Regeneration of a key not possible

[Plaenkler]

🔍 Problem Description:

I am encountering a problem where I cannot recover the QR code after restarting my application. The key.Secret() is the only information that is permanently stored. However, it seems impossible to recover the same key using only the secret.

💡 Relevant Code Snippet:

In the code section below from totp.go (lines 182-192), I initially believed that the key could be recovered only from the secret.

v := url.Values{}
if len(opts.Secret) != 0 {
    v.Set("secret", b32NoPadding.EncodeToString(opts.Secret))
} else {
    secret := make([]byte, opts.SecretSize)
    _, err := opts.Rand.Read(secret)
    if err != nil {
        return nil, err
    }
    v.Set("secret", b32NoPadding.EncodeToString(secret))
}

However, recent tests have contradicted this assumption.

🔬 Test Results:

fmt.Println(keySecret)
key, err := totp.Generate(totp.GenerateOpts{
    Issuer:      issuer,
    AccountName: accountName,
    Secret:      []byte(keySecret),
})
fmt.Println(key.Secret())

keySecret: JRKDOQRSJJBDIVCRLJJEQS22KBGQINGFQMRUGZHDGNKYINBUSRKI
key.Secret(): KKKJFUIT2RKJJUUSSCIREVMQ2SJRFEURKRKMZDES2CI5IQJFHEORSRJVJFKR22JBCEOTSLLFEU4QSVKNJEWS

🛠️ Environment:

Operating System: Windows X86@64
Golang Version: 1.21
Library version used: pqerna/otp v1.4.0

🏁 Expected Outcome:

My expectation is to get a key with the exact same secret when entering the secret.

🚫 Actual Outcome:

Because the entered secret is encoded again, another secret is created.

v.Set("secret", b32NoPadding.EncodeToString(opts.Secret))

📝 Solution:

I assume that this decision was made on purpose so I provide a wrapper that decrypts the entered secret first.

func Regenerate(opts GenerateOpts) (*otp.Key, error) {
	if opts.SecretSize == 0 {
		return nil, otp.ErrRegenerateMissingSecret
	}
	secret := make([]byte, base32.StdEncoding.DecodedLen(len(opts.Secret)))
	_, err := base32.StdEncoding.Decode(secret, opts.Secret)
	if err != nil {
		return nil, err
	}
	opts.Secret = secret
	return Generate(opts)
}

[pquerna]

I'm not sure this needs a new function, named Regenerate, but rather more clarity about the code flows for serialization and deserialization.

[Plaenkler]

I'm not sure this needs a new function, named Regenerate, but rather more clarity about the code flows for serialization and deserialization.

Of course, a note in the documentation and an example could help here. However, I think the user should not be responsible for the decoding of the secret. Basically, as a user, I would assume that the set option "Secret", which is also a field of the key is used unprocessed. The function does not introduce any breaking changes, which is why I would prefer it to a correction in the documentation.

[pquerna]

Need to be clear here, its not "encrypted"; Its base32 encoded.

[Plaenkler]

I had made a mistake in my choice of words. Regardless of whether it is encoded or encrypted, the library should provide a way to recover the key from the secret.

[DavesPlanet]

Plaenkler, thanks for saving my hide with your example! I would never have figured that out.
One minor tweek, the code does not seem to run without allocating space for the buffer

var secret []byte

needed to become this for me to get it to work

secret := make([]byte, base32.StdEncoding.DecodedLen(len(opts.Secret)))

[Plaenkler]

Plaenkler, thanks for saving my hide with your example! I would never have figured that out. One minor tweek, the code does not seem to run without allocating space for the buffer

var secret []byte

needed to become this for me to get it to work

secret := make([]byte, base32.StdEncoding.DecodedLen(len(opts.Secret)))

Thanks for the info have adjusted it in the pull request