go/cryptojs/pkcs7.go

package cryptojs

import (
	"bytes"
	"fmt"
)

// Appends padding.
func pkcs7Pad(data []byte, blocklen int) ([]byte, error) {
	if blocklen <= 0 {
		return nil, fmt.Errorf("invalid blocklen %d", blocklen)
	}
	padlen := uint8(1)
	for ((len(data) + int(padlen)) % blocklen) != 0 {
		padlen++
	}

	if int(padlen) > blocklen {
		panic(fmt.Sprintf("generated invalid padding length %v for block length %v", padlen, blocklen))
	}
	pad := bytes.Repeat([]byte{byte(padlen)}, int(padlen))
	return append(data, pad...), nil
}

// Returns slice of the original data without padding.
func pkcs7Unpad(data []byte, blocklen int) ([]byte, error) {
	if blocklen <= 0 {
		return nil, fmt.Errorf("invalid blocklen %d", blocklen)
	}
	if len(data)%blocklen != 0 || len(data) == 0 {
		return nil, fmt.Errorf("invalid data len %d", len(data))
	}

	padlen := int(data[len(data)-1])
	if padlen > blocklen || padlen == 0 {
		// Not padded
		return data, nil
	}
	// check padding
	pad := data[len(data)-padlen:]
	for i := 0; i < padlen; i++ {
		if pad[i] != byte(padlen) {
			return data, nil
		}
	}

	return data[:len(data)-padlen], nil
}