64 lines
1.7 KiB
Go
64 lines
1.7 KiB
Go
// Package pkcs7 implements PKCS#7 padding
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//
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// This is a standard way of encoding variable length buffers into
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// buffers which are a multiple of an underlying crypto block size.
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package pkcs7
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import "errors"
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// Errors Unpad can return
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var (
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ErrorPaddingNotFound = errors.New("bad PKCS#7 padding - not padded")
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ErrorPaddingNotAMultiple = errors.New("bad PKCS#7 padding - not a multiple of blocksize")
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ErrorPaddingTooLong = errors.New("bad PKCS#7 padding - too long")
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ErrorPaddingTooShort = errors.New("bad PKCS#7 padding - too short")
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ErrorPaddingNotAllTheSame = errors.New("bad PKCS#7 padding - not all the same")
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)
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// Pad buf using PKCS#7 to a multiple of n.
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//
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// Appends the padding to buf - make a copy of it first if you don't
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// want it modified.
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func Pad(n int, buf []byte) []byte {
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if n <= 1 || n >= 256 {
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panic("bad multiple")
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}
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length := len(buf)
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padding := n - (length % n)
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for i := 0; i < padding; i++ {
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buf = append(buf, byte(padding))
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}
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if (len(buf) % n) != 0 {
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panic("padding failed")
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}
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return buf
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}
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// Unpad buf using PKCS#7 from a multiple of n returning a slice of
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// buf or an error if malformed.
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func Unpad(n int, buf []byte) ([]byte, error) {
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if n <= 1 || n >= 256 {
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panic("bad multiple")
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}
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length := len(buf)
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if length == 0 {
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return nil, ErrorPaddingNotFound
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}
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if (length % n) != 0 {
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return nil, ErrorPaddingNotAMultiple
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}
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padding := int(buf[length-1])
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if padding > n {
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return nil, ErrorPaddingTooLong
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}
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if padding == 0 {
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return nil, ErrorPaddingTooShort
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}
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for i := 0; i < padding; i++ {
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if buf[length-1-i] != byte(padding) {
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return nil, ErrorPaddingNotAllTheSame
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}
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}
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return buf[:length-padding], nil
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}
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