From af8e2f91e2de1ef0ed0d38578d45cc343c3487a2 Mon Sep 17 00:00:00 2001
From: Alexander Schau <alexander@schau.email>
Date: Sun, 30 Oct 2022 13:45:03 +0000
Subject: [PATCH] Initial commit

---
 README.md |  1 +
 base62.go | 43 +++++++++++++++++++++++++++++++++++++++++++
 crc32.go  | 14 ++++++++++++++
 go.mod    |  3 +++
 main.go   | 44 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 105 insertions(+)
 create mode 100644 README.md
 create mode 100644 base62.go
 create mode 100644 crc32.go
 create mode 100644 go.mod
 create mode 100644 main.go

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..4e1e0c7
--- /dev/null
+++ b/README.md
@@ -0,0 +1 @@
+# Go Access Token
\ No newline at end of file
diff --git a/base62.go b/base62.go
new file mode 100644
index 0000000..564e57f
--- /dev/null
+++ b/base62.go
@@ -0,0 +1,43 @@
+package accessToken
+
+import (
+	"crypto/rand"
+)
+
+func generateRandomBytes(n int) ([]byte, error) {
+	b := make([]byte, n)
+	_, err := rand.Read(b)
+	// Note that err == nil only if we read len(b) bytes.
+	if err != nil {
+		return nil, err
+	}
+
+	return b, nil
+}
+func randomBase62String(n int) (string, error) {
+	const letters = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+	bytes, err := generateRandomBytes(n)
+	if err != nil {
+		return "", err
+	}
+	for i, b := range bytes {
+		bytes[i] = letters[b%byte(len(letters))]
+	}
+	return string(bytes), nil
+}
+
+func EncodeBase62(n int) (string, error) {
+	letters := "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+	if n == 0 {
+		return "0", nil
+	}
+
+	var base62 string
+
+	for n > 0 {
+		base62 = letters[n%62:n%62+1] + base62
+		n = n / 62
+	}
+
+	return base62, nil
+}
diff --git a/crc32.go b/crc32.go
new file mode 100644
index 0000000..8e258ad
--- /dev/null
+++ b/crc32.go
@@ -0,0 +1,14 @@
+package accessToken
+
+import "hash/crc32"
+
+func getChecksum(text string) (string, error) {
+	checksum, err := EncodeBase62(int(crc32.ChecksumIEEE([]byte(text))))
+	if err != nil {
+		return "", err
+	}
+	checksum = "000000" + checksum
+	checksum = checksum[len(checksum)-6:]
+
+	return checksum, nil
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..8652448
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module git.alxs.xyz/alexander/go_access_token
+
+go 1.19
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..2dd860c
--- /dev/null
+++ b/main.go
@@ -0,0 +1,44 @@
+package accessToken
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+)
+
+func Generate(prefix string, secret string) (string, error) {
+
+	if len(prefix) != 3 {
+		return "", errors.New("prefix is not 3 characters long")
+	}
+
+	randomString, err := randomBase62String(30)
+	if err != nil {
+		return "", err
+	}
+
+	firstPart := fmt.Sprintf("%s_%s", prefix, randomString)
+
+	checksum, err := getChecksum(firstPart + secret)
+	if err != nil {
+		return "", err
+	}
+
+	return firstPart + checksum, nil
+}
+
+func Validate(token string, secret string) (bool, error) {
+
+	exp := regexp.MustCompile("([abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ]{3})_([0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ]{30})([0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ]{6})")
+
+	if !exp.Match([]byte(token)) {
+		return false, errors.New("wrong token format")
+	}
+
+	checksum, err := getChecksum(token[:len(token)-6] + secret)
+	if err != nil {
+		return false, err
+	}
+
+	return token[len(token)-6:] == checksum, nil
+}