Re-design of the original gocryptfs code to work as a library.
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libgocryptfs/internal/nametransform/pad16.go

64 lines
1.6 KiB

package nametransform
import (
"crypto/aes"
"errors"
"fmt"
"log"
)
// pad16 - pad data to AES block size (=16 byte) using standard PKCS#7 padding
// https://tools.ietf.org/html/rfc5652#section-6.3
func pad16(orig []byte) (padded []byte) {
oldLen := len(orig)
if oldLen == 0 {
log.Panic("Padding zero-length string makes no sense")
}
padLen := aes.BlockSize - oldLen%aes.BlockSize
if padLen == 0 {
padLen = aes.BlockSize
}
newLen := oldLen + padLen
padded = make([]byte, newLen)
copy(padded, orig)
padByte := byte(padLen)
for i := oldLen; i < newLen; i++ {
padded[i] = padByte
}
return padded
}
// unPad16 - remove padding
func unPad16(padded []byte) ([]byte, error) {
oldLen := len(padded)
if oldLen == 0 {
return nil, errors.New("Empty input")
}
if oldLen%aes.BlockSize != 0 {
return nil, errors.New("Unaligned size")
}
// The last byte is always a padding byte
padByte := padded[oldLen-1]
// The padding byte's value is the padding length
padLen := int(padByte)
// Padding must be at least 1 byte
if padLen == 0 {
return nil, errors.New("Padding cannot be zero-length")
}
// Padding more than 16 bytes make no sense
if padLen > aes.BlockSize {
return nil, fmt.Errorf("Padding too long, padLen=%d > 16", padLen)
}
// Padding cannot be as long as (or longer than) the whole string,
if padLen >= oldLen {
return nil, fmt.Errorf("Padding too long, oldLen=%d >= padLen=%d", oldLen, padLen)
}
// All padding bytes must be identical
for i := oldLen - padLen; i < oldLen; i++ {
if padded[i] != padByte {
return nil, fmt.Errorf("Padding byte at i=%d is invalid", i)
}
}
newLen := oldLen - padLen
return padded[0:newLen], nil
}