Merge pull request #20523 from m-kay

* pr/20523:
  Polish "Configure signature algorithm when using public key jwt decoder"
  Configure signature algorithm when using public key jwt decoder

Closes gh-20523

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/servlet/OAuth2ResourceServerJwtConfiguration.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -78,7 +78,8 @@ class OAuth2ResourceServerJwtConfiguration {
 		JwtDecoder jwtDecoderByPublicKeyValue() throws Exception {
 			RSAPublicKey publicKey = (RSAPublicKey) KeyFactory.getInstance("RSA")
 					.generatePublic(new X509EncodedKeySpec(getKeySpec(this.properties.readPublicKey())));
-			return NimbusJwtDecoder.withPublicKey(publicKey).build();
+			return NimbusJwtDecoder.withPublicKey(publicKey)
+					.signatureAlgorithm(SignatureAlgorithm.from(this.properties.getJwsAlgorithm())).build();
 		}
 
 		private byte[] getKeySpec(String keyValue) {

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/security/oauth2/resource/servlet/OAuth2ResourceServerAutoConfigurationTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -190,6 +190,16 @@ class OAuth2ResourceServerAutoConfigurationTests {
 						.hasMessageContaining("Public key location does not exist"));
 	}
 
+	@Test
+	void autoConfigurationShouldFailIfAlgorithmIsInvalid() {
+		this.contextRunner
+				.withPropertyValues(
+						"spring.security.oauth2.resourceserver.jwt.public-key-location=classpath:public-key-location",
+						"spring.security.oauth2.resourceserver.jwt.jws-algorithm=NOT_VALID")
+				.run((context) -> assertThat(context).hasFailed().getFailure()
+						.hasMessageContaining("signatureAlgorithm cannot be null"));
+	}
+
 	@Test
 	void autoConfigurationWhenSetUriKeyLocationAndIssuerUriPresentShouldUseSetUri() {
 		this.contextRunner