diff --git a/README.md b/README.md
index 15897bb..eae690c 100644
--- a/README.md
+++ b/README.md
@@ -1,23 +1,35 @@
-# [hmac_sha256](https://github.com/h5p9sl/hmac_sha256)
-*A SHA256 HMAC implementation in C/C++*
+# hmac_sha256
+*Minimal HMAC-SHA256 implementation in C / C++*
+
+This repository provides minimal HMAC-Sha256 code you can copy into your own projects.
+The `hmac_sha256` function looks like this:
+```cpp
+size_t // Returns the number of bytes written to `out`
+hmac_sha256(
+    // [in]: The key and its length.
+    //      Should be at least 32 bytes long for optimal security.
+    const void* key, const size_t keylen,
+
+    // [in]: The data to hash alongside the key.
+    const void* data, const size_t datalen,
+
+    // [out]: The output hash.
+    //      Should be 32 bytes long. If it's less than 32 bytes,
+    //      the resulting hash will be truncated to the specified length.
+    void* out, const size_t outlen
+);
+```
+
+## Contributing
+All contributions are welcome, feature requests, or issues.
+I aim to tailor this code not only for myself, but for other's use cases too.
 
 ## Usage Example (C++)
+https://github.com/h5p9sl/hmac_sha256/blob/79a57d2a85aaab32449e5179a4f08f37e38cdee5/examples/simple_example.cpp#L13-L26
+
 ```cpp
-#include "../hmac_sha256.h"
-
-#include <vector>
-#include <string>
-#include <iostream>
-#include <sstream>
-#include <iomanip>
-#include <cassert>
-
-#define SHA256_HASH_SIZE 32
-
-int main() {
     const std::string str_data = "Hello World!";
     const std::string str_key = "super-secret-key";
-    std::stringstream ss_result;
 
     // Allocate memory for the HMAC
     std::vector<uint8_t> out(SHA256_HASH_SIZE);
@@ -28,23 +40,7 @@ int main() {
         str_data.data(), str_data.size(),
         out.data(),  out.size()
     );
-
-    // Convert `out` to string with std::hex
-    for (uint8_t x : out) {
-        ss_result << std::hex << std::setfill('0') << std::setw(2) << (int)x;
-    }
-
-    // Print out the result
-    std::cout << "Message: " << str_data << std::endl;
-    std::cout << "HMAC: " << ss_result.str() << std::endl;
-
-    // This assertion fails if something went wrong
-    assert(
-        ss_result.str() ==
-        "4b393abced1c497f8048860ba1ede46a23f1ff5209b18e9c428bddfbb690aad8"
-    );
-    return 0;
-}
 ```
-
-Thank you to the [WjCryptLib](https://github.com/WaterJuice/WjCryptLib) for providing the Sha256 implementation!
+## Sha256 Implementation
+Big thank you to [WjCryptLib](https://github.com/WaterJuice/WjCryptLib) for providing the Sha256 implementation of which this project is based off.
+If you need more public domain cryptographic functions in C (sha, aes, md5), check them out.
diff --git a/examples/simple_example.cpp b/examples/simple_example.cpp
index 231e5e8..71809c3 100644
--- a/examples/simple_example.cpp
+++ b/examples/simple_example.cpp
@@ -31,6 +31,7 @@ int main() {
 
     // Print out the result
     std::cout << "Message: " << str_data << std::endl;
+    std::cout << "Key: " << str_key << std::endl;
     std::cout << "HMAC: " << ss_result.str() << std::endl;
 
     // This assertion fails if something went wrong
diff --git a/examples/tests.cpp b/examples/tests.cpp
index 88f669c..272605e 100644
--- a/examples/tests.cpp
+++ b/examples/tests.cpp
@@ -12,22 +12,29 @@
 
 typedef std::vector<std::tuple<std::string, std::string, std::string>> TestData_t;
 
