package padding import ( "errors" "github.com/emmansun/gmsm/internal/alias" ) // Add a single bit with value 1 to the end of the data. // Then if necessary add bits with value 0 to the end of the data until the padded data is a multiple of n. // // https://en.wikipedia.org/wiki/ISO/IEC_9797-1 // also GB/T 17964-2021 C.2 Padding method 2 type iso9797M2Padding uint func (pad iso9797M2Padding) BlockSize() int { return int(pad) } func (pad iso9797M2Padding) Pad(src []byte) []byte { overhead := pad.BlockSize() - len(src)%pad.BlockSize() ret, out := alias.SliceForAppend(src, overhead) out[0] = 0x80 clear(out[1:overhead]) return ret } // Unpad decrypted plaintext, non-constant-time func (pad iso9797M2Padding) Unpad(src []byte) ([]byte, error) { srcLen := len(src) if srcLen == 0 || srcLen%pad.BlockSize() != 0 { return nil, errors.New("padding: src length is not multiple of block size") } tail := src[srcLen-pad.BlockSize():] allZero := true padStart := 0 for i := pad.BlockSize() - 1; i >= 0; i-- { if tail[i] == 0x80 { padStart = i break } if tail[i] != 0 { allZero = false break } } if !allZero { return nil, errors.New("padding: inconsistent padding bytes") } return src[:srcLen-pad.BlockSize()+padStart], nil }