diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 83501c9..8679ed0 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -10,13 +10,13 @@ permissions:
   contents: read
 
 jobs:
-  build:
+  test:
     runs-on: ${{ matrix.os }}
 
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        ghc: ["9.8", "9.6"]
+        ghc:  ['9.12', '9.10', '9.8', '9.6', '9.4', '9.2', '9.0']
 
     steps:
       - name: Check out repository
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3539f13..6f565e4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,4 +8,5 @@ and this project adheres to the
 
 ## Unreleased
 
-## 0.1.0.0 - YYYY-MM-DD
+## 0.1.0.0 - 2025-01-15
+- First release of `clockwork-base32` library.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..af63bff
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,13 @@
+## Contributing as a Developer
+
+- When creating a bug report: Please follow the template and provide detailed information.
+- When fixing a feature: Create a Pull Request (PR) with accompanying test code.
+- When adding a feature: First, propose the feature in an Issue.
+
+## Contributing Outside of Coding
+
+The following actions help boost my motivation:
+
+- Giving a GitHub Star
+- Promoting the application
+- Becoming a GitHub Sponsor
diff --git a/README.md b/README.md
index ff8727b..91850bd 100644
--- a/README.md
+++ b/README.md
@@ -1,11 +1,33 @@
+[![Test](https://github.com/nao1215/clockwork-base32/actions/workflows/test.yml/badge.svg)](https://github.com/nao1215/clockwork-base32/actions/workflows/test.yml)
+
 # clockwork-base32
 
 Clockwork Base32 is a simple variant of Base32 inspired by Crockford's Base32. See [Clockwork Base32 Specification](https://gist.github.com/szktty/228f85794e4187882a77734c89c384a8).
 
+## Supported Haskell Versions & OS
+- GHC 9.0 or later
+- Linux, macOS, Windows
+
 ## Usage
 
 ```haskell
--- TODO: Write usage examples here
+module Main where
+
+import ClockworkBase32 (encode, decode)
+
+main :: IO ()
+main = do
+    -- encode example
+    let originalString = "foobar"
+    putStrLn $ "Original string: " ++ originalString
+    let encodedString = encode originalString
+    putStrLn $ "Encoded (Base32): " ++ encodedString
+
+    -- decode example
+    let decodedResult = decode encodedString
+    case decodedResult of
+        Right decodedString -> putStrLn $ "Decoded string: " ++ decodedString
+        Left err            -> putStrLn $ "Error decoding: " ++ err
 ```
 
 ## Other Implementations
@@ -27,3 +49,20 @@ Clockwork Base32 is a simple variant of Base32 inspired by Crockford's Base32. S
 - Rust: [hnakamur/rs-clockwork-base32](https://github.com/hnakamur/rs-clockwork-base32)
 - Go: [shogo82148/go-clockwork-base32](https://github.com/shogo82148/go-clockwork-base32)
 - TypeScript: [niyari/base32-ts](https://github.com/niyari/base32-ts)
+
+
+## Contributing
+
+First off, thanks for taking the time to contribute! ❤️  See [CONTRIBUTING.md](./CONTRIBUTING.md) for more information.
+Contributions are not only related to development. For example, GitHub Star motivates me to develop!
+
+## Contact
+
+If you would like to send comments such as "find a bug" or "request for additional features" to the developer, please use one of the following contacts.
+
+- [GitHub Issue](https://github.com/nao1215/clockwork-base32/issues)
+
+
+## LICENSE
+
+[MIT License](./LICENSE)
diff --git a/clockwork-base32.cabal b/clockwork-base32.cabal
index c86ca48..949a3d6 100644
--- a/clockwork-base32.cabal
+++ b/clockwork-base32.cabal
@@ -10,7 +10,7 @@ license-file:        LICENSE
 author:              Naohiro CHIKAMATSU
 maintainer:          n.chika156@gmail.com
 copyright:           2025 Naohiro CHIKAMATSU
-category:            Web
+category:            Base32
 build-type:          Simple
 extra-source-files:  README.md
                      CHANGELOG.md
diff --git a/src/ClockworkBase32.hs b/src/ClockworkBase32.hs
index a20bb22..ff459c3 100644
--- a/src/ClockworkBase32.hs
+++ b/src/ClockworkBase32.hs
@@ -1,8 +1,88 @@
 module ClockworkBase32
-  ( maxInList
+  ( encode,
+    decode
   ) where
 