-void do_tests(const TestData_t& test_vectors) {
-    // Perform tests
+static void verify_result(const std::vector<uint8_t>& result, const std::string& expected) {
+    std::stringstream ss;
+    for (uint8_t i : result) { ss << std::hex << std::setfill('0') << std::setw(2) << (int)i; }
+    if (expected != ss.str()) {
+        std::cout << "*** TEST FAILED ***: \n\t" << ss.str() << " != \n\t" << expected << std::endl;
+    } else {
+        std::cout << "Test successful" << std::endl;
+    }
+}
+
+static void do_tests(const TestData_t& test_vectors) {
     for (auto tvec : test_vectors) {
-        std::stringstream ss_result;
-        std::vector<uint8_t> out(SHA256_HASH_SIZE);
+        std::vector<uint8_t> out(
+            std::get<2>(tvec).size() / 2
+        );
+
         hmac_sha256(
             std::get<0>(tvec).data(), std::get<0>(tvec).size(),
             std::get<1>(tvec).data(), std::get<1>(tvec).size(),
             out.data(),  out.size()
         );
-        for (uint8_t i : out) { ss_result << std::hex << std::setfill('0') << std::setw(2) << (int)i; }
-        if (std::get<2>(tvec) != ss_result.str()) {
-            std::cout << "TEST FAILED: \n\t" << ss_result.str() << " != \n\t" << std::get<2>(tvec) << std::endl;
-        } else {
-            std::cout << "Test successful" << std::endl;
-        }
+
+        verify_result(out, std::get<2>(tvec));
     }
 }
 
@@ -37,21 +44,43 @@ int main() {
         // Key      Data      HMAC
         {
             "\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b",
-            "Hi There",
+            "\x48\x69\x20\x54\x68\x65\x72\x65",
             "b0344c61d8db38535ca8afceaf0bf12b881dc200c9833da726e9376c2e32cff7",
         }, {
-            "Jefe",
-            "what do ya want for nothing?",
+            /* Test with a key shorter than the length of the HMAC output. */
+            "\x4a\x65\x66\x65",
+            "\x77\x68\x61\x74\x20\x64\x6f\x20\x79\x61\x20\x77\x61\x6e\x74\x20\x66\x6f\x72\x20\x6e\x6f\x74\x68\x69\x6e\x67\x3f",
             "5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843",
         }, {
+            /* Test with a combined length of key and data that is larger than 64
+               bytes (= block-size of SHA-224 and SHA-256). */
             "\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa",
             "\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd\xdd",
             "773ea91e36800e46854db8ebd09181a72959098b3ef8c122d9635514ced565fe",
         }, {
+            /* Test with a combined length of key and data that is larger than 64
+               bytes (= block-size of SHA-224 and SHA-256). */
             "\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19",
             "\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd\xcd",
             "82558a389a443c0ea4cc819899f2083a85f0faa3e578f8077a2e3ff46729665b",
-        },
+        }, {
+            /* Test with a truncation of output to 128 bits. */
+            "\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c\x0c",
+            "\x54\x65\x73\x74\x20\x57\x69\x74\x68\x20\x54\x72\x75\x6e\x63\x61\x74\x69\x6f\x6e",
+            "a3b6167473100ee06e0c796c2955552b",
+        }, {
+            /* Test with a key larger than 128 bytes (= block-size of SHA-384 and
+               SHA-512). */
+           "\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa",
+           "\x54\x65\x73\x74\x20\x55\x73\x69\x6e\x67\x20\x4c\x61\x72\x67\x65\x72\x20\x54\x68\x61\x6e\x20\x42\x6c\x6f\x63\x6b\x2d\x53\x69\x7a\x65\x20\x4b\x65\x79\x20\x2d\x20\x48\x61\x73\x68\x20\x4b\x65\x79\x20\x46\x69\x72\x73\x74",
+           "60e431591ee0b67f0d8a26aacbf5b77f8e0bc6213728c5140546040f0ee37f54",
+        }, {
+            /* Test with a key and data that is larger than 128 bytes (= block-size
+               of SHA-384 and SHA-512). */
+           "\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa\xaa",
+           "\x54\x68\x69\x73\x20\x69\x73\x20\x61\x20\x74\x65\x73\x74\x20\x75\x73\x69\x6e\x67\x20\x61\x20\x6c\x61\x72\x67\x65\x72\x20\x74\x68\x61\x6e\x20\x62\x6c\x6f\x63\x6b\x2d\x73\x69\x7a\x65\x20\x6b\x65\x79\x20\x61\x6e\x64\x20\x61\x20\x6c\x61\x72\x67\x65\x72\x20\x74\x68\x61\x6e\x20\x62\x6c\x6f\x63\x6b\x2d\x73\x69\x7a\x65\x20\x64\x61\x74\x61\x2e\x20\x54\x68\x65\x20\x6b\x65\x79\x20\x6e\x65\x65\x64\x73\x20\x74\x6f\x20\x62\x65\x20\x68\x61\x73\x68\x65\x64\x20\x62\x65\x66\x6f\x72\x65\x20\x62\x65\x69\x6e\x67\x20\x75\x73\x65\x64\x20\x62\x79\x20\x74\x68\x65\x20\x48\x4d\x41\x43\x20\x61\x6c\x67\x6f\x72\x69\x74\x68\x6d\x2e",
+           "9b09ffa71b942fcb27635fbcd5b0e944bfdc63644f0713938a7f51535c3a35e2",
+        }
     };
     do_tests(test_vectors);
     return 0;
diff --git a/hmac_sha256.c b/hmac_sha256.c
index 568e9e7..ab246ec 100644
--- a/hmac_sha256.c
+++ b/hmac_sha256.c
@@ -9,76 +9,82 @@
 #include <stdlib.h>
 #include <string.h>
 
