### Swift Package Dependency for libtor

Source: https://docs.21.dev/data/documentation/openssl/choosinglibcryptovsopenssl

Example of how a Swift package (`swift-tor`) can depend on `libcrypto` and `libssl` products from the `swift-openssl` package. This is common for C-based projects that need to link against a specific OpenSSL version.

```swift
dependencies: [
    .package(url: "https://github.com/21-DOT-DEV/swift-openssl.git", branch: "main"),
    .package(url: "https://github.com/21-DOT-DEV/swift-event.git", branch: "main"),
],
targets: [
    .target(
        name: "libtor",
        dependencies: [
            .product(name: "libcrypto", package: "swift-openssl"),
            .product(name: "libssl", package: "swift-openssl"),
            .product(name: "libevent", package: "swift-event"),
        ],
        // ...
    ),
],
```

--------------------------------

### SHA-256 Hashing and Runtime Version

Source: https://docs.21.dev/data/documentation/openssl

Demonstrates how to compute a SHA-256 hash for a string and retrieve the OpenSSL runtime version. Ensure the OpenSSL module is imported.

```swift
let digest = SHA256.hash(string: "Hello, World!")
print(digest.hexString)
// dffd6021bb2bd5b0af676290809ec3a53191dd81c7f70a4b28688a362182986f

print(SSL.versionString)
// OpenSSL 3.6.2 ...
```

--------------------------------

### Parse RSA Keys from PEM Format

Source: https://docs.21.dev/data/documentation/openssl/gettingstarted

This snippet demonstrates parsing RSA keys from PEM format. Note that signing and verification functionalities are not yet implemented in this MVP. Use `pemData` to round-trip the original bytes.

```swift

```

--------------------------------

### OpenSSL Package Swift Products

Source: https://docs.21.dev/data/documentation/openssl/choosinglibcryptovsopenssl

Defines the three public library products available in the OpenSSL Swift package. Note the warning about API stability for pre-1.0 versions.

```swift
products: [
    // WARNING: These APIs should not be considered stable and may change at any time.
    .library(name: "libcrypto", targets: ["libcrypto"]),
    .library(name: "libssl", targets: ["libssl"]),
    .library(name: "OpenSSL", targets: ["OpenSSL"])
],
```

--------------------------------

### Audit Runtime OpenSSL Version

Source: https://docs.21.dev/data/documentation/openssl/securityconsiderations

Use `SSL.versionString` to report the statically compiled OpenSSL version at runtime. This is crucial for cross-referencing against OpenSSL security advisories before release builds. Run this check in a release-gate CI step to fail builds if the version is below the minimum acceptable patch level.

```swift
import OpenSSL
print(SSL.versionString)
// OpenSSL 3.6.2 ...
```

--------------------------------

### Add swift-openssl Package Dependency

Source: https://docs.21.dev/data/documentation/openssl/gettingstarted

Add the swift-openssl package as a Swift Package Manager dependency. Pin an exact version to avoid breaking changes due to the package being pre-1.0.

```swift
.package(url: "https://github.com/21-DOT-DEV/swift-openssl.git", exact: "0.1.0"),
```

```swift
.target(name: "<target>", dependencies: [
    .product(name: "OpenSSL", package: "swift-openssl"),
]),
```

--------------------------------

### Encode Data to Base64URL

Source: https://docs.21.dev/data/documentation/openssl/gettingstarted

Encode data into a Base64URL string using the `encode(_:)` method. This method never produces '+', '/', or '=', ensuring URL-safe output.

```swift
let encoded = Base64URL.encode(data)
print(encoded) // "a3a59327f7c30530341741334939764204152242271924128380000000000000"
```

