Swift-Sodium

Swift-Sodium provides a safe and easy to use interface to perform common cryptographic operations on iOS and macOS.

It leverages the Sodium library, and although Swift is the primary target, the framework can also be used in Objective-C applications.

Please help!

The current Swift-Sodium documentation is not great. Your help to improve it and make it awesome would be very appreciated!

Usage

Add Sodium.framework as a dependency to your project, and import the module:

import Sodium

The Sodium library itself doesn't have to be installed on the system: the repository already includes a precompiled library for armv7, armv7s, arm64, as well as for the iOS simulator.

It targets Swift 4.2, introduced in Xcode 10.

The libsodium-ios.a file has been generated by the dist-build/ios.sh script. The libsodium-osx.a file has been generated by the dist-build/osx.sh script. The libsodium-watchos.a file has been generated by the dist-build/watchos.sh script.

Running these scripts on Xcode 10.1 (10B61) on the revision 7d418f8203897a967cce3dc0dcf8925f894e3428 of libsodium generates files identical to the ones present in this repository.

Secret-key cryptography

Messages are encrypted and decrypted using the same secret key.

That key can be generated using the key() method, derived from a password using the Password Hashing API, or computed using a secret key and the peer's public key with the Key Exchange API.

Authenticated encryption for a sequence of messages

let sodium = Sodium()
let message1 = "Message 1".bytes
let message2 = "Message 2".bytes
let message3 = "Message 3".bytes

let secretkey = sodium.secretStream.xchacha20poly1305.key()

/* stream encryption */

let stream_enc = sodium.secretStream.xchacha20poly1305.initPush(secretKey: secretkey)!
let header = stream_enc.header()
let encrypted1 = stream_enc.push(message: message1)!
let encrypted2 = stream_enc.push(message: message2)!
let encrypted3 = stream_enc.push(message: message3, tag: .FINAL)!

/* stream decryption */

let stream_dec = sodium.secretStream.xchacha20poly1305.initPull(secretKey: secretkey, header: header)!
let (message1_dec, tag1) = stream_dec.pull(cipherText: encrypted1)!
let (message2_dec, tag2) = stream_dec.pull(cipherText: encrypted2)!
let (message3_dec, tag3) = stream_dec.pull(cipherText: encrypted3)!

A stream is a sequence of messages, that will be encrypted as they arrive, and that are expected to be received in the same order as they were received.

Streams can be arbitrary long. This API can thus be used for file encryption, by splitting files into small chunks so that the whole content doesn't need to fit in memory.

It can also be used to exchange a sequence of messages between two peers.

The decryption function automatically checks that chunks have been received without modification, and truncation or reordering.

A tag is attached to each message, and can be used to signal the end of a sub-sequence (PUSH), or the end of the string (FINAL).

Authenticated encryption for independent messages

let sodium = Sodium()
let message = "My Test Message".bytes
let secretKey = sodium.secretBox.key()
let encrypted: Bytes = sodium.secretBox.seal(message: message, secretKey: secretKey)!
if let decrypted = sodium.secretBox.open(nonceAndAuthenticatedCipherText: encrypted, secretKey: secretKey) {
    // authenticator is valid, decrypted contains the original message
}

This API encrypts a message. The decryption process check that they haven't been tampered with before decrypting them.

Messages encrypted that way are independent: if multiple messages are sent that way, the recipient cannot detect if some messages have been duplicated, deleted or reordered without including additional data to each message.

Public-key Cryptography

With public-key cryptography, each peer has two keys: a secret key, that has to remain secret, and a public key that anyone can use to send an encrypted message to that peer. That public key can be only be used to encrypt a message. The related secret is required to decrypt it.

Authenticated Encryption

let sodium = Sodium(())!
let aliceKeyPair = sodium.box.keyPair()!
let bobKeyPair = sodium.box.keyPair()!
let message = "My Test Message".bytes

let encryptedMessageFromAliceToBob: Bytes =
    sodium.box.seal(message: message,
                    recipientPublicKey: bobKeyPair.publicKey,
                    senderSecretKey: aliceKeyPair.secretKey)!

let messageVerifiedAndDecryptedByBob =
    sodium.box.open(nonceAndAuthenticatedCipherText: encryptedMessageFromAliceToBob,
                    senderPublicKey: aliceKeyPair.publicKey,
                    recipientSecretKey: bobKeyPair.secretKey)

This operation encrypts and sends a message to someone using their public key.

