rsa

func DecryptOAEP ¶

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

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 is legacy and ignored, and it can be nil.

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

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", 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(random io.Reader, priv *PrivateKey, ciphertext []byte) ([]byte, error)

DecryptPKCS1v15 decrypts a plaintext using RSA and the padding scheme from PKCS #1 v1.5. The random parameter is legacy and ignored, and it can be nil.

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(random io.Reader, priv *PrivateKey, ciphertext []byte, key []byte) error

DecryptPKCS1v15SessionKey decrypts a session key using RSA and the padding scheme from PKCS #1 v1.5. The random parameter is legacy and ignored, and it can be nil.

DecryptPKCS1v15SessionKey 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.

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 also defeats the point of this function. Using at least a 16-byte key will protect against this attack.

This method implements protections against Bleichenbacher chosen ciphertext attacks [0] described in RFC 3218 Section 2.3.2 [1]. While these protections make a Bleichenbacher attack significantly more difficult, the protections are only effective if the rest of the protocol which uses DecryptPKCS1v15SessionKey is designed with these considerations in mind. In particular, if any subsequent operations which use the decrypted session key leak any information about the key (e.g. whether it is a static or random key) then the mitigations are defeated. This method must be used extremely carefully, and typically should only be used when absolutely necessary for compatibility with an existing protocol (such as TLS) that is designed with these properties in mind.

• [0] “Chosen Ciphertext Attacks Against Protocols Based on the RSA Encryption Standard PKCS #1”, Daniel Bleichenbacher, Advances in Cryptology (Crypto '98)
• [1] RFC 3218, Preventing the Million Message Attack on CMS, https://www.rfc-editor.org/rfc/rfc3218.html

// 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", plaintext)

Output:

func EncryptOAEP ¶

func EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, label []byte) ([]byte, 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. Most applications should use crypto/rand.Reader as random.

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.

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)

Output:

func EncryptPKCS1v15 ¶

func EncryptPKCS1v15(random io.Reader, pub *PublicKey, msg []byte) ([]byte, 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 random parameter is used as a source of entropy to ensure that encrypting the same message twice doesn't result in the same ciphertext. Most applications should use crypto/rand.Reader as random. Note that the returned ciphertext does not depend deterministically on the bytes read from random, and may change between calls and/or between versions.

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

func SignPKCS1v15 ¶

func SignPKCS1v15(random io.Reader, priv *PrivateKey, hash crypto.Hash, hashed []byte) ([]byte, 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.

The random parameter is legacy and ignored, and it can be nil.

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.

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)

Output:

func SignPSS ¶ added in go1.2

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

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.

The signature is randomized depending on the message, key, and salt size, using bytes from rand. Most applications should use crypto/rand.Reader as rand.

func VerifyPKCS1v15 ¶

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

The inputs are not considered confidential, and may leak through timing side channels, or if an attacker has control of part of the inputs.

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.

Output:

func VerifyPSS ¶ added in go1.2

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

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.

The inputs are not considered confidential, and may leak through timing side channels, or if an attacker has control of part of the inputs.