--- returns the maximum element in a list, if it exists
-maxInList :: (Ord a) => [a] -> Maybe a
-maxInList [] = Nothing
-maxInList xs = Just (maximum xs)
+import           Data.Bits             (shiftL, shiftR, (.&.), (.|.))
+import qualified Data.ByteString       as BS
+import qualified Data.ByteString.Char8 as BSC
+import           Data.Word             (Word8)
+
+-- clockwork base32 symbols
+base32Symbols :: String
+base32Symbols = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"
+
+-- | Encode a string into Clockwork Base32 format.
+-- This function encodes a given input string into the Clockwork Base32 encoding
+-- by processing it in chunks of 5 bytes. The input is converted into a sequence
+-- of Base32 symbols using the Clockwork Base32 alphabet.
+--
+-- For example:
+-- encode "foobar" => "CSQPYRK1E8"
+encode :: String -> String
+encode input = processChunks inputBytes
+  where
+    inputBytes = BS.unpack $ BSC.pack input
+    processChunks [] = ""
+    processChunks bytes = encodeChunk (take 5 bytes) ++ processChunks (drop 5 bytes)
+
+-- | Encode a chunk of 5 bytes into Base32 symbols.
+-- This function takes a chunk of 1 to 5 bytes and converts it into the corresponding
+-- Base32 symbols. If the chunk is smaller than 5 bytes, the number of output symbols
+-- will be adjusted accordingly.
+--
+-- For example:
+-- encodeChunk [102] => "CR"
+encodeChunk :: [Word8] -> String
+encodeChunk chunk = take outputLength $ map (base32Symbols !!) indices
+  where
+    -- 5 bytes -> 40 bits
+    val :: Integer
+    val = foldl (\acc (b, i) -> acc .|. (fromIntegral b `shiftL` (32 - 8 * i))) 0 (zip chunk [0..])
+    -- split 40 bits into 8 5-bit indices
+    indices = [fromIntegral ((val `shiftR` (35 - 5 * i)) .&. 0x1F) | i <- [0..7]]
+    -- determine the number of symbols to output based on the input size
+    outputLength = case length chunk of
+      1 -> 2
+      2 -> 4
+      3 -> 5
+      4 -> 7
+      5 -> 8
+      _ -> 0
+
+-- | Decode a Clockwork Base32 encoded string into the original string.
+decode :: String -> Either String String
+decode input = case mapM decodeChar input of
+  Left err      -> Left err
+  Right symbols -> processChunks symbols []
+  where
+    -- convert a Base32 symbol into a value
+    decodeChar :: Char -> Either String Word8
+    decodeChar c = case lookup c decodeMap of
+      Just val -> Right val
+      Nothing  -> Left ("Invalid character: " ++ [c])
+
+    -- process the input symbols in chunks of 8 characters
+    processChunks :: [Word8] -> [Word8] -> Either String String
+    processChunks [] acc = Right (BSC.unpack (BS.pack (reverse acc)))
+    processChunks symbols acc = case decodeChunk (take 8 symbols) of
+      []    -> processChunks (drop 8 symbols) acc
+      chunk -> processChunks (drop 8 symbols) (reverse chunk ++ acc)
+
+    -- decode a chunk of 8 Base32 symbols into 5 bytes
+    decodeChunk :: [Word8] -> [Word8]
+    decodeChunk chunk = [fromIntegral byte | byte <- [v1, v2, v3, v4, v5], byte /= 0]
+      where
+        -- 8 symbols -> 40 bits
+        val :: Integer
+        val = foldl (\acc (b, i) -> acc .|. (fromIntegral b `shiftL` (35 - 5 * i))) 0 (zip chunk [0..])
+        -- split 40 bits into 5 8-bit values
+        v1 = (val `shiftR` 32) .&. 0xFF
+        v2 = (val `shiftR` 24) .&. 0xFF
+        v3 = (val `shiftR` 16) .&. 0xFF
+        v4 = (val `shiftR` 8) .&. 0xFF
+        v5 = val .&. 0xFF
+
+    -- create a map for decoding Base32 symbols
+    decodeMap :: [(Char, Word8)]
+    decodeMap = zip base32Symbols [0..]
diff --git a/test/Spec.hs b/test/Spec.hs
index b3a0d81..c5a11ca 100644
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -1,14 +1,80 @@
 module Main (main) where
 
-import           ClockworkBase32        (maxInList)
+import           ClockworkBase32 (decode, encode)
 import           Test.Hspec
 