The recipient has to know the sender's public key as well, and will reject a message that doesn't appear to be valid for the expected public key.

seal() automatically generates a nonce and prepends it to the ciphertext. open() extracts the nonce and decrypts the ciphertext.

Optionally, Box provides the ability to utilize a user-defined nonce via seal(message: recipientPublicKey: senderSecretKey: nonce:).

The Box class also provides alternative functions and parameters to deterministically generate key pairs, to retrieve the nonce and/or the authenticator, and to detach them from the original message.

Anonymous Encryption (Sealed Boxes)

let sodium = Sodium()
let bobKeyPair = sodium.box.keyPair()!
let message = "My Test Message".bytes

let encryptedMessageToBob =
    sodium.box.seal(message: message, recipientPublicKey: bobKeyPair.publicKey)!

let messageDecryptedByBob =
    sodium.box.open(anonymousCipherText: encryptedMessageToBob,
                    recipientPublicKey: bobKeyPair.publicKey,
                    recipientSecretKey: bobKeyPair.secretKey)

seal() generates an ephemeral keypair, uses the ephemeral secret key in the encryption process, combines the ephemeral public key with the ciphertext, then destroys the keypair.

The sender can not decrypt the resulting ciphertext. open() extracts the public key and decrypts using the recipient's secret key. Message integrity is verified, but the sender's identity cannot be correlated to the ciphertext.

Key exchange

let sodium = Sodium()
let aliceKeyPair = sodium.keyExchange.keyPair()!
let bobKeyPair = sodium.keyExchange.keyPair()!

let sessionKeyPairForAlice = sodium.keyExchange.sessionKeyPair(publicKey: aliceKeyPair.publicKey,
    secretKey: aliceKeyPair.secretKey, otherPublicKey: bobKeyPair.publicKey, side: .CLIENT)!
let sessionKeyPairForBob = sodium.keyExchange.sessionKeyPair(publicKey: bobKeyPair.publicKey,
    secretKey: bobKeyPair.secretKey, otherPublicKey: aliceKeyPair.publicKey, side: .SERVER)!

let aliceToBobKeyEquality = sodium.utils.equals(sessionKeyPairForAlice.tx, sessionKeyPairForBob.rx) // true
let bobToAliceKeyEquality = sodium.utils.equals(sessionKeyPairForAlice.rx, sessionKeyPairForBob.tx) // true

This operation computes a shared secret key using a secret key and a peer's public key.

Public-key signatures

Signatures allow multiple parties to verify the authenticity of a public message, using the public key of the author's message.

This can be especially useful to sign software updates.

Detached signatures

let sodium = Sodium()
let message = "My Test Message".bytes
let keyPair = sodium.sign.keyPair()!
let signature = sodium.sign.signature(message: message, secretKey: keyPair.secretKey)!
if sodium.sign.verify(message: message,
                      publicKey: keyPair.publicKey,
                      signature: signature) {
    // signature is valid
}

Attached signatures

let sodium = Sodium()
let message = "My Test Message".bytes
let keyPair = sodium.sign.keyPair()!
let signedMessage = sodium.sign.sign(message: message, secretKey: keyPair.secretKey)!
if let unsignedMessage = sodium.sign.open(signedMessage: signedMessage, publicKey: keyPair.publicKey) {
    // signature is valid
}

Hashing

Deterministic hashing

let sodium = Sodium()
let message = "My Test Message".bytes
let h = sodium.genericHash.hash(message: message)

Keyed hashing

let sodium = Sodium()
let message = "My Test Message".bytes
let key = "Secret key".bytes
let h = sodium.genericHash.hash(message: message, key: key)

Streaming

let sodium = Sodium()
let message1 = "My Test ".bytes
let message2 = "Message".bytes
let key = "Secret key".bytes
let stream = sodium.genericHash.initStream(key: key)!
stream.update(input: message1)
stream.update(input: message2)
let h = stream.final()

Short-output hashing (SipHash)

let sodium = Sodium()
let message = "My Test Message".bytes
let key = sodium.randomBytes.buf(length: sodium.shortHash.KeyBytes)!
let h = sodium.shortHash.hash(message: message, key: key)

Random numbers generation

let sodium = Sodium()
let randomBytes = sodium.randomBytes.buf(length: 1000)!
let seed = "0123456789abcdef0123456789abcdef".bytes
let stream = sodium.randomBytes.deterministic(length: 1000, seed: seed)!

