Nov 22, 2019 eccrypto: isomorphic implementation of ECDSA, ECDH and ECIES for both browserify and node (uses elliptic for browser and secp256k1-node for node) LICENSE This software is licensed under the MIT License. Please delete colons ':' and new lines for the private key and the public key and fill 'EC private key (hex)' and 'EC public key (hex)' in above form and choose proper.
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Elliptic Curve Diffie Hellman (ECDH) is an Elliptic Curve variant of the standard Diffie Hellman algorithm. See Elliptic Curve Cryptography for an overview of the basic concepts behind Elliptic Curve algorithms.
ECDH is used for the purposes of key agreement. Suppose two people, Alice and Bob, wish to exchange a secret key with each other. Alice will generate a private key dA and a public key QA=dAG (where G is the generator for the curve). Similarly Bob has his private key dB and a public key QB=dBG. If Bob sends his public key to Alice then she can calculate dAQB=dAdBG. Similarly if Alice sends her public key to Bob, then he can calculate dbQA=dAdBG. The shared secret is the x co-ordinate of the calculated point dAdBG. Any eavesdropper would only know QA and QB, and would be unable to calculate the shared secret.
Using ECDH in OpenSSL[edit]
In order for two peers to exchange a shared secret they need to first agree on the parameters to be used. In Elliptic Curve Cryptography this is typically done through the use of named curves. A named curve is simply a well defined and well known set of parameters that define an elliptic curve. OpenSSL has support for a wide variety of different well known named curves. In the example below the ANSI X9.62 Prime 256v1 curve is used.
The example below shows how to set up the parameters based on the use of a named curve, how to generate a public/private key pair for those parameters and subsequently how to derive a shared secret. The details of how to obtain the other party's key (the peer key) are omitted, as this is specific to your particular situation. Note that you do not necessarily need to generate a new private/public key pair for every exchange (although you may choose to do so). Also note that the derived shared secret is not suitable for use directly as a shared key. Typically the shared secret is passed through some hash function first in order to generate a key.
See below for the example code.
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You should also refer to the EVP Key Agreement page for general information on the key agreement API in OpenSSL.
Using the Low Level APIs[edit]
Users of the OpenSSL library are expected to normally use the EVP method for working with Elliptic Curve Diffie Hellman as described above and on the EVP Key Agreement page. The EVP API is implemented by a lower level ECDH API. In some circumstances, expert users may need to use the low level API. This is not recommended for most users. However, if you need to use this then an example of use is shown below.
As noted in the high level EVP section of this page, you should never use a shared secret directly. It must be passed through some form of key derivation function (KDF) first. The last argument to
ECDH_compute_key
can optionally pass a function pointer for such a KDF. The shared secret will then be passed through this function and the value returned in the output buffer will be suitable for direct use as a key.The function below is taken from
apps/speed.c
in the OpenSSL codebase, and shows an example of a KDF based on the hash function SHA1.SHA1 may not be appropriate if the key length required is longer than the number of bits provided as output from the hash function. A standards based KDF which can be used to derive longer keys is described in: http://www.secg.org/collateral/sec1.pdf (see section 3.6.1)
ECDH and Named Curves[edit]
If you want to save a key and later load it with SSL_CTX_use_PrivateKey_file, then you must set the OPENSSL_EC_NAMED_CURVE flag on the key. You do that by calling EC_KEY_set_asn1_flag(ecKey, OPENSSL_EC_NAMED_CURVE). Failure to do so will result in a SSL error of 0x1408a0c1 (no shared cipher) at the server.
As an example, the following creates a elliptic curve key and saves it using a named curve rather than an expanded list of group paramters:
If you want to detect the flags after reading a key or certificate from disk, then use the following code:
The certificates below were dumped with openssl x509 -in server-ecdsa-cert.pem -text -noout. The certificate on the left was created with a key using OPENSSL_EC_NAMED_CURVE, while the certificate on the right was not. Notice the certificate on the left includes ASN1 OID: prime256v1. The certificate on the left can be used with SSL server using ECDSA, but the certificate on the right cannot because it will result in 0x1408a0c1 at the server.
