Source: poly1305.go in package vendor/golang.org/x/crypto/poly1305

Package poly1305 implements Poly1305 one-time message authentication code as specified in https://cr.yp.to/mac/poly1305-20050329.pdf. Poly1305 is a fast, one-time authentication function. It is infeasible for an attacker to generate an authenticator for a message without the key. However, a key must only be used for a single message. Authenticating two different messages with the same key allows an attacker to forge authenticators for other messages with the same key. Poly1305 was originally coupled with AES in order to make Poly1305-AES. AES was used with a fixed key in order to generate one-time keys from an nonce. However, in this package AES isn't used and the one-time key is specified directly.

package poly1305 // import "golang.org/x/crypto/poly1305"

import "crypto/subtle"

TagSize is the size, in bytes, of a poly1305 authenticator.

const TagSize = 16

Sum generates an authenticator for msg using a one-time key and puts the 16-byte result into out. Authenticating two different messages with the same key allows an attacker to forge messages at will.

func Sum(out *[16]byte, m []byte, key *[32]byte) {
	h := New(key)
	h.Write(m)
	h.Sum(out[:0])
}

Verify returns true if mac is a valid authenticator for m with the given key.

func Verify(mac *[16]byte, m []byte, key *[32]byte) bool {
	var tmp [16]byte
	Sum(&tmp, m, key)
	return subtle.ConstantTimeCompare(tmp[:], mac[:]) == 1
}

New returns a new MAC computing an authentication tag of all data written to it with the given key. This allows writing the message progressively instead of passing it as a single slice. Common users should use the Sum function instead. The key must be unique for each message, as authenticating two different messages with the same key allows an attacker to forge messages at will.

func New(key *[32]byte) *MAC {
	m := &MAC{}
	initialize(key, &m.macState)
	return m
}

MAC is an io.Writer computing an authentication tag of the data written to it. MAC cannot be used like common hash.Hash implementations, because using a poly1305 key twice breaks its security. Therefore writing data to a running MAC after calling Sum or Verify causes it to panic.

type MAC struct {
	mac // platform-dependent implementation

	finalized bool
}

Size returns the number of bytes Sum will return.

func (h *MAC) Size() int { return TagSize }

Write adds more data to the running message authentication code. It never returns an error. It must not be called after the first call of Sum or Verify.

func (h *MAC) Write(p []byte) (n int, err error) {
	if h.finalized {
		panic("poly1305: write to MAC after Sum or Verify")
	}
	return h.mac.Write(p)
}

Sum computes the authenticator of all data written to the message authentication code.

func (h *MAC) Sum(b []byte) []byte {
	var mac [TagSize]byte
	h.mac.Sum(&mac)
	h.finalized = true
	return append(b, mac[:]...)
}

Verify returns whether the authenticator of all data written to the message authentication code matches the expected value.

func (h *MAC) Verify(expected []byte) bool {
	var mac [TagSize]byte
	h.mac.Sum(&mac)
	h.finalized = true
	return subtle.ConstantTimeCompare(expected, mac[:]) == 1