Allow empty plaintext for transit encryption.

We now allow construction and encryption of empty Plaintext objects. Empty plaintext can make sense from a security perspective.

Closes gh-223.

spring-vault-core/src/main/java/org/springframework/vault/support/Plaintext.java

@@ -18,7 +18,6 @@ package org.springframework.vault.support;
 import lombok.EqualsAndHashCode;
 
 import org.springframework.util.Assert;
-import org.springframework.util.ObjectUtils;
 
 /**
  * Value object representing plaintext with an optional {@link VaultTransitContext}.
@@ -44,13 +43,12 @@ public class Plaintext {
 	/**
 	 * Factory method to create {@link Plaintext} from a byte sequence.
 	 *
-	 * @param plaintext the plaintext to encrypt, must not be {@literal null} or empty.
+	 * @param plaintext the plaintext to encrypt, must not be {@literal null}.
 	 * @return the {@link Plaintext} for {@code plaintext}.
 	 */
 	public static Plaintext of(byte[] plaintext) {
 
-		Assert.isTrue(!ObjectUtils.isEmpty(plaintext),
-				"Plaintext must not be null or empty");
+		Assert.notNull(plaintext, "Plaintext must not be null");
 
 		return new Plaintext(plaintext, VaultTransitContext.empty());
 	}
@@ -60,12 +58,12 @@ public class Plaintext {
 	 * {@link String} is encoded to {@code byte} using the default
 	 * {@link java.nio.charset.Charset}.
 	 *
-	 * @param plaintext the plaintext to encrypt, must not be {@literal null} or empty.
+	 * @param plaintext the plaintext to encrypt, must not be {@literal null}.
 	 * @return the {@link Plaintext} for {@code plaintext}.
 	 */
 	public static Plaintext of(String plaintext) {
 
-		Assert.hasText(plaintext, "Plaintext must not be null or empty");
+		Assert.notNull(plaintext, "Plaintext must not be null");
 
 		return of(plaintext.getBytes());
 	}

spring-vault-core/src/test/java/org/springframework/vault/core/VaultTransitTemplateIntegrationTests.java

@@ -293,6 +293,24 @@ public class VaultTransitTemplateIntegrationTests extends IntegrationTestSupport
 		assertThat(ciphertext).startsWith("vault:v1:");
 	}
 
+	@Test
+	public void encryptShouldEncryptEmptyValues() {
+
+		transitOperations.createKey("mykey", VaultTransitKeyCreationRequest.builder()
+				.convergentEncryption(true).derived(true).build());
+
+		VaultTransitContext context = VaultTransitContext.builder()
+				.context("blubb".getBytes()) //
+				.nonce("123456789012".getBytes()) //
+				.build();
+
+		Ciphertext ciphertext = transitOperations.encrypt("mykey",
+				Plaintext.of("").with(context));
+
+		assertThat(ciphertext.getCiphertext()).startsWith("vault:v1:");
+		assertThat(ciphertext.getContext()).isEqualTo(context);
+	}
+
 	@Test
 	public void encryptShouldCreateWrappedCiphertextWithNonceAndContext() {