diff --git a/library.properties b/library.properties
index be924c7..23703d3 100644
--- a/library.properties
+++ b/library.properties
@@ -1,5 +1,5 @@
 name=TOTP library
-version=1.0.0
+version=1.0.1
 author=Luca Dentella <luca@dentella.it>
 maintainer=Luca Dentella <luca@dentella.it>
 sentence=Library to generate Time-based One-Time Passwords
diff --git a/src/sha1.cpp b/src/sha1.cpp
new file mode 100644
index 0000000..15b5ac3
--- /dev/null
+++ b/src/sha1.cpp
@@ -0,0 +1,152 @@
+#include <string.h>
+#include <avr/io.h>
+#include <avr/pgmspace.h>
+#include "sha1.h"
+
+#define SHA1_K0 0x5a827999
+#define SHA1_K20 0x6ed9eba1
+#define SHA1_K40 0x8f1bbcdc
+#define SHA1_K60 0xca62c1d6
+
+const uint8_t sha1InitState[] PROGMEM = {
+  0x01,0x23,0x45,0x67, // H0
+  0x89,0xab,0xcd,0xef, // H1
+  0xfe,0xdc,0xba,0x98, // H2
+  0x76,0x54,0x32,0x10, // H3
+  0xf0,0xe1,0xd2,0xc3  // H4
+};
+
+void Sha1Class::init(void) {
+  memcpy_P(state.b,sha1InitState,HASH_LENGTH);
+  byteCount = 0;
+  bufferOffset = 0;
+}
+
+uint32_t Sha1Class::rol32(uint32_t number, uint8_t bits) {
+  return ((number << bits) | (number >> (32-bits)));
+}
+
+void Sha1Class::hashBlock() {
+  uint8_t i;
+  uint32_t a,b,c,d,e,t;
+
+  a=state.w[0];
+  b=state.w[1];
+  c=state.w[2];
+  d=state.w[3];
+  e=state.w[4];
+  for (i=0; i<80; i++) {
+    if (i>=16) {
+      t = buffer.w[(i+13)&15] ^ buffer.w[(i+8)&15] ^ buffer.w[(i+2)&15] ^ buffer.w[i&15];
+      buffer.w[i&15] = rol32(t,1);
+    }
+    if (i<20) {
+      t = (d ^ (b & (c ^ d))) + SHA1_K0;
+    } else if (i<40) {
+      t = (b ^ c ^ d) + SHA1_K20;
+    } else if (i<60) {
+      t = ((b & c) | (d & (b | c))) + SHA1_K40;
+    } else {
+      t = (b ^ c ^ d) + SHA1_K60;
+    }
+    t+=rol32(a,5) + e + buffer.w[i&15];
+    e=d;
+    d=c;
+    c=rol32(b,30);
+    b=a;
+    a=t;
+  }
+  state.w[0] += a;
+  state.w[1] += b;
+  state.w[2] += c;
+  state.w[3] += d;
+  state.w[4] += e;
+}
+
+void Sha1Class::addUncounted(uint8_t data) {
+  buffer.b[bufferOffset ^ 3] = data;
+  bufferOffset++;
+  if (bufferOffset == BLOCK_LENGTH) {
+    hashBlock();
+    bufferOffset = 0;
+  }
+}
+
+__WRITE_RESULT Sha1Class::write(uint8_t data) {
+  ++byteCount;
+  addUncounted(data);
+
+  __WRITE_RETURN(1);
+}
+
+void Sha1Class::pad() {
+  // Implement SHA-1 padding (fips180-2 §5.1.1)
+
+  // Pad with 0x80 followed by 0x00 until the end of the block
+  addUncounted(0x80);
+  while (bufferOffset != 56) addUncounted(0x00);
+
+  // Append length in the last 8 bytes
+  addUncounted(0); // We're only using 32 bit lengths
+  addUncounted(0); // But SHA-1 supports 64 bit lengths
+  addUncounted(0); // So zero pad the top bits
+  addUncounted(byteCount >> 29); // Shifting to multiply by 8
+  addUncounted(byteCount >> 21); // as SHA-1 supports bitstreams as well as
+  addUncounted(byteCount >> 13); // byte.
