Documentation ¶
Overview ¶
Package captcha implements generation and verification of image and audio CAPTCHAs.
A captcha solution is the sequence of digits 0-9 with the defined length. There are two captcha representations: image and audio.
An image representation is a PNG-encoded image with the solution printed on it in such a way that makes it hard for computers to solve it using OCR.
An audio representation is a WAVE-encoded (8 kHz unsigned 8-bit) sound with the spoken solution (currently in English, Russian, Chinese, and Japanese). To make it hard for computers to solve audio captcha, the voice that pronounces numbers has random speed and pitch, and there is a randomly generated background noise mixed into the sound.
This package doesn't require external files or libraries to generate captcha representations; it is self-contained.
To make captchas one-time, the package includes a memory storage that stores captcha ids, their solutions, and expiration time. Used captchas are removed from the store immediately after calling Verify or VerifyString, while unused captchas (user loaded a page with captcha, but didn't submit the form) are collected automatically after the predefined expiration time. Developers can also provide custom store (for example, which saves captcha ids and solutions in database) by implementing Store interface and registering the object with SetCustomStore.
Captchas are created by calling New, which returns the captcha id. Their representations, though, are created on-the-fly by calling WriteImage or WriteAudio functions. Created representations are not stored anywhere, but subsequent calls to these functions with the same id will write the same captcha solution. Reload function will create a new different solution for the provided captcha, allowing users to "reload" captcha if they can't solve the displayed one without reloading the whole page. Verify and VerifyString are used to verify that the given solution is the right one for the given captcha id.
Server provides an http.Handler which can serve image and audio representations of captchas automatically from the URL. It can also be used to reload captchas. Refer to Server function documentation for details, or take a look at the example in "capexample" subdirectory.
Index ¶
- Constants
- Variables
- func New(ctx context.Context) string
- func NewLen(ctx context.Context, length int) (id string)
- func RandomDigits(length int) []byte
- func Reload(ctx context.Context, id string) bool
- func Server(imgWidth, imgHeight int) http.Handler
- func SetCustomStore(s Store)
- func Verify(ctx context.Context, id string, digits []byte) bool
- func VerifyString(ctx context.Context, id string, digits string) bool
- func WriteAudio(ctx context.Context, w io.Writer, id string, lang string) error
- func WriteImage(ctx context.Context, w io.Writer, id string, width, height int) error
- type Audio
- type Image
- type Store
Constants ¶
const ( // Default number of digits in captcha solution. DefaultLen = 6 // The number of captchas created that triggers garbage collection used // by default store. CollectNum = 100 // Expiration time of captchas used by default store. Expiration = 10 * time.Minute )
const ( // Standard width and height of a captcha image. StdWidth = 240 StdHeight = 80 )
Variables ¶
var (
ErrNotFound = errors.New("captcha: id not found")
)
Functions ¶
func New ¶
New creates a new captcha with the standard length, saves it in the internal storage and returns its id.
func RandomDigits ¶
RandomDigits returns a byte slice of the given length containing pseudorandom numbers in range 0-9. The slice can be used as a captcha solution.
func Reload ¶
Reload generates and remembers new digits for the given captcha id. This function returns false if there is no captcha with the given id.
After calling this function, the image or audio presented to a user must be refreshed to show the new captcha representation (WriteImage and WriteAudio will write the new one).
func Server ¶
Server returns a handler that serves HTTP requests with image or audio representations of captchas. Image dimensions are accepted as arguments. The server decides which captcha to serve based on the last URL path component: file name part must contain a captcha id, file extension — its format (PNG or WAV).
For example, for file name "LBm5vMjHDtdUfaWYXiQX.png" it serves an image captcha with id "LBm5vMjHDtdUfaWYXiQX", and for "LBm5vMjHDtdUfaWYXiQX.wav" it serves the same captcha in audio format.
To serve a captcha as a downloadable file, the URL must be constructed in such a way as if the file to serve is in the "download" subdirectory: "/download/LBm5vMjHDtdUfaWYXiQX.wav".
To reload captcha (get a different solution for the same captcha id), append "?reload=x" to URL, where x may be anything (for example, current time or a random number to make browsers refetch an image instead of loading it from cache).
By default, the Server serves audio in English language. To serve audio captcha in one of the other supported languages, append "lang" value, for example, "?lang=ru".
func SetCustomStore ¶
func SetCustomStore(s Store)
SetCustomStore sets custom storage for captchas, replacing the default memory store. This function must be called before generating any captchas.
func Verify ¶
Verify returns true if the given digits are the ones that were used to create the given captcha id.
The function deletes the captcha with the given id from the internal storage, so that the same captcha can't be verified anymore.
func VerifyString ¶
VerifyString is like Verify, but accepts a string of digits. It removes spaces and commas from the string, but any other characters, apart from digits and listed above, will cause the function to return false.
func WriteAudio ¶
WriteAudio writes WAV-encoded audio representation of the captcha with the given id and the given language. If there are no sounds for the given language, English is used.
Types ¶
type Audio ¶
type Audio struct {
// contains filtered or unexported fields
}
func NewAudio ¶
NewAudio returns a new audio captcha with the given digits, where each digit must be in range 0-9. Digits are pronounced in the given language. If there are no sounds for the given language, English is used.
Possible values for lang are "en", "ja", "ru", "zh".
func (*Audio) EncodedLen ¶
EncodedLen returns the length of WAV-encoded audio captcha.
type Image ¶
type Store ¶
type Store interface { // Set sets the digits for the captcha id. Set(ctx context.Context, id string, digits []byte) // Get returns stored digits for the captcha id. Clear indicates // whether the captcha must be deleted from the store. Get(ctx context.Context, id string, clear bool) (digits []byte) }
Store An object implementing Store interface can be registered with SetCustomStore function to handle storage and retrieval of captcha ids and solutions for them, replacing the default memory store.
It is the responsibility of an object to delete expired and used captchas when necessary (for example, the default memory store collects them in Set method after the certain amount of captchas has been stored.)
func NewMemoryStore ¶
NewMemoryStore returns a new standard memory store for captchas with the given collection threshold and expiration time (duration). The returned store must be registered with SetCustomStore to replace the default one.
Source Files ¶
Directories ¶
Path | Synopsis |
---|---|
example of HTTP server that uses the captcha package.
|
example of HTTP server that uses the captcha package. |
capgen is an utility to test captcha generation.
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capgen is an utility to test captcha generation. |
generate is a tool to generate sounds.go from WAVE files.
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generate is a tool to generate sounds.go from WAVE files. |