Password hashing

let sodium = Sodium()
let password = "Correct Horse Battery Staple".bytes
let hashedStr = sodium.pwHash.str(passwd: password,
                                  opsLimit: sodium.pwHash.OpsLimitInteractive,
                                  memLimit: sodium.pwHash.MemLimitInteractive)!

if sodium.pwHash.strVerify(hash: hashedStr, passwd: password) {
    // Password matches the given hash string
} else {
    // Password doesn't match the given hash string
}

if sodium.pwHash.strNeedsRehash(hash: hashedStr,
                                opsLimit: sodium.pwHash.OpsLimitInteractive,
                                memLimit: sodium.pwHash.MemLimitInteractive) {
    // Previously hashed password should be recomputed because the way it was
    // hashed doesn't match the current algorithm and the given parameters.
}

Authentication tags

The sodium.auth.tag() function computes an authentication tag (HMAC) using a message and a key. Parties knowing the key can then verify the authenticity of the message using the same parameters and the sodium.auth.verify() function.

Authentication tags are not signatures: the same key is used both for computing and verifying a tag. Therefore, verifiers can also compute tags for arbitrary messages.

let sodium = Sodium()
let input = "test".bytes
let key = sodium.auth.key()
let tag = sodium.auth.tag(message: input, secretKey: key)!
let tagIsValid = sodium.auth.verify(message: input, secretKey: key, tag: tag)

Key derivation

The sodium.keyDerivation.derive() function generates a subkey using an input (master) key, an index, and a 8 bytes string identifying the context. Up to (2^64) - 1 subkeys can be generated for each context, by incrementing the index.

let sodium = Sodium()
let secretKey = sodium.keyDerivation.keygen()!

let subKey1 = sodium.keyDerivation.derive(secretKey: secretKey,
                                          index: 0, length: 32,
                                          context: "Context!")
let subKey2 = sodium.keyDerivation.derive(secretKey: secretKey,
                                          index: 1, length: 32,
                                          context: "Context!")

Utilities

Zeroing memory

let sodium = Sodium()
var dataToZero = "Message".bytes
sodium.utils.zero(&dataToZero)

Constant-time comparison

let sodium = Sodium()
let secret1 = "Secret key".bytes
let secret2 = "Secret key".bytes
let equality = sodium.utils.equals(secret1, secret2)

Padding

let sodium = Sodium()
var bytes = "test".bytes

// make bytes.count a multiple of 16
sodium.utils.pad(bytes: &bytes, blockSize: 16)!

// restore original size
sodium.utils.unpad(bytes: &bytes, blockSize: 16)!

Padding can be useful to hide the length of a message before it is encrypted.

Constant-time hexadecimal encoding

let sodium = Sodium()
let bytes = "Secret key".bytes
let hex = sodium.utils.bin2hex(bytes)

Hexadecimal decoding

let sodium = Sodium()
let data1 = sodium.utils.hex2bin("deadbeef")
let data2 = sodium.utils.hex2bin("de:ad be:ef", ignore: " :")

Constant-time base64 encoding

let sodium = Sodium()
let b64 = sodium.utils.bin2base64("data".bytes)!
let b64_2 = sodium.utils.bin2base64("data".bytes, variant: .URLSAFE_NO_PADDING)!

Base64 decoding

let data1 = sodium.utils.base642bin(b64)
let data2 = sodium.utils.base642bin(b64, ignore: " \n")
let data3 = sodium.utils.base642bin(b64_2, variant: .URLSAFE_NO_PADDING, ignore: " \n")

Helpers to build custom constructions

Only use the functions below if you know that you absolutely need them, and know how to use them correctly.

Unauthenticated encryption

The sodium.stream.xor() function combines (using the XOR operation) an arbitrary-long input with the output of a deterministic key stream derived from a key and a nonce. The same operation applied twice produces the original input.

No authentication tag is added to the output. The data can be tampered with; an adversary can flip arbitrary bits.

In order to encrypt data using a secret key, the SecretBox class is likely to be what you are looking for.

In order to generate a deterministic stream out of a seed, the RandomBytes.deterministic_rand() function is likely to be what you need.

let sodium = Sodium()
let input = "test".bytes
let key = sodium.stream.key()
let (output, nonce) = sodium.stream.xor(input: input, secretKey: key)!
let twice = sodium.stream.xor(input: output, nonce: nonce, secretKey: key)!

XCTAssertEqual(input, twice)