How to use cryptographic hash functions in CryptoKit for iOS security

Cryptographic hash functions are essential tools in secure application development, commonly used for verifying data integrity, securely storing passwords, and generating unique identifiers. Apple's CryptoKit framework offers a range of hashing functions that are efficient, secure, and easy to implement within Swift.

In this guide, we’ll explore how to work with CryptoKit’s hash functions, such as SHA-256 and SHA-512, to improve the security of your iOS apps.

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What is a cryptographic hash function?

A cryptographic hash function generates a fixed-length hash (or digest) from input data of any size. This hash acts like a digital fingerprint, uniquely identifying the input without revealing the original data. Key features of a secure cryptographic hash function include:

• one-way function - meaning that it's practically irreversible
• deterministic - meaning that the same message always results in the same hash value
• unique - meaning that it's practically not possible to find two different messages with the same hash value

These characteristics make hash functions ideal for storing hashed passwords, validating data integrity, and ensuring message authenticity.

For example, when a user enters a password into our app to login, we hash the password and send it to the server for verification. The passwords stored on the server are also computed hash values of the original passwords. This way, we never store the original password or send it over the network but still are able to verify it.

Hash functions in CryptoKit

CryptoKit provides several hashing algorithms, the main two algorithms are:

• SHA-256: A commonly used 256-bit hash function providing a secure balance between performance and cryptographic strength.
• SHA-512: A 512-bit version of SHA, offering a higher level of security, typically used for applications requiring greater protection.

For most iOS applications, SHA-256 is a preferred choice because of its strength and efficiency.

Let's look at how we can hash a password with the SHA-256 algorithm:

func hashPassword(_ password: String) -> String {
    let hash = SHA256.hash(data: Data(password.utf8))
    return hash.compactMap { String(format: "%02x", $0) }.joined()
}

The compactMap line in this function is used to convert each byte in the SHA-256 hash into a hexadecimal String representation, returning a single, readable hash string.