Question on PKCS7 Padding

[matthiasgeihs]

CryptoSwift/Sources/CryptoSwift/PKCS/PKCS7Padding.swift

Lines 30 to 35 in db51c40

	let padding = UInt8(blockSize - (bytes.count % blockSize))
	var withPadding = bytes
	if padding == 0 {
	// If the original data is a multiple of N bytes, then an extra block of bytes with value N is added.
	withPadding += Array<UInt8>(repeating: UInt8(blockSize), count: Int(blockSize))
	} else {

PKCS7 padding has an if branch that seems unreachable, if I see it correctly.
In the above example, padding = blockSize - (bytes.count % blockSize).
For blockSize > 0, blockSize is strictly greater a % blockSize, for any integer a.
Hence, padding can never be zero.

==> The branch if padding == 0 should be removed.

Right?

(Might relate to #953.)

[krzyzanowskim]

it's 0 for bytes.count == blockSize, isn't it

[matthiasgeihs]

Assume blockSize > 0.

If bytes.count == blockSize, then bytes.count % blockSize == 0.
Hence, padding = blockSize - (bytes.count % blockSize) = blockSize.
Since blockSize > 0, padding != 0.