secretive/SecretKit/Common/OpenSSH/OpenSSHKeyWriter.swift

import Foundation
import CryptoKit

// For the moment, only supports ecdsa-sha2-nistp256 and ecdsa-sha2-nistp386 keys
public struct OpenSSHKeyWriter {

    public init() {
    }

    public func data<SecretType: Secret>(secret: SecretType) -> Data {
        lengthAndData(of: curveType(for: secret.algorithm, length: secret.keySize).data(using: .utf8)!) +
            lengthAndData(of: curveIdentifier(for: secret.algorithm, length: secret.keySize).data(using: .utf8)!) +
            lengthAndData(of: secret.publicKey)
    }

    public func openSSHString<SecretType: Secret>(secret: SecretType) -> String {
        "\(curveType(for: secret.algorithm, length: secret.keySize)) \(data(secret: secret).base64EncodedString())"
    }

    public func openSSHFingerprint<SecretType: Secret>(secret: SecretType) -> String {
        Insecure.MD5.hash(data: data(secret: secret))
            .compactMap { ("0" + String($0, radix: 16, uppercase: false)).suffix(2) }
            .joined(separator: ":")
    }

}

extension OpenSSHKeyWriter {

    public func lengthAndData(of data: Data) -> Data {
        let rawLength = UInt32(data.count)
        var endian = rawLength.bigEndian
        return Data(bytes: &endian, count: UInt32.bitWidth/8) + data
    }

    public func curveIdentifier(for algorithm: Algorithm, length: Int) -> String {
        switch algorithm {
        case .ellipticCurve:
            return "nistp" + String(describing: length)
        }
    }

    public func curveType(for algorithm: Algorithm, length: Int) -> String {
        switch algorithm {
        case .ellipticCurve:
            return "ecdsa-sha2-nistp" + String(describing: length)
        }
    }
}