SLAE 7: Creating your own crypter using golang

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

package main

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/rand"
    "crypto/sha256"
    "encoding/hex"
    "flag"
    "fmt"
    "io"
    "os"
    "os/user"
    "path/filepath"
)

type cliOptions struct {
    encrypt   bool
    decrypt   bool
    shellcode string
}

func processArgs() cliOptions {
    opts := cliOptions{}
    flag.BoolVar(&opts.decrypt, "decrypt", false, "Decrypt your shellcode")
    flag.BoolVar(&opts.decrypt, "d", false, "Decrypt your shellcode")
    flag.BoolVar(&opts.encrypt, "encrypt", false, "Encrypt your shellcode")
    flag.BoolVar(&opts.encrypt, "e", false, "Encrypt your shellcode")
    flag.StringVar(&opts.shellcode, "shellcode", "", "Your shellcode")
    flag.StringVar(&opts.shellcode, "s", "", "Your shellcode")
    flag.Parse()

    return opts
}

func init() {
    flag.Usage = func() {
        h := "\nEncrypt your shellcode! Very nice! Made by @dubs3c.\n\n"

        h += "Usage:\n"
        h += "  cyrptoBoot [shellcode] [options]\n\n"

        h += "Options:\n"
        h += "  -e,  --encrypt      Encrypt shellcode\n"
        h += "  -d,  --decrypt      Decrypt shellcode\n"
        h += "  -s,  --shellcode    Shellcode\n"
        h += "  -v,  --version      Show version\n"

        h += "\nExamples:\n"
        h += "  cryptoBoot -e -s \"<shellcode>\"\n"
        h += "  cryptoBoot -d -s \"<shellcode>\"\n"

        fmt.Fprintf(os.Stderr, h)
    }
}

func encrypt(shellcode string, envKey []byte) (encryptedShellcode []byte, err error) {
    plaintext := []byte(shellcode)

    block, err := aes.NewCipher(envKey)
    if err != nil {
        panic(err.Error())
    }

    // Never use more than 2^32 random nonces with a given key because of the risk of a repeat.
    nonce := make([]byte, 12)
    if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
        panic(err.Error())
    }

    aesgcm, err := cipher.NewGCM(block)
    if err != nil {
        panic(err.Error())
    }

    ciphertext := aesgcm.Seal(nil, nonce, plaintext, nil)
    ciphertextWithNonce := append(nonce, ciphertext...)
    encodedCiphertext := make([]byte, hex.EncodedLen(len(ciphertextWithNonce)))
    hex.Encode(encodedCiphertext, ciphertextWithNonce)
    return encodedCiphertext, nil
}

func decrypt(ciphertext string, envKey []byte) (plaintext []byte, err error) {
    ciphertxt, _ := hex.DecodeString(ciphertext[24:])
    nonce, _ := hex.DecodeString(ciphertext[:24])

    block, err := aes.NewCipher(envKey)
    if err != nil {
        return []byte{}, err
    }

    aesgcm, err := cipher.NewGCM(block)
    if err != nil {
        return []byte{}, err
    }

    plaintxt, err := aesgcm.Open(nil, nonce, ciphertxt, nil)
    if err != nil {
        return []byte{}, err
    }

    return plaintxt, nil
}

func main() {

    if len(os.Args) <= 1 {
        flag.Usage()
        os.Exit(0)
    }

    opts := processArgs()

    currentUser := &user.User{}
    currentUser, err := user.Current()
    envPath := "/etc/vmware-tools/scripts/vmware/network"
    envKey := sha256.Sum256([]byte(envPath + currentUser.Username))

    if opts.encrypt {
        if opts.shellcode == "" {
            fmt.Println("[-] Please specify your shellcode")
            os.Exit(1)
        }

        ciphertext, err := encrypt(opts.shellcode, envKey[:])
        if err != nil {
            fmt.Println("[-] Something went wrong encrypting your shellcode. Error: ", err)
            os.Exit(1)
        }

        fmt.Printf("[+] Your encrypted shellcode: %s\n", ciphertext)
        return
    }

    if opts.decrypt {

        if opts.shellcode == "" {
            fmt.Println("[-] Please specify your encrypted shellcode")
            os.Exit(1)
        }

        encryptedShellcode := opts.shellcode

        err = filepath.Walk("/etc/", func(path string, info os.FileInfo, err error) error {
            envKey := sha256.Sum256([]byte(path + currentUser.Username))
            plaintext, err := decrypt(encryptedShellcode, envKey[:])
            if err != nil {
                // Disregard errors
                return nil
            } else {
                fmt.Printf("[+] Decrypted shellcode: %s\n", plaintext)
            }
            return nil
        })

        if err != nil {
            fmt.Printf("error walking the path %q: %v\n", "/etc/", err)
            return
        }
    }
}