func()

in v3/internal/pkcs7_padder.go [48:79]

• 23 lines of code
• 5 McCabe index (conditional complexity)

func (padder pkcs7Padder) Unpad(buf []byte) ([]byte, error) {
	if len(buf) == 0 {
		return nil, errPKCS7Padding
	}

	// Here be dragons. We're attempting to check the padding in constant
	// time. The only piece of information here which is public is len(buf).
	// This code is modeled loosely after tls1_cbc_remove_padding from
	// OpenSSL.
	padLen := buf[len(buf)-1]
	toCheck := pkcs7MaxPaddingSize
	good := 1
	if toCheck > len(buf) {
		toCheck = len(buf)
	}
	for i := 0; i < toCheck; i++ {
		b := buf[len(buf)-1-i]

		outOfRange := subtle.ConstantTimeLessOrEq(int(padLen), i)
		equal := subtle.ConstantTimeByteEq(padLen, b)
		good &= subtle.ConstantTimeSelect(outOfRange, 1, equal)
	}

	good &= subtle.ConstantTimeLessOrEq(1, int(padLen))
	good &= subtle.ConstantTimeLessOrEq(int(padLen), len(buf))

	if good != 1 {
		return nil, errPKCS7Padding
	}

	return buf[:len(buf)-int(padLen)], nil
}