fix: use more compatible com.google.common.io.BaseEncoding

Use it instead of java.util.Base64 (requires newer android)
and also instead of com.google.api.client.util.Base64 (which is deprecated)

Legacy android doesn't support java.util.Base64. It was added in API level 26 - see https://developer.android.com/reference/java/util/Base64?hl=en

oauth2_http/java/com/google/auth/oauth2/DefaultPKCEProvider.java

@@ -34,7 +34,7 @@
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.security.SecureRandom;
-import java.util.Base64;
+import com.google.common.io.BaseEncoding;
 
 /**
  * Implements PKCE using only the Java standard library. See https://www.rfc-editor.org/rfc/rfc7636.
@@ -50,7 +50,7 @@ private String createCodeVerifier() {
     SecureRandom sr = new SecureRandom();
     byte[] code = new byte[MAX_CODE_VERIFIER_LENGTH];
     sr.nextBytes(code);
-    return Base64.getUrlEncoder().encodeToString(code);
+    return BaseEncoding.base64().base64Url().encode(code);
   }
 
   private CodeChallenge createCodeChallenge(String codeVerifier) {
@@ -90,7 +90,7 @@ private class CodeChallenge {
 
         byte[] digest = md.digest();
 
-        this.codeChallenge = Base64.getUrlEncoder().encodeToString(digest);
+        this.codeChallenge = BaseEncoding.base64().base64Url().encode(digest);
         this.codeChallengeMethod = "S256";
       } catch (NoSuchAlgorithmException e) {
         this.codeChallenge = codeVerifier;

oauth2_http/java/com/google/auth/oauth2/TokenVerifier.java

@@ -39,7 +39,7 @@
 import com.google.api.client.http.HttpTransport;
 import com.google.api.client.json.GenericJson;
 import com.google.api.client.json.webtoken.JsonWebSignature;
-import com.google.api.client.util.Base64;
+import com.google.common.io.BaseEncoding;
 import com.google.api.client.util.Clock;
 import com.google.api.client.util.ExponentialBackOff;
 import com.google.api.client.util.Key;
@@ -395,8 +395,8 @@ private PublicKey buildRs256PublicKey(JsonWebKey key)
       Preconditions.checkNotNull(key.e);
       Preconditions.checkNotNull(key.n);
 
-      BigInteger modulus = new BigInteger(1, Base64.decodeBase64(key.n));
-      BigInteger exponent = new BigInteger(1, Base64.decodeBase64(key.e));
+      BigInteger modulus = new BigInteger(1, BaseEncoding.base64().decode(key.n));
+      BigInteger exponent = new BigInteger(1, BaseEncoding.base64().decode(key.e));
 
       RSAPublicKeySpec spec = new RSAPublicKeySpec(modulus, exponent);
       KeyFactory factory = KeyFactory.getInstance("RSA");
@@ -408,8 +408,8 @@ private PublicKey buildEs256PublicKey(JsonWebKey key)
       Preconditions.checkArgument("EC".equals(key.kty));
       Preconditions.checkArgument("P-256".equals(key.crv));
 
-      BigInteger x = new BigInteger(1, Base64.decodeBase64(key.x));
-      BigInteger y = new BigInteger(1, Base64.decodeBase64(key.y));
+      BigInteger x = new BigInteger(1, BaseEncoding.base64().decode(key.x));
+      BigInteger y = new BigInteger(1, BaseEncoding.base64().decode(key.y));
       ECPoint pubPoint = new ECPoint(x, y);
       AlgorithmParameters parameters = AlgorithmParameters.getInstance("EC");
       parameters.init(new ECGenParameterSpec("secp256r1"));

oauth2_http/javatests/com/google/auth/oauth2/DefaultPKCEProviderTest.java

@@ -35,7 +35,7 @@
 
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
-import java.util.Base64;
+import com.google.common.io.BaseEncoding;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -52,7 +52,7 @@ public void testPkceExpected() throws NoSuchAlgorithmException {
 
     byte[] digest = md.digest();
 
-    String expectedCodeChallenge = Base64.getUrlEncoder().encodeToString(digest);
+    String expectedCodeChallenge = BaseEncoding.base64().base64Url().encode(digest);
     String expectedCodeChallengeMethod = "S256";
 
     assertEquals(pkce.getCodeChallenge(), expectedCodeChallenge);