chore: Add ECDH examples

Examples/runtimes/java/DynamoDbEncryption/src/test/java/software/amazon/cryptography/examples/keyring/TestKmsEcdhKeyringExample.java

@@ -12,8 +12,10 @@ public class TestKmsEcdhKeyringExample {
 
   @Test
   public void TestKmsEcdhKeyringExampleStatic() {
-    // You may provide your own ECC public keys at EXAMPLE_ECC_PUBLIC_KEY_SENDER_FILENAME
-    // and EXAMPLE_ECC_PUBLIC_KEY_RECIPIENT_FILENAME.
+    // You may provide your own ECC public keys at
+    // - EXAMPLE_ECC_PUBLIC_KEY_SENDER_FILENAME
+    // - EXAMPLE_ECC_PUBLIC_KEY_RECIPIENT_FILENAME.
+    // If you provide these, the keys MUST be on curve P256
     // This must be the public key for the ECC key represented at eccKeyArn
     // If this file is not present, this will write a UTF-8 encoded PEM file for you.
     if (shouldGetNewPublicKeys()) {

Examples/runtimes/java/DynamoDbEncryption/src/test/java/software/amazon/cryptography/examples/keyring/TestRawEcdhKeyringExample.java

@@ -13,6 +13,7 @@ public void TestStaticRawEcdhKeyringExample() {
     // You may provide your own ECC Key pairs in the files located at
     // - EXAMPLE_ECC_PRIVATE_KEY_FILENAME_SENDER
     // - EXAMPLE_ECC_PUBLIC_KEY_FILENAME_RECIPIENT
+    // If you provide this, the keys MUST be on curve P256
     // If these files are not present, this will generate a pair for you.
     // For this example we will use the curve P256.
     if (RawEcdhKeyringExample.shouldGenerateNewEccKeyPairs()) {
@@ -31,6 +32,7 @@ public void TestStaticRawEcdhKeyringExample() {
   public void TestEphemeralRawEcdhKeyringExample() {
     // You may provide your own ECC Public Key in the files located at
     // - EXAMPLE_ECC_PUBLIC_KEY_FILENAME_RECIPIENT
+    // If you provide this, the keys MUST be on curve P256
     // If these files are not present, this will generate a pair for you.
     // For this example we will use the curve P256.
     if (RawEcdhKeyringExample.shouldGenerateNewEccKeyPairs()) {
@@ -50,6 +52,7 @@ public void TestDiscoveryRawEcdhKeyringExample() {
     // You may provide your own ECC Public Key in the files located at
     // - EXAMPLE_ECC_PUBLIC_KEY_FILENAME_RECIPIENT
     // - EXAMPLE_ECC_PRIVATE_KEY_FILENAME_RECIPIENT
+    // If you provide this, the keys MUST be on curve P256
     // If these files are not present, this will generate a pair for you.
     // For this example we will use the curve P256.
     if (RawEcdhKeyringExample.shouldGenerateNewEccKeyPairs()) {