diff options
Diffstat (limited to 'vendor/golang.org/x/crypto/chacha20/chacha_generic.go')
| -rw-r--r-- | vendor/golang.org/x/crypto/chacha20/chacha_generic.go | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/vendor/golang.org/x/crypto/chacha20/chacha_generic.go b/vendor/golang.org/x/crypto/chacha20/chacha_generic.go index 098ec9f6..7c498e90 100644 --- a/vendor/golang.org/x/crypto/chacha20/chacha_generic.go +++ b/vendor/golang.org/x/crypto/chacha20/chacha_generic.go @@ -136,6 +136,33 @@ func quarterRound(a, b, c, d uint32) (uint32, uint32, uint32, uint32) { return a, b, c, d } +// SetCounter sets the Cipher counter. The next invocation of XORKeyStream will +// behave as if (64 * counter) bytes had been encrypted so far. +// +// To prevent accidental counter reuse, SetCounter panics if counter is +// less than the current value. +func (s *Cipher) SetCounter(counter uint32) { + // Internally, s may buffer multiple blocks, which complicates this + // implementation slightly. When checking whether the counter has rolled + // back, we must use both s.counter and s.len to determine how many blocks + // we have already output. + outputCounter := s.counter - uint32(s.len)/blockSize + if counter < outputCounter { + panic("chacha20: SetCounter attempted to rollback counter") + } + + // In the general case, we set the new counter value and reset s.len to 0, + // causing the next call to XORKeyStream to refill the buffer. However, if + // we're advancing within the existing buffer, we can save work by simply + // setting s.len. + if counter < s.counter { + s.len = int(s.counter-counter) * blockSize + } else { + s.counter = counter + s.len = 0 + } +} + // XORKeyStream XORs each byte in the given slice with a byte from the // cipher's key stream. Dst and src must overlap entirely or not at all. // |