Cryptopasta

TL;DR- Copy & paste your crypto code from here instead of Stack Overflow.

This library demonstrates a suite of basic cryptography from the Go standard library. To the extent possible, it tries to hide complexity and help you avoid common mistakes. The recommendations were chosen as a compromise between cryptographic qualities, the Go standard lib, and my existing use cases.

Some particular design choices I've made:

1. SHA-512/256 has been chosen as the default hash for the examples. It's faster on 64-bit machines and immune to length extension. If it doesn't work in your case, replace instances of it with ordinary SHA-256.

2. The specific ECDSA parameters were chosen to be compatible with RFC7518[1] while using the best implementation of ECDSA available. Go's P-256 is constant-time (which prevents certain types of attacks) while its P-384 and P-521 are not.

3. Key parameters are arrays rather than slices so the compiler can help you avoid mixing up the arguments. The signing and marshaling functions use the crypto/ecdsa key types directly for the same reason.

4. Public/private keypairs for signing are marshaled into and out of PEM format, making them relatively portable to other crypto software you're likely to use (openssl, cfssl, etc).

5. Key generation functions will panic if they can't read enough random bytes to generate the key. Key generation is critical, and if crypto/rand fails at that stage then you should stop doing cryptography on that machine immediately.

6. The license is a CC0 public domain dedication, with the intent that you can just copy bits of this directly into your code and never be required to acknowledge my copyright, provide source code, or do anything else commonly associated with open licenses.

The specific recommendations are:

Encryption - 256-bit AES-GCM with random 96-bit nonces

Using AES-GCM (instead of AES-CBC, AES-CFB, or AES-CTR, all of which Go also offers) provides authentication in addition to confidentiality. This means that the content of your data is hidden and that any modification of the encrypted data will result in a failure to decrypt. This rules out entire classes of possible attacks. Randomized nonces remove the choices around nonce generation and management, which are another common source of error in crypto implementations.

The interfaces in this library allow only the use of 256-bit keys.

Hashing - HMAC-SHA512/256

Using hash functions directly is fraught with various perils – it's common for developers to accidentally write code that is subject to easy collision or length extension attacks. HMAC is a function built on top of hashes and it doesn't have those problems. Using SHA-512/256 as the underlying hash function means the process will be faster on 64-bit machines, but the output will be the same length as the more familiar SHA-256.

This interface encourages you to scope your hashes with an English-language string (a "tag") that describes the purpose of the hash. Tagged hashes are a common "security hygiene" measure to ensure that hashing the same data for different purposes will produce different outputs.

Password hashing - bcrypt with work factor 14

Use this to store users' passwords and check them for login (e.g. in a web backend). While they both have "hashing" in the name, password hashing is an entirely different situation from ordinary hashing and requires its own specialized algorithm. bcrypt is a hash function designed for password storage. It can be made selectively slower (based on a "work factor") to increase the difficulty of brute-force password cracking attempts.

As of 2016, a work factor of 14 should be well on the side of future-proofing over performance. If it turns out to be too slow for your needs, you can try using 13 or even 12. You should not go below work factor 12.

Symmetric Signatures / Message Authentication - HMAC-SHA512/256

When two parties share a secret key, they can use message authentication to make sure that a piece of data hasn't been altered. You can think of it as a "symmetric signature" - it proves both that the data is unchanged and that someone who knows the shared secret key generated it. Anyone who does not know the secret key can neither validate the data nor make valid alterations.

This comes up most often in the context of web stuff, such as:

1. Authenticating requests to your API. The most widely known example is probably the Amazon AWS API, which requires you to sign requests with HMAC-SHA256. In this type of use, the "secret key" is a token that the API provider issues to authorized API users.

2. Validating authenticated tokens (cookies, JWTs, etc) that are issued by a service but are stored by a user. In this case, the service wants to ensure that a user doesn't modify the data contained in the token.

As with encryption, you should always use a 256-bit random key to authenticate messages.

Asymmetric Signatures - ECDSA on P-256 with SHA-256 message digests

These are the classic public/private keypair signatures that you probably think of when you hear the word "signature". The holder of a private key can sign data that anyone who has the corresponding public key can verify.

Go takes very good care of us here. In particular, the Go implementation of P-256 is constant time to protect against side-channel attacks, and the Go implementation of ECDSA generates safe nonces to protect against the type of repeated-nonce attack that broke the PS3.

In terms of JWTs, this algorithm is called "ES256". The functions "EncodeSignatureJWT" and "DecodeSignatureJWT" will convert the basic signature format to and from the encoding specified by RFC7515[2]

[1] https://tools.ietf.org/html/rfc7518#section-3.1 [2] https://tools.ietf.org/html/rfc7515#appendix-A.3