Generate Diffie-hellman With Keys Openssl C

That's what is done for encryption algorithms such as RSA. But Diffie-Hellman is a key exchange algorithm, not an encryption algorithm. DH isn't for doing asymmetric encryption or signature, it generates a shared secret. The shared secret can be used for encryption, but then it's symmetric encryption (e.g.

The Diffie-Hellman algorithm provides the capability for two communicating parties to agree upon a shared secret between them. Its an agreement scheme because both parties add material used to derive the key (as opposed to transport, where one party selects the key). The shared secret can then be used as the basis for some encryption key to be used for further communication.

If Alice and Bob wish to communicate with each other, they first agree between them a large prime number p, and a generator (or base) g (where 0 < g < p).

Diffie Hellman Algorithm

I think the point is that you need to load Diffie-Hellman parameters before you can generate a DH key. DH parameters define the playground in which you'll be able to generate keys and perform a key exchange. (You need to make sure both parties are using the same parameters.).
Generating DH parameters. The first task for setting up Diffie-Hellman is to generate a set of DH parameters that consist of two numbers p (a large prime number) and g (the generator value, which is always 2 for OpenSSL). Parameter generation is CPU expensive, and is therefore normally done once in advance.
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.
Python M2Crypto - generating a DSA key pair and separating public/private components; Generate EC Diffie-Hellman public and private key pair; Generate Private and Public key OpenSSL; Can't set public/private key in OpenSSL; Private and public key separately; public key cryptograpy with Java and OpenSSL; Private key parsing with PemReader.
Create a Diffie-Hellman public key BLOB by calling the CryptExportKey function, passing PUBLICKEYBLOB in the dwBlobType parameter and the handle to the Diffie-Hellman key in the hKey parameter. This function call causes the calculation of the public key value, (G^X) mod P.

Alice chooses a secret integer a (her private key) and then calculates g^a mod p (which is her public key).Bob chooses his private key b, and calculates his public key in the same way.

Alice and Bob then send each other their public keys. Alice now knows a and Bob's public key g^b mod p. She is not able to calculate the value b from Bob's public key as this is a hard mathematical problem (known as the discrete logarithm problem). She can however calculate (g^b)^a mod p = g^ab mod p.

Bob knows b and g^a, so he can calculate (g^a)^b mod p = g^ab mod p. Therefore both Alice and Bob know a shared secret g^ab mod p. Eve who was listening in on the communication knows p, g, Alice's public key (g^a mod p) and Bob's public key (g^b mod p). She is unable to calculate the shared secret from these values.

In static-static mode both Alice and Bob retain their private/public keys over multiple communications. Therefore the resulting shared secret will be the same every time. In ephemeral-static mode one party will generate a new private/public key every time, thus a new shared secret will be generated.

Diffie-Hellman Standards[edit]

There are a number of standards relevant to Diffie-Hellman key agreement. Some of the key ones are:

PKCS 3 defines the basic algorithm and data formats to be used.
ANSI X9.42 is a later standard than PKCS 3 and provides further guidance on its use (note OpenSSL does not support ANSI X9.42 in the released versions - support is available in the as yet unreleased 1.0.2 and 1.1.0)
RFC 2631 summarizes the key points of ANSI X9.42
RFC 5114 defines 3 standard sets of parameters for use with Diffie-Hellman (OpenSSL will have built-in support for these parameters from OpenSSL 1.0.2 - not yet released)
NIST SP 800-56A is a NIST publication that provides recommendations on the implementation of X9.42

Diffie-Hellman in SSL/TLS[edit]

There are three versions of Diffie-Hellman used in SSL/TLS.

Anonymous Diffie-Hellman
Fixed Diffie-Hellman
Ephemeral Diffie-Hellman

Anonymous Diffie-Hellman uses Diffie-Hellman, but without authentication. Because the keys used in the exchange are not authenticated, the protocol is susceptible to Man-in-the-Middle attacks. Note: if you use this scheme, a call to SSL_get_peer_certificate will return NULL because you have selected an anonymous protocol. This is the only time SSL_get_peer_certificate is allowed to return NULL under normal circumstances.

Generate Diffie-hellman With Keys Openssl C

You should not use Anonymous Diffie-Hellman. You can prohibit its use in your code by using '!ADH' in your call to SSL_set_cipher_list.

Fixed Diffie-Hellman embeds the server's public parameter in the certificate, and the CA then signs the certificate. That is, the certificate contains the Diffie-Hellman public-key parameters, and those parameters never change.

Ephemeral Diffie-Hellman uses temporary, public keys. Each instance or run of the protocol uses a different public key. The authenticity of the server's temporary key can be verified by checking the signature on the key. Because the public keys are temporary, a compromise of the server's long term signing key does not jeopardize the privacy of past sessions. This is known as Perfect Forward Secrecy (PFS).

You should always use Ephemeral Diffie-Hellman because it provides PFS. You can specify ephemeral methods by providing 'kEECDH:kEDH' in your call to SSL_set_cipher_list.

Working with Parameters and Generating Keys[edit]

The first step with the Diffie-Hellman algorithm is to ensure that both parties are using the same set of parameters (i.e. the same values for p and g). Since parameter generation can be an expensive process this is normally done once in advance and then the same set of parameters are used over many key exchanges. A new set of parameters can be generated by OpenSSL, or alternatively there is support for built-in standard sets of parameters.

To generate your own parameters refer to EVP Key and Parameter Generation.

Note: The function DH_get_2048_256 is scheduled for release in OpenSSL 1.0.2; it is not available in 1.0.1e or earlier.

Generating a Shared Secret[edit]

Having obtained a private/public key pair you need to also obtain the public key of the other communicating party. Refer to EVP Key Agreement for information on how to agree a shared secret.

Using the Low Level APIs[edit]

Diffie Hellman Algorithm Example

Users of the OpenSSL library are expected to normally use the EVP method for working with Diffie Hellman as described above and on the EVP Key Agreement page. The EVP api is implemented by a lower level Diffie Hellman 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. The manual page for the low level API is available here: Manual:dh(3)

There are (currently) no DH_ level routines to read and writea public OR private key, but the generic PUBKEY andPrivateKey routines do so as an X.509 SubjectPublickKeyInfo structure (aka SPKI or PKCS#8). This includes the parameters plus the public key (and the private key for the PrivateKey routines) (see Manual:Pem(3)).