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diff --git a/include/sha256.h b/include/sha256.h
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+// This file is part of a crypto library, exam for the Algorithms class.
+// All of these files are under MIT license.
+//
+// SHA declaration document:
+// https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf
+
+#ifndef HMACSHA256_SHA256_H_
+#define HMACSHA256_SHA256_H_
+
+#include<cstdint>
+#include<array>
+
+namespace hmacsha256 {
+class SHA256 {
+public:
+    // create a new SHA256 thanks an array of bytes
+    SHA256(const uint8_t*, uint32_t);
+
+    SHA256(const std::string&);
+
+    // get digest as array of bytes
+    uint8_t* digest();
+
+    std::string hexdigest();
+private:
+    uint8_t* digest_;
+
+    // work variables: a, b, c, d, e, f, g, h
+    uint32_t vars_[8];
+
+    // data message, in bytes
+    // M,  "message to be hashed"
+    uint8_t message_[64];
+
+    // length L of the original message
+    uint32_t message_l_;
+
+    // lengh of bits
+    uint64_t bit_len_;
+
+    // first 80 costansts 64bit words, paragraph 4.2.2
+    std::array<uint32_t, 64> K = {
+        0x428a2f98,0x71374491,0xb5c0fbcf,0xe9b5dba5,
+        0x3956c25b,0x59f111f1,0x923f82a4,0xab1c5ed5,
+        0xd807aa98,0x12835b01,0x243185be,0x550c7dc3,
+        0x72be5d74,0x80deb1fe,0x9bdc06a7,0xc19bf174,
+        0xe49b69c1,0xefbe4786,0x0fc19dc6,0x240ca1cc,
+        0x2de92c6f,0x4a7484aa,0x5cb0a9dc,0x76f988da,
+        0x983e5152,0xa831c66d,0xb00327c8,0xbf597fc7,
+        0xc6e00bf3,0xd5a79147,0x06ca6351,0x14292967,
+        0x27b70a85,0x2e1b2138,0x4d2c6dfc,0x53380d13,
+        0x650a7354,0x766a0abb,0x81c2c92e,0x92722c85,
+        0xa2bfe8a1,0xa81a664b,0xc24b8b70,0xc76c51a3,
+        0xd192e819,0xd6990624,0xf40e3585,0x106aa070,
+        0x19a4c116,0x1e376c08,0x2748774c,0x34b0bcb5,
+        0x391c0cb3,0x4ed8aa4a,0x5b9cca4f,0x682e6ff3,
+        0x748f82ee,0x78a5636f,0x84c87814,0x8cc70208,
+        0x90befffa,0xa4506ceb,0xbef9a3f7,0xc67178f2
+    };
+
+    // the constructor
+    void init(const uint8_t* data, uint32_t length);
+
+    // Ch(x, y, z) = (x | y) ^ (~x & z)
+    static uint32_t ch(uint32_t, uint32_t, uint32_t);
+
+    // Maj(x, y, z) = (x | y) ^ (x & z) ^ (y & z)
+    static uint32_t maj(uint32_t, uint32_t, uint32_t);
+
+    // circular right shift. Takes two params: `x` a w-bit word and `n` an int
+    // with 0 <= n < w.
+    // w, in our case is 32
+    //
+    // rots is also defined in the NIST document as 
+    // rotr(x) = (x >> n) or (x << w - n)
+    static uint32_t rotr(uint32_t, uint32_t);
+
+    // function defined in 4.4
+    static uint32_t big_sigma0(uint32_t);
+
+    // function defined in 4.5
+    static uint32_t big_sigma1(uint32_t);
+
+    // function defined in 4.6
+    static uint32_t sigma0(uint32_t);
+
+    // function defined in 4.7
+    static uint32_t sigma1(uint32_t);
+
+    // padding
+    void pad();
+
+    // main function
+    void transform();
+
+    // transform bytes in big-endian, because SHA256 wants big endian ordering
+    // Takes 1 parameter, the array of bytes that we want to reorder
+    void bigendian(uint8_t*);
+};
+
+} // namespace hmacsha256
+
+#endif // HMACSHA256_SHA256_H_