github.com/google/osv-scalibr@v0.4.1/veles/secrets/github/checksum/checksum.go (about)

     1  // Copyright 2025 Google LLC
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //      http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package checksum contains the checksum validation logic for github tokens
    16  package checksum
    17  
    18  import (
    19  	"bytes"
    20  	"hash/crc32"
    21  )
    22  
    23  // Validate validates a GitHub token
    24  func Validate(token []byte) bool {
    25  	_, suf, ok := bytes.Cut(token, []byte("_"))
    26  	if !ok {
    27  		return false
    28  	}
    29  
    30  	if len(suf) <= 6 {
    31  		return false
    32  	}
    33  
    34  	// Split content and checksum
    35  	splitIdx := len(suf) - 6
    36  	content, checksum := suf[:splitIdx], suf[splitIdx:]
    37  
    38  	// Compute CRC32 on ASCII bytes of the content (not decoded base62)
    39  	crc := crc32.ChecksumIEEE(content)
    40  
    41  	// Encode checksum in base62 (ignoring a possible overflow)
    42  	got := base62Encode(crc, 6)
    43  
    44  	return bytes.Equal(got, checksum)
    45  }
    46  
    47  const base62Chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
    48  
    49  func base62Encode(n uint32, size int) []byte {
    50  	result := make([]byte, size)
    51  	for i := size - 1; i >= 0; i-- {
    52  		result[i] = base62Chars[n%62]
    53  		n /= 62
    54  	}
    55  	// ignore the overflow
    56  	return result
    57  }