-#define SIZEOFARRAY(x) sizeof(x) / sizeof(x[0])
+#define SIZEOFARRAY(x) (sizeof(x) / sizeof(x[0]))
 #define SHA256_BLOCK_SIZE 64
 
 /* LOCAL FUNCTIONS */
 
-// wrapper for sha256 digest functions
-void sha256(const void *data, const unsigned datalen, void *out);
-// concatonate src & dest then sha2 digest them
-void concat_and_hash(const void *dest, const unsigned destlen, const void *src,
-        const unsigned srclen, void *out, const unsigned outlen);
+// Concatenate X & Y, return hash.
+static void* H(const void *x, const size_t xlen,
+        const void *y, const size_t ylen,
+        void *out, const size_t outlen
+);
+// Wrapper for sha256
+static void* sha256(const void *data, const size_t datalen,
+    void *out, const size_t outlen
+);
 
 // Declared in hmac_sha256.h
-void hmac_sha256(const void *key, const unsigned keylen, const void *data,
-        const unsigned datalen, void *out, const unsigned outlen) {
-    uint8_t k[SHA256_BLOCK_SIZE]; // block-sized key derived from 'key' parameter
+size_t hmac_sha256(const void *key, const size_t keylen,
+        const void *data, const size_t datalen,
+        void *out, const size_t outlen) {
+    uint8_t k[SHA256_BLOCK_SIZE];
     uint8_t k_ipad[SHA256_BLOCK_SIZE];
     uint8_t k_opad[SHA256_BLOCK_SIZE];
-    uint8_t hash0[SHA256_HASH_SIZE];
-    uint8_t hash1[SHA256_HASH_SIZE];
+    uint8_t ihash[SHA256_HASH_SIZE];
+    uint8_t ohash[SHA256_HASH_SIZE];
+    size_t sz;
     int i;
 
-    // Fill 'k' with zero bytes
     memset(k, 0, SIZEOFARRAY(k));
+    memset(k_ipad, 0x36, SHA256_BLOCK_SIZE);
+    memset(k_opad, 0x5c, SHA256_BLOCK_SIZE);
+
     if (keylen > SHA256_BLOCK_SIZE) {
-        // If the key is larger than the hash algorithm's block size, we must
-        // digest it first.
-        sha256(key, keylen, k);
+        // If the key is larger than the hash algorithm's
+        // block size, we must digest it first.
+        sha256(key, keylen, k, SIZEOFARRAY(k));
     } else {
         memcpy(k, key, keylen);
     }
 
-    // Create outer & inner padded keys
-    memset(k_ipad, 0x36, SHA256_BLOCK_SIZE);
-    memset(k_opad, 0x5c, SHA256_BLOCK_SIZE);
     for (i = 0; i < SHA256_BLOCK_SIZE; i++) {
         k_ipad[i] ^= k[i];
         k_opad[i] ^= k[i];
     }
 
