---
product: Chilkat
product_version: 11.7.0
language: js
language_label: JavaScript
class: Dh
license: bundle
deprecated: false
reference_html: https://www.chilkatsoft.com/refdoc/jsDhRef.html
---
# Chilkat Dh — JavaScript — v11.7.0

> Chilkat 11.7.0 · language=js · class=Dh
> Other versions: replace `11.7.0` in this URL. Other languages: `https://www.chilkatsoft.com/ai-docs/v/11.7.0/<lang>/classes/Dh.md`.
> Language-wide rules (install, unlock, error model, strings/bytes): `https://www.chilkatsoft.com/ai-docs/v/11.7.0/js/guide.md`.

## Purpose

**Perform Diffie-Hellman shared-secret key exchange.**

`Chilkat.Dh` provides the core operations needed for Diffie-Hellman shared-secret exchange. It allows two parties to agree on a shared secret over an insecure channel by using common Diffie-Hellman parameters, generating public exchange values, and computing the same shared secret from the other party's public value.

**Choose known parameters**

Use `UseKnownPrime` when the application wants to use a predefined Diffie-Hellman prime group.

**Generate parameters**

Use `GenPG` to generate Diffie-Hellman parameters when custom prime and generator values are required.

**Set custom P and G**

Use `SetPG` when the prime and generator are already known or have been received from another party or protocol.

**Create the exchange value**

Use `CreateE` to create the public Diffie-Hellman value that is sent to the other party.

**Compute the shared secret**

Use `FindK` with the other party's public exchange value to compute the shared secret.

**Use the result carefully**

The shared secret is typically used as input to a key derivation step before being used for encryption or authentication.

**Common pattern:** Select or generate the Diffie-Hellman parameters, create your public exchange value with `CreateE`, exchange public values with the other party, then call `FindK` to compute the shared secret. The primary methods are `UseKnownPrime`, `GenPG`, `SetPG`, `CreateE`, and `FindK`.

For an extended overview, see [Dh Class Overview](https://www.chilkatsoft.com/dh_class_overview.asp).

## Object creation, unlocking and error handling

**Object Creation**

Note: This is intended for running within a [Chilkat.Js embedded JavaScript engine](https://www.chilkatsoft.com/chilkat_javascript_engine.asp).

```
var obj = new CkDh();
```

Call `Global.UnlockBundle` once at the start of the program, before using this or any other Chilkat class. Any string unlocks a fully functional 30-day trial; a purchased unlock code is used in production. See `guide.md` for the exact code in JavaScript.

How failure is reported depends on the method's return type and is stated in the note under each method below. In every case the diagnostic text is in the `LastErrorText` property of the object whose method was called.

## Members common to every Chilkat class

Documented once per language in `common-members.md`; listed here so this page is self-contained.

| Property | Signature | Access | Meaning |
|---|---|---|---|
| `DebugLogFilePath` | `DebugLogFilePath` | read/write | If set to a file path, this property logs the LastErrorText of each Chilkat method or property call to the specified file. |
| `LastErrorHtml` | `LastErrorHtml` | read-only | Provides HTML-formatted information about the last called method or property. |
| `LastErrorText` | `LastErrorText` | read-only | Provides plain text information about the last called method or property. |
| `LastErrorXml` | `LastErrorXml` | read-only | Provides XML-formatted information about the last called method or property. |
| `LastMethodSuccess` | `LastMethodSuccess` | read/write | Indicates the success or failure of the most recent method call: `true` means success, `false` means failure. |
| `VerboseLogging` | `VerboseLogging` | read/write | If set to `true`, then the contents of LastErrorText (or LastErrorXml, or LastErrorHtml) may contain more verbose information. |
| `Version` | `Version` | read-only | Version of the component/library, such as "10.1.0" |

## Member index

| Member | Kind | Signature |
|---|---|---|
| `G` | property, read-only | `G` |
| `P` | property, read-only | `P` |
| `CreateE` | method | `var strVal = CreateE(numBits);` |
| `FindK` | method | `var strVal = FindK(E);` |
| `GenPG` | method | `var success = GenPG(numBits, G);` |
| `SetPG` | method | `var success = SetPG(p, g);` |
| `UseKnownPrime` | method | `UseKnownPrime(index);` |

## Properties

### Dh.G

`Chilkat 11.7.0 · js · Dh · G · property · read-only`

```javascript
G
```

· `int`, `read-only`

Returns the current Diffie-Hellman generator, `g`. The generator is the small public integer used with the prime modulus `p` in the modular exponentiation operations that create the public exchange value and shared secret.