-
 main :: IO ()
 main = hspec $ do
-  describe "maxInList" $ do
-    it "returns Nothing for an empty list" $ do
-      maxInList ([] :: [Int]) `shouldBe` Nothing
+  -- testcase from
+  -- https://github.com/szktty/go-clockwork-base32/blob/c2cac4daa7ad2045089b943b377b12ac57e3254e/base32_test.go#L36-L44
+  -- https://github.com/shiguredo/erlang-base32/blob/0cc88a702ce1d8ca345e516a05a9a85f7f23a718/test/base32_clockwork_test.erl#L7-L18
+  describe "encode" $ do
+    it "returns '' when input value is ''" $ do
+      encode "" `shouldBe` ""
+
+    it "returns 'CR' when input value is 'f'" $ do
+      encode "f" `shouldBe` "CR"
+
+    it "returns 'CSQG' when input value is 'fo'" $ do
+      encode "fo" `shouldBe` "CSQG"
+
+    it "returns 'CSQPY' when input value is 'foo'" $ do
+      encode "foo" `shouldBe` "CSQPY"
+
+    it "returns 'CSQPYRG' when input value is 'foob'" $ do
+      encode "foob" `shouldBe` "CSQPYRG"
+
+    it "returns 'CSQPYRK1' when input value is 'fooba'" $ do
+      encode "fooba" `shouldBe` "CSQPYRK1"
+
+    it "returns 'CSQPYRK1E8' when input value is 'foobar'" $ do
+      encode "foobar" `shouldBe` "CSQPYRK1E8"
+
+    it "returns '91JPRV3F5GG7EVVJDHJ22' when input value is 'Hello, world!'" $ do
+      encode "Hello, world!" `shouldBe` "91JPRV3F5GG7EVVJDHJ22"
+
+    it "returns 'AHM6A83HENMP6TS0C9S6YXVE41K6YY10D9TPTW3K41QQCSBJ41T6GS90DHGQMY90CHQPEBG' when input value is 'The quick brown fox jumps over the lazy dog.'" $ do
+      encode "The quick brown fox jumps over the lazy dog." `shouldBe` "AHM6A83HENMP6TS0C9S6YXVE41K6YY10D9TPTW3K41QQCSBJ41T6GS90DHGQMY90CHQPEBG"
+
+    it "returns '07EKWRQY2N7DEAVD5MJ3JX36KM' when input value is '\x01\xdd\x3e\x62\xfe\x15\x4e\xd7\x2b\x6d\x2d\x24\x39\x74\x66\x9d'" $ do
+      encode "\x01\xdd\x3e\x62\xfe\x15\x4e\xd7\x2b\x6d\x2d\x24\x39\x74\x66\x9d" `shouldBe` "07EKWRQY2N7DEAVD5MJ3JX36KM"
+
+    it "returns 'AXQQEB10D5T20WK5C5P6RY90EXQQ4TVK44' when input value is 'Wow, it really works!'" $ do
+      encode "Wow, it really works!" `shouldBe` "AXQQEB10D5T20WK5C5P6RY90EXQQ4TVK44"
+
+  describe "decode" $ do
+    it "returns Right '' when input value is ''" $ do
+      decode "" `shouldBe` Right ""
+
+    it "returns Right 'f' when input value is 'CR'" $ do
+      decode "CR" `shouldBe` Right "f"
+
+    it "returns Right 'fo' when input value is 'CSQG'" $ do
+      decode "CSQG" `shouldBe` Right "fo"
+
+    it "returns Right 'foo' when input value is 'CSQPY'" $ do
+      decode "CSQPY" `shouldBe` Right "foo"
+
+    it "returns Right 'foob' when input value is 'CSQPYRG'" $ do
+      decode "CSQPYRG" `shouldBe` Right "foob"
+
+    it "returns Right 'fooba' when input value is 'CSQPYRK1'" $ do
+      decode "CSQPYRK1" `shouldBe` Right "fooba"
+
+    it "returns Right 'foobar' when input value is 'CSQPYRK1E8'" $ do
+      decode "CSQPYRK1E8" `shouldBe` Right "foobar"
+
+    it "returns Right 'Hello, world!' when input value is '91JPRV3F5GG7EVVJDHJ22'" $ do
+      decode "91JPRV3F5GG7EVVJDHJ22" `shouldBe` Right "Hello, world!"
+
+    it "returns Right 'The quick brown fox jumps over the lazy dog.' when input value is 'AHM6A83HENMP6TS0C9S6YXVE41K6YY10D9TPTW3K41QQCSBJ41T6GS90DHGQMY90CHQPEBG'" $ do
+      decode "AHM6A83HENMP6TS0C9S6YXVE41K6YY10D9TPTW3K41QQCSBJ41T6GS90DHGQMY90CHQPEBG" `shouldBe` Right "The quick brown fox jumps over the lazy dog."
+
+    it "returns Right '\x01\xdd\x3e\x62\xfe\x15\x4e\xd7\x2b\x6d\x2d\x24\x39\x74\x66\x9d' when input value is '07EKWRQY2N7DEAVD5MJ3JX36KM'" $ do
+      decode "07EKWRQY2N7DEAVD5MJ3JX36KM" `shouldBe` Right "\x01\xdd\x3e\x62\xfe\x15\x4e\xd7\x2b\x6d\x2d\x24\x39\x74\x66\x9d"
+
+    it "returns Right 'Wow, it really works!' when input value is 'AXQQEB10D5T20WK5C5P6RY90EXQQ4TVK44'" $ do
+      decode "AXQQEB10D5T20WK5C5P6RY90EXQQ4TVK44" `shouldBe` Right "Wow, it really works!"
 
-    it "returns the maximum element for a non-empty list" $ do
-      maxInList ([1, 2, 3] :: [Int]) `shouldBe` Just 3
+    it "returns Left 'Invalid character: ~' when input value is '~'" $ do
+      decode "~" `shouldBe` Left "Invalid character: ~"