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Simple generically-derived cryptographic digestion

Gastronomy provides a range of common cryptographic operations through a simple, typesafe and immutable API.


  • hashing of simple and primitive Scala types
  • generically-derived digests for all product and coproduct types
  • supports SHA-256, SHA-1 and MD5 hash algorithms
  • symmetric encryption with AES
  • asymmetric encryption/decryption using RSA
  • signing with DSA
  • AES, RSA and DSA key generation
  • calculation of HMACs for SHA-256, SHA-1 and MD5
  • encoding into Hex, BASE-64, and URL-safe BASE-64
  • serializers and parsers for PEM-encoded data

Availability Plan

Gastronomy has not yet been published. The medium-term plan is to build Gastronomy with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Gastronomy.

Subsequently, Gastronomy will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

Getting Started

All Gastronomy terms and types are defined in the gastronomy package:

import gastronomy.*

Gastronomy provides representations of public, private and symmetric keys which offer a number of cryptographic methods:

  • PublicKey provides:
    • verify for verifying signatures
    • encrypt for encrypting data
    • pem to provide the public key as a PEM-encoded string
  • PrivateKey provides:
    • sign for signing data
    • decrypt for decrypting encrypted data
    • public to derive a PublicKey from the PrivateKey
    • pem to provide the private key as a PEM-encoded string
  • SymmetricKey provides verify, encrypt, pem, sign and decrypt in a single key.

Additionally, the extension methods, digest and hmac are provided for any value which can be serialized to bytes.

The objects PrivateKey and SymmetricKey both have generate methods which will generate new random keys.


Given, for example, a PrivateKey[Dsa[1024]] instance, key, data may be signed with, for example,

val key: PrivateKey[Dsa[1024]] = ???
val signature: Signature[Dsa[1024]] = key.sign(data)

This works for any value, data, that has an appropriate ByteCodec instance. The type parameter of the signature will depend on the type parameter of the private key.

Verifying a signature

A public key, pubKey, which could, for example, be derived from the private key in the previous example,

val pubKey = key.public

may be used to verify a signature of type Signature[Dsa[1024]] with:

val valid: Boolean = pubKey.verify(data, signature)

Here, data must be the same object that was used (with the private key) to produce the signature, and may be any type that has a contextual ByteCodec instance.


A public key instance, for example, pubKey of type PublicKey[Rsa[2048]], can encrypt some data by calling,

val encrypted: Message[Rsa[2048]] = pubKey.encrypt(data)


An encrypted message may conversely be decrypted using the corresponding PrivateKey[Rsa[2048]] instance, key:

val data: String = key.decrypt[String](encrypted)

The return type (String in the above example) must be specified as a parameter to the decrypt method, and may be any type for which a corresponding ByteCodec exists in context. However, the type should be the same as the type of the object that was originally encrypted, otherwise it may fail to decode.


A cryptographic digest (or hash) of any value may be calculated by calling digest[A] on that value, for an appropriate choice of A, provided a Hashable instance is in context for that type of object. Hashable instances exist for Strings, primitive types, sequences of these types, and product and coproduct types consisting of just other hashable types.

Cryptographic digests have the type Digest[A] where A is the algorithm type.

For example,

val digest: Digest[Sha2[384]] = (10, "alpha", 'z').digest[Sha2[384]]


Any value whose type has a corresponding ByteCodec instance in context may have an HMAC value calculated, of type Hmac[A] (where A is the cryptographic algorithm). As a parameter, this needs an IArray[Byte] representing (in some form) the key to be used.

Here is an example using SHA-512:

val hmac: Hmac[Sha2[512]] = "Hello world".hmac("secret".bytes)

Type inference

Whenever an expression is used in a position with an expected type, the type parameters of the methods decrypt, digest and hmac may be omitted, for example given the case class,

case class Block(digest: Digest[Sha2[256]], json: Json, hmac: Hmac[Sha2[512]])

we can instantiate it with just,

val block = Block(data.digest, data.decrypt, value.hmac)

Alternatively, a particular given may be imported directly into the current scope to prioritize it, such that it may be used in preference to the alternatives.