Figure 1: Key with OPENSSL_EC_NAMED_CURVE | Figure 2: Key without OPENSSL_EC_NAMED_CURVE |
If you use a key or certificate without without the OPENSSL_EC_NAMED_CURVE flag (i.e., one that looks like the image on the right), then the SSL connection will fail with the following symptoms:
Note that OpenSSL's X509_verify, X509_verify_cert, SSL_CTX_check_private_key, SSL_CTX_use_PrivateKey_file, and SSL_CTX_use_certificate_chain_file will not return a failure when using a key or certificate in the wrong format.
See also[edit]
Retrieved from 'https://wiki.openssl.org/index.php?title=Elliptic_Curve_Diffie_Hellman&oldid=1558'
-->Definition
Provides a Cryptography Next Generation (CNG) implementation of the Elliptic Curve Diffie-Hellman (ECDH) algorithm. This class is used to perform cryptographic operations.
ECDiffieHellmanCng
Examples
The following example shows how to use the ECDiffieHellmanCng class to establish a key exchange and how to use that key to encrypt a message that can be sent over a public channel and decrypted by the receiver.
Remarks
The ECDiffieHellmanCng class enables two parties to exchange private key material even if they are communicating through a public channel. Both parties can calculate the same secret value, which is referred to as the secret agreement in the managed Diffie-Hellman classes. The secret agreement can then be used for a variety of purposes, including as a symmetric key. However, instead of exposing the secret agreement directly, the ECDiffieHellmanCng class does some post-processing on the agreement before providing the value. This post processing is referred to as the key derivation function (KDF); you can select which KDF you want to use and set its parameters through a set of properties on the instance of the Diffie-Hellman object.
Key derivation function | Properties |
---|---|
Hash | HashAlgorithm - The hash algorithm that is used to process the secret agreement. SecretPrepend - An optional byte array to prepend to the secret agreement before hashing it. SecretAppend - An optional byte array to append to the secret agreement before hashing it. |
Hmac | HashAlgorithm - The hash algorithm that is used to process the secret agreement. SecretPrepend- An optional byte array to prepend to the secret agreement before hashing it. SecretAppend - An optional byte array to append to the secret agreement before hashing it. |
Tls | Label - The label for key derivation. Seed - The seed for key derivation. |
The result of passing the secret agreement through the key derivation function is a byte array that may be used as key material for your application. The number of bytes of key material generated is dependent on the key derivation function; for example, SHA-256 will generate 256 bits of key material, whereas SHA-512 will generate 512 bits of key material.The basic flow of an ECDH key exchange is as follows:
- Alice and Bob create a key pair to use for the Diffie-Hellman key exchange operation
- Alice and Bob configure the KDF using parameters the agree on.
- Alice sends Bob her public key.
- Bob sends Alice his public key.
- Alice and Bob use each other's public keys to generate the secret agreement, and apply the KDF to the secret agreement to generate key material.
Constructors
ECDiffieHellmanCng() | Initializes a new instance of the ECDiffieHellmanCng class with a random key pair. |
ECDiffieHellmanCng(CngKey) | Initializes a new instance of the ECDiffieHellmanCng class by using the specified CngKey object. |
ECDiffieHellmanCng(ECCurve) | Creates a new instance of the ECDiffieHellmanCng class whose public/private key pair is generated over the specified curve. |
ECDiffieHellmanCng(Int32) | Initializes a new instance of the ECDiffieHellmanCng class with a random key pair, using the specified key size. |
Fields
KeySizeValue | Represents the size, in bits, of the key modulus used by the asymmetric algorithm. (Inherited from AsymmetricAlgorithm) |
LegalKeySizesValue | Specifies the key sizes that are supported by the asymmetric algorithm. (Inherited from AsymmetricAlgorithm) |
Properties
