Encrypting a string with AES and Base64

This is based on the NewCFBEncrypter / NewCFBDecrypter examples and seems to do what you require:

EDIT: Based on Kluyg’s comment regarding IV creation I’ve modified the example code to use the recommended method of creating the IV from the ciphertext same method as the linked example to create the IV from the ciphertext. (In production code the IV should be generated seperately each time. Thanks to RoundSparrow hilltx for pointing this out.)

I think the problem you’re encountering is due to an invalid key length, but I’m not 100% sure.

package main

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/rand"
    "encoding/base64"
    "errors"
    "fmt"
    "io"
    "log"
)

func main() {
    key := []byte("a very very very very secret key") // 32 bytes
    plaintext := []byte("some really really really long plaintext")
    fmt.Printf("%s\n", plaintext)
    ciphertext, err := encrypt(key, plaintext)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%0x\n", ciphertext)
    result, err := decrypt(key, ciphertext)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%s\n", result)
}

// See alternate IV creation from ciphertext below
//var iv = []byte{35, 46, 57, 24, 85, 35, 24, 74, 87, 35, 88, 98, 66, 32, 14, 05}

func encrypt(key, text []byte) ([]byte, error) {
    block, err := aes.NewCipher(key)
    if err != nil {
        return nil, err
    }
    b := base64.StdEncoding.EncodeToString(text)
    ciphertext := make([]byte, aes.BlockSize+len(b))
    iv := ciphertext[:aes.BlockSize]
    if _, err := io.ReadFull(rand.Reader, iv); err != nil {
        return nil, err
    }
    cfb := cipher.NewCFBEncrypter(block, iv)
    cfb.XORKeyStream(ciphertext[aes.BlockSize:], []byte(b))
    return ciphertext, nil
}

func decrypt(key, text []byte) ([]byte, error) {
    block, err := aes.NewCipher(key)
    if err != nil {
        return nil, err
    }
    if len(text) < aes.BlockSize {
        return nil, errors.New("ciphertext too short")
    }
    iv := text[:aes.BlockSize]
    text = text[aes.BlockSize:]
    cfb := cipher.NewCFBDecrypter(block, iv)
    cfb.XORKeyStream(text, text)
    data, err := base64.StdEncoding.DecodeString(string(text))
    if err != nil {
        return nil, err
    }
    return data, nil
}

Produces:

some really really really long plaintext
54618bd6bb10612a7b590c53192df214501e01b685540b012581a0ed9ff3ddaa1f4177cc6186b501fb8cce0c2eb764daff475aab724d4d33e614d7d89cf556d8512fd920018c090f
some really really really long plaintext

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