`package `**rsa**
Import Path
crypto/rsa* (on golang.org and go.dev)*
Dependency Relation
imports 10 packages, and imported by 2 packages
Involved Source Files
pkcs1v15.go
pss.go
Package rsa implements RSA encryption as specified in PKCS #1 and RFC 8017.
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 2, 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 necessary, 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 type implements the
Decrypter and Signer interfaces from the crypto package.
The RSA operations in this package are not implemented using constant-time algorithms.
Code Examples
{
ciphertext, _ := hex.DecodeString("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")
label := []byte("orders")
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))
}
{
rng := rand.Reader
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 {
fmt.Fprintf(os.Stderr, "Error from RSA decryption: %s\n", err)
return
}
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 {
fmt.Fprintf(os.Stderr, "Error decrypting: %s\n", err)
return
}
fmt.Printf("Plaintext: %s\n", string(plaintext))
}
{
secretMessage := []byte("send reinforcements, we're going to advance")
label := []byte("orders")
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
}
fmt.Printf("Ciphertext: %x\n", ciphertext)
}
{
rng := rand.Reader
message := []byte("message to be signed")
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)
}
{
message := []byte("message to be signed")
signature, _ := hex.DecodeString("ad2766728615cc7a746cc553916380ca7bfa4f8983b990913bc69eb0556539a350ff0f8fe65ddfd3ebe91fe1c299c2fac135bc8c61e26be44ee259f2f80c1530")
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
}
}
Package-Level Type Names* (total 7)*
CRTValue contains the precomputed Chinese remainder theorem values.
// R·Coeff ≡ 1 mod Prime.
// D mod (prime-1).
// product of primes prior to this (inc p and q).
OAEPOptions is an interface for passing options to OAEP decryption using the
crypto.Decrypter interface.
Hash is the hash function that will be used when generating the mask.
Label is an arbitrary byte string that must be equal to the value
used when encrypting.
PKCS1v15DecrypterOpts is for passing options to PKCS #1 v1.5 decryption using
the crypto.Decrypter interface.
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.
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.
// D mod (P-1) (or mod Q-1)
// D mod (P-1) (or mod Q-1)
// Q^-1 mod P
A PrivateKey represents an RSA key
// private exponent
Precomputed contains precomputed values that speed up private
operations, if available.
// prime factors of N, has >= 2 elements.
// public part.
// public exponent
// modulus
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.
Equal reports whether priv and x have equivalent values. It ignores
Precomputed values.
Precompute performs some calculations that speed up private key operations
in the future.
Public returns the public key corresponding to priv.
Sign signs digest 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. digest must be the result of hashing the input message using
opts.HashFunc().
This method implements crypto.Signer, which is an interface 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 directly.
Size returns the modulus size in bytes. Raw signatures and ciphertexts
for or by this public key will have the same size.
Validate performs basic sanity checks on the key.
It returns nil if the key is valid, or else an error describing a problem.
*T : crypto.Decrypter
*T : crypto.Signer
func GenerateKey(random io.Reader, bits int) (***PrivateKey**, error)
func GenerateMultiPrimeKey(random io.Reader, nprimes int, bits int) (***PrivateKey**, error)
func crypto/x509.ParsePKCS1PrivateKey(der []byte) (***PrivateKey**, error)
func DecryptOAEP(hash hash.Hash, random io.Reader, priv ***PrivateKey**, ciphertext []byte, label []byte) ([]byte, error)
func DecryptPKCS1v15(rand io.Reader, priv ***PrivateKey**, ciphertext []byte) ([]byte, error)
func DecryptPKCS1v15SessionKey(rand io.Reader, priv ***PrivateKey**, ciphertext []byte, key []byte) error
func SignPKCS1v15(rand io.Reader, priv ***PrivateKey**, hash crypto.Hash, hashed []byte) ([]byte, error)
func SignPSS(rand io.Reader, priv ***PrivateKey**, hash crypto.Hash, digest []byte, opts *PSSOptions) ([]byte, error)
func crypto/x509.MarshalPKCS1PrivateKey(key ***PrivateKey**) []byte
PSSOptions contains options for creating and verifying PSS signatures.
Hash is the hash function used to generate the message digest. If not
zero, it overrides the hash function passed to SignPSS. It's required
when using PrivateKey.Sign.
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.
HashFunc returns opts.Hash so that PSSOptions implements crypto.SignerOpts.
*T : crypto.SignerOpts
func SignPSS(rand io.Reader, priv *PrivateKey, hash crypto.Hash, digest []byte, opts ***PSSOptions**) ([]byte, error)
func VerifyPSS(pub *PublicKey, hash crypto.Hash, digest []byte, sig []byte, opts ***PSSOptions**) error
A PublicKey represents the public part of an RSA key.
// public exponent
// modulus
Equal reports whether pub and x have the same value.
Size returns the modulus size in bytes. Raw signatures and ciphertexts
for or by this public key will have the same size.
func crypto/x509.ParsePKCS1PublicKey(der []byte) (***PublicKey**, error)
func EncryptOAEP(hash hash.Hash, random io.Reader, pub ***PublicKey**, msg []byte, label []byte) ([]byte, error)
func EncryptPKCS1v15(rand io.Reader, pub ***PublicKey**, msg []byte) ([]byte, error)
func VerifyPKCS1v15(pub ***PublicKey**, hash crypto.Hash, hashed []byte, sig []byte) error
func VerifyPSS(pub ***PublicKey**, hash crypto.Hash, digest []byte, sig []byte, opts *PSSOptions) error
func crypto/x509.MarshalPKCS1PublicKey(key ***PublicKey**) []byte
Package-Level Functions* (total 11)*
DecryptOAEP decrypts ciphertext using 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, 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.
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.
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.
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 encrypt 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 minus
twice the hash length, minus a further 2.
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.
GenerateKey generates an RSA keypair of the given bit size using the
random source random (for example, crypto/rand.Reader).
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
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.
SignPSS calculates the signature of digest using PSS.
digest 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. If opts.Hash is set, it overrides hash.
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.
VerifyPSS verifies a PSS signature.
A valid signature is indicated by returning a nil error. digest 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. opts.Hash is
ignored.
Package-Level Variables* (total 3)*
ErrDecryption represents a failure to decrypt a message.
It is deliberately vague to avoid adaptive attacks.
ErrMessageTooLong is returned when attempting to encrypt a message which is
too large for the size of the public key.
ErrVerification represents a failure to verify a signature.
It is deliberately vague to avoid adaptive attacks.
Package-Level Constants* (total 2)*
PSSSaltLengthAuto causes the salt in a PSS signature to be as large
as possible when signing, and to be auto-detected when verifying.
PSSSaltLengthEqualsHash causes the salt length to equal the length
of the hash used in the signature.

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