为什么crypto / rsa库中的Go函数EncryptOAEP需要一个随机的io.Reader？

Question

I'm writing a file server that encrypts data on the client side, sends the data over TCP, and decrypts it on server side using asymmetric RSA-OAEP encryption. There are two main functions I have been trying to use, which take the following arguments per the documentation:

EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, label []byte) (out []byte, err error)
DecryptOAEP(hash hash.Hash, random io.Reader, priv *PrivateKey, ciphertext []byte, label []byte) (msg []byte, err error)

Each requires a random io.Reader, and the test file uses rand.Reader from crypto/rand. However, whenever I encrypt a message with rand.Reader on the client side, the message never is decrypted properly on the server side which has a separate instance of rand.Reader.

1. What is the purpose of the random io.Reader?
2. How can I ensure that the encrypted message is transferred and decrypted properly by the server? Would I also need to transfer some information about the rand.Reader used by the client as well to the server for the message to be properly decrypted?

Answer 1

Using rsa_test.go as a base I managed to create a small end to end decryption program. From the informations you provided is hard to tell where the error is, but I hope reading this program you can find the bug yourself. Probably you don't need to provide a Reader in the DecryptOAEP.

http://play.golang.org/p/7VVCHJOB7R

package main

import (
    "bytes"
    "crypto/rsa"
    "crypto/sha1"
    "log"
    "math/big"
)

func main() {
    sha1 := sha1.New()
    n := new(big.Int)
    d := new(big.Int)

    rsa_modulus := "a8b3b284af8eb50b387034a860f146c4919f318763cd6c5598c8ae4811a1e0abc4c7e0b082d693a5e7fced675cf4668512772c0cbc64a742c6c630f533c8cc72f62ae833c40bf25842e984bb78bdbf97c0107d55bdb662f5c4e0fab9845cb5148ef7392dd3aaff93ae1e6b667bb3d4247616d4f5ba10d4cfd226de88d39f16fb"
    rsa_d := "53339cfdb79fc8466a655c7316aca85c55fd8f6dd898fdaf119517ef4f52e8fd8e258df93fee180fa0e4ab29693cd83b152a553d4ac4d1812b8b9fa5af0e7f55fe7304df41570926f3311f15c4d65a732c483116ee3d3d2d0af3549ad9bf7cbfb78ad884f84d5beb04724dc7369b31def37d0cf539e9cfcdd3de653729ead5d1"

    n.SetString(rsa_modulus, 16)
    d.SetString(rsa_d, 16)
    public := rsa.PublicKey{n, 65537}
    d.SetString(rsa_d, 16)
    private := new(rsa.PrivateKey)
    private.PublicKey = public
    private.D = d

    seed := []byte{0x18, 0xb7, 0x76, 0xea, 0x21, 0x06, 0x9d, 0x69,
        0x77, 0x6a, 0x33, 0xe9, 0x6b, 0xad, 0x48, 0xe1, 0xdd,
        0xa0, 0xa5, 0xef,
    }
    randomSource := bytes.NewReader(seed)

    in := []byte("Hello World")

    encrypted, err := rsa.EncryptOAEP(sha1, randomSource, &public, in, nil)
    if err != nil {
        log.Println("error: %s", err)
    }

    plain, err := rsa.DecryptOAEP(sha1, nil, private, encrypted, nil)
    if err != nil {
        log.Println("error: %s", err)
    }

    log.Println(string(plain))
}