HashAlgorithm | Gets or sets the hash algorithm to use when generating key material. |
HmacKey | Gets or sets the Hash-based Message Authentication Code (HMAC) key to use when deriving key material. |
Key | Specifies the CngKey that is used by the current object for cryptographic operations. |
KeyDerivationFunction | Gets or sets the key derivation function for the ECDiffieHellmanCng class. |
KeyExchangeAlgorithm | Gets the name of the key exchange algorithm. (Inherited from ECDiffieHellman) |
KeySize | Gets or sets the size, in bits, of the key modulus used by the asymmetric algorithm. |
Label | Gets or sets the label value that is used for key derivation. |
LegalKeySizes | Gets the key sizes that are supported by the asymmetric algorithm. (Inherited from AsymmetricAlgorithm) |
PublicKey | Gets the public key that can be used by another ECDiffieHellmanCng object to generate a shared secret agreement. |
SecretAppend | Gets or sets a value that will be appended to the secret agreement when generating key material. |
SecretPrepend | Gets or sets a value that will be added to the beginning of the secret agreement when deriving key material. |
Seed | Gets or sets the seed value that will be used when deriving key material. |
SignatureAlgorithm | Gets the name of the signature algorithm. (Inherited from ECDiffieHellman) |
UseSecretAgreementAsHmacKey | Gets a value that indicates whether the secret agreement is used as a Hash-based Message Authentication Code (HMAC) key to derive key material. |
Methods
Clear() | Releases all resources used by the AsymmetricAlgorithm class. (Inherited from AsymmetricAlgorithm) |
DeriveKeyFromHash(ECDiffieHellmanPublicKey, HashAlgorithmName) | Performs key derivation using a specified hash algorithm. (Inherited from ECDiffieHellman) |
DeriveKeyFromHash(ECDiffieHellmanPublicKey, HashAlgorithmName, Byte[], Byte[]) | Performs key derivation using a specified hash algorithm with optional prepended or appended data. |
DeriveKeyFromHmac(ECDiffieHellmanPublicKey, HashAlgorithmName, Byte[]) | Performs key derivation using a specified HMAC (Hash-based Message Authentication Code) algorithm. (Inherited from ECDiffieHellman) |
DeriveKeyFromHmac(ECDiffieHellmanPublicKey, HashAlgorithmName, Byte[], Byte[], Byte[]) | Performs key derivation using a specified HMAC (Hash-based Message Authentication Code) algorithm with optional prepended or appended data. |
DeriveKeyMaterial(CngKey) | Derives the key material that is generated from the secret agreement between two parties, given a CngKey object that contains the second party's public key. |
DeriveKeyMaterial(ECDiffieHellmanPublicKey) | Derives the key material that is generated from the secret agreement between two parties, given an ECDiffieHellmanPublicKey object that contains the second party's public key. |
DeriveKeyTls(ECDiffieHellmanPublicKey, Byte[], Byte[]) | Performs key derivation using the TLS (Transport Layer Security) 1.1 PRF (Pseudo-Random Function). |
DeriveSecretAgreementHandle(CngKey) | Gets a handle to the secret agreement generated between two parties, given a CngKey object that contains the second party's public key. |
DeriveSecretAgreementHandle(ECDiffieHellmanPublicKey) | Gets a handle to the secret agreement generated between two parties, given an ECDiffieHellmanPublicKey object that contains the second party's public key. |
Dispose() | Releases all resources used by the current instance of the AsymmetricAlgorithm class. (Inherited from AsymmetricAlgorithm) |
Dispose(Boolean) | Releases the unmanaged resources used by the AsymmetricAlgorithm class and optionally releases the managed resources. (Inherited from AsymmetricAlgorithm) |
Equals(Object) | Determines whether the specified object is equal to the current object. (Inherited from Object) |
ExportECPrivateKey() | Exports the current key in the ECPrivateKey format. Avast pro 2015 key generator. (Inherited from ECDiffieHellman) |
ExportEncryptedPkcs8PrivateKey(ReadOnlySpan<Byte>, PbeParameters) | Exports the current key in the PKCS#8 EncryptedPrivateKeyInfo format with a byte-based password. (Inherited from AsymmetricAlgorithm) |
