Chacha20 (Chacha20 v1.0.4)

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Chacha20 symmetric stream cipher

https://tools.ietf.org/html/rfc7539

The calling semantics are still sub-optimal and no performance tuning has been done.

Summary

Types

chacha_parameters()

The parameters and state of the current session

key()

The shared encryption key.

nonce()

The shared per-session nonce.

Functions

block(k, n, b)

Return an arbitrary block

crypt(m, k, n, c \\ 0)

The crypt function suitable for a complete message.

crypt_bytes(m, p, acc)

The crypt function suitable for streaming

Types

chacha_parameters()

@type chacha_parameters() :: {key(), nonce(), non_neg_integer(), binary()}

The parameters and state of the current session

  • The shared key
  • The session nonce
  • The next block number
  • The unused portion of the current block

To start from block 0, the initial state is {k,n,0,""}

key()

@type key() :: binary()

The shared encryption key.

nonce()

@type nonce() :: binary()

The shared per-session nonce.

By spec, this nonce may be used to encrypt a stream of up to 256GiB

An eight-byte nonce is compatible with the original reference implementation.

Functions

block(k, n, b)

@spec block(key(), nonce(), non_neg_integer()) :: binary()

Return an arbitrary block

This is probably most useful in fast-forwarding a stream.

crypt(m, k, n, c \\ 0)

@spec crypt(binary(), key(), nonce(), non_neg_integer()) :: binary()

The crypt function suitable for a complete message.

This is a convenience wrapper when the full message is ready for processing.

The operations are symmetric, so if crypt(m,k,n) = c, then crypt(c,k,n) = m

crypt_bytes(m, p, acc)

@spec crypt_bytes(binary(), chacha_parameters(), [binary()]) ::
  {binary(), chacha_parameters()}

The crypt function suitable for streaming

Use an initial state of {k,n,0,""} The returned parameters can be used for the next available bytes. Any previous emitted binary can be included in the acc, if desired.