rsa

package
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 Go to latest Published: Feb 13, 2016 License: BSD-3-Clause Imports: 8 Imported by: 0

Documentation

Overview

Package rsa implements RSA encryption as specified in PKCS#1.

RSA is a single, fundamental operation that is used in this package to implement either public-key encryption or public-key signatures.

The original specification for encryption and signatures with RSA is PKCS#1 and the terms "RSA encryption" and "RSA signatures" by default refer to PKCS#1 version 1.5. However, that specification has flaws and new designs should use version two, usually called by just OAEP and PSS, where possible.

Two sets of interfaces are included in this package. When a more abstract interface isn't neccessary, there are functions for encrypting/decrypting with v1.5/OAEP and signing/verifying with v1.5/PSS. If one needs to abstract over the public-key primitive, the PrivateKey struct implements the Decrypter and Signer interfaces from the crypto package.

Index

Examples

Constants

View Source 
const (
	// PSSSaltLengthAuto causes the salt in a PSS signature to be as large
	// as possible when signing, and to be auto-detected when verifying.
	PSSSaltLengthAuto = 0
	// PSSSaltLengthEqualsHash causes the salt length to equal the length
	// of the hash used in the signature.
	PSSSaltLengthEqualsHash = -1
)

Variables

View Source 
var ErrDecryption = errors.New("crypto/rsa: decryption error")

ErrDecryption represents a failure to decrypt a message. It is deliberately vague to avoid adaptive attacks.

View Source 
var ErrMessageTooLong = errors.New("crypto/rsa: message too long for RSA public key size")

ErrMessageTooLong is returned when attempting to encrypt a message which is too large for the size of the public key.

View Source 
var ErrVerification = errors.New("crypto/rsa: verification error")

ErrVerification represents a failure to verify a signature. It is deliberately vague to avoid adaptive attacks.

Functions

func DecryptOAEP 

func DecryptOAEP(hash hash.Hash, random io.Reader, priv *PrivateKey, ciphertext []byte, label []byte) (msg []byte, err error)

OAEP is parameterised by a hash function that is used as a random oracle. Encryption and decryption of a given message must use the same hash function and sha256.New() is a reasonable choice.

The random parameter, if not nil, is used to blind the private-key operation and avoid timing side-channel attacks. Blinding is purely internal to this function – the random data need not match that used when encrypting.

The label parameter must match the value given when encrypting. See EncryptOAEP for details.

Example 
ciphertext, _ := hex.DecodeString("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")
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.

Output:

func DecryptPKCS1v15 

func DecryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (out []byte, err error)

DecryptPKCS1v15 decrypts a plaintext using RSA and the padding scheme from PKCS#1 v1.5. If rand != nil, it uses RSA blinding to avoid timing side-channel attacks.

Note that whether this function returns an error or not discloses secret information. If an attacker can cause this function to run repeatedly and learn whether each instance returned an error then they can decrypt and forge signatures as if they had the private key. See DecryptPKCS1v15SessionKey for a way of solving this problem.

func DecryptPKCS1v15SessionKey 

func DecryptPKCS1v15SessionKey(rand io.Reader, priv *PrivateKey, ciphertext []byte, key []byte) (err error)

DecryptPKCS1v15SessionKey decrypts a session key using RSA and the padding scheme from PKCS#1 v1.5. If rand != nil, it uses RSA blinding to avoid timing side-channel attacks. It returns an error if the ciphertext is the wrong length or if the ciphertext is greater than the public modulus. Otherwise, no error is returned. If the padding is valid, the resulting plaintext message is copied into key. Otherwise, key is unchanged. These alternatives occur in constant time. It is intended that the user of this function generate a random session key beforehand and continue the protocol with the resulting value. This will remove any possibility that an attacker can learn any information about the plaintext. See “Chosen Ciphertext Attacks Against Protocols Based on the RSA Encryption Standard PKCS #1”, Daniel Bleichenbacher, Advances in Cryptology (Crypto '98).

