使用 libsodium 加密，需要使用 crypto_box_keypair 生成公钥和私钥

Question

I have been working with the libsodium library to implement Shamir secret sharing and trying to test the implementation done by dark crystal

https://gitlab.com/dark-crystal-javascript/key-backup-crypto/-/blob/master/example.js

Implementation is something like this

function encryptionKeypair () {
    const keypair = {
      publicKey: sodium.sodium_malloc(sodium.crypto_box_PUBLICKEYBYTES),
      secretKey: sodium.sodium_malloc(sodium.crypto_box_SECRETKEYBYTES)
    }
    sodium.crypto_box_keypair(keypair.publicKey, keypair.secretKey)
    return keypair
  },

 function oneWayBox (message, publicKey) {
    console.log('in one way box');
    const curvePublicKey = sodium.sodium_malloc(sodium.crypto_box_PUBLICKEYBYTES)
    // console.log('curvePublicKey', curvePublicKey.toString('hex'));
    console.log('curvePublicKey', curvePublicKey.length);
    console.log('publicKey', publicKey.length);
    
    sodium.crypto_sign_ed25519_pk_to_curve25519(curvePublicKey, publicKey)

    // console.log('curvePublicKey', curvePublicKey.toString('hex'));


    console.log('in one way box');
    console.log('\n');

    const ephemeral = this.encryptionKeypair()
    const nonce = this.randomBytes(sodium.crypto_box_NONCEBYTES)
    const cipherText = sodium.sodium_malloc(message.length + sodium.crypto_box_MACBYTES)
  
    sodium.crypto_box_easy(cipherText, message, nonce, curvePublicKey, ephemeral.secretKey)
    zero(ephemeral.secretKey)
    zero(message)
    return Buffer.concat([nonce, ephemeral.publicKey, cipherText])
  },

below is Secret-sharing-generation.js

const secrets = require('secret-sharing')
const s = require('.')

const secret = 'My secret key'
const label = ''

console.log('Secret to share:', secret.toString('hex'))

console.log(`Packing with label: '${label}'`)
const packedSecret = s.packLabel(secret, label)
console.log(`Packed secret: ${packedSecret.toString('hex')}`)
console.log(`Length of packed secret is ${packedSecret.length} bytes.`)
const signingKeypair = s.keypair()
const encryptionKeypair = s.signingKeypairToEncryptionKeypair(signingKeypair)

const custodians = []
for (let i = 0; i < 5; i++) {
  custodians.push(s.encryptionKeypair())
}


console.log('custodians', custodians);

console.log('Creating 5 shares, 3 needed to recover')
secrets.share(packedSecret, 5, 3).then((shards) => {
  console.log('Shards:')
  console.log(shards.map(s => s.toString('hex')))
  console.log('Signed shards:')
  const signedShards = s.signShards(shards, signingKeypair)
  console.log(signedShards.map(s => s.toString('hex')))

  const boxedShards = signedShards.map((shard, i) => {
    return s.oneWayBox(shard, custodians[i].publicKey)
  })

  console.log('Boxed shards:')
  console.log(boxedShards.map(s => s.toString('hex')))
  console.log(`Length of boxed shards are ${boxedShards[0].length} bytes.`)
  secrets.combine(shards.slice(2)).then((result) => {
    console.log('Result of recombining 3 shares:', result.toString())
  })
})

Now the problem is when I am using encryptionKeypair function to generate key pair for and then after that when I am trying to generate to do crypto_sign_ed25519_sk_to_curve25519 using the key pair generated in this encryptionKeypair function I am getting

UnhandledPromiseRejectionWarning: Error: ENOMEM, Cannot allocate memory

I have checked my swap space it is completely free

           total        used        free      shared  buff/cache   available
Mem:           3138          83        2896           0         158        2908
Swap:          5119           0        5119

I am not able to understand what is the issue.

Answer 1

It's not clear to me why you want to convert a key pair created with encryptionKeypair() with crypto_sign_ed25519_sk_to_curve25519() or crypto_sign_ed25519_pk_to_curve25519() .

The latter two methods convert a secret or public Ed25519 key (used in the context of signing) to a secret or public X25519 key (used in the context of key exchange).

encryptionKeypair() applies crypto_box_keypair() and thus already creates an X25519 key pair, so conversion is not necessary (and not possible).

A working use of the conversion methods would be, eg using crypto_sign_keypair() , which generates an Ed25519 key pair:

var sodium = require('sodium-native');
...
var ed25519KeyPair = signingKeypair() // Create an Ed25519 keypair
var x25519KeyPair = {
    publicKey: sodium.sodium_malloc(sodium.crypto_box_PUBLICKEYBYTES),
    secretKey: sodium.sodium_malloc(sodium.crypto_box_SECRETKEYBYTES)
}
sodium.crypto_sign_ed25519_pk_to_curve25519(x25519KeyPair.publicKey, ed25519KeyPair.publicKey) // Convert the public Ed25519 into a public X25519 key
sodium.crypto_sign_ed25519_sk_to_curve25519(x25519KeyPair.secretKey, ed25519KeyPair.secretKey) // Convert the secret Ed25519 into a secret X25519 key
console.log(x25519KeyPair.publicKey)
console.log(x25519KeyPair.secretKey)

function signingKeypair () {
    const keypair = {
        publicKey: sodium.sodium_malloc(sodium.crypto_sign_PUBLICKEYBYTES),
        secretKey: sodium.sodium_malloc(sodium.crypto_sign_SECRETKEYBYTES)
    }
    sodium.crypto_sign_keypair(keypair.publicKey, keypair.secretKey)
    return keypair
}

Also, I can't reproduce the posted error message. When using encryptionKeypair() instead of signingKeypair() , I get the following error message Error: public key conversion failed .

Edit:

In the 2nd code snippet, the custodians ' key pairs are created with s.encryptionKeypair() , which produces X25519 key pairs. In the later called s.oneWayBox() it is then tried to convert the public keys with crypto_sign_ed25519_pk_to_curve25519() , which must fail as described above.

Presumably this is a bug! A possible fix is to generate the custodians ' key pairs with s.keypair() (or signingKeypair() ), which creates Ed25519 key pairs. The public keys can then be successfully converted to X25519 keys in s.oneWayBox() . With this change, oneWayBox() runs without any errors on my machine.

This change to Ed25519 key pairs is also consistent with the description of encryptionKeypair() , which states that this method is only used when generating ephemeral keys, eg in oneWayBox() . In all other cases it is internally derived from Ed25519 keys.