For this class, `g` is expected to be `2` or `5`. Its value is established by `UseKnownPrime`, `GenPG`, or `SetPG`.

**Public parameter:** The generator is not secret. Both parties must use exactly the same `p` and `g` values for the exchange.

### Dh.P

`Chilkat 11.7.0 · js · Dh · P · property · read-only`

```javascript
P
```

· `string`, `read-only`

Returns the current Diffie-Hellman prime modulus, `p`, as a hex-encoded SSH1-format bignum. The value is established by `UseKnownPrime`, `GenPG`, or `SetPG`.

The modulus is a *safe prime*: `p = 2q + 1`, where `q` is also prime. Safe-prime groups are commonly used for finite-field Diffie-Hellman because their subgroup structure is well understood.

**Encoding note:** The returned string is the hexadecimal encoding of an SSH1 bignum, which includes the SSH1 bit-length prefix. It is not simply the conventional hexadecimal representation of the integer `p`.

**Public parameter:** The prime modulus is not secret and may be transmitted to the other party. Both parties must use the same `p` and `g`.

## Methods

### Dh.CreateE

`Chilkat 11.7.0 · js · Dh · CreateE · method`

```javascript
var strVal = CreateE(numBits);
```

· return: `string`  
· numBits: `int`

Creates this party's public Diffie-Hellman exchange value, conventionally written as `E = gx mod p`. Before calling this method, initialize the common Diffie-Hellman parameters by calling `UseKnownPrime`, `GenPG`, or `SetPG`.

`numBits` (`numBits`) controls the size of the randomly generated private exponent retained internally by this `Dh` object. Chilkat recommends setting it to twice the desired strength of the symmetric key that will ultimately be derived. For example, use `256` when the protocol will derive a 128-bit key.

The returned `E` value is a hex-encoded SSH1-format bignum. It is public and is sent to the other party. The size of the eventual shared-secret integer is determined primarily by `p`; `numBits` does not make the returned shared secret a fixed-length symmetric key.

**Object state:** Call `FindK` on this same `Dh` object after receiving the other party's exchange value. `CreateE` stores the corresponding private value internally, and `FindK` needs it to compute the shared secret. Use a separate `Dh` object for each party and generate a fresh exchange value for each session.

**Authentication required:** Basic Diffie-Hellman does not authenticate either party and is vulnerable to a man-in-the-middle attack. The surrounding protocol must authenticate the exchanged parameters and public values.

For language bindings where a string return cannot directly communicate failure, check `LastMethodSuccess`. If it is `FALSE`, inspect `LastErrorText`.

Returns `null` on failure

### Dh.FindK

`Chilkat 11.7.0 · js · Dh · FindK · method`

```javascript
var strVal = FindK(E);
```

· return: `string`  
· E: `string`

Computes the Diffie-Hellman shared secret from the other party's public exchange value. `E` is the peer's `E` value, encoded as the hexadecimal representation of an SSH1-format bignum.

This method must be called on the same `Dh` object that previously called `CreateE`. Both parties must also be using the same `p` and `g` parameters. Internally, the method computes `K = Ex mod p`, where `x` is the private value generated by `CreateE`.

The result is the shared-secret integer `K`, returned as a hex-encoded SSH1-format bignum. When the exchange is performed correctly, both parties compute the same value.

**Do not use the returned text directly as an encryption key:** It is an encoded bignum, not a uniformly formatted symmetric key. Feed the shared-secret value into the key-derivation procedure required by the protocol, together with any required transcript, salt, labels, or context. Do not invent an ad hoc conversion when interoperability or cryptographic security matters.

**Key agreement is not authentication:** Matching shared secrets do not by themselves prove the identity of the peer. Authenticate the exchange to prevent man-in-the-middle attacks.