Byte data

Representations of binary data are common with low-level cryptographic operations. All operations in Gastronomy use the immutable IArray[Byte] type as the underlying representation of binary data, but typically wrap the data in a type which more precisely indicates the content of that data.

These types include the key types, PublicKey, PrivateKey and SymmetricKey, and result types, Signature, Hmac, Digest and Message.

These types are all further refined with a type parameter representing the cryptographic algorithm associated with that data. For example, an MD5 digest is typed as, Digest[Md5] and a 384-bit SHA-2 HMAC has the type, Hmac[Sha2[384]].

In order to make it easier to share these values, they can be encoded to and from Strings using a number of different encodings:

  • binary (Binary)
  • hexadecimal (Hex)
  • BASE-64 (Base64)
  • URL-safe BASE-64 (Base64Url)

The encode method, which exists as an extension on IArray[Byte], as well as (directly) on all types representing byte data. It takes one of these as a type parameter to produce a String of that data, encoded with the specified encoding.


Gastronomy's cryptographic functions are implemented through different algorithms, which are represented by types. Their names follow the conventions of other Scala types, hence:

Additionally, the types Base64, Base64Url, Hex and Binary represent non-cryptographic byte-to-string encodings.

Generating keys

The PrivateKey object provides the generate[A]() method, where A is Rsa[B], Dsa[B] or Aes[B] for an appropriate choice of B.

The algorithm Aes[B] can also be used with the SymmetricKey object to get a symmetric key which has the functionality of both a public and private key.

Byte codecs

Any object which can be serialized to bytes may be digested, signed, verified, HMACked or encrypted, and can be returned from a decryption operation, provided a corresponding ByteCodec instance is available for that type.

ByteCodecs are provided for IArray[Byte] (trivially) and for Strings (assuming a UTF-8 encoding).

PEM encoding

The Privacy-Enhanced Mail format is commonly used for exchanging cryptographic values safely in ASCII-only environments.

A Pem type is provided for reading, writing and representing this data. The case class Pem has two fields: kind, which is the label that appears after the words BEGIN and END in the serialized format, and data, which is an IArray[Byte] of the byte data.

The serialize method will produce a String of the data, properly encoded as BASE-64, and delimited.

The method Pem.parse will attempt to parse a String containing PEM-encoded data.

All Gastronomy's key types offer a pem method which will return an appropriately-labelled Pem value containing that key, however to avoid the risk of accidentally exposing a private key, the pem method of PrivateKey must be called with a special singleton value, like so:


Other Cryptographic Algorithms

Gastronomy may be easily extended to support other cryptographic algorithms. The existing implementations of Rsa, Dsa, Aes, Sha1, Sha2 and Md5 should be studied to investigate this possibility.


Gastronomy is classified as fledgling. For reference, Scala One projects are categorized into one of the following five stability levels:

  • embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
  • fledgling: of proven utility, seeking contributions, but liable to significant redesigns
  • maturescent: major design decisions broady settled, seeking probatory adoption and refinement
  • dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
  • adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Gastronomy is designed to be small. Its entire source code currently consists of 513 lines of code.


Gastronomy will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Gastronomy?".

  1. Copy the sources into your own project

    Read the fury file in the repository root to understand Gastronomy's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

    The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

  2. Build with Wrath

    Wrath is a bootstrapping script for building Gastronomy and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

    Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

    Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of gastronomy. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Gastronomy's dependencies.

    If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.


Contributors to Gastronomy are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Gastronomy easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.


Gastronomy was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.


Gastronomy is named after the art and science of "good eating", which leads to digestion, since the library consumes data to produce digests (but has subsequently grown in scope).

In general, Scala One project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.


The logo shows a slice of avocado, a gastronomic delight.


Gastronomy is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.