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v/vlib/crypto/README.md

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## Description:
`crypto` is a module that exposes cryptographic algorithms to V programs.
Each submodule implements things differently, so be sure to consider the documentation
of the specific algorithm you need, but in general, the method is to create a `cipher`
struct using one of the module functions, and then to call the `encrypt` or `decrypt`
method on that struct to actually encrypt or decrypt your data.
This module is a work-in-progress. For example, the AES implementation currently requires you
to create a destination buffer of the correct size to receive the decrypted data, and the AesCipher
`encrypt` and `decrypt` functions only operate on the first block of the `src`.
The implementations here are loosely based on [Go's crypto package](https://pkg.go.dev/crypto).
## Examples:
### AES:
```v
import crypto.aes
import crypto.rand
fn main() {
// remember to save this key somewhere if you ever want to decrypt your data
key := rand.bytes(32)?
println('KEY: $key')
// this data is one block (16 bytes) big
mut data := 'THIS IS THE DATA'.bytes()
println('generating cipher')
cipher := aes.new_cipher(key)
println('performing encryption')
mut encrypted := []u8{len: aes.block_size}
cipher.encrypt(mut encrypted, data)
println(encrypted)
println('performing decryption')
mut decrypted := []u8{len: aes.block_size}
cipher.decrypt(mut decrypted, encrypted)
println(decrypted)
assert decrypted == data
}
```
### JWT:
```v
import crypto.hmac
import crypto.sha256
import encoding.base64
import json
import time
struct JwtHeader {
alg string
typ string
}
struct JwtPayload {
sub string
name string
iat int
}
fn main() {
sw := time.new_stopwatch()
secret := 'your-256-bit-secret'
token := make_token(secret)
ok := auth_verify(secret, token)
dt := sw.elapsed().microseconds()
println('token: $token')
println('auth_verify(secret, token): $ok')
println('Elapsed time: $dt uS')
}
fn make_token(secret string) string {
header := base64.url_encode(json.encode(JwtHeader{'HS256', 'JWT'}).bytes())
payload := base64.url_encode(json.encode(JwtPayload{'1234567890', 'John Doe', 1516239022}).bytes())
signature := base64.url_encode(hmac.new(secret.bytes(), '${header}.$payload'.bytes(),
sha256.sum, sha256.block_size))
jwt := '${header}.${payload}.$signature'
return jwt
}
fn auth_verify(secret string, token string) bool {
token_split := token.split('.')
signature_mirror := hmac.new(secret.bytes(), '${token_split[0]}.${token_split[1]}'.bytes(),
sha256.sum, sha256.block_size)
signature_from_token := base64.url_decode(token_split[2])
return hmac.equal(signature_from_token, signature_mirror)
}
```