Add API to retreive private key from wallet address

src/Features/Blockcore.Features.Wallet/Api/Controllers/WalletController.cs

@@ -1181,6 +1181,39 @@ public IActionResult GetExtPubKey([FromQuery] GetExtPubKeyModel request)
             }
         }
 
+        /// <summary>
+        /// Gets the private key of a specified wallet address.
+        /// </summary>
+        /// <param name="request">An object containing the necessary parameters to retrieve.</param>
+        /// <param name="cancellationToken">The Cancellation Token</param>
+        /// <returns>A JSON object containing the private key of the address in WIF representation.</returns>
+        /// <response code="200">Returns private key</response>
+        /// <response code="400">Invalid request, or unexpected exception occurred</response>
+        /// <response code="500">Request is null</response>
+        [Route("privatekey")]
+        [HttpPost]
+        public IActionResult RetrievePrivateKey([FromBody] RetrievePrivateKeyModel request)
+        {
+            Guard.NotNull(request, nameof(request));
+
+            // checks the request is valid
+            if (!this.ModelState.IsValid)
+            {
+                return ModelStateErrors.BuildErrorResponse(this.ModelState);
+            }
+
+            try
+            {
+                string result = this.walletManager.RetrievePrivateKey(request.Password, request.WalletName, request.AccountName, request.Address);
+                return this.Json(result);
+            }
+            catch (Exception e)
+            {
+                this.logger.LogError("Exception occurred: {0}", e.ToString());
+                return ErrorHelpers.BuildErrorResponse(HttpStatusCode.BadRequest, e.Message, e.ToString());
+            }
+        }
+
         /// <summary>
         /// Requests the node resyncs from a block specified by its block hash.
         /// Internally, the specified block is taken as the new wallet tip

src/Features/Blockcore.Features.Wallet/Api/Models/RequestModels.cs

@@ -654,6 +654,40 @@ public GetExtPubKeyModel()
         public string AccountName { get; set; }
     }
 
+    /// <summary>
+    /// A class containing the necessary parameters for a private key retrieval request.  
+    /// </summary>
+    public class RetrievePrivateKeyModel : RequestModel
+    {
+        public RetrievePrivateKeyModel()
+        {
+            this.AccountName = WalletManager.DefaultAccount;
+        }
+
+        /// <summary>
+        /// The password for the wallet.
+        /// </summary>
+        [Required]
+        public string Password { get; set; }
+
+        /// <summary>
+        /// The name of the wallet from which to get the private key.
+        /// </summary>
+        [Required]
+        public string WalletName { get; set; }
+
+        /// <summary>
+        /// The name of the account for which to get the private key.
+        /// <summary>
+        public string AccountName { get; set; }
+
+        /// <summary>
+        /// The address to retrieve the private key for.
+        /// </summary>
+        [Required]
+        public string Address { get; set; }
+    }
+
     /// <summary>
     /// A class containing the necessary parameters for a new account request.
     /// </summary>

src/Features/Blockcore.Features.Wallet/Interfaces/IWalletManager.cs

@@ -81,6 +81,16 @@ public interface IWalletManager
         /// <returns>A mnemonic defining the wallet's seed used to generate addresses.</returns>
         Mnemonic CreateWallet(string password, string name, string passphrase = null, Mnemonic mnemonic = null, int? coinType = null);
 
+        /// <summary>
+        /// Gets the private key associated with an address in the wallet.
+        /// </summary>
+        /// <param name="password">The user's password.</param>
+        /// <param name="walletName">The name of the wallet.</param>
+        /// <param name="accountName">The name of the account.</param>
+        /// <param name="address">Address to extract the private key of.</param>
+        /// <returns>The private key associated with the given address, in WIF representation.</returns>
+        string RetrievePrivateKey(string password, string walletName, string accountName, string address);
+
         /// <summary>
         /// Signs a string message.
         /// </summary>

src/Features/Blockcore.Features.Wallet/WalletManager.cs

@@ -319,6 +319,22 @@ public Mnemonic CreateWallet(string password, string name, string passphrase, Mn
             return mnemonic;
         }
 
+        /// <inheritdoc />
+        public string RetrievePrivateKey(string password, string walletName, string accountName, string address)
+        {
+            Guard.NotEmpty(password, nameof(password));
+            Guard.NotEmpty(walletName, nameof(walletName));
+            Guard.NotEmpty(address, nameof(address));
+
+            Types.Wallet wallet = this.GetWallet(walletName);
+
+            // Locate the address based on its base58 string representation.
+            HdAddress hdAddress = wallet.GetAddress(address, account => account.Name.Equals(accountName));
+
+            ISecret privateKey = wallet.GetExtendedPrivateKeyForAddress(password, hdAddress).PrivateKey.GetWif(this.network);
+            return privateKey.ToString();
+        }
+
         /// <inheritdoc />
         public SignMessageResult SignMessage(string password, string walletName, string accountName, string externalAddress, string message)
         {