-    // Perform HMAC algorithm H(K XOR opad, H(K XOR ipad, text))
-    // https://tools.ietf.org/html/rfc2104
-    concat_and_hash(k_ipad, SIZEOFARRAY(k_ipad), data, datalen, hash0,
-            SIZEOFARRAY(hash0));
-    concat_and_hash(k_opad, SIZEOFARRAY(k_opad), hash0, SIZEOFARRAY(hash0), hash1,
-            SIZEOFARRAY(hash1));
+    // Perform HMAC algorithm: ( https://tools.ietf.org/html/rfc2104 )
+    //      `H(K XOR opad, H(K XOR ipad, data))`
+    H(k_ipad, SIZEOFARRAY(k_ipad),
+        data, datalen,
+        ihash, SIZEOFARRAY(ihash)
+    );
+    H(k_opad, SIZEOFARRAY(k_opad),
+        ihash, SIZEOFARRAY(ihash),
+        ohash, SIZEOFARRAY(ohash)
+    );
 
-    // Copy the resulting hash the output buffer
-    // Trunacate sha256 hash if needed
-    unsigned sz = (SHA256_HASH_SIZE <= outlen) ? SHA256_HASH_SIZE : outlen;
-    memcpy(out, hash1, sz);
+    sz = (outlen > SHA256_HASH_SIZE) ? SHA256_HASH_SIZE : outlen;
+    memcpy(out, ohash, sz);
+    return sz;
 }
 
-void concat_and_hash(const void *dest, const unsigned destlen, const void *src,
-        const unsigned srclen, void *out, const unsigned outlen) {
-    uint8_t buf[destlen + srclen];
-    uint8_t hash[SHA256_HASH_SIZE];
+static void* H(const void *x, const size_t xlen,
+        const void *y, const size_t ylen,
+        void *out, const size_t outlen
+) {
+    const size_t buflen = xlen + ylen;
+    uint8_t buf[buflen];
 
-    memcpy(buf, dest, destlen);
-    memcpy(buf + destlen, src, srclen);
-
-    // Hash 'buf' and store into into another buffer
-    sha256(buf, SIZEOFARRAY(buf), hash);
-
-    // Copy the resulting hash to the output buffer
-    // Truncate hash if needed
-    unsigned sz = (SHA256_HASH_SIZE <= outlen) ? SHA256_HASH_SIZE : outlen;
-    memcpy(out, hash, SHA256_HASH_SIZE);
+    memcpy(buf, x, xlen);
+    memcpy(buf + xlen, y, ylen);
+    return sha256(buf, buflen * sizeof(uint8_t), out, outlen);
 }
 
-void sha256(const void *data, const unsigned datalen, void *out) {
+static void* sha256(const void *data, const size_t datalen,
+    void *out, const size_t outlen
+) {
+    size_t sz;
     Sha256Context ctx;
     SHA256_HASH hash;
 
@@ -86,5 +92,7 @@ void sha256(const void *data, const unsigned datalen, void *out) {
     Sha256Update(&ctx, data, datalen);
     Sha256Finalise(&ctx, &hash);
 
-    memcpy(out, hash.bytes, SHA256_HASH_SIZE);
+    sz = (outlen > SHA256_HASH_SIZE) ? SHA256_HASH_SIZE : outlen;
+    return memcpy(out, hash.bytes, sz);
 }
+
diff --git a/hmac_sha256.h b/hmac_sha256.h
index 1c1e5f3..df6add8 100644
--- a/hmac_sha256.h
+++ b/hmac_sha256.h
@@ -10,15 +10,21 @@
 extern "C" {
 #endif // __cplusplus
 
-#include <stdint.h>
+#include <stddef.h>
 
-void hmac_sha256(
-    // [in]: The key and it's length. Should be at least 32 bytes long for optimal security.
-    const void* key, const unsigned keylen,
-     // [in]: The data to hash along with the key.
-    const void* data, const unsigned datalen,
-    // [out]: The output hash. Should be 32 bytes long, but if it's less than 32 bytes, the function will truncate the resulting hash.
-    void* out, const unsigned outlen
+size_t // Returns the number of bytes written to `out`
+hmac_sha256(
+    // [in]: The key and its length.
+    //      Should be at least 32 bytes long for optimal security.
+    const void* key, const size_t keylen,
+
+    // [in]: The data to hash alongside the key.
+    const void* data, const size_t datalen,
+
+    // [out]: The output hash.
+    //      Should be 32 bytes long. If it's less than 32 bytes,
+    //      the resulting hash will be truncated to the specified length.
+    void* out, const size_t outlen
 );
 
 #ifdef __cplusplus