--------------------------------

### SHA256Digest Structure

Source: https://docs.21.dev/data/documentation/openssl/sha256/sha256digest

The SHA256Digest structure is a fixed-size wrapper for 32 bytes (256 bits) of SHA-256 hash output. It is typically generated by the `hash(data:)` or `hash(string:)` methods on the SHA256 type. Direct initialization using `init(rawValue:)` is less common but available for round-tripping digest values.

```APIDOC
## SHA256Digest

### Description
A fixed-size SHA-256 digest produced by [`hash(data:)`](/documentation/OpenSSL/SHA256/hash(data:)) or [`hash(string:)`](/documentation/OpenSSL/SHA256/hash(string:)).

### Structure Definition
```swift
struct SHA256Digest
```

### Overview
A `SHA256Digest` wraps exactly 32 raw bytes (256 bits) — the output length fixed by [FIPS PUB 180-4](https://csrc.nist.gov/publications/detail/fips/180/4/final) and carried verbatim from `SHA256_Final`’s `md` buffer. The byte order matches the C routine: index 0 holds the most significant output byte, index 31 the least, matching every published test vector (RFC 6234, NIST CAVP).

Instances are typically produced by the `static` `hash(_:)` family on [`SHA256`](/documentation/OpenSSL/SHA256). Callers rarely need [`init(rawValue:)`](/documentation/OpenSSL/SHA256/SHA256Digest/init(rawValue:)) directly, but it exists for round-tripping a stored digest (e.g. one that was base64-decoded from a Nostr event ID) back into a typed value. Digests are value types: equality is bytewise, and `Sendable` conformance allows safe sharing across concurrency domains.

> Note: `Equatable` on this type defers to `Data`’s built-in equality, which is **not** constant-time. Do not compare secret-derived digests with `==` on a side-channel-sensitive path; use a constant-time comparison primitive from your HMAC or AEAD layer instead. See <doc://OpenSSL/documentation/OpenSSL/SecurityConsiderations>.

> SeeAlso: ``doc://OpenSSL/documentation/OpenSSL/SHA256/hash(data:)``, ``doc://OpenSSL/documentation/OpenSSL/SHA256/hash(string:)``, ``doc://OpenSSL/documentation/OpenSSL/SHA256/SHA256Digest/hexString``
```

--------------------------------

### SHA256 Hashing

Source: https://docs.21.dev/data/documentation/openssl/sha256

The SHA256 enum provides static methods for hashing data. The primary entry points are `hash(data:)` and `hash(string:)`, both returning a SHA256Digest.

```APIDOC
## SHA256 Hashing

### Description
Provides static methods for hashing data using the SHA-256 algorithm. The namespace is caseless and cannot be instantiated.

### Methods
- `hash(data:)`
- `hash(string:)`

### Returns
- `SHA256Digest`: The resulting SHA-256 hash.

### See Also
- `SHA256.SHA256Digest`
```

--------------------------------

### Compute SHA-256 Digest from String

Source: https://docs.21.dev/data/documentation/openssl/gettingstarted

Use the `hash(string:)` method to compute the SHA-256 digest for UTF-8 strings. Be aware that Unicode normalization differences can alter the resulting digest.

```swift
let digest = try SHA256.hash(string: "hello world")
print(digest.hexString) // "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
```

--------------------------------

### Compute SHA-256 Digest from Data

Source: https://docs.21.dev/data/documentation/openssl/gettingstarted

Use the `hash(data:)` method to compute the SHA-256 digest for arbitrary bytes. This method is byte-exact and aligns with RFC 6234 test vectors.

```swift
let digest = try SHA256.hash(data: data)
print(digest.hexString) // "a3a59327f7c30530341741334939764204152242271924128380000000000000"
```

--------------------------------

### RSA.PrivateKey Structure

Source: https://docs.21.dev/data/documentation/openssl/rsa/privatekey

Represents an RSA private key parsed from PEM text. It accepts input in PKCS#1 or PKCS#8 format and is a Sendable value type.

```APIDOC
## RSA.PrivateKey

### Description
A typed container for an RSA private key parsed from PEM text.

### Overview
`PrivateKey` accepts PEM input in either the traditional PKCS#1 form ([RFC 8017](https://datatracker.ietf.org/doc/html/rfc8017), `-----BEGIN RSA PRIVATE KEY-----`) or the PKCS#8 form ([RFC 5958](https://datatracker.ietf.org/doc/html/rfc5958), `-----BEGIN PRIVATE KEY-----`). Instances are `Sendable` value types: constructing one copies the PEM bytes into the struct, so the resulting value can be shared across actors and tasks safely.

Today this type is primarily a typed wrapper for key material flowing through application code. It is produced by [`init(pemRepresentation:)`](/documentation/OpenSSL/RSA/PrivateKey/init(pemRepresentation:)) and round-tripped via [`pemData`](/documentation/OpenSSL/RSA/PrivateKey/pemData).

> Warning: **Signing is not yet functional.** Key parsing validates the outer PEM frame, but `PrivateKey` cannot currently produce signatures — that path requires the OpenSSL provider layer (`OSSL_PROVIDER_load`, `EVP_PKEY_sign_init`), which is not integrated in this MVP. Any calling code that reaches `EVP_PKEY_sign` through a future API will fail until providers land. See <doc://OpenSSL/documentation/OpenSSL/SecurityConsiderations> for the complete gap list.

> SeeAlso: ``doc://OpenSSL/documentation/OpenSSL/RSA/PublicKey``, ``doc://OpenSSL/documentation/OpenSSL/RSA/PrivateKey/pemData``, ``doc://OpenSSL/documentation/OpenSSL/RSA/PrivateKey/init(pemRepresentation:)``
```

