1 files changed, 157 insertions, 0 deletions
diff --git a/src/crypto/rsa/example_test.go b/src/crypto/rsa/example_test.go
new file mode 100644
index 0000000..d07ee7d
--- /dev/null
+++ b/src/crypto/rsa/example_test.go
@@ -0,0 +1,157 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package rsa_test
+
+import (
+	"crypto"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"os"
+)
+
+// RSA is able to encrypt only a very limited amount of data. In order
+// to encrypt reasonable amounts of data a hybrid scheme is commonly
+// used: RSA is used to encrypt a key for a symmetric primitive like
+// AES-GCM.
+//
+// Before encrypting, data is “padded” by embedding it in a known
+// structure. This is done for a number of reasons, but the most
+// obvious is to ensure that the value is large enough that the
+// exponentiation is larger than the modulus. (Otherwise it could be
+// decrypted with a square-root.)
+//
+// In these designs, when using PKCS #1 v1.5, it's vitally important to
+// avoid disclosing whether the received RSA message was well-formed
+// (that is, whether the result of decrypting is a correctly padded
+// message) because this leaks secret information.
+// DecryptPKCS1v15SessionKey is designed for this situation and copies
+// the decrypted, symmetric key (if well-formed) in constant-time over
+// a buffer that contains a random key. Thus, if the RSA result isn't
+// well-formed, the implementation uses a random key in constant time.
+func ExampleDecryptPKCS1v15SessionKey() {
+	// The hybrid scheme should use at least a 16-byte symmetric key. Here
+	// we read the random key that will be used if the RSA decryption isn't
+	// well-formed.
+	key := make([]byte, 32)
+	if _, err := rand.Read(key); err != nil {
+		panic("RNG failure")
+	}
+
+	rsaCiphertext, _ := hex.DecodeString("aabbccddeeff")
+
+	if err := rsa.DecryptPKCS1v15SessionKey(nil, rsaPrivateKey, rsaCiphertext, key); err != nil {
+		// Any errors that result will be “public” – meaning that they
+		// can be determined without any secret information. (For
+		// instance, if the length of key is impossible given the RSA
+		// public key.)
+		fmt.Fprintf(os.Stderr, "Error from RSA decryption: %s\n", err)
+		return
+	}
+
+	// Given the resulting key, a symmetric scheme can be used to decrypt a
+	// larger ciphertext.
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		panic("aes.NewCipher failed: " + err.Error())
+	}
+
+	// Since the key is random, using a fixed nonce is acceptable as the
+	// (key, nonce) pair will still be unique, as required.
+	var zeroNonce [12]byte
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		panic("cipher.NewGCM failed: " + err.Error())
+	}
+	ciphertext, _ := hex.DecodeString("00112233445566")
+	plaintext, err := aead.Open(nil, zeroNonce[:], ciphertext, nil)
+	if err != nil {
+		// The RSA ciphertext was badly formed; the decryption will
+		// fail here because the AES-GCM key will be incorrect.
+		fmt.Fprintf(os.Stderr, "Error decrypting: %s\n", err)
+		return
+	}
+
+	fmt.Printf("Plaintext: %s\n", string(plaintext))
+}
+
+func ExampleSignPKCS1v15() {
+	message := []byte("message to be signed")
+
+	// Only small messages can be signed directly; thus the hash of a
+	// message, rather than the message itself, is signed. This requires
+	// that the hash function be collision resistant. SHA-256 is the
+	// least-strong hash function that should be used for this at the time
+	// of writing (2016).
+	hashed := sha256.Sum256(message)
+
+	signature, err := rsa.SignPKCS1v15(nil, rsaPrivateKey, crypto.SHA256, hashed[:])
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error from signing: %s\n", err)
+		return
+	}
+
+	fmt.Printf("Signature: %x\n", signature)
+}
+
+func ExampleVerifyPKCS1v15() {
+	message := []byte("message to be signed")
+	signature, _ := hex.DecodeString("ad2766728615cc7a746cc553916380ca7bfa4f8983b990913bc69eb0556539a350ff0f8fe65ddfd3ebe91fe1c299c2fac135bc8c61e26be44ee259f2f80c1530")
+
+	// Only small messages can be signed directly; thus the hash of a
+	// message, rather than the message itself, is signed. This requires
+	// that the hash function be collision resistant. SHA-256 is the
+	// least-strong hash function that should be used for this at the time
+	// of writing (2016).
+	hashed := sha256.Sum256(message)
+
+	err := rsa.VerifyPKCS1v15(&rsaPrivateKey.PublicKey, crypto.SHA256, hashed[:], signature)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error from verification: %s\n", err)
+		return
+	}
+
+	// signature is a valid signature of message from the public key.
+}
+
+func ExampleEncryptOAEP() {
+	secretMessage := []byte("send reinforcements, we're going to advance")
+	label := []byte("orders")
+
+	// crypto/rand.Reader is a good source of entropy for randomizing the
+	// encryption function.
+	rng := rand.Reader
+
+	ciphertext, err := rsa.EncryptOAEP(sha256.New(), rng, &test2048Key.PublicKey, secretMessage, label)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error from encryption: %s\n", err)
+		return
+	}
+
+	// Since encryption is a randomized function, ciphertext will be
+	// different each time.
+	fmt.Printf("Ciphertext: %x\n", ciphertext)
+}
+
+func ExampleDecryptOAEP() {
+	ciphertext, _ := hex.DecodeString("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")
+	label := []byte("orders")
+
+	plaintext, err := rsa.DecryptOAEP(sha256.New(), nil, test2048Key, ciphertext, label)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error from decryption: %s\n", err)
+		return
+	}
+
+	fmt.Printf("Plaintext: %s\n", string(plaintext))
+
+	// Remember that encryption only provides confidentiality. The
+	// ciphertext should be signed before authenticity is assumed and, even
+	// then, consider that messages might be reordered.
+}