### Generate otpauth URI for QR code

Source: https://hexdocs.pm/nimble_totp

Creates an otpauth URI string and demonstrates how to convert it into an SVG QR code using EQRCode.

```elixir
uri = NimbleTOTP.otpauth_uri("Acme:alice", secret, issuer: "Acme")
#=> "otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme"
uri |> EQRCode.encode() |> EQRCode.svg()
#=> "<?xml version=\"1.0\" standalone=\"yes\"?>\n<svg version=\"1.1\" ..."
```

--------------------------------

### Generate OTP Auth URI

Source: https://hexdocs.pm/nimble_totp

Generates the URI string for a QR code, used for setting up TOTP authentication. Requires a label, secret, and optional issuer parameter.

```elixir
iex> NimbleTOTP.otpauth_uri("Acme:alice", "abcd", issuer: "Acme")
"otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme"
```

--------------------------------

### Generate and validate verification codes

Source: https://hexdocs.pm/nimble_totp

Computes the current verification code and demonstrates how to validate it against a secret.

```elixir
NimbleTOTP.verification_code(secret)
#=> "569777"
```

```elixir
NimbleTOTP.valid?(secret, "569777")
#=> true

NimbleTOTP.valid?(secret, "012345")
#=> false
```

--------------------------------

### NimbleTOTP Functions

Source: https://hexdocs.pm/nimble_totp

Core functions for managing TOTP lifecycle including secret generation, URI creation, code generation, and validation.

```APIDOC
## otpauth_uri(label, secret, uri_params)

### Description
Generate the URI to be encoded in the QR code.

### Parameters
- **label** (String.t) - Required - The label for the TOTP entry.
- **secret** (String.t) - Required - The secret key.
- **uri_params** (keyword) - Optional - Additional parameters for the URI.

## secret(size)

### Description
Generate a binary composed of random bytes.

### Parameters
- **size** (non_neg_integer) - Optional - The number of bytes (default 20).

## valid?(secret, otp, opts)

### Description
Checks if the given otp code matches the given secret.

### Parameters
- **secret** (binary) - Required - The secret key.
- **otp** (String.t) - Required - The code to validate.
- **opts** (list) - Optional - Options including :time, :since, and :period.

## verification_code(secret, opts)

### Description
Generate Time-Based One-Time Password (TOTP).

### Parameters
- **secret** (binary) - Required - The secret key.
- **opts** (list) - Optional - Options including :time and :period.
```

--------------------------------

### Generate Random Secret

Source: https://hexdocs.pm/nimble_totp

Generates a binary composed of random bytes for TOTP secrets. Defaults to 20 bytes as per RFC.

```elixir
NimbleTOTP.secret()
#=> <<178, 117, 46, 7, 172, 202, 108, 127, 186, 180, ...>>
```

--------------------------------

### Generate a random secret

Source: https://hexdocs.pm/nimble_totp

Generates a binary secret of 20 random bytes for use in TOTP.

```elixir
secret = NimbleTOTP.secret()
#=> <<178, 117, 46, 7, 172, 202, 108, 127, 186, 180, ...>>
```

--------------------------------

### Validate TOTP Code with Grace Period

Source: https://hexdocs.pm/nimble_totp

Checks if a given OTP code is valid against a secret, allowing for a grace period by checking against the current time and 30 seconds prior. This helps ensure the previous code is still valid.

```elixir
def valid_code?(secret, otp) do
  time = System.os_time(:second)

  NimbleTOTP.valid?(secret, otp, time: time) or
    NimbleTOTP.valid?(secret, otp, time: time - 30)
end
```

--------------------------------

### Generate TOTP Verification Code

Source: https://hexdocs.pm/nimble_totp

Generates a Time-Based One-Time Password (TOTP) code from a secret. The code is valid for a default period of 30 seconds.

```elixir
secret = Base.decode32!("PTEPUGZ7DUWTBGMW4WLKB6U63MGKKMCA")
NimbleTOTP.verification_code(secret)
#=> "569777"
```

--------------------------------

### Prevent code reuse

Source: https://hexdocs.pm/nimble_totp

Validates a TOTP code while ensuring it has not been reused by checking against the last successful usage timestamp.

```elixir
NimbleTOTP.valid?(user.totp_secret, code, since: user.last_totp_at)
```

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