--------------------------------

### RSA.PrivateKey Structure Definition

Source: https://docs.21.dev/data/documentation/openssl/rsa/privatekey

Defines the structure for a typed container of an RSA private key parsed from PEM text.

```swift
struct PrivateKey
```

--------------------------------

### Decode Base64URL String

Source: https://docs.21.dev/data/documentation/openssl/gettingstarted

Decode a Base64URL string into Data using the `decode(_:)` method. It accepts both padded and unpadded inputs and returns nil for malformed strings.

```swift
let decoded = Base64URL.decode("a3a59327f7c30530341741334939764204152242271924128380000000000000")
print(decoded! as NSData)
// <a3a59327 f7c30530 34174133 49397642 04152242 27192412 83800000 00000000>
```

--------------------------------

### Base64URL Enum Declaration

Source: https://docs.21.dev/data/documentation/openssl/base64url

Declares the Base64URL enumeration, which serves as a namespace for URL-safe unpadded Base64 encoding operations.

```swift
enum Base64URL
```

--------------------------------

### RSA.PublicKey Structure Definition

Source: https://docs.21.dev/data/documentation/openssl/rsa/publickey

Defines the PublicKey structure for OpenSSL's RSA module. This structure is a typed container for RSA public keys parsed from PEM text.

```swift
struct PublicKey

```

--------------------------------

### SHA256Digest Structure

Source: https://docs.21.dev/data/documentation/openssl/sha256/sha256digest

Defines the structure for a SHA256 digest, which is a fixed-size 32-byte output.

```swift
struct SHA256Digest
```

--------------------------------

### OpenSSL SSL Enum Declaration

Source: https://docs.21.dev/data/documentation/openssl/ssl

Declares the SSL enumeration, which serves as a namespace for utility bindings. This enum is caseless and cannot be instantiated, with all members being static.

```swift
enum SSL
```

--------------------------------

### SHA256 Enum Definition

Source: https://docs.21.dev/data/documentation/openssl/sha256

Defines the SHA256 namespace for the SHA-256 hashing algorithm.

```swift
enum SHA256
```

--------------------------------

### OpenSSLError Enumeration

Source: https://docs.21.dev/data/documentation/openssl/opensslerror

The OpenSSLError enumeration serves as the single failure surface for the OpenSSL Swift module. Each case includes a human-readable String describing the specific failure, which is surfaced via `errorDescription` to Foundation's `LocalizedError`.

```APIDOC
## Enumeration: OpenSSLError

### Description
The unified error type thrown by every `OpenSSL` Swift API. It provides a single failure surface for the module, with each case carrying a human-readable `String` describing the specific failure condition.

### Associated Values
Instances are produced exclusively by the package's Swift code. Equality compares the case and the associated `String`.

### Future Cases
Some cases are reserved for future API surface and are not produced by any code path shipping today. They are included to allow for non-breaking additions of corresponding operations later.

- `signingFailed(_:)`
- `verificationFailed(_:)
- `invalidSignature(_:)

### See Also
- `doc://OpenSSL/documentation/OpenSSL/RSA/PrivateKey/init(pemRepresentation:)`
- `doc://OpenSSL/documentation/OpenSSL/SHA256/SHA256Digest/init(rawValue:)`
- [Foundation `LocalizedError`](https://developer.apple.com/documentation/foundation/localizederror)
```

--------------------------------

### OpenSSLError Enumeration Definition

Source: https://docs.21.dev/data/documentation/openssl/opensslerror

Defines the unified error type for all OpenSSL Swift APIs. Each case includes a human-readable string for error descriptions.

```swift
enum OpenSSLError
```

--------------------------------

### RSA Namespace Declaration

Source: https://docs.21.dev/data/documentation/openssl/rsa

Declares the RSA namespace, which groups RSA-related cryptographic types and operations within the OpenSSL framework. This is a caseless enum and cannot be instantiated.

```swift
enum RSA
```

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