Note that if the session key is too small then it may be possible for an attacker to brute-force it. If they can do that then they can learn whether a random value was used (because it'll be different for the same ciphertext) and thus whether the padding was correct. This defeats the point of this function. Using at least a 16-byte key will protect against this attack.

Example

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. 

// 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))

Output:

func EncryptOAEP 

func EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, label []byte) (out []byte, err error)

EncryptOAEP encrypts the given message with RSA-OAEP.

OAEP is parameterised by a hash function that is used as a random oracle. Encryption and decryption of a given message must use the same hash function and sha256.New() is a reasonable choice.

The random parameter is used as a source of entropy to ensure that encrypting the same message twice doesn't result in the same ciphertext.

The label parameter may contain arbitrary data that will not be encrypted, but which gives important context to the message. For example, if a given public key is used to decrypt two types of messages then distinct label values could be used to ensure that a ciphertext for one purpose cannot be used for another by an attacker. If not required it can be empty.

The message must be no longer than the length of the public modulus less twice the hash length plus 2.

Example 
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)

Output:

func EncryptPKCS1v15 

func EncryptPKCS1v15(rand io.Reader, pub *PublicKey, msg []byte) (out []byte, err error)

EncryptPKCS1v15 encrypts the given message with RSA and the padding scheme from PKCS#1 v1.5. The message must be no longer than the length of the public modulus minus 11 bytes.

The rand parameter is used as a source of entropy to ensure that encrypting the same message twice doesn't result in the same ciphertext.

WARNING: use of this function to encrypt plaintexts other than session keys is dangerous. Use RSA OAEP in new protocols.

func SignPKCS1v15 

func SignPKCS1v15(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []byte) (s []byte, err error)

SignPKCS1v15 calculates the signature of hashed using RSASSA-PKCS1-V1_5-SIGN from RSA PKCS#1 v1.5. Note that hashed must be the result of hashing the input message using the given hash function. If hash is zero, hashed is signed directly. This isn't advisable except for interoperability.

If rand is not nil then RSA blinding will be used to avoid timing side-channel attacks.

This function is deterministic. Thus, if the set of possible messages is small, an attacker may be able to build a map from messages to signatures and identify the signed messages. As ever, signatures provide authenticity, not confidentiality.

Example 
// 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)

Output:

func SignPSS 

func SignPSS(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []byte, opts *PSSOptions) (s []byte, err error)

SignPSS calculates the signature of hashed using RSASSA-PSS [1]. Note that hashed must be the result of hashing the input message using the given hash function. The opts argument may be nil, in which case sensible defaults are used.

func VerifyPKCS1v15 

func VerifyPKCS1v15(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte) (err error)

VerifyPKCS1v15 verifies an RSA PKCS#1 v1.5 signature. hashed is the result of hashing the input message using the given hash function and sig is the signature. A valid signature is indicated by returning a nil error. If hash is zero then hashed is used directly. This isn't advisable except for interoperability.

Example 
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.

Output:

func VerifyPSS 

func VerifyPSS(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte, opts *PSSOptions) error

VerifyPSS verifies a PSS signature. hashed is the result of hashing the input message using the given hash function and sig is the signature. A valid signature is indicated by returning a nil error. The opts argument may be nil, in which case sensible defaults are used.

Types

type CRTValue 

type CRTValue struct {
	Exp   *big.Int // D mod (prime-1).
	Coeff *big.Int // R·Coeff ≡ 1 mod Prime.
	R     *big.Int // product of primes prior to this (inc p and q).
}

CRTValue contains the precomputed Chinese remainder theorem values.

type OAEPOptions 

type OAEPOptions struct {
	// Hash is the hash function that will be used when generating the mask.
	Hash crypto.Hash
	// Label is an arbitrary byte string that must be equal to the value
	// used when encrypting.
	Label []byte
}

OAEPOptions is an interface for passing options to OAEP decryption using the crypto.Decrypter interface.

type PKCS1v15DecryptOptions 

type PKCS1v15DecryptOptions struct {
	// SessionKeyLen is the length of the session key that is being
	// decrypted. If not zero, then a padding error during decryption will
	// cause a random plaintext of this length to be returned rather than
	// an error. These alternatives happen in constant time.
	SessionKeyLen int
}

