error aesGCMOpen got cipher: message authentication failed: golang decrypt gcm

Question

I'm trying to rewrite rails activesupport decryption to golang, but got this error

err aesGCMOpen got cipher: message authentication failed

Here is the rails code from active support decryption

class Crypton
  SECRET_KEY_BASE = ENV["SECRET_KEY_BASE"]
  class << self
    def encrypt text
        raise 'Encypt failed, secret_key_base not found' unless SECRET_KEY_BASE.present?
        text = text.to_s unless text.is_a? String
  
        len   = ActiveSupport::MessageEncryptor.key_len
        salt  = SecureRandom.hex len
        key   = ActiveSupport::KeyGenerator.new(SECRET_KEY_BASE).generate_key salt, len
        crypt = ActiveSupport::MessageEncryptor.new key
        encrypted_data = crypt.encrypt_and_sign text
        "#{salt}$$#{encrypted_data}"
    end
    def decrypt text
      raise 'Decrypt failed, secret_key_base not found' unless SECRET_KEY_BASE.present?
      salt, data = text.split "$$"

      len   = ActiveSupport::MessageEncryptor.key_len
      key   = ActiveSupport::KeyGenerator.new(SECRET_KEY_BASE).generate_key salt, len
      crypt = ActiveSupport::MessageEncryptor.new key
      crypt.decrypt_and_verify data
    end
  end
end

and here is my golang code, I tried to rewrite from rails decryption activesupport

// DecryptGCM
// reference on Rails 5.2-stable:
// https://github.com/rails/rails/blob/5-2-stable/activesupport/lib/active_support/message_encryptor.rb#L183
func DecryptGCM(encryptedText string, secretKeyBase string) (string, error) {
    encryptText := strings.Split(encryptedText, "$$")
    saltHex := encryptText[0]
    encodedText := encryptText[1]

    splitEncodedText := strings.Split(encodedText, "--")
    encodedText = splitEncodedText[0]
    ivText := splitEncodedText[1]
    authTagText := splitEncodedText[2]

    decodeText, err := base64.StdEncoding.DecodeString(encodedText)
    if err != nil {
        return "", fmt.Errorf(`err b64 decode text got %v`, err)
    }

    ivDecodeText, err := base64.StdEncoding.DecodeString(ivText)
    if err != nil {
        return "", fmt.Errorf(`err b64 iv got %v`, err)
    }

    authTagTextDecoded, err := base64.StdEncoding.DecodeString(authTagText)
    if err != nil {
        return "", fmt.Errorf(`err b64 auth tag got %v`, err)
    }

    key := GenerateKey(secretKeyBase, saltHex)

    block, err := aes.NewCipher(key)
    if err != nil {
        return "", fmt.Errorf(`err aesNewCipher got %v`, err)
    }

    aesGCM, err := cipher.NewGCM(block)
    if err != nil {
        return "", fmt.Errorf(`err chipperNewGCM got %v`, err)
    }

    plaintext, err := aesGCM.Open(nil, ivDecodeText, decodeText, authTagTextDecoded)
    if err != nil {
        return "", fmt.Errorf(`err aesGCMOpen got %v`, err)
    }

    return string(plaintext), nil
}

func GenerateKey(secretKeyBase string, saltHex string) []byte {
    key := pbkdf2.Key([]byte(secretKeyBase), []byte(saltHex), 65536, 32, sha1.New)
    return key
}

func TestMain(t *testing.T) {
    encryptText := "7c7eb6202943398b0d0619d008d226372f1b3b341eb39500eab71c3b67b7f641$$hDJ5e+6QkoCjk4cqT+hAY9c7Jj7Hxg==--t9hrqWnzQeeJTffr--1bHoguSmIkYQrpI1cd/KRQ=="


    decrypted, err := DecryptGCM(encryptText, SECRET_KEY)
    if err != nil {
        fmt.Println("Error:", err)
    } else {
        fmt.Println("Decrypted:", decrypted)
    }
}

Here is the key:

SECRET_KEY_BASE="3ae9b0ce19316f877554a0427044180e27267fb9798db9147feeb318865b3a52f79824201608f6e4e10dc8e3f29e5bf4b83e46c4103ff8d98b99903d054d720i"

I generated the encrypted data from this rails command

Crypton.encrypt("hello, world")

If you found any solution please leave a comment,

thank you

Can decrypt encrypted data from rails encryption

Answer 1

In the aesGCM.Open() call of the Go code, ciphertext and authentication tag are passed in the third and fourth parameters respectively. This is wrong. Both data must be concatenated and passed in the third parameter. The fourth parameter needs to be nil. This parameter is for the additional data that is not used here.
In addition, the deserialization of the decrypted data is missing in the Go code. For this, an appropriate library must be applied, e.g. ruby-marshal.

Overall, the following changes are required:

import (
    "bytes"
    rbmarshal "github.com/dozen/ruby-marshal"
    ...
)

...

func DecryptGCM(encryptedText string, secretKeyBase string) (string, error) {
    
    ...
    
    plaintext, err := aesGCM.Open(nil, ivDecodeText, append(decodeText, authTagTextDecoded...), nil) // Fix 1
    if err != nil {
        return "", fmt.Errorf(`err aesGCMOpen got %v`, err)
    }

    var v string
    rbmarshal.NewDecoder(bytes.NewReader(plaintext)).Decode(&v) // Fix 2
    return string(v), nil
}

With these changes, the data encrypted with the Rails code can be successfully decrypted with the Go code.