57 lines
1.2 KiB
Go
57 lines
1.2 KiB
Go
package symm
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import (
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"crypto/cipher"
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"errors"
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)
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var (
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ErrInvalidCTROffset = errors.New("ctr offset must be non-negative")
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ErrCTRCounterOverflow = errors.New("ctr counter overflow")
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)
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func xorCTRAtOffset(block cipher.Block, data, iv []byte, offset int64) ([]byte, error) {
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if offset < 0 {
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return nil, ErrInvalidCTROffset
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}
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if len(iv) != block.BlockSize() {
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return nil, errors.New("iv length must match block size")
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}
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counter := make([]byte, len(iv))
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copy(counter, iv)
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blockSize := int64(block.BlockSize())
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blockOffset := uint64(offset / blockSize)
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byteOffset := int(offset % blockSize)
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if err := addUint64ToCounter(counter, blockOffset); err != nil {
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return nil, err
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}
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stream := cipher.NewCTR(block, counter)
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if byteOffset > 0 {
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skip := make([]byte, byteOffset)
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stream.XORKeyStream(skip, skip)
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}
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out := make([]byte, len(data))
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stream.XORKeyStream(out, data)
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return out, nil
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}
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func addUint64ToCounter(counter []byte, inc uint64) error {
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if inc == 0 {
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return nil
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}
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for i := len(counter) - 1; i >= 0 && inc > 0; i-- {
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sum := uint64(counter[i]) + (inc & 0xff)
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counter[i] = byte(sum)
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inc = (inc >> 8) + (sum >> 8)
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}
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if inc > 0 {
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return ErrCTRCounterOverflow
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}
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return nil
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}
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