PKCS1v15DecrypterOpts is for passing options to PKCS#1 v1.5 decryption using the crypto.Decrypter interface.

type PSSOptions 

type PSSOptions struct {
	// SaltLength controls the length of the salt used in the PSS
	// signature. It can either be a number of bytes, or one of the special
	// PSSSaltLength constants.
	SaltLength int

	// Hash, if not zero, overrides the hash function passed to SignPSS.
	// This is the only way to specify the hash function when using the
	// crypto.Signer interface.
	Hash crypto.Hash
}

PSSOptions contains options for creating and verifying PSS signatures.

func (*PSSOptions) HashFunc 

func (pssOpts *PSSOptions) HashFunc() crypto.Hash

HashFunc returns pssOpts.Hash so that PSSOptions implements crypto.SignerOpts.

type PrecomputedValues 

type PrecomputedValues struct {
	Dp, Dq *big.Int // D mod (P-1) (or mod Q-1)
	Qinv   *big.Int // Q^-1 mod P

	// CRTValues is used for the 3rd and subsequent primes. Due to a
	// historical accident, the CRT for the first two primes is handled
	// differently in PKCS#1 and interoperability is sufficiently
	// important that we mirror this.
	CRTValues []CRTValue
}

type PrivateKey 

type PrivateKey struct {
	PublicKey            // public part.
	D         *big.Int   // private exponent
	Primes    []*big.Int // prime factors of N, has >= 2 elements.

	// Precomputed contains precomputed values that speed up private
	// operations, if available.
	Precomputed PrecomputedValues
}

A PrivateKey represents an RSA key

func GenerateKey 

func GenerateKey(random io.Reader, bits int) (priv *PrivateKey, err error)

GenerateKey generates an RSA keypair of the given bit size using the random source random (for example, crypto/rand.Reader).

func GenerateMultiPrimeKey 

func GenerateMultiPrimeKey(random io.Reader, nprimes int, bits int) (priv *PrivateKey, err error)

GenerateMultiPrimeKey generates a multi-prime RSA keypair of the given bit size and the given random source, as suggested in [1]. Although the public keys are compatible (actually, indistinguishable) from the 2-prime case, the private keys are not. Thus it may not be possible to export multi-prime private keys in certain formats or to subsequently import them into other code.

Table 1 in [2] suggests maximum numbers of primes for a given size.

[1] US patent 4405829 (1972, expired) [2] http://www.cacr.math.uwaterloo.ca/techreports/2006/cacr2006-16.pdf

func (*PrivateKey) Decrypt 

func (priv *PrivateKey) Decrypt(rand io.Reader, ciphertext []byte, opts crypto.DecrypterOpts) (plaintext []byte, err error)

Decrypt decrypts ciphertext with priv. If opts is nil or of type *PKCS1v15DecryptOptions then PKCS#1 v1.5 decryption is performed. Otherwise opts must have type *OAEPOptions and OAEP decryption is done.

func (*PrivateKey) Precompute 

func (priv *PrivateKey) Precompute()

Precompute performs some calculations that speed up private key operations in the future.

func (*PrivateKey) Public 

func (priv *PrivateKey) Public() crypto.PublicKey

Public returns the public key corresponding to priv.

func (*PrivateKey) Sign 

func (priv *PrivateKey) Sign(rand io.Reader, msg []byte, opts crypto.SignerOpts) ([]byte, error)

Sign signs msg with priv, reading randomness from rand. If opts is a *PSSOptions then the PSS algorithm will be used, otherwise PKCS#1 v1.5 will be used. This method is intended to support keys where the private part is kept in, for example, a hardware module. Common uses should use the Sign* functions in this package.

func (*PrivateKey) Validate 

func (priv *PrivateKey) Validate() error

Validate performs basic sanity checks on the key. It returns nil if the key is valid, or else an error describing a problem.

type PublicKey 

type PublicKey struct {
	N *big.Int // modulus
	E int      // public exponent
}

A PublicKey represents the public part of an RSA key.

Source Files

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