From 04ee35a71bb3eaedfac08ccab42f6167cc777365 Mon Sep 17 00:00:00 2001
From: fallenoak <git@fallenoak.me>
Date: Mon, 6 Feb 2023 17:00:19 -0600
Subject: [PATCH] feat(sha1): add SHA1_InterleaveHash

---
 common/SHA1.cpp | 55 +++++++++++++++++++++++++++++++++++++++++++++++++
 common/SHA1.hpp |  2 ++
 test/SHA1.cpp   | 32 ++++++++++++++++++++++++++++
 3 files changed, 89 insertions(+)

diff --git a/common/SHA1.cpp b/common/SHA1.cpp
index 8384a2a..2caa899 100644
--- a/common/SHA1.cpp
+++ b/common/SHA1.cpp
@@ -1,4 +1,5 @@
 #include "common/SHA1.hpp"
+#include <cstdlib>
 #include <cstring>
 
 #define rol(value, bits) (((value) << (bits)) | ((value) >> (32 - (bits))))
@@ -61,6 +62,60 @@ void SHA1_Init(SHA1_CONTEXT* context) {
     context->count[1] = 0;
 }
 
+uint8_t* SHA1_InterleaveHash(uint8_t* const digest, const uint8_t* data, uint32_t len) {
+    // Terminate data at first null
+    uint32_t l;
+    for (l = len; l; l--) {
+        if (*data) {
+            break;
+        }
+
+        data++;
+    }
+
+    if (l & 1) {
+        data++;
+        l--;
+    }
+
+    SHA1_CONTEXT ctx;
+    uint8_t scratchDigest[SHA1_DIGEST_SIZE];
+
+    auto scratchLen = l / 2;
+    auto scratch = static_cast<uint8_t*>(alloca(scratchLen));
+    if (!scratch) {
+        return nullptr;
+    }
+
+    // Even half
+    for (uint32_t i = 0; i < scratchLen; i++) {
+        scratch[i] = data[i * 2];
+    }
+
+    SHA1_Init(&ctx);
+    SHA1_Update(&ctx, scratch, scratchLen);
+    SHA1_Final(scratchDigest, &ctx);
+
+    for (uint32_t i = 0; i < sizeof(scratchDigest); i++) {
+        digest[i * 2] = scratchDigest[i];
+    }
+
+    // Odd half
+    for (uint32_t i = 0; i < scratchLen; i++) {
+        scratch[i] = data[i * 2 + 1];
+    }
+
+    SHA1_Init(&ctx);
+    SHA1_Update(&ctx, scratch, scratchLen);
+    SHA1_Final(scratchDigest, &ctx);
+
+    for (uint32_t i = 0; i < sizeof(scratchDigest); i++) {
+        digest[i * 2 + 1] = scratchDigest[i];
+    }
+
+    return digest;
+}
+
 void SHA1_Transform(uint32_t state[5], const uint8_t buffer[64]) {
     uint32_t a, b, c, d, e;
     typedef union {
diff --git a/common/SHA1.hpp b/common/SHA1.hpp
index 56606a9..3f1afae 100644
--- a/common/SHA1.hpp
+++ b/common/SHA1.hpp
@@ -15,6 +15,8 @@ void SHA1_Final(uint8_t* const digest, SHA1_CONTEXT* context);
 
 void SHA1_Init(SHA1_CONTEXT* context);
 
+uint8_t* SHA1_InterleaveHash(uint8_t* digest, const uint8_t* data, uint32_t len);
+
 void SHA1_Update(SHA1_CONTEXT* context, const uint8_t* data, uint32_t len);
 
 #endif
diff --git a/test/SHA1.cpp b/test/SHA1.cpp
index 5d3847e..05047bf 100644
--- a/test/SHA1.cpp
+++ b/test/SHA1.cpp
@@ -55,3 +55,35 @@ TEST_CASE("SHA1_Final", "[util]") {
         REQUIRE(memcmp(digest, expected, sizeof(digest)) == 0);
     }
 }
+
+TEST_CASE("SHA1_InterleaveHash", "[sha1]") {
+    SECTION("correctly computes interleaved digest of empty string") {
+        auto input = "";
+        uint8_t digest[SHA1_DIGEST_SIZE * 2];
+        uint8_t expected[SHA1_DIGEST_SIZE * 2] = { 0xda, 0xda, 0x39, 0x39, 0xa3, 0xa3, 0xee, 0xee, 0x5e, 0x5e, 0x6b, 0x6b, 0x4b, 0x4b, 0x0d, 0x0d, 0x32, 0x32, 0x55, 0x55, 0xbf, 0xbf, 0xef, 0xef, 0x95, 0x95, 0x60, 0x60, 0x18, 0x18, 0x90, 0x90, 0xaf, 0xaf, 0xd8, 0xd8, 0x07, 0x07, 0x09, 0x09 };
+
+        SHA1_InterleaveHash(digest, reinterpret_cast<const uint8_t*>(input), strlen(input));
+
+        REQUIRE(memcmp(digest, expected, sizeof(digest)) == 0);
+    }
+
+    SECTION("correctly computes interleaved digest of 'xyz'") {
+        auto input = "xyz";
+        uint8_t digest[SHA1_DIGEST_SIZE * 2];
+        uint8_t expected[SHA1_DIGEST_SIZE * 2] = { 0x95, 0x39, 0xcb, 0x5d, 0x0b, 0xf8, 0xfd, 0xf7, 0x29, 0xc5, 0x77, 0x1f, 0xc7, 0x00, 0x61, 0x70, 0x29, 0x19, 0x8d, 0xcb, 0x96, 0x30, 0x24, 0x20, 0xe4, 0x1c, 0xb4, 0x49, 0xd4, 0xe8, 0xc7, 0x84, 0x2a, 0xb4, 0x39, 0x6b, 0x97, 0x92, 0x4a, 0xfa };
+
+        SHA1_InterleaveHash(digest, reinterpret_cast<const uint8_t*>(input), strlen(input));
+
+        REQUIRE(memcmp(digest, expected, sizeof(digest)) == 0);
+    }
+
+    SECTION("correctly computes interleaved digest of 'foobar'") {
+        auto input = "foobar";
+        uint8_t digest[SHA1_DIGEST_SIZE * 2];
+        uint8_t expected[SHA1_DIGEST_SIZE * 2] = { 0xba, 0xdf, 0x06, 0x40, 0x23, 0x4f, 0xbd, 0x14, 0xc6, 0x5f, 0x7c, 0x1c, 0x26, 0xe8, 0xd2, 0x31, 0x05, 0x06, 0xbb, 0x50, 0x58, 0x88, 0xac, 0x33, 0xc0, 0xf5, 0xaa, 0xe3, 0x9d, 0x03, 0xe6, 0x71, 0x6b, 0xdf, 0xa6, 0x84, 0x0c, 0x40, 0x24, 0xba };
+
+        SHA1_InterleaveHash(digest, reinterpret_cast<const uint8_t*>(input), strlen(input));
+
+        REQUIRE(memcmp(digest, expected, sizeof(digest)) == 0);
+    }
+}