For language bindings where a string return cannot directly communicate failure, check `LastMethodSuccess`. If it is `FALSE`, inspect `LastErrorText`.

Returns `null` on failure

### Dh.GenPG

`Chilkat 11.7.0 · js · Dh · GenPG · method`

```javascript
var success = GenPG(numBits, G);
```

· return: `boolean`; `true` for success, `false` for failure  
· numBits: `int`  
· G: `int`

Generates new finite-field Diffie-Hellman parameters and stores them in this object. `numBits` specifies the size of the safe prime `p` in bits, and `G` specifies the generator `g`, which should be `2` or `5`.

On success, the generated values are available through the `P` and `G` properties. The prime generation and validation process is computationally expensive, especially for large groups.

**Prefer standardized groups:** Most applications should use `UseKnownPrime` or parameters mandated by the surrounding protocol instead of generating a new group. Standardized groups improve interoperability and avoid the cost and risk of custom parameter generation.

**Modern key sizes:** The legacy 768-, 1024-, and 1536-bit MODP groups are not appropriate for new designs. Use at least a 2048-bit standardized group when compatibility requires finite-field DH; a 3072-bit group is commonly selected when approximately 128-bit classical security is required.

Returns `TRUE` if the parameters were generated successfully; otherwise returns `FALSE`. Check `LastErrorText` for failure details.

Returns true for success, false for failure.

### Dh.SetPG

`Chilkat 11.7.0 · js · Dh · SetPG · method`

```javascript
var success = SetPG(p, g);
```

· return: `boolean`; `true` for success, `false` for failure  
· p: `string`  
· g: `int`

Sets explicit finite-field Diffie-Hellman parameters for this object. `p` is the prime modulus `p`, supplied as a hex-encoded SSH1-format bignum, and `g` is the generator `g`, normally `2` or `5`.

The method validates that the supplied values satisfy the requirements expected by this Diffie-Hellman implementation. It returns `TRUE` if the parameters are accepted; otherwise it returns `FALSE`.

**Encoding requirement:** `p` is not ordinary integer hex. It must contain the hexadecimal encoding of the SSH1 bignum representation, including its bit-length prefix. The `P` property of another Chilkat `Dh` object is already in the required form.

**Interoperability:** Both parties must use exactly the same `p` and `g`. These values are public, but untrusted or nonstandard parameters should not be accepted unless the protocol explicitly permits them and their origin is authenticated.

Returns true for success, false for failure.

### Dh.UseKnownPrime

`Chilkat 11.7.0 · js · Dh · UseKnownPrime · method`

```javascript
UseKnownPrime(index);
```

· return: `undefined`  
· index: `int`

Selects one of eight built-in safe-prime MODP groups and sets the object's `P` and `G` properties. `index` is a Chilkat selector from `1` through `8`; it is **not** the IKE group number.

| `index` | MODP group | Prime bits | Definition | G |
| :--- | :--- | :--- | :--- | :--- |
| `1` | Group 1 | 768 | RFC 2409, section 6.1 | `2` |
| `2` | Group 2 | 1024 | RFC 2409, section 6.2 | `2` |
| `3` | Group 5 | 1536 | RFC 3526, section 2 | `2` |
| `4` | Group 14 | 2048 | RFC 3526, section 3 | `2` |
| `5` | Group 15 | 3072 | RFC 3526, section 4 | `2` |
| `6` | Group 16 | 4096 | RFC 3526, section 5 | `2` |
| `7` | Group 17 | 6144 | RFC 3526, section 6 | `2` |
| `8` | Group 18 | 8192 | RFC 3526, section 7 | `2` |

**Security guidance:** Selectors `1`, `2`, and `3` are legacy groups and should not be chosen for new systems. Selector `4` is the 2048-bit group 14; selector `5` is the 3072-bit group 15 and is commonly used when approximately 128-bit classical security is required. Always use the group required by the protocol or the communicating peer.

**Performance:** Larger groups require more CPU time and produce larger public values. Choose a group based on the required security level and interoperability requirements, rather than automatically selecting the largest group.