+  addUncounted(byteCount >> 5);
+  addUncounted(byteCount << 3);
+}
+
+
+uint8_t* Sha1Class::result(void) {
+  // Pad to complete the last block
+  pad();
+  
+  // Swap byte order back
+  for (int i=0; i<5; i++) {
+    uint32_t a,b;
+    a=state.w[i];
+    b=a<<24;
+    b|=(a<<8) & 0x00ff0000;
+    b|=(a>>8) & 0x0000ff00;
+    b|=a>>24;
+    state.w[i]=b;
+  }
+  
+  // Return pointer to hash (20 characters)
+  return state.b;
+}
+
+#define HMAC_IPAD 0x36
+#define HMAC_OPAD 0x5c
+
+void Sha1Class::initHmac(const uint8_t* key, int keyLength) {
+  uint8_t i;
+  memset(keyBuffer,0,BLOCK_LENGTH);
+  if (keyLength > BLOCK_LENGTH) {
+    // Hash long keys
+    init();
+    for (;keyLength--;) write(*key++);
+    memcpy(keyBuffer,result(),HASH_LENGTH);
+  } else {
+    // Block length keys are used as is
+    memcpy(keyBuffer,key,keyLength);
+  }
+  // Start inner hash
+  init();
+  for (i=0; i<BLOCK_LENGTH; i++) {
+    write(keyBuffer[i] ^ HMAC_IPAD);
+  }
+}
+
+uint8_t* Sha1Class::resultHmac(void) {
+  uint8_t i;
+  // Complete inner hash
+  memcpy(innerHash,result(),HASH_LENGTH);
+  // Calculate outer hash
+  init();
+  for (i=0; i<BLOCK_LENGTH; i++) write(keyBuffer[i] ^ HMAC_OPAD);
+  for (i=0; i<HASH_LENGTH; i++) write(innerHash[i]);
+  return result();
+}
+Sha1Class Sha1;
diff --git a/src/sha1.h b/src/sha1.h
new file mode 100644
index 0000000..79c94dc
--- /dev/null
+++ b/src/sha1.h
@@ -0,0 +1,51 @@
+#ifndef Sha1_h
+#define Sha1_h
+
+#include <inttypes.h>
+#include "Print.h"
+
+#if ARDUINO < 100
+#define __WRITE_RESULT void
+#define __WRITE_RETURN(x) return;
+#else
+#define __WRITE_RESULT size_t
+#define __WRITE_RETURN(x) return x;
+#endif
+
+#define HASH_LENGTH 20
+#define BLOCK_LENGTH 64
+
+union _buffer {
+  uint8_t b[BLOCK_LENGTH];
+  uint32_t w[BLOCK_LENGTH/4];
+};
+union _state {
+  uint8_t b[HASH_LENGTH];
+  uint32_t w[HASH_LENGTH/4];
+};
+
+class Sha1Class : public Print
+{
+  public:
+    void init(void);
+    void initHmac(const uint8_t* secret, int secretLength);
+    uint8_t* result(void);
+    uint8_t* resultHmac(void);
+    virtual __WRITE_RESULT write(uint8_t);
+    using Print::write;
+  private:
+    void pad();
+    void addUncounted(uint8_t data);
+    void hashBlock();
+    uint32_t rol32(uint32_t number, uint8_t bits);
+    _buffer buffer;
+    uint8_t bufferOffset;
+    _state state;
+    uint32_t byteCount;
+    uint8_t keyBuffer[BLOCK_LENGTH];
+    uint8_t innerHash[HASH_LENGTH];
+    
+};
+extern Sha1Class Sha1;
+
+#endif