aes-256-ctr

README

aes-256-ctr example

AES-256 CTR (Counter) mode, which has similar characteristics to OFB, but also allows a random access property during decryption.

I have the following AES-256 mode examples,

• aes-256 No Mode
• aes-256-cbc Cipher Block Chaining
• aes-256-cfb Cipher FeedBack Mode
• aes-256-ctr Counter Mode (You are here)
• aes-256-gcm Galois/Counter Mode (I like this one)
• aes-256-ofb Output FeedBack Mode

GitHub Webpage

RUN

run aes-ctr.go

You output should be,

Original Text:           This is AES-256 CTR (32 Bytes)!!

The 32-byte Key:         myverystrongpasswordo32bitlength
The Nonce:               25ce7cd9388daf451864bb2212b9fbc7

Encrypted Text:          17b697b077d086692632adaae6fc935b245060ed3758fd3d936e4aaf81f31161
Decrypted Text:          This is AES-256 CTR (32 Bytes)!!

HOW IT WORKS

• The Counter (CTR) mode has similar characteristics to OFB
• Allows a random access property during decryption
• Well suited to operate on a multi-processor machine where blocks can be encrypted in parallel
• plaintext should be multiple of AES block size, hence padding would have to be added when encrypting (Not in this example)
• Because of the symmetry of the XOR operation, encryption and decryption are exactly the same.

For simplicity I did not include the nonce in the cipherText.

Encryption,

// GET CIPHER BLOCK USING KEY
block, err := aes.NewCipher(keyByte)
checkErr(err)

// GET CTR
ctr := cipher.NewCTR(block, nonce)

// ENCRYPT DATA
ctr.XORKeyStream(cipherTextByte, plainTextByte)

// RETURN HEX
cipherText := hex.EncodeToString(cipherTextByte)

Decryption,

// GET CIPHER BLOCK USING KEY
block, err := aes.NewCipher(keyByte)
checkErr(err)

// GET CTR
ctr := cipher.NewCTR(block, nonce)

// DECRYPT DATA
ctr.XORKeyStream(plainTextByte, cipherTextByte)

// RETURN STRING
plainText := string(plainTextByte[:])

This illustration may help,