rand.Read() or io.ReadFull(rand.Reader)?

[Dmitry Chestnykh]

There's a convenience function in crypto/rand called Read, which calls rand.Reader.Read.

rand.Reader.Read in rand_unix.go is implemented via bufio, and bufio's documentation states:

"Read reads data into p. It returns the number of bytes read into p. It calls Read at most once on the underlying Reader, hence n may be less than len(p). At EOF, the count will be zero and err will be os.EOF."

Does this mean that doing this on Unix:

b := make([]byte, 10)

if _, err := rand.Read(b); err != nil {

   // report error

}

doesn't guarantee that it reads 10 bytes into b, unless you check for returned length or use io.ReadFull?

I see that ecdsa packages doesn't check the returned length:

http://weekly.goneat.org/src/pkg/crypto/ecdsa/ecdsa.go#L40

(actually, it's a bug, since it uses whatever random Reader you pass to the function, I'll file it later).

while dsa package uses io.ReadFull:

http://weekly.goneat.org/src/pkg/crypto/dsa/dsa.go#L202

(As for how /dev/random may not return the bytes requested, I found this: https://groups.google.com/d/msg/obstcp/zxfXPqb7AtA/SuhgLIkDZQQJ)

-Dmitry

[Russ Cox]

Code should probably call io.ReadFull.

There's no security problem with the current code's behavior,
because the rand.Reader is a bufio.Reader, and it
behaves like io.ReadFull on ordinary reads.
But that is a detail of one implementation,
not something to rely on.

Russ