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 *    * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
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 * in the documentation and/or other materials provided with the
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 */

package com.google.auth.oauth2;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Base64;

/**
 * Implements PKCE using only the Java standard library. See https://www.rfc-editor.org/rfc/rfc7636.
 *
 * <p>https://developers.google.com/identity/protocols/oauth2/native-app#step1-code-verifier.
 */
public class DefaultPKCEProvider implements PKCEProvider {
  private String codeVerifier;
  private CodeChallenge codeChallenge;
  private static final int MAX_CODE_VERIFIER_LENGTH = 127;

  private String createCodeVerifier() {
    SecureRandom sr = new SecureRandom();
    byte[] code = new byte[MAX_CODE_VERIFIER_LENGTH];
    sr.nextBytes(code);
    return Base64.getUrlEncoder().encodeToString(code);
  }

  private CodeChallenge createCodeChallenge(String codeVerifier) {
    return new DefaultPKCEProvider.CodeChallenge(codeVerifier);
  }

  public DefaultPKCEProvider() {
    this.codeVerifier = createCodeVerifier();
    this.codeChallenge = createCodeChallenge(this.codeVerifier);
  }

  @Override
  public String getCodeVerifier() {
    return codeVerifier;
  }

  @Override
  public String getCodeChallenge() {
    return codeChallenge.getCodeChallenge();
  }

  @Override
  public String getCodeChallengeMethod() {
    return codeChallenge.getCodeChallengeMethod();
  }

  /** Class representing the Code Challenge derived from a Code Verifier string. */
  private class CodeChallenge {
    private String codeChallenge;
    private String codeChallengeMethod;

    CodeChallenge(String codeVerifier) {
      try {
        byte[] bytes = codeVerifier.getBytes();
        MessageDigest md = MessageDigest.getInstance("SHA-256");
        md.update(bytes);

        byte[] digest = md.digest();

        this.codeChallenge = Base64.getUrlEncoder().encodeToString(digest);
        this.codeChallengeMethod = "S256";
      } catch (NoSuchAlgorithmException e) {
        this.codeChallenge = codeVerifier;
        this.codeChallengeMethod = "plain";
      }
    }

    public String getCodeChallenge() {
      return codeChallenge;
    }

    public String getCodeChallengeMethod() {
      return codeChallengeMethod;
    }
  }
}