// 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 import ( "crypto" "crypto/aes" "crypto/cipher" "crypto/rand" "crypto/sha256" "encoding/hex" "fmt" "io" "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() { // crypto/rand.Reader is a good source of entropy for blinding the RSA // operation. rng := rand.Reader // 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 := io.ReadFull(rng, key); err != nil { panic("RNG failure") } rsaCiphertext, _ := hex.DecodeString("aabbccddeeff") if err := DecryptPKCS1v15SessionKey(rng, 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() { // crypto/rand.Reader is a good source of entropy for blinding the RSA // operation. rng := rand.Reader 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 := SignPKCS1v15(rng, 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 := 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 := 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("4d1ee10e8f286390258c51a5e80802844c3e6358ad6690b7285218a7c7ed7fc3a4c7b950fbd04d4b0239cc060dcc7065ca6f84c1756deb71ca5685cadbb82be025e16449b905c568a19c088a1abfad54bf7ecc67a7df39943ec511091a34c0f2348d04e058fcff4d55644de3cd1d580791d4524b92f3e91695582e6e340a1c50b6c6d78e80b4e42c5b4d45e479b492de42bbd39cc642ebb80226bb5200020d501b24a37bcc2ec7f34e596b4fd6b063de4858dbf5a4e3dd18e262eda0ec2d19dbd8e890d672b63d368768360b20c0b6b8592a438fa275e5fa7f60bef0dd39673fd3989cc54d2cb80c08fcd19dacbc265ee1c6014616b0e04ea0328c2a04e73460") label := []byte("orders") // crypto/rand.Reader is a good source of entropy for blinding the RSA // operation. rng := rand.Reader plaintext, err := DecryptOAEP(sha256.New(), rng, 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. }