&xof, dst, dst_len);
 }
 
+void kmac128(
+  const kmac_params_t params,
+  const uint8_t * const msg, const size_t msg_len,
+  uint8_t * const dst, const size_t dst_len
+) {
+  static const uint8_t PAD[CSHAKE128_XOF_RATE] = { 0 };
+  static const uint8_t NAME[4] = { 'K', 'M', 'A', 'C' };
+
+  // build cshake128 params
+  const cshake_params_t cshake_params = {
+    .name = NAME,
+    .name_len = sizeof(NAME),
+    .custom = params.custom,
+    .custom_len = params.custom_len,
+  };
+
+  // build key prefix
+  uint8_t key_buf[9] = { 0 };
+  const size_t key_buf_len = encode_string_prefix(key_buf, params.key_len);
+
+  // build bytepad prefix
+  const bytepad_t bp = bytepad(key_buf_len + params.key_len, CSHAKE128_XOF_RATE);
+
+  // init xof
+  sha3_xof_t xof;
+  cshake128_xof_init(&xof, cshake_params);
+
+  // absorb bytepad prefix
+  (void) cshake128_xof_absorb(&xof, bp.prefix, bp.prefix_len);
+
+  // absorb key
+  (void) cshake128_xof_absorb(&xof, key_buf, key_buf_len);
+  if (params.key_len > 0) {
+    (void) cshake128_xof_absorb(&xof, params.key, params.key_len);
+  }
+
+  // absorb padding
+  for (size_t ofs = 0; ofs < bp.pad_len; ofs += sizeof(PAD)) {
+    const size_t len = MIN(bp.pad_len - ofs, sizeof(PAD));
+    (void) cshake128_xof_absorb(&xof, PAD, len);
+  }
+
+  // absorb message
+  (void) cshake128_xof_absorb(&xof, msg, msg_len);
+
+  // build output length suffix
+  uint8_t suffix_buf[9] = { 0 };
+  const size_t suffix_buf_len = right_encode(suffix_buf, dst_len << 3);
+
+  // absorb output length suffix
+  (void) cshake128_xof_absorb(&xof, suffix_buf, suffix_buf_len);
+
+  // squeeze
+  cshake128_xof_squeeze(&xof, dst, dst_len);
+}
+
 #ifdef SHA3_TEST
 #include <stdio.h> // printf()
 
@@ -2444,6 +2500,10 @@ static void test_cshake256(void) {
   }
 }
 
+static void test_kmac128(void) {
+  // TODO
+}
+
 int main(void) {
   test_theta();
   test_rho();
@@ -2467,6 +2527,7 @@ int main(void) {
   test_bytepad();
   test_cshake128();
   test_cshake256();
+  test_kmac128();
   printf("ok\n");
 }
 
diff --git a/sha3.h b/sha3.h
index 99e15b5..aa75cd6 100644
--- a/sha3.h
+++ b/sha3.h
@@ -37,9 +37,9 @@ void shake256_xof_squeeze(sha3_xof_t * const xof, uint8_t * const dst, const siz
 void shake256_xof_once(const uint8_t * const src, const size_t src_len, uint8_t * const dst, const size_t dst_len);
 
 typedef struct {
-  const uint8_t * const name; // NIST function name
+  const uint8_t *name; // NIST function name
   const size_t name_len; // length of NIST function name, in bytes
-  const uint8_t * const custom; // customization string
+  const uint8_t *custom; // customization string
   const size_t custom_len; // length of customization string, in bytes
 } cshake_params_t;
 
@@ -54,6 +54,13 @@ void cshake256_xof_init(sha3_xof_t * const xof, const cshake_params_t params);
 _Bool cshake256_xof_absorb(sha3_xof_t * const xof, const uint8_t * const msg, const size_t len);
 void cshake256_xof_squeeze(sha3_xof_t * const xof, uint8_t * const dst, const size_t len);
 
+typedef struct {
+  const uint8_t *key; // key string
+  const size_t key_len; // length of key string, in bytes
+  const uint8_t *custom; // customization string
+  const size_t custom_len; // length of customization string, in bytes
+} kmac_params_t;
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */
-- 
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