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42 lines
1.0 KiB
Go
42 lines
1.0 KiB
Go
package padding
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import (
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"errors"
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"github.com/emmansun/gmsm/internal/alias"
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)
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// https://datatracker.ietf.org/doc/html/rfc5652#section-6.3
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type pkcs7Padding uint
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func (pad pkcs7Padding) BlockSize() int {
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return int(pad)
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}
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func (pad pkcs7Padding) Pad(src []byte) []byte {
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overhead := pad.BlockSize() - len(src)%pad.BlockSize()
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ret, out := alias.SliceForAppend(src, overhead)
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for i := 0; i < overhead; i++ {
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out[i] = byte(overhead)
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}
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return ret
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}
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// Unpad decrypted plaintext, non-constant-time
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func (pad pkcs7Padding) Unpad(src []byte) ([]byte, error) {
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srcLen := len(src)
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if srcLen == 0 || srcLen%pad.BlockSize() != 0 {
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return nil, errors.New("padding: src length is not multiple of block size")
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}
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paddedLen := src[srcLen-1]
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if paddedLen == 0 || int(paddedLen) > pad.BlockSize() {
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return nil, errors.New("padding: invalid padding byte/length")
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}
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for _, b := range src[srcLen-int(paddedLen) : srcLen-1] {
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if b != paddedLen {
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return nil, errors.New("padding: inconsistent padding bytes")
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}
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}
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return src[:srcLen-int(paddedLen)], nil
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}
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