example_test.go

     1  // Copyright 2016 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package rsa
     6  
     7  import (
     8  	"crypto"
     9  	"crypto/aes"
    10  	"crypto/cipher"
    11  	"crypto/rand"
    12  	"crypto/sha256"
    13  	"encoding/hex"
    14  	"fmt"
    15  	"io"
    16  	"os"
    17  )
    18  
    19  // RSA is able to encrypt only a very limited amount of data. In order
    20  // to encrypt reasonable amounts of data a hybrid scheme is commonly
    21  // used: RSA is used to encrypt a key for a symmetric primitive like
    22  // AES-GCM.
    23  //
    24  // Before encrypting, data is “padded” by embedding it in a known
    25  // structure. This is done for a number of reasons, but the most
    26  // obvious is to ensure that the value is large enough that the
    27  // exponentiation is larger than the modulus. (Otherwise it could be
    28  // decrypted with a square-root.)
    29  //
    30  // In these designs, when using PKCS #1 v1.5, it's vitally important to
    31  // avoid disclosing whether the received RSA message was well-formed
    32  // (that is, whether the result of decrypting is a correctly padded
    33  // message) because this leaks secret information.
    34  // DecryptPKCS1v15SessionKey is designed for this situation and copies
    35  // the decrypted, symmetric key (if well-formed) in constant-time over
    36  // a buffer that contains a random key. Thus, if the RSA result isn't
    37  // well-formed, the implementation uses a random key in constant time.
    38  func ExampleDecryptPKCS1v15SessionKey() {
    39  	// crypto/rand.Reader is a good source of entropy for blinding the RSA
    40  	// operation.
    41  	rng := rand.Reader
    42  
    43  	// The hybrid scheme should use at least a 16-byte symmetric key. Here
    44  	// we read the random key that will be used if the RSA decryption isn't
    45  	// well-formed.
    46  	key := make([]byte, 32)
    47  	if _, err := io.ReadFull(rng, key); err != nil {
    48  		panic("RNG failure")
    49  	}
    50  
    51  	rsaCiphertext, _ := hex.DecodeString("aabbccddeeff")
    52  
    53  	if err := DecryptPKCS1v15SessionKey(rng, rsaPrivateKey, rsaCiphertext, key); err != nil {
    54  		// Any errors that result will be “public” – meaning that they
    55  		// can be determined without any secret information. (For
    56  		// instance, if the length of key is impossible given the RSA
    57  		// public key.)
    58  		fmt.Fprintf(os.Stderr, "Error from RSA decryption: %s\n", err)
    59  		return
    60  	}
    61  
    62  	// Given the resulting key, a symmetric scheme can be used to decrypt a
    63  	// larger ciphertext.
    64  	block, err := aes.NewCipher(key)
    65  	if err != nil {
    66  		panic("aes.NewCipher failed: " + err.Error())
    67  	}
    68  
    69  	// Since the key is random, using a fixed nonce is acceptable as the
    70  	// (key, nonce) pair will still be unique, as required.
    71  	var zeroNonce [12]byte
    72  	aead, err := cipher.NewGCM(block)
    73  	if err != nil {
    74  		panic("cipher.NewGCM failed: " + err.Error())
    75  	}
    76  	ciphertext, _ := hex.DecodeString("00112233445566")
    77  	plaintext, err := aead.Open(nil, zeroNonce[:], ciphertext, nil)
    78  	if err != nil {
    79  		// The RSA ciphertext was badly formed; the decryption will
    80  		// fail here because the AES-GCM key will be incorrect.
    81  		fmt.Fprintf(os.Stderr, "Error decrypting: %s\n", err)
    82  		return
    83  	}
    84  
    85  	fmt.Printf("Plaintext: %s\n", string(plaintext))
    86  }
    87  
    88  func ExampleSignPKCS1v15() {
    89  	// crypto/rand.Reader is a good source of entropy for blinding the RSA
    90  	// operation.
    91  	rng := rand.Reader
    92  
    93  	message := []byte("message to be signed")
    94  
    95  	// Only small messages can be signed directly; thus the hash of a
    96  	// message, rather than the message itself, is signed. This requires
    97  	// that the hash function be collision resistant. SHA-256 is the
    98  	// least-strong hash function that should be used for this at the time
    99  	// of writing (2016).
   100  	hashed := sha256.Sum256(message)
   101  
   102  	signature, err := SignPKCS1v15(rng, rsaPrivateKey, crypto.SHA256, hashed[:])
   103  	if err != nil {
   104  		fmt.Fprintf(os.Stderr, "Error from signing: %s\n", err)
   105  		return
   106  	}
   107  
   108  	fmt.Printf("Signature: %x\n", signature)
   109  }
   110  
   111  func ExampleVerifyPKCS1v15() {
   112  	message := []byte("message to be signed")
   113  	signature, _ := hex.DecodeString("ad2766728615cc7a746cc553916380ca7bfa4f8983b990913bc69eb0556539a350ff0f8fe65ddfd3ebe91fe1c299c2fac135bc8c61e26be44ee259f2f80c1530")
   114  
   115  	// Only small messages can be signed directly; thus the hash of a
   116  	// message, rather than the message itself, is signed. This requires
   117  	// that the hash function be collision resistant. SHA-256 is the
   118  	// least-strong hash function that should be used for this at the time
   119  	// of writing (2016).
   120  	hashed := sha256.Sum256(message)
   121  
   122  	err := VerifyPKCS1v15(&rsaPrivateKey.PublicKey, crypto.SHA256, hashed[:], signature)
   123  	if err != nil {
   124  		fmt.Fprintf(os.Stderr, "Error from verification: %s\n", err)
   125  		return
   126  	}
   127  
   128  	// signature is a valid signature of message from the public key.
   129  }
   130  
   131  func ExampleEncryptOAEP() {
   132  	secretMessage := []byte("send reinforcements, we're going to advance")
   133  	label := []byte("orders")
   134  
   135  	// crypto/rand.Reader is a good source of entropy for randomizing the
   136  	// encryption function.
   137  	rng := rand.Reader
   138  
   139  	ciphertext, err := EncryptOAEP(sha256.New(), rng, &test2048Key.PublicKey, secretMessage, label)
   140  	if err != nil {
   141  		fmt.Fprintf(os.Stderr, "Error from encryption: %s\n", err)
   142  		return
   143  	}
   144  
   145  	// Since encryption is a randomized function, ciphertext will be
   146  	// different each time.
   147  	fmt.Printf("Ciphertext: %x\n", ciphertext)
   148  }
   149  
   150  func ExampleDecryptOAEP() {
   151  	ciphertext, _ := hex.DecodeString("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")
   152  	label := []byte("orders")
   153  
   154  	// crypto/rand.Reader is a good source of entropy for blinding the RSA
   155  	// operation.
   156  	rng := rand.Reader
   157  
   158  	plaintext, err := DecryptOAEP(sha256.New(), rng, test2048Key, ciphertext, label)
   159  	if err != nil {
   160  		fmt.Fprintf(os.Stderr, "Error from decryption: %s\n", err)
   161  		return
   162  	}
   163  
   164  	fmt.Printf("Plaintext: %s\n", string(plaintext))
   165  
   166  	// Remember that encryption only provides confidentiality. The
   167  	// ciphertext should be signed before authenticity is assumed and, even
   168  	// then, consider that messages might be reordered.
   169  }
   170
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