ExportEncryptedPkcs8PrivateKey(ReadOnlySpan<Char>, PbeParameters) | Exports the current key in the PKCS#8 EncryptedPrivateKeyInfo format with a char-based password. (Inherited from AsymmetricAlgorithm) |
ExportExplicitParameters(Boolean) | Exports the key and explicit curve parameters used by the ECCurve object into an ECParameters object. |
ExportParameters(Boolean) | Exports the key used by the ECCurve object into an ECParameters object. |
ExportPkcs8PrivateKey() | Exports the current key in the PKCS#8 PrivateKeyInfo format. (Inherited from AsymmetricAlgorithm) |
ExportSubjectPublicKeyInfo() | Exports the public-key portion of the current key in the X.509 SubjectPublicKeyInfo format. (Inherited from AsymmetricAlgorithm) |
FromXmlString(String) | This method is not implemented. |
FromXmlString(String, ECKeyXmlFormat) | Deserializes the key information from an XML string by using the specified format. |
GenerateKey(ECCurve) | Generates a new ephemeral public/private key pair for the specified curve. |
GetHashCode() | Serves as the default hash function. (Inherited from Object) |
GetType() | Gets the Type of the current instance. (Inherited from Object) |
ImportECPrivateKey(ReadOnlySpan<Byte>, Int32) | Imports the public/private keypair from an ECPrivateKey structure, replacing the keys for this object. (Inherited from ECDiffieHellman) |
ImportEncryptedPkcs8PrivateKey(ReadOnlySpan<Byte>, ReadOnlySpan<Byte>, Int32) | Imports the public/private keypair from a PKCS#8 EncryptedPrivateKeyInfo structure after decrypting with a byte-based password, replacing the keys for this object. (Inherited from ECDiffieHellman) |
ImportEncryptedPkcs8PrivateKey(ReadOnlySpan<Char>, ReadOnlySpan<Byte>, Int32) | Imports the public/private keypair from a PKCS#8 EncryptedPrivateKeyInfo structure after decrypting with a char-based password, replacing the keys for this object. 40 bit encryption key generator. (Inherited from ECDiffieHellman) |
ImportParameters(ECParameters) | Imports the specified parameters for an ECCurve object as a key into the current instance. |
ImportPkcs8PrivateKey(ReadOnlySpan<Byte>, Int32) | Imports the public/private keypair from a PKCS#8 PrivateKeyInfo structure after decryption, replacing the keys for this object. (Inherited from ECDiffieHellman) |
ImportSubjectPublicKeyInfo(ReadOnlySpan<Byte>, Int32) | Imports the public key from an X.509 SubjectPublicKeyInfo structure after decryption, replacing the keys for this object. (Inherited from ECDiffieHellman) |
MemberwiseClone() | Creates a shallow copy of the current Object. (Inherited from Object) |
ToString() | Returns a string that represents the current object. (Inherited from Object) |
ToXmlString(Boolean) | This method is not implemented. |
ToXmlString(ECKeyXmlFormat) | Serializes the key information to an XML string by using the specified format. |
TryExportECPrivateKey(Span<Byte>, Int32) | Attempts to export the current key in the ECPrivateKey format into a provided buffer. (Inherited from ECDiffieHellman) |
TryExportEncryptedPkcs8PrivateKey(ReadOnlySpan<Byte>, PbeParameters, Span<Byte>, Int32) | Attempts to export the current key in the PKCS#8 EncryptedPrivateKeyInfo format into a provided buffer, using a byte-based password. (Inherited from ECDiffieHellman) |
TryExportEncryptedPkcs8PrivateKey(ReadOnlySpan<Char>, PbeParameters, Span<Byte>, Int32) | Attempts to export the current key in the PKCS#8 EncryptedPrivateKeyInfo format into a provided buffer, using a char-based password. (Inherited from ECDiffieHellman) |
TryExportPkcs8PrivateKey(Span<Byte>, Int32) | Attempts to export the current key in the PKCS#8 PrivateKeyInfo format into a provided buffer. (Inherited from ECDiffieHellman) |
TryExportSubjectPublicKeyInfo(Span<Byte>, Int32) | Attempts to export the current key in the X.509 SubjectPublicKeyInfo format into a provided buffer. (Inherited from ECDiffieHellman) |
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Explicit Interface Implementations
IDisposable.Dispose() | For a description of this member, see Dispose(). (Inherited from AsymmetricAlgorithm) |